YODAR,

The "TRICK" to TV 'type' Coax Cable [Shielded] SWL Loop Antennas {RHF}
http://groups.yahoo.com/group/Shortw...a/message/1626

I too tried to build a Coax Cable [Shielded] Loop Antenna
that was made of "TV type Coax" that had a 'Shield' made
of Foil and about 33% Braid. The Results were very poor.

Then I Got An IDEA "*" ! { K I S S }

Why not use "Pre-Made" Parts ) WHAT ?

* One Piece of TV type Coax Cable with the "F" Connectors
on both Ends. This can should be these Pre-Made Lengths
Three Feet (3 Ft); Six Feet (6 Ft); Nine Feet (9 Ft);
Twelve Feet (12 Ft); and Fifteen Feet (15 Ft); ETC.

* One "F" Connector to PL-259 Plug (SO-239 Adapter)
RadioShack Catalog # 278-258

* One TV type Two-to-One Splitter with "F" Connectors.
Get the one with the two Output "F" Connectors on
'opposite' Sides and the Input "F" Connector on the
Bottom (looks like a 'T').

THE SECRET IS TO BUILD A NEXUS LOOP-HUB:
Using the Two-to-One Splitter. Remove the 'Seal' Plate
from the Body of the Splitter. Remove the Ferrite Core
and Wires from inside the Splitter. Next clean a 'Spot'
inside the Body of the Splitter for a Solder Ground-Point.
Use a Solder Gun (150W) to make this Solder Ground-Point;
get in and get out quickly so that you do not melt the
plastic insulation inside the "F" Connectors. Wrap a
Wire around the Left "F" Connector Terminal. Route this
Wire to the Solder Ground-Point on the Body of the Splitter.
Using a Soldering Iron (35W) and Solder the Left Terminal
and the Ground-Point. Next 'wrap' a Wire around the
Bottom "F" Connector Terminal; and 'route' the Wire to
the Right "F" Connector Terminal. Then you need to make
a 'new' "Metal Cover" for the Splitter. I used one of
those old AOL CD "Tins" that come in the Mail. They are
thin enough to 'cut' with a Heavy Pair of Scissors .
Cut and Trim a piece until it "Fits". Ensure your have
good Metal-to-Metal 'contact' between the Cover and
the Spliter and Glue them together for Security. You
now have the NEXUS "LOOP-HUB" of your Coax Cable Loop
Antenna.

The Three Feet (3 Ft) piece of Coax Cable is your
Feed-in-Line.

MAKING YOUR "INTERCHANGABLE" LOOP ANTENNA ELEMENTS:
The Six Feet (6 Ft); Nine Feet (9 Ft); Twelve Feet
(12 Ft) and Fifteen Feet (15 Ft) Coax Cable are your
[Shielded] Loop Antenna Elements. Fold them End-to-End
and 'mark' a One Inch (1") Area at the Fold as their
Mid-Point. Very Carefully 'cut' the Outer Insulation
and then the Foil and Wire Braid away and out from this
One Inch Area. {Do Not Damage the Inner Insulation and
Center Wire of the Coax Cable.) Tape this One Inch Area
and an Inch on each side for Strength and Security.

TIP: If you have a Plastic Hula-Hoop or some 5/8" or
3/4" (White) Low Pressure Water Line Polyethylene Tubing
as a "Form" to hold the Loops in a Circle Shape. This
Tubing must 'slip-over' the Coax Cable's "F" Connectors.

THE LOOP ANTENNA "HOOK-UP":
* Slide the Coax Cable Loop Antenna Element into the
Plastic Tubing.
* Connect your Coax Cable Loop Antenna Element to the
LOOP-HUB {Splitter}.
* Connect your Coax Cable Feed-in-Line to the LOOP-HUB
{Splitter}.
* Install the "F" Connector to PL-259 Plug to the
'other end' of your Coax Cable Feed-in-Line and Connect
to the your Radio/Receiver.
* Shape and Position your Loop Antenna Element.

CAUTION: When Installing the Coax Cable "F" Connectors
into the "F" Connectors of the LOOP-HUB: Be careful to
make sure that the Center Wire is NOT Bent and Shorted-Out.

SIZING: This is a Low Noise SWL InDoor Loop Antenna that
allows the 'changing' of the Loop Antenna Element for the
Coverage of different Shortwave Bands. These Loops are 'cut'
"Sized" to roughly One Tenth of a Wave Length (1/10WL)
* Twenty-Five Feet (25 Ft) roughly the 120 Meter SW Band
* Twenty Feet (20 Ft) roughly the 90 Meter SW Band
* Fifteen Feet (15 Ft) roughly the 75 Meter SW Band
* Twelve Feet (12 Ft) roughly the 60 Meter SW Band
* Nine Feet (9 Ft) roughly the 41 Meter SW Band
* Six Feet (6 Ft) roughly the 31 Meter SW Band
* Five Feet (5 Ft) roughly the 25 Meter SW Band
* Four Feet (4 Ft) roughly the 19 Meter SW Band
* Three Feet (3 Ft) roughly the 13 Meter SW Band
The above Lengths and SW Bands are 'based' on the low
cost common {RadioShack} TV type RG-59 Coax Cable with
a Velocity Factor of 0.66.
NOTE: If you use RG6 "Quad Shield" Coax Cable that has
a Velocity Factor of 0.78 to 0.82: Then every thing would
shift down one SW Band 120M90M, 90M75M, 75M60M, ETC.

PACK-AND-GO "TRAVEL" LOOP ANTENNA:
This Design-Set-Up makes for a 'portable' Travel SWL Loop
Antenna that can be Taken-Down and Put-Up in a Minute or two.

