TopBand: Advice on double Shunt-Feed
Donald B. Hall
donhall@myriad.net
Mon, 15 Dec 1997 07:32:45 -0800
Alberto,
I have used multiple shunt feeds on my tower for many years with
good results. I drop them from separate corners of the triangular
tower to individual tuning capacitors.
My tower is only 54 feet so the 160M feed is at the top and the
tow feeds for 80/75 are at the second tower section, about 35 feet.
There appears to be no interaction and I can tune any one of them
for resonance individually.
I have recently replaced two of the boxes with plastic rather than
metal and eliminated some arcing which seems to be caused by
corrosion around the feed-thru insulators. Electrical boxes from the
home improvement store work fine.
I recommend separate feeds.
73 Don K5AQ
----------
> From: Alberto Perello <ea3cr@redestb.es>
> To: 'topband@contesting.com'
> Subject: TopBand: Advice on double Shunt-Feed
> Date: Sunday, December 14, 1997 12:02 PM
>
>
> One year ago I install a shunt-fed in my self-supported tower 65 feet
high, in order to work the band of 160m. Up of the tower is installed a KLM
20M-5.
>
> The gamma-match is mounted with number 10 wire a distance of the tower of
22" and it goes from the top of the tower (65 feet) until the box of the
air variable capacitor (1/14"- 250pf) that is 24" of the ground.
>
> It is necessary to add that I have an earth of excellent conductivity and
a good ground system.
>
> The SWR is 1:1 with the capacitor working to about 100 pf and the
bandwidth is reasonable. I have arcing only with my 3CX1500A to full power
and when it rains. If I reduce a little the power the arcing disappears
even raining.
>
> This installation supplemented with an EWE for 160/ 80m in reception, has
provided me good results and this me encouraged to prove of making a shunt
feed to the tower for the band of 80 meters and it is here where I need
advice.
>
> In the band of 80 meters the wire of number 10 is parallel to the wire of
160m in a distance of the tower of 15" and it goes from the level of 28
feet until the same box of the air capacitor for 160 meters, but using
other air capacitor 1/ 14"- 250pf.
>
> The line of feeding (RG-213) is common for the two bands (160/ 80). The
design is similar that of W6SAI in their book "Beam Antenna Handbook " page
65 "The Tri Match Gamma".
>
> In definitive the shunt feeds are in parallel. I know that this system
could create any problem of harmonics.
>
> The result is that seemingly the antenna of 160 meters has experienced no
modification in their parameters (remain analyze the diagram of radiation).
>
> I have measured with my MFJ-259 to the exit of the box of the gamma
before and after installing the shunt-feed of 80m and the SWR continue
being 1:1 and of course, with a resistance of 50 ohms.
>
> Adjusting the air capacitor to minimum SWR it remain at about 100pf in
3,700 and the SWR to 1:4, and the resistance is only 10 ohms. The correct
resonance is in 5,300 with the air capacitor working with a few pf, almost
in their residual.
>
> I have proven of adapting the impedance with a second air capacitor
(omega match), without any result
>
> I have also moved the gamma wire closer and farther from the tower, only
for the part of the connection to the gamma box, without any apparent
result.
>
> With these data could somebody suggest me for where continuing?
>
> Going up or lowering the taking of the gamma in the tower ?, exist
interaction with the wire for 160m and this change the measures of the
gamma for 80 meters?, maybe there is others alternatives?
>
> Thank you in advance for the advice.
>
> 73 from Barcelona.
>
> Alberto Perello
> EA3CR
>
>
>
>
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