TopBand: Advice on double Shunt-Feed
Alberto Perello
ea3cr@redestb.es
Sun, 14 Dec 1997 21:02:28 +0100
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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