Posted by
vk4tux on
Apr 05, 2012; 7:28am
URL: http://elecraft.85.s1.nabble.com/K3-power-reduction-due-to-high-SWR-tp7439053.html
I disagree, your coax suggestion has 32% loss against a good wl installation
which would provide 9% loss.
Use of RF ammeter on each leg can be used to balance the loading via antenna
leg adjustments if required.
A dedicated eg: Johnson matchbox or GQ balanced tuner etc at TX would give
good results.
The 450 ohm to 72 ohm connection is of no issue centre fed. The ladder line
length will have an impedance transformation effect.
Trying for common multiple of half wavelengths on FL length -vf at the
frequencies used will be a good idea if possible to represent the feedpoint
impedance at the TX end if possible. In any case the tuner will take care of
the transformation.
Any decent low loss coax on this length will cost lots.
I assume that the guy can't get the antenna closer, trees in the distance
etc probably, but I have seen many good examples from the USA of successful
long distance 450 ohm wl runs with wire antenna's.
A ladder line setup here will outperform your RG11 etc by 1.2dB.
When wet your RG11 setup will have a 1.4dB advantage. I guess it all depends
on the weather, and the guys bank balance.
213 is ordinary for coax, I much prefer lmr400, which I use with my BD
4115-ocf.
vk4tux
-----Original Message-----
From:
[hidden email]
[mailto:
[hidden email]] On Behalf Of Jim Brown
Sent: Thursday, 5 April 2012 3:26 PM
To:
[hidden email]
Subject: Re: [Elecraft] K3 power reduction due to high SWR
> If I have to use your antenna, then my move would be to replace the 250'
> of75 ohm coax with 250' of 450 ohm ladder line, check the fan dipole
> legs are exactly the same to provide a balanced load.
You're on target about loss in the feedline, but ladder line is not a good
move here.David has resonant dipoles, so they are a pretty good match to 50
or 75 ohm cable. Folks think QRP, so they think small coax, But when the
line is as long as these are, the loss certainly gets out of hand unless
it's bigger coax. If I were to make ANY change in the feedline, it would be
to a decent RG8 or RG11. I would also look at ways to reduce the length of
the line by finding a shorter route to the shack. You don't need expensive
coax -- Davis 213 is about as good as it gets for the HF bands, and it's
fairly inexpensive.
Window line is NOT the cure for all ills -- it is as lossy as small coax
when it gets wet, and there's some loss (and cost) in any impedance
transforming balun. Further, most real antennas are unbalanced by their
surroundings, so they need a good ferrite common mode choke at the feedpoint
whether they're fed by coax or parallel wire line.
73, Jim Brown K9YC
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