MEGA-SIZED FIFTY FOOT LOOP FOR THE AM/MW BROADCAST BAND:
Take a Fifty Foot (50 Ft) piece of Coax Cable and Mount
it OutSide Doors
- 540 kHz to 1700 kHz with a MidPoint of 1120 kHz
- Fifty Feet (50 Ft0 [1/10WL] VF=0.66 using RG59 Coax Cable.
- Square shape 12.5 Feet per Side with 17.5 Cross-Arms.
- Round shape 16 Diameter Circle.
- Coax Cable Lead-in-Line 5-10 Feet.
This is roughly the "Sizing" for the AM/MW Broadcast Band
For DX-Peditions throw a 50 Foot Coax Loop Antenna Element
Over a Tree Limb about 15 Feet High and stretch-out the two
bottom side to form an Equilateral Triangle 17 Feet on a side.
Hook-Up everything and sit-down and Listen )


iane ~ RHF
..
Some Say: On A Clear Day You Can See Forever.
http://groups.yahoo.com/group/Shortw...na/message/502
I BELIEVE: On A Clear Night...
You Can Hear Forever and Beyond - The Beyond !
..
..
= = = In ,
= = = "YODAR" yodar@in O'do wrote:
I MADE MY FIRST ONE AND FLOPPED because I used TV Coax
(RG 58?) and there was no braid, it was foil and I
could make no solder connection...ARRGGGG

The RG-8 second try was so ungainly and costly and
unyielding of ANY signal (perhaps I had failed with
a connection) , that I got irrigated (kind way of
saying P***ed OFF!) and threw it out.

If I mess with Coax again it will be for an attic
SNAKE ANTENNA

Yodar in O'do
who is still using SLINKIES

..

"RHF" wrote in message
om...
YODAR,

The "TRICK" to TV 'type' Coax Cable [Shielded] SWL Loop Antennas {RHF}
http://groups.yahoo.com/group/Shortw...a/message/1626

I too tried to build a Coax Cable [Shielded] Loop Antenna
that was made of "TV type Coax" that had a 'Shield' made
of Foil and about 33% Braid. The Results were very poor.

Then I Got An IDEA "*" ! { K I S S }

Why not use "Pre-Made" Parts ) WHAT ?
I used a similar idea for my 3.3' and 7' loops constructed from 3/4" CATV
hardline. A CATV junction box for the termination and wideband preamp ( no
tuning) and the standard 3/4" CATV connectors.

Here's some pix:
http://www.parelectronics.com/pics_new/P1010023.JPG
http://www.parelectronics.com/pics_new/P1010024.JPG 1
meter version
http://www.parelectronics.com/pics_new/P1010025.JPG
http://www.parelectronics.com/pics_new/P1010026.JPG
close up of hub and preamp
http://www.parelectronics.com/pics_new/P1010027.JPG gap
assembly
http://www.parelectronics.com/pics_new/P1010028.JPG 7'
version


--
Dale W4OP
for PAR Electronics, Inc.

(RHF) wrote in message . com...
YODAR,

The "TRICK" to TV 'type' Coax Cable [Shielded] SWL Loop Antennas {RHF}
http://groups.yahoo.com/group/Shortw...a/message/1626

I too tried to build a Coax Cable [Shielded] Loop Antenna
that was made of "TV type Coax" that had a 'Shield' made
of Foil and about 33% Braid. The Results were very poor.

Hummm... It *should* have worked just fine. Besides being lousy to
solder to, there is nothing wrong with a foil shield. Thats solid
coverage, and better than strands of wire. The coax shield for a SW or
MW loop should be fairly non critical. The only purpose is to ensure
balance. MK

In article ,
(Mark Keith) wrote:

(RHF) wrote in message
. com...
YODAR,

The "TRICK" to TV 'type' Coax Cable [Shielded] SWL Loop Antennas {RHF}
http://groups.yahoo.com/group/Shortw...a/message/1626

I too tried to build a Coax Cable [Shielded] Loop Antenna
that was made of "TV type Coax" that had a 'Shield' made
of Foil and about 33% Braid. The Results were very poor.

Hummm... It *should* have worked just fine. Besides being lousy to
solder to, there is nothing wrong with a foil shield. Thats solid
coverage, and better than strands of wire. The coax shield for a SW or
MW loop should be fairly non critical. The only purpose is to ensure
balance. MK

"The Results were very poor" is not very much information to go on to
any conclusion or solution.

Maybe he was expecting more output from the loop than he got. A shielded
loop that is not very large is not going to have a big output. Maybe he
screwed up and did not have a shield gap.

--
Telamon
Ventura, California

GOTCHA!
(printed for use)

Thanks a million

yodar




RHF wrote:
YODAR,

The "TRICK" to TV 'type' Coax Cable [Shielded] SWL Loop Antennas {RHF}
http://groups.yahoo.com/group/Shortw...a/message/1626

I too tried to build a Coax Cable [Shielded] Loop Antenna
that was made of "TV type Coax" that had a 'Shield' made
of Foil and about 33% Braid. The Results were very poor.

Then I Got An IDEA "*" ! { K I S S }

Why not use "Pre-Made" Parts ) WHAT ?

= = = Telamon wrote in message
= = = ...
In article ,
(Mark Keith) wrote:

(RHF) wrote in message
. com...
YODAR,

The "TRICK" to TV 'type' Coax Cable [Shielded] SWL Loop Antennas {RHF}
http://groups.yahoo.com/group/Shortw...a/message/1626

I too tried to build a Coax Cable [Shielded] Loop Antenna
that was made of "TV type Coax" that had a 'Shield' made
of Foil and about 33% Braid. The Results were very poor.

Hummm... It *should* have worked just fine. Besides being lousy to
solder to, there is nothing wrong with a foil shield. Thats solid
coverage, and better than strands of wire. The coax shield for a SW or
MW loop should be fairly non critical. The only purpose is to ensure
balance. MK

"The Results were very poor" is not very much information to go on to
any conclusion or solution.

