80 m compact loop - progress report
 

"Frank" wrote in message
news:GbRLf.5174$M52.1045@edtnps89...

"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

PPS I just noticed the 27 kV P-P voltage is across the tuning capacitor.

80 m compact loop - progress report
 

Most people put the series tuning capacitor in the top side and the feed
point at the bottom side. Since the capacitor, motor, and their mounting
hardware is quite heavy, I am currently planning to do the reverse of the
above, i.e., tuning capacitor at bottom and feed point at the top. It is not
clear to me if there is any difference between these two configurations.

John, N9JG

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

[snip]
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.
[snip]

80 m compact loop - progress report
 

"John, N9JG" wrote in message
news:ij%Lf.797528$_o.574035@attbi_s71...
Most people put the series tuning capacitor in the top side and the feed
point at the bottom side. Since the capacitor, motor, and their mounting
hardware is quite heavy, I am currently planning to do the reverse of the
above, i.e., tuning capacitor at bottom and feed point at the top. It is
not clear to me if there is any difference between these two
configurations.

John, N9JG

I have not attempted to model the gamma match, but doubt it will change the
current distribution. Feeding at the top places the minimum current at the
loop base which seems to effect the high angle radiation only. The gain
straight up is about + 2 dBi, and -3 dBi when reversed. Low angle
radiation, for example at 10 deg elevation, is - 3 dBi in both cases.
Removing the capacitor totally reverses the current distribution. Maximum
gain occurs parallel to the loop plane.

Frank, VE6CB

80 m compact loop - progress report
 

"Frank's" wrote in message
news:3z0Mf.1838$dg.1085@clgrps13...

"John, N9JG" wrote in message
news:ij%Lf.797528$_o.574035@attbi_s71...
Most people put the series tuning capacitor in the top side and the feed
point at the bottom side. Since the capacitor, motor, and their mounting
hardware is quite heavy, I am currently planning to do the reverse of the
above, i.e., tuning capacitor at bottom and feed point at the top. It is
not clear to me if there is any difference between these two
configurations.

John, N9JG

I have not attempted to model the gamma match, but doubt it will change
the current distribution. Feeding at the top places the minimum current
at the loop base which seems to effect the high angle radiation only. The
gain straight up is about + 2 dBi, and -3 dBi when reversed. Low angle
radiation, for example at 10 deg elevation, is - 3 dBi in both cases.
Removing the capacitor totally reverses the current distribution. Maximum
gain occurs parallel to the loop plane.

Frank, VE6CB

Also top feed increases the input impedance to 2.664 - j0.949, with the
capacitive load 0.1 -j858 at 3.75 MHz. (NEC 4.1 double precision).