[Elecraft] open wire feeders

[Robert G. Strickland]

Don...

Many hams - as I have done/do - use coax to get out of the house, 
connect it to a balun, and then connect twinlead from the balun to an 
antenna. There is a lot of commentary on this setup, but perhaps another 
time through would be helpful. The questions arise:
- if the coax is short, say under ten feet, is this setup more or less 
equivalent to running the twinlead all the way from the antenna to the 
transmitter?
- again, if the coax is short, will RG8 or 213 be sufficient to the task?
- is there any advantage of one balun ratio to another [1:1, 4:1, 9:1]?

Happy New Year, and thanks for your contributions here on the Elecraft 
reflector.

...robert

On 12/31/2011 17:42, Don Wilhelm wrote:
> George,
>
> Short question, long answer follows --
>
> Do to constraints at home, I no longer use open wire or ladder line
> feeders, but when I did use them, I found several things were true if
> you did not want them to radiate (and create RF in the Shack).
> My first rule is to use balanced antennas - off center fed antennas are
> famous for feedline radiation and RF in the shack.
> The second rule is to run the feedline away from the antenna at right
> angles for as great a length as you can manage, but certainly for a
> quarterwavelength - The feedline can pick up radiation from the antenna
> if this rule is not followed.
> Third is to run the feedline correctly - use nice gentle bends if you
> must change direction, support it using as few hangers as possible (if
> you can put the feedline under tension, you can get away with very few
> supports) but support it so it is stable even in the wind.  Do not run
> it parallel to other conductors, but you may cross a conductor at right
> angles if necessary.  The line should be spaced away from other objects
> by at least 3 times the spacing of the conductors.
>
> Lastly, If I could, use a true balanced tuner, link coupled is best, so
> if you see a Johnson Matchbox at a hamfest, get it.  If you must use an
> unbalanced tuner, use a good balun at the output (see K9AY's info on
> baluns).  BTW, do not assume that a 4:1 balun is the thing to use, the
> feedpoint impedance in the shack can vary wildly from very low to very high.
>
> If you do encounter a high impedance feedpoint on any band, that will
> place a high RF voltage point at the shack end - add or subtract some
> feedline to bring the feedpoint impedance down.
> If you do not understand how the  feedpoint impedance changes with the
> length, take a look at the Antenna article on my website www.w3fpr.com
>
> 73,
> Don W3FPR
>
> On 12/31/2011 11:25 AM, W2bpi1 at aol.com wrote:
>> Those of you using open wire feed lines. How do you keep RF out of the
>> shack? 73 George/W2BPI K2/100
>>
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-- 
Robert G. Strickland, PhD, ABPH - KE2WY
rcrgs at verizon.net
Syracuse, New York, USA