Again, with a properly installed single antenna, coax length does not
matter.

-SSB

This thread was about a ......................

1. mag mount
2. on a vehilcle
3. with a recommended coax length

Does this thread represent a "properly installed single antenna"?

wrote:

Again, with a properly installed single antenna, coax length does not
matter.

-SSB

This thread was about a ......................

1. mag mount
2. on a vehilcle
3. with a recommended coax length

Does this thread represent a "properly installed single antenna"?

It usually represents a disagreement no matter how it's worded.

--
http://NewsReader.Com
30GB/Month

On 05 Nov 2004 13:08:42 GMT, Steveo
wrote:

wrote:

Again, with a properly installed single antenna, coax length does not
matter.

-SSB

This thread was about a ......................

1. mag mount
2. on a vehilcle
3. with a recommended coax length

Does this thread represent a "properly installed single antenna"?

It usually represents a disagreement no matter how it's worded.

Yes. Coax length creates static. It's an electrical fact.

Hello All:

I have been reading most of the thread here and its quit good reading.

Great inputs from Frank, Sideband, Jimmie Lancer, Tnom, and others.

One main consideration on the magnetic mount antennas is that the coax
is being used as a counter poise and a lead for capacitance to ground.
So I can see where a manufacture will specify a 3/4 wavelength length of
coax, 18 feet.

Anytime you use the coax as a counter poise or other end of the antenna
coax lengths will effect the SWR, as the coax is no longer a simply a
transmission line, but now also a antenna. I would love to have the time
to do more testing.

You will not see this type of design in Military, Commercial, or Amateur
Antennas.

I got a old Wilson 5000 magnetmatic mount antenna that I rebuilt and use
on my car. I replaced the Krapy RG58C coax with "18" Ft of RG142B Teflon
type coax that is bullet proof. It works great. I get it surplus at times.

But I wanted to add 3 or 4 each, 9 foot ground plane radial wires, being
run over the body of the car, from the base of the antenna, connected to
the shield of the coax. And do a few tests. Food for thought.

One thing I may do as a quick add on improvement is add in a Toroid Core
in the coax about 9 feet down the coax from the antenna. Wrapping the
coax around a 2.4 inch OD Amidon Toroid Core, will choke the outer coax
shield currents from going farther down the coax, but allowing a 1/4
wave of coax to be a counter poise. Worth a try I think. It will also
reduce some interference noise heard by the radio, that are coming in
from the ungrounded shield of the coax. Neat-Oh!

Suggested Toroid Core Amidon FT-240-61
Amidon is at: http://www.amidoncorp.com/aai_productselection.htm

Jay in the Mojave

Kreedentials:
Rock n Roll Fan
Owner 1967 Ford F250, 4x4, with worlds loudest PA System
Took Electronics in 8th grade, way more better than that ackademic crap
that schools now teach kids now a days ancient European history, liberal
arts, and in incoming paper clips, in place of industrial arts.
Paid Member of Mojave Desert Radio Club, 9 dollars now.
Radio enthusiast type of guy

..

"sideband" wrote in message
om...
tnom:

Explain this, then... CB radio in my Peterbilt, with a mirror mounted
Hustler HQ-27 on the driver's side.. 6 feet of coax shows a 1:1 on 19,
1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same readings...
18 feet of coax shows.. guess what? the SAME RESULTS.

Nobody said it would be any different ,go back and read my post. I was very
clear on why manufactureers say to use 18 ft and why you do not have to.

UNLESS SOMETHING IS FUBAR CHANGING COAX LENGTH DOES NOT CHANGE SWR. JUST THE
IMPEDANCE THE RADIO SEES.SWR IS DETERMINED SOLEY BY THE IMPEDANCE OF THE
ANTENNA AND THE IMPEDANCE OF THE TRANSMISSION LINE.
YOUR STATEMENT PROVES NOTHING.

IF something is FUBAR making it read right by changing coax length will not
fix it.
You just sound like another ham that likes to hang around the CB group so
they can sound important. Nobody even made the claims you are refuting.

"Lancer" wrote in message
ews.com...
On Thu, 04 Nov 2004 21:59:02 GMT, sideband wrote:

tnom:

Explain this, then... CB radio in my Peterbilt, with a mirror mounted
Hustler HQ-27 on the driver's side.. 6 feet of coax shows a 1:1 on 19,
1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same
readings... 18 feet of coax shows.. guess what? the SAME RESULTS.

IT IS possible to get near "perfect world" results with a little work,
and attention to detail.

