This winter I have been working on the design and construction of a compact
loop for 80 meters. The loop will be constructed from 72 ft of 3/4 in copper
pipe and will be in the shape of an octagon. The plane of the loop will be
vertical, and the loop will be remotely tuned by a vacuum variable capacitor
coupled to a geared stepper motor.

I have just finished constructing and testing the driver for the stepper
motor. See
http://tinyurl.com/bwobb for pictures and some information about the system.
Constructive comments will be appreciated.

John, N9JG

"John, N9JG" wrote in message
newshGLf.796098$_o.157184@attbi_s71...
This winter I have been working on the design and construction of a
compact loop for 80 meters. The loop will be constructed from 72 ft of 3/4
in copper pipe and will be in the shape of an octagon. The plane of the
loop will be vertical, and the loop will be remotely tuned by a vacuum
variable capacitor coupled to a geared stepper motor.

I have just finished constructing and testing the driver for the stepper
motor. See
http://tinyurl.com/bwobb for pictures and some information about the
system. Constructive comments will be appreciated.

John, N9JG

John, how high above ground is the antenna?

Frank, VE6CB

Initially, at least, the loop will be just high enough so that a lawnmower
can get underneath the lowest side of the octagon. It the loop works fairly
well, I might install a 40 foot fiberglass flagpole and suspend the top of
the loop from the top of the flagpole. Supposedly, that might cut down on
some loss due to ground currents.

"Frank's" wrote in message
news:4WJLf.1213$dg.9@clgrps13...
"John, N9JG" wrote in message
newshGLf.796098$_o.157184@attbi_s71...
This winter I have been working on the design and construction of a
compact loop for 80 meters. The loop will be constructed from 72 ft of
3/4 in copper pipe and will be in the shape of an octagon. The plane of
the loop will be vertical, and the loop will be remotely tuned by a
vacuum variable capacitor coupled to a geared stepper motor.

I have just finished constructing and testing the driver for the stepper
motor. See
http://tinyurl.com/bwobb for pictures and some information about the
system. Constructive comments will be appreciated.

John, N9JG

John, how high above ground is the antenna?

Frank, VE6CB

This winter I have been working on the design and construction of a
compact loop for 80 meters. The loop will be constructed from 72 ft of
3/4 in copper pipe and will be in the shape of an octagon. The plane of
the loop will be vertical, and the loop will be remotely tuned by a
vacuum variable capacitor coupled to a geared stepper motor.

I have just finished constructing and testing the driver for the stepper
motor. See
http://tinyurl.com/bwobb for pictures and some information about the
system. Constructive comments will be appreciated.

John, N9JG

John, how high above ground is the antenna?

Frank, VE6CB

John, just for interest I placed the top of the antenna at 30 ft; which puts
the base at about 8' 3". Using NEC's Sommerfeld/Norton ground with: sigma =
5 mS/m, and Er = 13. The input impedance computes to 6.57 + j855.6 ohms.
Since this is very close to the edge of the Smith Chart, your 1 ohm figure
is probably within a reasonable margin of error. However I cannot see how
you can match such an impedance with a single element.

The structure appears to be a very efficient, physically small, antenna.
NEC computes 0.84% copper loss.

Frank

"John, N9JG" wrote in message
news:L3LLf.796414$_o.66941@attbi_s71...
Initially, at least, the loop will be just high enough so that a lawnmower
can get underneath the lowest side of the octagon. It the loop works
fairly well, I might install a 40 foot fiberglass flagpole and suspend the
top of the loop from the top of the flagpole. Supposedly, that might cut
down on some loss due to ground currents.

Thanks John, I guess my estimate of 8ft at the base is a bit high -- see
following post.

Frank

I plan to feed the loop at the side opposite to the side containing the
tuning capacitor. Note that the capacitor is inserted in series with the
loop and is used to tune the loop to resonance at the operating frequency. A
gamma match is used to connect the coax to the loop, and the loop is NOT cut
on the feed side.

"Frank's" wrote in message
news:r5LLf.1227$dg.71@clgrps13...
This winter I have been working on the design and construction of a
compact loop for 80 meters. The loop will be constructed from 72 ft of
3/4 in copper pipe and will be in the shape of an octagon. The plane of
the loop will be vertical, and the loop will be remotely tuned by a
vacuum variable capacitor coupled to a geared stepper motor.

I have just finished constructing and testing the driver for the stepper
motor. See
http://tinyurl.com/bwobb for pictures and some information about the
system. Constructive comments will be appreciated.

John, N9JG

John, how high above ground is the antenna?

