[Elecraft] open wire feeders

[Sandy]

The only LARGE problem that rears it's ugly head is a large amount of 
inductive or capacitive reactance often times present that the actual balun 
has to cope with.  This dissipates power no mater what the VSWR on the open 
wire line happens to be.  You WILL NOT be transforming a 50 ohm line to a 
200 ohm line (4:1 transformer) OR a 50 ohm line to a 450 ohm line (9:1) 
There will be always some reactance present.  If you run higher power 
(500-1000 watts or more) this may actually ultimately destroy the balun 
transformer itself!  This effect doesn't seem to be as radical with "choke" 
type (ferrite beads over a run of coax) compared to a "transformer" type 
balun.  I think you would be better off in the long run, in this instance 
with a "choke" type balun and use a coupler between to rig "power source" 
and the "load" Choke balun/ladder line/open wire feeder.  The idea is to 
keep the coax part as short as possible and let the tuner deal with the 
oddball reactances that occur on the line.

Over the years I have had troubles and seen other with same syndrome trying 
to let a transformer balun compensate for a impedance transformation under 
the duress of a HIGH reactance present which seems to destroy things 
eventually, AND radiate less useful power rather than it would other wise if 
the reactance was "tuned out".

I hope I am making myself clear.  In my old setup before my XYL had a 
stroke, causing me to stop using a homebrewed balance line tuner (ladder 
line feeder entering the shack directly) worked most effectively.  The 
dipole was 135' long at 50' and fed with about 110 feet of 450 ohm ladder 
line.  I am unable to erect a similar antenna from the master bedroom where 
the rig is now and had to resort to using an end fed wire again.

73,

Sandy W5TVW

-----Original Message----- 
From: Robert G. Strickland
Sent: Saturday, December 31, 2011 12:40 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] open wire feeders

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
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