Maybe he was expecting more output from the loop than he got. A shielded
loop that is not very large is not going to have a big output. Maybe he
screwed up and did not have a shield gap.


MK & TELAMON,

My reason for stating that the performance was 'poor' had to do with
may small problems:

* Learning Curve - It was my first Coax Cable [Shielded] Loop
Antenna.

* Questionable 'quality' of the old RG-59 Coax Cable being used.
[ Learned to use 'new' better quality Coax Cable. ]

* Trouble getting a good 'connection' at the Bottom-Joint between
the Aluminum Braid.
[ Learned to use more exposed Braid to make 3-4 complete Wraps
around the other section of Exposed Braid at the Bottom-Joint. ]

IDEA: Got the Idea for the LOOP-HUB using a 'modified' TV
type Two-Way Splitter.

* The "F" Connector' End had a Center Wire that was 'short' and
resulted in an intermittent connection when moved.
[ Learned to make sure that the Center Wire was Long Enough
and that I did NOT Bend them and Short-them-Out when installing
them. ]


The basic design of the "GreerTech" Coax Cable [Shielded] Loop
Antenna is good and very practical.
LOOP= http://www.greertech.com/hfloop/mymagloop.html


My 'second' Coax Cable [Shielded] Loop Antenna was made from
Thirteen Feet (13 Ft) of standard size RG-8 Coax Cable.

* Big and difficult to work with.

* Easy to get a good 'connection' at the Bottom-Joint.


The most recent Coax Cable [Shielded] Loop Antenna was made from
a standard 'pre-made' Twelve Feet (12 Ft) of RG-6 'Quad Shield'
Coax Cable.

* RG-6 is 'easy' to work with.

* Pre-Made Lengths with "F" Connectors already attached.

* Just 'cut' the One Inch "Gap" at the Center of the Length.

* Easy to get a good 'connection' at the Bottom-Joint using the
LOOP-HUB made from a TV type Two Way Splitter.

RESULTS: Very Good Performance. (Equal to the 13 Ft RG-8 Loop.)

NOTE: Lessons Learned.


iane ~ RHF
..
Some Say: On A Clear Day You Can See Forever.
http://groups.yahoo.com/group/Shortw...na/message/502
I BELIEVE: On A Clear Night . . .
You Can Hear Forever and Beyond - The Beyond !
..
..

(RHF) wrote in message


MK & TELAMON,

My reason for stating that the performance was 'poor' had to do with
may small problems:

* Learning Curve - It was my first Coax Cable [Shielded] Loop
Antenna.

* Questionable 'quality' of the old RG-59 Coax Cable being used.
[ Learned to use 'new' better quality Coax Cable. ]

The coax shouldn't matter much at all...

* Trouble getting a good 'connection' at the Bottom-Joint between
the Aluminum Braid.
[ Learned to use more exposed Braid to make 3-4 complete Wraps
around the other section of Exposed Braid at the Bottom-Joint. ]

That could cause a problem...

IDEA: Got the Idea for the LOOP-HUB using a 'modified' TV
type Two-Way Splitter.

* The "F" Connector' End had a Center Wire that was 'short' and
resulted in an intermittent connection when moved.
[ Learned to make sure that the Center Wire was Long Enough
and that I did NOT Bend them and Short-them-Out when installing
them. ]


The basic design of the "GreerTech" Coax Cable [Shielded] Loop
Antenna is good and very practical.
LOOP= http://www.greertech.com/hfloop/mymagloop.html

I notice he likes PVC...I do also...About the easiest frame you can
build for a loop...Here is one design I have used...
http://web.wt.net/~nm5k/loop5.jpg



My 'second' Coax Cable [Shielded] Loop Antenna was made from
Thirteen Feet (13 Ft) of standard size RG-8 Coax Cable.

Kinda heavy....:/

* Big and difficult to work with.

Yep...

* Easy to get a good 'connection' at the Bottom-Joint.


The most recent Coax Cable [Shielded] Loop Antenna was made from
a standard 'pre-made' Twelve Feet (12 Ft) of RG-6 'Quad Shield'
Coax Cable.

* RG-6 is 'easy' to work with.

* Pre-Made Lengths with "F" Connectors already attached.

* Just 'cut' the One Inch "Gap" at the Center of the Length.

* Easy to get a good 'connection' at the Bottom-Joint using the
LOOP-HUB made from a TV type Two Way Splitter.

RESULTS: Very Good Performance. (Equal to the 13 Ft RG-8 Loop.)

NOTE: Lessons Learned.

I've tried a few coax loops, and I guess they are ok, but in general,
I prefer the normal solenoid windings using plain wire... Much
lighter, and the performance *if properly balanced* , is the same. I
only use variable caps to tune the loop. My present loop that I use
the most is a diamond, appx 44 inches per side, and has 5 turns. "The
PVC frame is about 6 ft tall, counting the mast below the loop
winding, and the cross arm is 5 ft across" I use a multi gang
variable for tuning, and it has a switch to change to a small value
gang for upper freq range use. Also have some fixed caps to tack on if
I want to listen to LF. With all the cap variations, the tuning range
is about 250kc to about 2500 kc or so. But it's usually used on MW and
160m. It's indoors on a stand thats next to me on the floor and
rotates...MK

FO&A,

Connecting your TV 'type' Coax Cable [Shielded] SWL Loop Antenna
to your Table Top Shortwave Receiver or 'Portable' AM/FM/Shortwave
Radio.

