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To: wv5s@icon.net
Subject: Re: TopBand: 160/80 shunt fed tower?
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Hi Jim"
W8UVZ wrote:
>>"Jim
>>I have used my 78ft tower for years as a 160M shunt fed vertical. It has
>>plenty of top loading - 5 el 20M KLM yagi and a Mosley 2 el 40. Works
>>great on top band.
>>I decided to try it for 80 and put another gamma rod on the side opposite
>>the 160 M gamma. Using an omega match shorter than the other, I was able
>>to easily match it to 1:1 SWR but it just didn't play. We concluded that
>>it was substantial taller than a quarter wave on 80 and probably
>>approached a half wave. I finally tied the two gammas together for 160 to
>>increase the bandwidth.
>>Suspect u have the same "too tall problem" as we calculated with the top
>>loading u and I have on the towers, u'd need only abt 30 ft of tower to
>>reach an electrical quarter wave on 80 M. Make sense??
>>73 George
>>W8UVZ"
>Hello Topbanders,
>
>I have a 65 ft tower with a TH7 and 40-2CD on top and 16 radials from
>1/8 to 1/4 wavelength long. I have shunt fed it for 160m with a
>jumper across the 18 inch spaced shunt at abt 50 ft. It seems to
> work pretty good. The SWR is almost flat across the entire 160m band.
> Is that typical?
>
>I also tried to add an 80 meter shunt on the opposite side of the
>tower. I havn't been able to find an optimum jumper point on the
>shunt. The lowest SWR has been 2:1 and the bandwith is quite narrow
>(which is OK). The performance has been poor. I probably just need
> more time to refine it. My question regards interaction between the
>two shunts. It appears that the 160m performance has decreased since
> I added the 80 m shunt. Could this be true or is it just my biased
> judgement or band conditions?
>
>Have any of you had success with a combo 160/80 m shunt fed tower?
>Any guess where the 80 m jumper would be with 18 inch spacing?
>
>Thanks for reading my questions and for your response if you can
>help.
>
>73,
>
>Jim Hood, K5TT
>President, Oklahoma DX Association
>wv5s@icon.net
>Past operations: KC6SS, V63SH, /XE, /WH6, /NH7, /KP2
>Previous calls: WV5S, W0PUD, W7KDH, K5TWD
>USA-CA # 432
George W8UVZ
I looked up the tables from John True's article in May '75 Ham Radio
Magazine. He says about a 30 foot rod for 80 is OK.
The problem may be that you are electrically close to a half wave on
80 meters and the vertical wants to look like an inverted ground plane
with the antennas on top as radials, and a 1/4 wave radiator down to
the ground. Since you then have a voltage point at the real ground
plane, the RF might get confused by the geometry and either to up
through the top loading or just not go far.
You are probably better off hanging a vertical wire parallel to the
tower, maybe 4 - 6 feet out, and feeding it as a simple ground plane.
A piece of the UV resistant conduit makes a nice yard arm. If that
doesn't work I guess you will have to go to an antenna away from your
tower, or hang an inverted V off the tower at 55 feet and add a choke
to the coax feeder to not mess up 160. One of the guys in Chicago did
that quite successfully.
73 the other George in Battle Creek, K8GG.
--IMA.Boundary.997301948--
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