By the way, I've gotten similar results on two meters with a magnet
mount 5/8 2M Lakeview whip on my old Ford LTLA9000. Had to shorten the
coax because I was sick of the bundle of it in the cab... no change in
SWR over a four MHz bandwidth...

There goes the "magmount theory" as well..

If changing the length of the coax changes the SWR, then there is
something wrong with the antenna.

-SSB

SSB;
I know this was addressed to Tnom, hope you don't mind my 2-cents
worth.

Very few mag mounts have an adequate "RF" ground, at least on 11
meters or lower. There isn't enough capacitive coupling to the
vehicle body. In some cases the length of the coax can matter, not
from a matching point of view, but as a counterpoise for the antenna.
I have seen mag mount installations where you could see the SWR change
just by moving the coax around.

This is true but is totally beside the point of why manufacturers reccomend
18ft of coax. If you have the problem you mentioned fix the ground dont play
with the coax length. A good test for a CB antenna is to temporarily splice
in 6 ft of coax . If your SWR changes your ground is probably bad.Poorly
made SWR meters will cause this too.

"Jimmie" wrote in message
om...

"Lancer" wrote in message
ews.com...
On Thu, 04 Nov 2004 21:59:02 GMT, sideband wrote:

tnom:

Explain this, then... CB radio in my Peterbilt, with a mirror mounted
Hustler HQ-27 on the driver's side.. 6 feet of coax shows a 1:1 on 19,
1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same
readings... 18 feet of coax shows.. guess what? the SAME RESULTS.

IT IS possible to get near "perfect world" results with a little work,
and attention to detail.

By the way, I've gotten similar results on two meters with a magnet
mount 5/8 2M Lakeview whip on my old Ford LTLA9000. Had to shorten the
coax because I was sick of the bundle of it in the cab... no change in
SWR over a four MHz bandwidth...

There goes the "magmount theory" as well..

If changing the length of the coax changes the SWR, then there is
something wrong with the antenna.

-SSB

SSB;
I know this was addressed to Tnom, hope you don't mind my 2-cents
worth.

Very few mag mounts have an adequate "RF" ground, at least on 11
meters or lower. There isn't enough capacitive coupling to the
vehicle body. In some cases the length of the coax can matter, not
from a matching point of view, but as a counterpoise for the antenna.
I have seen mag mount installations where you could see the SWR change
just by moving the coax around.

This is true but is totally beside the point of why manufacturers
reccomend 18ft of coax. If you have the problem you mentioned fix the
ground dont play with the coax length. A good test for a CB antenna is to
temporarily splice in 6 ft of coax . If your SWR changes your ground is
probably bad.Poorly made SWR meters will cause this too.

The 18ft rule is also there for "just in case" . As you change the lenght of
the coax the impedance seen by the radio will also change. Assuming the
antenna is at resonance you will see a pure resistance at the radio for only
2 lengths, an odd 1/4wl(16ft) and an even 1/4wl. At all other length unless
the SWR is a perfect 1:1 match the impedances seen at the radio end will be
complex. This is not always a bad thing. However there are some complex
impedances that the radio may not like. It may dislike it so much the finals
fry, the modulation becomes mushy or rx is nearly dead or it may be no
problem at all for any length on your radio. There are a lot of different
radios out there so the manufacturer usually recommends something safe they
figure will work with all radios. 18 ft is a length that will normally allow
enough cable to reach just about anywhere on the vehicle and provide a radio
friendly impedance at the other end of the cable. Does it really need to be
18ft, probably not.. If you find the rare case where it does you can chase
your tail till hell freezes over.

On Fri, 05 Nov 2004 08:13:49 -0800, Jay in the Mojave
wrote in :

Hello All:

I have been reading most of the thread here and its quit good reading.

Great inputs from Frank, Sideband, Jimmie Lancer, Tnom, and others.

One main consideration on the magnetic mount antennas is that the coax
is being used as a counter poise and a lead for capacitance to ground.
So I can see where a manufacture will specify a 3/4 wavelength length of
coax, 18 feet.


The purpose of using 18 feet is not to effect a counterpoise but to
mirror the RF potential of the shield at both ends of the coax,
assuming that at least one end is properly grounded. IOW, if the radio
has a good RF ground but the antenna mount is poorly grounded (like a
mag-mount), using a 1/2 wave (as measured on the -outside- of the
shield) will prevent the antenna end from floating. Or vice-versa.


Anytime you use the coax as a counter poise or other end of the antenna
coax lengths will effect the SWR, as the coax is no longer a simply a
transmission line, but now also a antenna. I would love to have the time
to do more testing.

You will not see this type of design in Military, Commercial, or Amateur
Antennas.