Table Top Shortwave Receivers:
For "Receivers" with a SO-239 Jack for the External Antenna
you can use an "F" Jack to PL-259 Plug (SO-239 Adapter)
RadioShack Catalog # 278-258

'Portable' AM/FM/Shortwave Radios:
For "Radios" with a 1/8" Mono-Jack for the External Antenna
you can use an "F" Jack to 1/8" Plug (1/8" Mono-Jack Adapter)
RadioShack Catalog # 278-257

~ RHF
..
..
= = = (RHF) wrote in message
= = = . com...
YODAR,

The "TRICK" to TV 'type' Coax Cable [Shielded] SWL Loop Antennas {RHF}
http://groups.yahoo.com/group/Shortw...a/message/1626

I too tried to build a Coax Cable [Shielded] Loop Antenna
that was made of "TV type Coax" that had a 'Shield' made
of Foil and about 33% Braid. The Results were very poor.

Then I Got An IDEA "*" ! { K I S S }

Why not use "Pre-Made" Parts ) WHAT ?

* One Piece of TV type Coax Cable with the "F" Connectors
on both Ends. This can should be these Pre-Made Lengths
Three Feet (3 Ft); Six Feet (6 Ft); Nine Feet (9 Ft);
Twelve Feet (12 Ft); and Fifteen Feet (15 Ft); ETC.

* One "F" Connector to PL-259 Plug (SO-239 Adapter)
RadioShack Catalog # 278-258

* One TV type Two-to-One Splitter with "F" Connectors.
Get the one with the two Output "F" Connectors on
'opposite' Sides and the Input "F" Connector on the
Bottom (looks like a 'T').

THE SECRET IS TO BUILD A NEXUS LOOP-HUB:
Using the Two-to-One Splitter. Remove the 'Seal' Plate
from the Body of the Splitter. Remove the Ferrite Core
and Wires from inside the Splitter. Next clean a 'Spot'
inside the Body of the Splitter for a Solder Ground-Point.
Use a Solder Gun (150W) to make this Solder Ground-Point;
get in and get out quickly so that you do not melt the
plastic insulation inside the "F" Connectors. Wrap a
Wire around the Left "F" Connector Terminal. Route this
Wire to the Solder Ground-Point on the Body of the Splitter.
Using a Soldering Iron (35W) and Solder the Left Terminal
and the Ground-Point. Next 'wrap' a Wire around the
Bottom "F" Connector Terminal; and 'route' the Wire to
the Right "F" Connector Terminal. Then you need to make
a 'new' "Metal Cover" for the Splitter. I used one of
those old AOL CD "Tins" that come in the Mail. They are
thin enough to 'cut' with a Heavy Pair of Scissors .
Cut and Trim a piece until it "Fits". Ensure your have
good Metal-to-Metal 'contact' between the Cover and
the Spliter and Glue them together for Security. You
now have the NEXUS "LOOP-HUB" of your Coax Cable Loop
Antenna.

The Three Feet (3 Ft) piece of Coax Cable is your
Feed-in-Line.

MAKING YOUR "INTERCHANGABLE" LOOP ANTENNA ELEMENTS:
The Six Feet (6 Ft); Nine Feet (9 Ft); Twelve Feet
(12 Ft) and Fifteen Feet (15 Ft) Coax Cable are your
[Shielded] Loop Antenna Elements. Fold them End-to-End
and 'mark' a One Inch (1") Area at the Fold as their
Mid-Point. Very Carefully 'cut' the Outer Insulation
and then the Foil and Wire Braid away and out from this
One Inch Area. {Do Not Damage the Inner Insulation and
Center Wire of the Coax Cable.) Tape this One Inch Area
and an Inch on each side for Strength and Security.

TIP: If you have a Plastic Hula-Hoop or some 5/8" or
3/4" (White) Low Pressure Water Line Polyethylene Tubing
as a "Form" to hold the Loops in a Circle Shape. This
Tubing must 'slip-over' the Coax Cable's "F" Connectors.

THE LOOP ANTENNA "HOOK-UP":
* Slide the Coax Cable Loop Antenna Element into the
Plastic Tubing.
* Connect your Coax Cable Loop Antenna Element to the
LOOP-HUB {Splitter}.
* Connect your Coax Cable Feed-in-Line to the LOOP-HUB
{Splitter}.
* Install the "F" Connector to PL-259 Plug to the
'other end' of your Coax Cable Feed-in-Line and Connect
to the your Radio/Receiver.
* Shape and Position your Loop Antenna Element.

CAUTION: When Installing the Coax Cable "F" Connectors
into the "F" Connectors of the LOOP-HUB: Be careful to
make sure that the Center Wire is NOT Bent and Shorted-Out.

SIZING: This is a Low Noise SWL InDoor Loop Antenna that
allows the 'changing' of the Loop Antenna Element for the
Coverage of different Shortwave Bands. These Loops are 'cut'
"Sized" to roughly One Tenth of a Wave Length (1/10WL)
* Twenty-Five Feet (25 Ft) roughly the 120 Meter SW Band
* Twenty Feet (20 Ft) roughly the 90 Meter SW Band
* Fifteen Feet (15 Ft) roughly the 75 Meter SW Band
* Twelve Feet (12 Ft) roughly the 60 Meter SW Band
* Nine Feet (9 Ft) roughly the 41 Meter SW Band
* Six Feet (6 Ft) roughly the 31 Meter SW Band
* Five Feet (5 Ft) roughly the 25 Meter SW Band
* Four Feet (4 Ft) roughly the 19 Meter SW Band
* Three Feet (3 Ft) roughly the 13 Meter SW Band
The above Lengths and SW Bands are 'based' on the low
cost common {RadioShack} TV type RG-59 Coax Cable with
a Velocity Factor of 0.66.
NOTE: If you use RG6 "Quad Shield" Coax Cable that has
a Velocity Factor of 0.78 to 0.82: Then every thing would
shift down one SW Band 120M90M, 90M75M, 75M60M, ETC.

PACK-AND-GO "TRAVEL" LOOP ANTENNA:
This Design-Set-Up makes for a 'portable' Travel SWL Loop
Antenna that can be Taken-Down and Put-Up in a Minute or two.

