[TowerTalk] 30/40/80 DIPOLE (balun/strain relief ideas needed)

[Bill Coleman AA4LR]

On 6/22/00 10:04 AM, Mark . at n1lo@hotmail.com wrote:

>Your absolute best bang for the buck is an 80 meter doublet (homebrew) , 
>strung up in a flat-top configuration, and fed with parallel wire feedline, 
>such as 300 ohm tv twinlead (very cheap and light) or 450 ohm 'ladder line' 
>(much stronger and durable). This requires either:
>1) A tuner in the shack having open wire outputs, or
>2) A remote 4:1 balun, located at the entrance to the shack, such as the 
>Radio Works "remote balun", a coax jumper, and a tuner with coaxial output.

I generally agree here. And would add a few points:

* Mount the doublet has high as you can. The higher the better. 50-150 
feet if you can.

* Don't use TV twinlead. Use manufactured line with windowed slots (450 
ohm), or make open wire line yourself. Look up my Open Wire Spacers 
summaries in the May 2000 archives of TowerTalk. (See 
<http://www.contesting.com>) TV twinlead has a lot of loss.

* For the coax jumper with the remote balun, use RG-8 or RG-213 solid 
dielectric coax. Keep it as short as possible. Since this coax carries a 
complex impedance, high voltage points may appear on it, so using the 
larger, solid dielectric coax prevents potential arcing.

* Overbuild your remote balun. Since it has to handle complex impedances, 
there's little difference of 1:1 or 4:1 or other transformations. Use a 1 
kW balun for a 100 watt transmitter. 

* Use a good tuner. Roller-inductor tuners are more expensive, but much 
easier to get a good match. 


Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org
Quote: "Boot, you transistorized tormentor! Boot!"
            -- Archibald Asparagus, VeggieTales


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