[UK-CONTEST] Vertical Dipole - balun placement

[David G3YYD]

Best use of a 2nd Choke is a quarter wave along the coax. The close as 
possible to the antenna choke produces a very high impedance (>2K for a 
SEK choke) which by the quarter wave section is transformed to a very 
low impedance. Putting another high impedance  choke at this  point will 
produce the maximum attenuation of braid current and hence very low 
radiation from the coax. If there is a lot of coupling between antenna 
and coax then another choke at another quarter wave along coax would be 
good.

Using RG-58 to wind Ian's choke baluns is good for 400 watts continuous 
at 30MHz. Those of you from higher power countries would need to use 
RG-142, which is the same size as RG-58 but rated 3KW continuous at 
30MHz. RG-142 has the same loss as RG-58 but does not melt until at 
least 200c is reached due to the use of PTFE in place of the 
Polyethylene of RG-58.

73 David G3YYD

On 26/07/2011 14:34, Stewart Rolfe wrote:
> Jim,
>
> Are you using a choke balun at the feed point? That'll make a difference...
>
> I used a vertical Moxon for 20m in ARRL DX last February and experienced the same thing. Fortunately was able to keep the feeder almost perpendicular coming away but the choke balun (a la GM3SEK) made a big difference with 'touchiness'.
>
> I assume without the balun the feeder is, to a greater or lesser extent, acting as another radiating element and interacting when not at 90deg from the vertical element. With the balun you just have to worry about induction on the braid from your dipole, so keep it straight out if possible. Perhaps a second choke at the tx end?
>
> 73,
>
> Stewart, GW0ETF
>
>
> --- On Tue, 26/7/11, mm0bqi<mm0bqi at blueyonder.co.uk>  wrote:
>
>> Set them up in
>> the garden yesterday
>> and they are very touchy regarding the angle at which the
>> coax leaves the
>> feed point.  Changes the resonant frequency and
>> SWR.
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