MEGA-SIZED FIFTY FOOT LOOP FOR THE AM/MW BROADCAST BAND:
Take a Fifty Foot (50 Ft) piece of Coax Cable and Mount
it OutSide Doors
- 540 kHz to 1700 kHz with a MidPoint of 1120 kHz
- Fifty Feet (50 Ft0 [1/10WL] VF=0.66 using RG59 Coax Cable.
- Square shape 12.5 Feet per Side with 17.5 Cross-Arms.
- Round shape 16 Diameter Circle.
- Coax Cable Lead-in-Line 5-10 Feet.
This is roughly the "Sizing" for the AM/MW Broadcast Band
For DX-Peditions throw a 50 Foot Coax Loop Antenna Element
Over a Tree Limb about 15 Feet High and stretch-out the two
bottom side to form an Equilateral Triangle 17 Feet on a side.
Hook-Up everything and sit-down and Listen )


iane ~ RHF
.
Some Say: On A Clear Day You Can See Forever.
http://groups.yahoo.com/group/Shortw...na/message/502
I BELIEVE: On A Clear Night...
You Can Hear Forever and Beyond - The Beyond !
.
.
= = = In ,
= = = "YODAR" yodar@in O'do wrote:
I MADE MY FIRST ONE AND FLOPPED because I used TV Coax
(RG 58?) and there was no braid, it was foil and I
could make no solder connection...ARRGGGG

The RG-8 second try was so ungainly and costly and
unyielding of ANY signal (perhaps I had failed with
a connection) , that I got irrigated (kind way of
saying P***ed OFF!) and threw it out.

If I mess with Coax again it will be for an attic
SNAKE ANTENNA

Yodar in O'do
who is still using SLINKIES

.

This seems like a good place to insert a discussion about a
"shielded loop" being a myth for anything except for deeper
nulls in the directivity pattern: (these were posted about
18 months ago).
------------------------------------------------------------
------------------------------------------------------------



From: Roy Lewallen
Date: Fri, 21 Mar 2003 11:30:17 -0800
Newsgroups: sci.electronics.design,alt.engineering.electrical, rec.radio.amateur.antenna
Subject: Loop Antenna questions

It's fairly easy to see that what Tom wrote (the first paragraph of
quoted material below) is true. "Shielding" a loop simply moves the
feedpoint to the gap, and the outside of the "shield" becomes the loop.
It's no more or less immune to electric fields than any other loop. What
"shielding" does (and all it does) is to increase rejection of common
mode excitation, improving loop balance and the resulting pattern null
in practical installations. The sensitivity of a "shielded" loop to both
electric and magnetic fields is exactly the same as for an "unshielded"
loop.

The myth that "shielding" a loop somehow rejects the electric field
without affecting the magnetic field is one that's been kicking around
the amateur community for decades. I don't suppose it'll ever die.

Another thing that seems to be unknown or overlooked is that a small
(so-called "magnetic") loop antenna is actually more sensitive to an
electric field and less sensitive to a magnetic field than a short
("electric") dipole, at all distances from the antenna unless the
radiation source is very close -- within less than a wavelength.

Roy Lewallen, W7EL

JLB wrote:
The external "metal toroid" is then the antenna, for it is on the
outside of that "toroid" that antenna currents must exist, and not
inside it. The gap is then the feedpoint, to which the internal wire
couples and then acts as a simple feedline. To best reject response
to electric fields, the gap must be properly positioned...



No, the gap is usually opposite the feed point and the sheild is grounded.
It shields the loop from electric fields but it still responds to magnetic
fields. This is common practice and the idea has been around for many
decades. From personal experience I can say that it makes a tremendous
difference in the sensitivity pattern of the loop, even when extremely close
to other conductors (including AC house wiring).


References available on request.


Ok, what are they?

Jim
N8EE





From: Roy Lewallen
Date: Fri, 21 Mar 2003 22:52:21 -0800
Newsgroups: sci.electronics.design,alt.engineering.electrical, rec.radio.amateur.antenna
Subject: Loop Antenna questions

After some reflection, I see that I can't really justify the statement I
made. Thanks for questioning it.

Let me explain what I do know. The following discussion assumes that all
antennas are lossless, to eliminate a factor that would obscure the
subject under discussion.

If you put 100 watts into a very short dipole, the ratio of the E field
it produces to the H field it produces is called the wave impedance. At
a distance of considerably less than a wavelength, or greater, this
ratio is about 377 ohms (in air or free space), and E and H are in
phase. The same thing is true for a very small loop antenna, or any
other small antenna. If you get far enough away, the E/H ratio is the
same for any kind of antenna, although you might have to get a farther
from an antenna that's large in terms of wavelength. Invoking the
reciprocity principle, you find that the relative response of a small
loop to E and H fields is the same as for a small dipole if the field
originates some distance from the antenna. The so-called "magnetic" loop
responds just the same as any other antenna to fields originating beyond
a wavelength or so from the antenna.

What's interesting is the relative strength of radiated E and H fields
close to the antenna. My overstatement was the implication that the
ratio of E to H transmitted fields represent relative "sensitivity" to E
and H fields when receiving. Contrary to what the CFA and EH proponents
claim, it's not possible to generate E and H fields independently. And
contrary to what people who misunderstand "shielded" loops say, it's not
possible to isolate E and H fields. (It is possible to change the
impedance of a wave -- that is, the E/H ratio -- in a region, but it
always reverts back to the impedance of the medium -- 377 ohms for free
space -- within a pretty small distance of the disturbance.) So
sensitivity to an E or H field is somewhat problematic, since you can't
create either one in isolation for a test. Consequently, I'll just
discuss the E and H fields that the antennas produce when transmitting.
I believe that an antenna with high transmit E/H ratio will be
relatively insensitive to a wave with low E/H ratio (which would have to
be created nearby), and vice-versa. But I can't quantify the
relationship. I only know it involves the concept of transfer impedance,
and I haven't given the topic enough study to say more about it. It is
discussed in a number of texts, for those who are interested.

