Re: Topband: feeding a unipole

[Herb Schoenbohm <herbs@vitelcom.net>]

john wrote:

> Hi. Is there anyone who has used a folded unipole with sucess who would like to share with me, the proper method to feed it to get a decent match some say a L/C network if so what would be the amount of turns on the coil and size of the capacitor i'm trying to help a friend fireup a new 110' tower and would like to try a folded unipole approach..i'd appreciate any input..John K1GUN


John,

I am using a 3 wire cage which is dropped from the 50 foot tap on a 68 
foot Rohn 45. the tower has a cushcraft A3S at  75 feet and a 15 foot 
1.5 inch vertical stinger on top of that.  I tried for weeks to find the 
50 ohm tap point from the tower to the cage which is made from number 
#19 copper before giving up.  So I use a CLC "T" network to get 1:1 on 
the cage.  This consists of series (withthe feedline) 200pf input fixed 
15 KV ceramic door knob, a 30 uh coil to ground, and another 500pf 
vacumn variable to the cage wire.  I can get an excellent match by 
taping the coil.  Only problem is that the bandwidth is only good for 
about 20 Khz and I am sure that it would be much better is I could 
eliminate the flat wound coil and input capacitor .
IMHO I believe that the A3, with its insulated element and short boom, 
don't give me as much capacity loading that I require to have something 
close to a quarter wave.  I thought the 15 foot stinger about the A3 
would help but I might be kidding myself.  The good news is that the 
antenna works well against the 16 1/4 radials and my reports are better 
than ever before.  the bad news is the bad width which keeps me pinned 
down to a 2:1 edge limit 15 Khz inside the bandwithout using the inline 
tuner. I have a good bridge but my reading are over 400 ohms inductive 
reactance and 400 ohms impedence.  I am still confident that my antenna 
will broaden out with more toploading asit was with the 204BA 20 meter 4 
element up there.(This antenna was lost in the hurricane season this 
year.  I am hoping to get Earl, K6SE to model this for me beforethe 
contest so I can make things better.   My mistake was believing the 
theory opf the wire cage walone would compensate for the reduce height 
but that does not seem to be the case.  I think also if I but a small 
choke 20 turn coil from each side of the A3S element to the boom I may 
get some more top loading but probably not enough to bring the self 
resonance with in the 160 meter band. I have even considered putting up 
the 204BA boom alone under the A3 could help.

In short this setup work well for signal launching but the bandwidth is 
very narrow probably due to the shortness of the tower and the macthing 
circutry.  I have thought of going to an omega match but I am afraid 
after reading Tom's treatise that that will not give me much more. Maybe 
someone can stear me in the right direction so i can make the whole 
thing work with a single series cap rather than the CLC network.

Good luck on your project and let me know what you come up with.

73

herb Schoenbohm, KV4FZ

>  

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