[Elecraft] open wire feeders

[Don Wilhelm]

Robert,

When that is done, I recommend doing several things:
1 - prune the balanced feedline until the feedpoint impedance is close 
to the characteristic impedance of the coax.  You may not be able to 
accomplish that on all desired bands, so plan to switch in extra 
feedline to accomplish that goal.

2 - If the parallel line feedpoint impedance is in the 150 to 300 ohm 
range, use a 4:1 balun (make certain it is a common mode choke type as 
has been recently pointed out.

3 - If the parallel line impedance is in the 25 to 100 ohm range, use a 
1:1 balun (or more properly a common mode choke - see Jim Brown's Balun 
articles.

Actually, the only thing accomplished by this technique is that the coax 
can be run into the shack more readily without regard for keeping the 
line away from other objects.
It is just as easy to use two parallel lengths of coax with the shield 
grounded and the balanced feedline connected to the two center 
conductors.  That will produce a shielded balanced line that you can 
bring into the shack just like a single coax.  As I said, the best is a 
link coupled balanced tuner, and no balun is needed at all.  The link 
coupling takes away concerns about common mode current and does a lot to 
cure RF in the Shack - unfortunately such couplers are "scarce as hen's 
teeth", and although you can easily build your own on a piece of wood 
(no need for shielding), they are not readily adaptable to 
bandswitching.  Most of the ones I have built use plug-in coils.

73,
Don W3FPR

On 12/31/2011 1:40 PM, Robert G. Strickland wrote:
> 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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