Re: K3 power reduction due to high SWR

Posted by Vic Rosenthal on Apr 05, 2012; 4:28pm
URL: http://elecraft.85.s1.nabble.com/K3-power-reduction-due-to-high-SWR-tp7439053p7440544.html

Speaking of balanced lines and tuners, I recently did an experiment that surprised me.

I have a dipole fed with 600-ohm open-wire line (and some window line indoors). I usually
tune it with a Johnson Matchbox, a fully balanced link-coupled tuner.

I was lucky enough to get my hands on a massive, high-quality edge-wound rotary inductor
in a well-made aluminum enclosure. So I built a high-power T-network tuner with it. I
purchased a good (and expensive) 1:1 balun designed for use after a tuner from DX
Engineering, and compared the T-net + balun to the Matchbox. I didn't notice any
difference in the strength of received signals between the tuners.

But here is the surprising part: I expected that the Matchbox would provide better
balance, which would reduce radiation and pickup from the feedline. To test this, I
coupled a grid-dip oscillator to the feedline about 20 feet from the tuner.

To my great surprise, the signal from the oscillator was much weaker with the T-network +
balun than with the Matchbox!

Signals the same, 'noise' weaker. So much for the need for 'truly balanced' tuners!

On 4/5/2012 12:28 AM, Adrian wrote:

>
> 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

--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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