What I've done is look at the E/H ratio radiated by a small loop and a
small dipole at various distances, in the direction of the maximum far
field radiation (in the plane of the loop and broadside to the dipole).

The following analysis was done with EZNEC/4 using its double precision
NEC-4 calculating engine. This was necessary because of problems NEC
codes have with very small loops, and the size of loop I chose was too
small for analysis with NEC-2. However, it could probably be done with
MININEC, or it could be done with an NEC-2 based program like the EZNEC
demo if the loop is made a bit larger. It's vital to set the loss to
zero so that a negative feed point impedance isn't concealed by positive
loss resistance. I've gotten email from a person who was skeptical of
the results and did the calculations analytically. He came to the same
conclusion, which gives me added confidence in the results. The
following calculations were done at 3 MHz (100 meter wavelength).

Here's what happens. A small loop is sometimes called a "low impedance"
antenna because of the low E/H ratio close to the antenna. This might be
adequate justification for calling it a "magnetic" antenna as amateurs
have taken to doing, but the term unfortunately conveys the mistaken
impression that the antenna somehow responds only to or more strongly to
magnetic fields coming far from the antenna, which isn't true. But very
close to the antenna, the E/H ratio is indeed lower than the 377 ohm
value this ratio always has far from the antenna. In exactly
complementary fashion, a small dipole is a "high impedance" antenna,
having an E/H ratio higher than 377 ohms very close to the antenna. Here
are the values of the magnitude of E/H for a 2 meter (1/50 wavelength)
circumference octagonal loop and a 2 meter long dipole at 3 MHz. The
distance is measured from the center of the loop or dipole, and the
direction is in the plane of the loop or broadside to the dipole.

Distance (m) E/H (ohms)
Dipole Loop
2 2630 63.9
4 1360 110
6 862 169
8 597 241
10 440 326
12 345 414
14 291 490
16 265 538
18 257 555
20 258 552
22 264 538
24 273 522
26 281 506
28 290 491
30 297 478
50 342 415
100 367 386

The curious thing is that, while the dipole field impedance is high
close to the dipole, and low close to the loop, in both cases it
overshoots 377 ohms by a considerable amount, reaching a peak (lowest Z
for the dipole and highest for the loop) at about 18 meters, then
asymptotically approaches 377 ohms from the "wrong side" beyond that. At
all distances of 12 meters (about 1/8 wavelength) and beyond, the E/H
ratio of the loop is actually higher than that of the dipole! That was
the basis for my original statement. Whatever might be said about the
"sensitivity" of a "magnetic" loop to E and H fields, it certainly can't
be said to be more sensitive to H fields, or less sensitive to E fields,
than a small dipole, at any distance greater than about 1/8 wavelength.

The exact sizes of the loop and dipole aren't important. The near fields
at these distances are essentially equal for all small dipoles and for
all small loops for a given input power, provided that they're much
smaller than a wavelength in any dimension.

Roy Lewallen, W7EL

Reg Edwards wrote:
Roy says -


- - - - a small
(so-called "magnetic") loop antenna is actually more sensitive to an
electric field and less sensitive to a magnetic field than a short
("electric") dipole - - - -


-----------------------------------------------------

Roy,

What are your units of "sensitivity"?

How are things to be measured (even hypothetically) and compared?

In any case, the statement is meaningless unless loop diameter and dipole
length are incorporated.

Explanation please.

A simple learned reference will not be of much use since 99.9 percent of
readers will have no chance of ever getting their hands on it.

Yours, Reg, G4FGQ







From: Roy Lewallen
Date: Sat, 22 Mar 2003 13:15:46 -0800
Newsgroups: sci.electronics.design,alt.engineering.electrical, rec.radio.amateur.antenna
Subject: Loop Antenna questions

Richard Clark wrote:

If anything, your work contributes to the notion of these "notable"
differences rather than eliminates them. You forcefully detail how
different the short dipole is from a small loop to exactly the same
degree as offered by CFA/EH proponents as that being its boon - even
if they invert the rationale of what constitutes the sensitivity.
This is to say, that if they are entirely wrong about a magnetic
antenna being sensitive to the magnetic field (something you reject
and offer compelling numbers as evidence); they do have a point about
the magnitude of difference (they are not entirely wedded to the
principle as they are the effect - if in fact that effect is not
another figment of wish fulfillment).

My understanding of the CFA/EH antenna is undeniably vague, but I think
it goes something like this: The E and H fields are created
independently, one by a capacitor-like part of the antenna and the other
by a coil-like part of the antenna. This, as I said and as many others
have pointed out, can't be done. Creation of a time-varying E field
always results in a time-varying H field, and vice-versa. The E and H
fields thus "independently" created are done so in the ratio of 377
ohms. This being the ratio of the the components in the far field, the
direct synthesis results in the lack of a near field. This lack of near
field is credited somehow with the magical properties of these antennas
(high efficiency despite small size) by mechanisms that have never been
clear to me. At any rate, these wonderful properties have never been
demonstrated with either believable measurements or modeling despite
more than a decade of hype, so it sure looks like they're fiction also.
The EH antenna further distinguishes itself by requiring a magic
inductor whose current at one terminal is shifted in phase 90 degrees
from the current at the other terminal, adding yet another layer of
impossibility.

If I've misinterpreted the explanations for how these things are
supposed to work, I apologize -- it wasn't intentional.

The argument about being less sensitive to neighborhood noise for
magnetic antennas, small loops, or loops in general, seems to be
justified by your data supporting very different E/H ratios out to
about 10M, which for me is easily the most disruptive region to any
antenna in a noisy neighborhood and makes that case in some sense.