Frank, VE6CB

John, just for interest I placed the top of the antenna at 30 ft; which
puts the base at about 8' 3". Using NEC's Sommerfeld/Norton ground with:
sigma = 5 mS/m, and Er = 13. The input impedance computes to 6.57 +
j855.6 ohms. Since this is very close to the edge of the Smith Chart, your
1 ohm figure is probably within a reasonable margin of error. However I
cannot see how you can match such an impedance with a single element.

The structure appears to be a very efficient, physically small, antenna.
NEC computes 0.84% copper loss.

Frank

"John, N9JG" wrote in message
newsgLLf.796419$_o.494459@attbi_s71...
I plan to feed the loop at the side opposite to the side containing the
tuning capacitor. Note that the capacitor is inserted in series with the
loop and is used to tune the loop to resonance at the operating frequency.
A gamma match is used to connect the coax to the loop, and the loop is NOT
cut on the feed side.

Ok, thanks. I am sure the loop will work very well.

Frank

Frank,

Will you please forward the nec code?

Thanks - Dan

Frank's wrote:
This winter I have been working on the design and construction of a
compact loop for 80 meters. The loop will be constructed from 72 ft of
3/4 in copper pipe and will be in the shape of an octagon. The plane of
the loop will be vertical, and the loop will be remotely tuned by a
vacuum variable capacitor coupled to a geared stepper motor.

I have just finished constructing and testing the driver for the stepper
motor. See
http://tinyurl.com/bwobb for pictures and some information about the
system. Constructive comments will be appreciated.

John, N9JG

John, how high above ground is the antenna?

Frank, VE6CB


John, just for interest I placed the top of the antenna at 30 ft; which puts
the base at about 8' 3". Using NEC's Sommerfeld/Norton ground with: sigma =
5 mS/m, and Er = 13. The input impedance computes to 6.57 + j855.6 ohms.
Since this is very close to the edge of the Smith Chart, your 1 ohm figure
is probably within a reasonable margin of error. However I cannot see how
you can match such an impedance with a single element.

The structure appears to be a very efficient, physically small, antenna.
NEC computes 0.84% copper loss.

Frank

"dansawyeror" wrote in message
...
Frank,

Will you please forward the nec code?

Thanks - Dan

Hi Dan, this is essentially the initial code that I used. I have just
removed, and replaced, the "GM" card by a series of "GW" cards.

Note that John mentioned he is using a gamma match, and tunes out the
reactance of the loop on the opposite side. The diameter of the loop is a
nominal 20 ft, so it would make the most sense to tune, and feed, the
antenna on the two vertical elements. My model is 30 ft at the top, and
8.272 ft at the bottom, so even with the reduced height motorized tuning
makes a lot of sense. I am going to model a gamma match, and opposite side
tuning cap, so will update the code this weekend.

The antenna is virtually lossless, with a 99.8% efficiency, and even
exhibits quite a good low angle performance. The gain at 25 deg elevation
is 0 dBi. I have no experience with gamma matches, but if you can build a
practical model with 3/4" pipe, then I would guess its loss would be low.
Also note the input voltage is 27 kV with a kW input.

73,

Frank

Here is the code

CM Octagonal loop
CE
GW 1 9 0 0 30 9 0 30 0.03125
GW 2 9 9 0 30 15.364 0 23.636 0.03125
GW 3 9 15.364 0 23.636 15.364 0 14.636 0.03125
GW 4 9 15.364 0 14.636 9 0 8.272 0.03125
GW 5 9 9 0 8.272 0 0 8.272 0.03125
GW 6 9 0 0 8.272 -6.364 0 14.636 0.03125
GW 7 9 -6.364 0 14.636 -6.364 0 23.636 0.03125
GW 8 9 -6.364 0 23.636 0 0 30 0.03125
GS 0 0 0.304800
GE 1
GN 2 0 0 0 13.0000 0.0050
EX 0 5 5 00 1 0
LD 5 1 1 72 5.8001E7
FR 0 11 0 0 3.7 0.01
RP 0 181 1 1000 -90 0 1.00000 1.00000
EN

"Frank" wrote in message
news:6XQLf.5171$M52.5078@edtnps89...

"dansawyeror" wrote in message
...
Frank,

Will you please forward the nec code?

Thanks - Dan


PS, inserting a 50 pF capacitor on the opposite side to the feed point -- as
John mentions on his web site -- makes the free space input impedance 1.1
ohms. 30 ft, at the top, above an average Sommerfeld/Norton ground, the
input impedance rises to 2 ohms.

Frank