I got a old Wilson 5000 magnetmatic mount antenna that I rebuilt and use
on my car. I replaced the Krapy RG58C coax with "18" Ft of RG142B Teflon
type coax that is bullet proof. It works great. I get it surplus at times.

But I wanted to add 3 or 4 each, 9 foot ground plane radial wires, being
run over the body of the car, from the base of the antenna, connected to
the shield of the coax. And do a few tests. Food for thought.


Here's something to try: Set your GDO for 27MHz and run it over your
vehicle. Some places will dip more than others indicating better RF
grounds, and therefore better places to drill.


One thing I may do as a quick add on improvement is add in a Toroid Core
in the coax about 9 feet down the coax from the antenna. Wrapping the
coax around a 2.4 inch OD Amidon Toroid Core, will choke the outer coax
shield currents from going farther down the coax, but allowing a 1/4
wave of coax to be a counter poise. Worth a try I think. It will also
reduce some interference noise heard by the radio, that are coming in
from the ungrounded shield of the coax. Neat-Oh!

Suggested Toroid Core Amidon FT-240-61
Amidon is at: http://www.amidoncorp.com/aai_productselection.htm


A sleeve with a high perm (u=850) should also work at 27MHz. As
chokes I've seen them used at the radio end, but maybe using one at
the antenna end will improve the performance of a mag-mount....? I
might just try that myself someday.






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On Fri, 05 Nov 2004 19:09:51 GMT, "Jimmie"
wrote in :

snip
.... Assuming the
antenna is at resonance you will see a pure resistance at the radio for only
2 lengths, an odd 1/4wl(16ft) and an even 1/4wl. At all other length unless
the SWR is a perfect 1:1 match the impedances seen at the radio end will be
complex.


The input impedance to the antenna (and coax) is complex regardless of
resonance simply because the feedpoint is slightly out of phase with
ground (the antenna wouldn't work if it wasn't). Even if the antenna
-was- purely resistive at resonance the impedance would be on the
order of 36.5 ohms (ideal), which does -not- present a 1:1 match to a
50 ohm coax. If you are getting 1:1 SWR then either the antenna has
some sort of matching mechanism or it's a dummy load.

But you are right, using 18 feet is a quick-&-dirty fix for a poor
installation.






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Jay:

Thanks for your input.. A couple of things, though..

If the coax is being used as a counterpoise, it's not part of a
capacitive grounding system.

The capacitive ground comes from the metallic base/magnet mount to the
vehicle body, not from the coax.

-SSB

Jay in the Mojave wrote:

Hello All:

I have been reading most of the thread here and its quit good reading.

Great inputs from Frank, Sideband, Jimmie Lancer, Tnom, and others.

One main consideration on the magnetic mount antennas is that the coax
is being used as a counter poise and a lead for capacitance to ground.
So I can see where a manufacture will specify a 3/4 wavelength length of
coax, 18 feet.

Anytime you use the coax as a counter poise or other end of the antenna
coax lengths will effect the SWR, as the coax is no longer a simply a
transmission line, but now also a antenna. I would love to have the time
to do more testing.

You will not see this type of design in Military, Commercial, or Amateur
Antennas.

I got a old Wilson 5000 magnetmatic mount antenna that I rebuilt and use
on my car. I replaced the Krapy RG58C coax with "18" Ft of RG142B Teflon
type coax that is bullet proof. It works great. I get it surplus at times.

But I wanted to add 3 or 4 each, 9 foot ground plane radial wires, being
run over the body of the car, from the base of the antenna, connected to
the shield of the coax. And do a few tests. Food for thought.

One thing I may do as a quick add on improvement is add in a Toroid Core
in the coax about 9 feet down the coax from the antenna. Wrapping the
coax around a 2.4 inch OD Amidon Toroid Core, will choke the outer coax
shield currents from going farther down the coax, but allowing a 1/4
wave of coax to be a counter poise. Worth a try I think. It will also
reduce some interference noise heard by the radio, that are coming in
from the ungrounded shield of the coax. Neat-Oh!

Suggested Toroid Core Amidon FT-240-61
Amidon is at: http://www.amidoncorp.com/aai_productselection.htm

Jay in the Mojave

Kreedentials:
Rock n Roll Fan
Owner 1967 Ford F250, 4x4, with worlds loudest PA System
Took Electronics in 8th grade, way more better than that ackademic crap
that schools now teach kids now a days ancient European history, liberal
arts, and in incoming paper clips, in place of industrial arts.
Paid Member of Mojave Desert Radio Club, 9 dollars now.
Radio enthusiast type of guy