It might. Remember, though, that this is constant in terms of
wavelength, so the range drops by a factor of two at twice the
frequency. (But on the other hand, the range will be greater at 160
meters.) Also, note that the difference isn't very great even at a
distance of 10 meters. You have to get very close indeed to see a truly
major difference. I suspect that major differences in interference
rejection are as likely to be due to differences in balance (causing
better or worse nulls) or polarization.

It seems to me that the difference applies only to situations where
that first 10M is significant, and to what degree coupling of
interference within that region becomes meaningful.

Yes, that's the conclusion I reach.

Roy Lewallen, W7EL

GG,

Interesting Information, Thank You for the Repost.

Somewhere, I remember reading that a Coax Cable [Shielded] Loop
Antenna is simply an End-Fed Dipole that is Terminated on Itself.
The fact that it is a Dipole (Electrically Balanced) and the Loop
Shape (Physically Balanced) give it it's inherent properties.

~ RHF
..
..
= = = (Gene Gardner) wrote in message
= = = ...
This seems like a good place to insert a discussion about a
"shielded loop" being a myth for anything except for deeper
nulls in the directivity pattern: (these were posted about
18 months ago).
------------------------------------------------------------
------------------------------------------------------------



From: Roy Lewallen
Date: Fri, 21 Mar 2003 11:30:17 -0800
Newsgroups: sci.electronics.design,alt.engineering.electrical, rec.radio.amateur.antenna
Subject: Loop Antenna questions

It's fairly easy to see that what Tom wrote (the first paragraph of
quoted material below) is true. "Shielding" a loop simply moves the
feedpoint to the gap, and the outside of the "shield" becomes the loop.
It's no more or less immune to electric fields than any other loop. What
"shielding" does (and all it does) is to increase rejection of common
mode excitation, improving loop balance and the resulting pattern null
in practical installations. The sensitivity of a "shielded" loop to both
electric and magnetic fields is exactly the same as for an "unshielded"
loop.

The myth that "shielding" a loop somehow rejects the electric field
without affecting the magnetic field is one that's been kicking around
the amateur community for decades. I don't suppose it'll ever die.

Another thing that seems to be unknown or overlooked is that a small
(so-called "magnetic") loop antenna is actually more sensitive to an
electric field and less sensitive to a magnetic field than a short
("electric") dipole, at all distances from the antenna unless the
radiation source is very close -- within less than a wavelength.

Roy Lewallen, W7EL

JLB wrote:
The external "metal toroid" is then the antenna, for it is on the
outside of that "toroid" that antenna currents must exist, and not
inside it. The gap is then the feedpoint, to which the internal wire
couples and then acts as a simple feedline. To best reject response
to electric fields, the gap must be properly positioned...



No, the gap is usually opposite the feed point and the sheild is grounded.
It shields the loop from electric fields but it still responds to magnetic
fields. This is common practice and the idea has been around for many
decades. From personal experience I can say that it makes a tremendous
difference in the sensitivity pattern of the loop, even when extremely close
to other conductors (including AC house wiring).


References available on request.


Ok, what are they?

Jim
N8EE





From: Roy Lewallen
Date: Fri, 21 Mar 2003 22:52:21 -0800
Newsgroups: sci.electronics.design,alt.engineering.electrical, rec.radio.amateur.antenna
Subject: Loop Antenna questions

After some reflection, I see that I can't really justify the statement I
made. Thanks for questioning it.

Let me explain what I do know. The following discussion assumes that all
antennas are lossless, to eliminate a factor that would obscure the
subject under discussion.

If you put 100 watts into a very short dipole, the ratio of the E field
it produces to the H field it produces is called the wave impedance. At
a distance of considerably less than a wavelength, or greater, this
ratio is about 377 ohms (in air or free space), and E and H are in
phase. The same thing is true for a very small loop antenna, or any
other small antenna. If you get far enough away, the E/H ratio is the
same for any kind of antenna, although you might have to get a farther
from an antenna that's large in terms of wavelength. Invoking the
reciprocity principle, you find that the relative response of a small
loop to E and H fields is the same as for a small dipole if the field
originates some distance from the antenna. The so-called "magnetic" loop
responds just the same as any other antenna to fields originating beyond
a wavelength or so from the antenna.

What's interesting is the relative strength of radiated E and H fields
close to the antenna. My overstatement was the implication that the
ratio of E to H transmitted fields represent relative "sensitivity" to E
and H fields when receiving. Contrary to what the CFA and EH proponents
claim, it's not possible to generate E and H fields independently. And
contrary to what people who misunderstand "shielded" loops say, it's not
possible to isolate E and H fields. (It is possible to change the
impedance of a wave -- that is, the E/H ratio -- in a region, but it
always reverts back to the impedance of the medium -- 377 ohms for free
space -- within a pretty small distance of the disturbance.) So
sensitivity to an E or H field is somewhat problematic, since you can't
create either one in isolation for a test. Consequently, I'll just
discuss the E and H fields that the antennas produce when transmitting.
I believe that an antenna with high transmit E/H ratio will be
relatively insensitive to a wave with low E/H ratio (which would have to
be created nearby), and vice-versa. But I can't quantify the
relationship. I only know it involves the concept of transfer impedance,
and I haven't given the topic enough study to say more about it. It is
discussed in a number of texts, for those who are interested.

What I've done is look at the E/H ratio radiated by a small loop and a
small dipole at various distances, in the direction of the maximum far
field radiation (in the plane of the loop and broadside to the dipole).

