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Re: [TowerTalk] question about constructing a doublet

Subject: Re: [TowerTalk] question about constructing a doublet
From: Steve Hunt <>
Date: Wed, 12 Nov 2008 22:52:47 +0000
List-post: <">>

With a ladder-line / tuner arrangement you don't need to worry about 
cutting the antenna for a particular part of the band. But you do need 
to worry about whether the antenna/ladder-line combination will present 
an impedance which is outside the range of your tuner.

Your particular arrangement looks fine on 40m. The worst case is at the 
top end of 80m where the tuner will see an impedance of [1000+j1300] @ 
3.8MHz and [2500-j600] @ 4 MHz. I don't know your particular tuner - 
others may be able to advise on whether it will handle these values. A 
4:1 balun would help get the impedance down on 80m.

Shortening the antenna to 60ft per leg would avoid the worst of the 80m 
problems and still be fine on 40m.

Steve G3TXQ

k2vi wrote:
> Im building a doublet for primarily 75 and 40 meter phone. It is being fed 
> with 450K # 14 stranded ladder line, the good stuff. Im confused on the 
> lengths. The run of ladder line will be around 45 feet directly to the atr-30 
> tuner. no EXTERNAL BALUN. Also the length of the antenna im thinking 65 feet 
> per leg but that falls on 3.6 mhz but makes it a full wave at 7.2 dead smack 
> in the 40 phone band. What should i do cut the antenna for the center of the 
> phone band on 80 or keep it at 65' per leg. If anyone has an idea what length 
> of feed and antenna would make a perfect load for both bands would be greatly 
> appreciated. thanks guys.
> tony k2vi
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