[Elecraft] open wire feeders

[Don Wilhelm]

Sandy,

Take the information on K9YC's balun comments to heart.  Jim has done a 
wonderful job of measuring the isolation effect of baluns (more 
specifically common mode chokes).

Yes, the reactive and resistive components of the feedpoint impedance 
play a large part in how the whole system reacts, but that can be varied 
with a change in the feedline length (see antenna article at 
www.w3fpr.com).  Keep the feedpoint impedance within range and all will 
be well for baluns, tuners and all other parts of the antenna system.

Keep in mind that the length of an unmatched feedline is critical to the 
success or failure of such a system.  That is a factor that is seldom 
mentioned in posts that say a particular antenna "works well" and "loads 
well" - the simple fact is that the feedline type an length are critical 
elements in that antenna system - to say that a 100 foot dipole works 
great is not sufficient, one needs to state the type and length of the 
feedline as well as the length of the antenna.

73,
Don W3FPR

On 12/31/2011 9:59 PM, Sandy wrote:
> 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
>