The following analysis was done with EZNEC/4 using its double precision
NEC-4 calculating engine. This was necessary because of problems NEC
codes have with very small loops, and the size of loop I chose was too
small for analysis with NEC-2. However, it could probably be done with
MININEC, or it could be done with an NEC-2 based program like the EZNEC
demo if the loop is made a bit larger. It's vital to set the loss to
zero so that a negative feed point impedance isn't concealed by positive
loss resistance. I've gotten email from a person who was skeptical of
the results and did the calculations analytically. He came to the same
conclusion, which gives me added confidence in the results. The
following calculations were done at 3 MHz (100 meter wavelength).

Here's what happens. A small loop is sometimes called a "low impedance"
antenna because of the low E/H ratio close to the antenna. This might be
adequate justification for calling it a "magnetic" antenna as amateurs
have taken to doing, but the term unfortunately conveys the mistaken
impression that the antenna somehow responds only to or more strongly to
magnetic fields coming far from the antenna, which isn't true. But very
close to the antenna, the E/H ratio is indeed lower than the 377 ohm
value this ratio always has far from the antenna. In exactly
complementary fashion, a small dipole is a "high impedance" antenna,
having an E/H ratio higher than 377 ohms very close to the antenna. Here
are the values of the magnitude of E/H for a 2 meter (1/50 wavelength)
circumference octagonal loop and a 2 meter long dipole at 3 MHz. The
distance is measured from the center of the loop or dipole, and the
direction is in the plane of the loop or broadside to the dipole.

Distance (m) E/H (ohms)
Dipole Loop
2 2630 63.9
4 1360 110
6 862 169
8 597 241
10 440 326
12 345 414
14 291 490
16 265 538
18 257 555
20 258 552
22 264 538
24 273 522
26 281 506
28 290 491
30 297 478
50 342 415
100 367 386

The curious thing is that, while the dipole field impedance is high
close to the dipole, and low close to the loop, in both cases it
overshoots 377 ohms by a considerable amount, reaching a peak (lowest Z
for the dipole and highest for the loop) at about 18 meters, then
asymptotically approaches 377 ohms from the "wrong side" beyond that. At
all distances of 12 meters (about 1/8 wavelength) and beyond, the E/H
ratio of the loop is actually higher than that of the dipole! That was
the basis for my original statement. Whatever might be said about the
"sensitivity" of a "magnetic" loop to E and H fields, it certainly can't
be said to be more sensitive to H fields, or less sensitive to E fields,
than a small dipole, at any distance greater than about 1/8 wavelength.

The exact sizes of the loop and dipole aren't important. The near fields
at these distances are essentially equal for all small dipoles and for
all small loops for a given input power, provided that they're much
smaller than a wavelength in any dimension.

Roy Lewallen, W7EL

Reg Edwards wrote:
Roy says -


- - - - a small
(so-called "magnetic") loop antenna is actually more sensitive to an
electric field and less sensitive to a magnetic field than a short
("electric") dipole - - - -


-----------------------------------------------------

Roy,

What are your units of "sensitivity"?

How are things to be measured (even hypothetically) and compared?

In any case, the statement is meaningless unless loop diameter and dipole
length are incorporated.

Explanation please.

A simple learned reference will not be of much use since 99.9 percent of
readers will have no chance of ever getting their hands on it.

Yours, Reg, G4FGQ







From: Roy Lewallen
Date: Sat, 22 Mar 2003 13:15:46 -0800
Newsgroups: sci.electronics.design,alt.engineering.electrical, rec.radio.amateur.antenna
Subject: Loop Antenna questions

Richard Clark wrote:

If anything, your work contributes to the notion of these "notable"
differences rather than eliminates them. You forcefully detail how
different the short dipole is from a small loop to exactly the same
degree as offered by CFA/EH proponents as that being its boon - even
if they invert the rationale of what constitutes the sensitivity.
This is to say, that if they are entirely wrong about a magnetic
antenna being sensitive to the magnetic field (something you reject
and offer compelling numbers as evidence); they do have a point about
the magnitude of difference (they are not entirely wedded to the
principle as they are the effect - if in fact that effect is not
another figment of wish fulfillment).

My understanding of the CFA/EH antenna is undeniably vague, but I think
it goes something like this: The E and H fields are created
independently, one by a capacitor-like part of the antenna and the other
by a coil-like part of the antenna. This, as I said and as many others
have pointed out, can't be done. Creation of a time-varying E field
always results in a time-varying H field, and vice-versa. The E and H
fields thus "independently" created are done so in the ratio of 377
ohms. This being the ratio of the the components in the far field, the
direct synthesis results in the lack of a near field. This lack of near
field is credited somehow with the magical properties of these antennas
(high efficiency despite small size) by mechanisms that have never been
clear to me. At any rate, these wonderful properties have never been
demonstrated with either believable measurements or modeling despite
more than a decade of hype, so it sure looks like they're fiction also.
The EH antenna further distinguishes itself by requiring a magic
inductor whose current at one terminal is shifted in phase 90 degrees
from the current at the other terminal, adding yet another layer of
impossibility.

If I've misinterpreted the explanations for how these things are
supposed to work, I apologize -- it wasn't intentional.

The argument about being less sensitive to neighborhood noise for
magnetic antennas, small loops, or loops in general, seems to be
justified by your data supporting very different E/H ratios out to
about 10M, which for me is easily the most disruptive region to any
antenna in a noisy neighborhood and makes that case in some sense.

It might. Remember, though, that this is constant in terms of
wavelength, so the range drops by a factor of two at twice the
frequency. (But on the other hand, the range will be greater at 160
meters.) Also, note that the difference isn't very great even at a
distance of 10 meters. You have to get very close indeed to see a truly
major difference. I suspect that major differences in interference
rejection are as likely to be due to differences in balance (causing
better or worse nulls) or polarization.

It seems to me that the difference applies only to situations where
that first 10M is significant, and to what degree coupling of
interference within that region becomes meaningful.

Yes, that's the conclusion I reach.

Roy Lewallen, W7EL