Posted by
Don Wilhelm-4 on
Dec 09, 2011; 2:53am
URL: http://elecraft.85.s1.nabble.com/KAT500-update-tp7072932p7076788.html
I read the W8JI comments presented in that link on the Elecraft website.
I was familiar with Tom's work and comments from emailing with him on
this particular subject many years ago.
The one thing that struck me in W8JI's paper is at the end and embodied
in Tom's comment that --
"The irony is, moving the balun to the input mostly works only when the
balun is not needed!"
For those who want a really high efficiency balanced tuner, go to ARRL
publications back several years to find a link coupled tuner. With
switching between series and parallel tuning, it can handle impedances
from very low to very high. The input is DC isolated from the output,
and the input can be either unbalanced or balanced. The output can be
either balanced or unbalanced depending on where you connect the output
tuned circuit. The venerable Johnson Matchbox is an example of a link
coupled tuner. The taps onto the output tank circuit were created by
the differential capacitor in the output tank circuit - that allowed it
to be boxed up into a bandswitchable box without need for connecting the
antenna feeders to taps on the tuner coil. That works quite well , but
it restricts the range of matching impedances to significantly less than
would be obtained by the basic tuner with taps on the inductor. Yes, I
do use these tuners, and they are configured for single band use at the
antenna - 3 coax feeds for HF and 3 for VHF/UHF give me access to all my
antennas The nearest antenna is 200 feet of coax away from the
hamshack, and that is why I use tower mounted preamps for VHF/UHF.
I will never give up my Matchbox until it is pried from my cold dead
hands - I use mine mostly as a test instrument - as bandpass filter as
well as an impedance transformation device. After experimentation asnd
development, the permanent tuners are designed and installed in the
antenna field - I have tuned coax fed antennas for each band from 160
meters through the 432 MHz bands. I use the Matchbox tuner only for
experimental antennas until I can develop a permanent and dedicated
tuner for any one antenna.
OK, those are the advantages - the drawbacks are that that the link
coupled tuner arrangement does not lend itself well to bandswitching,
but i would suggest it be the tuner of choice for situations where the
antenna is used for only a single band - put the tuner on the antenna
feedline and tune it to resonance (and minimum SWR) once and be done
with the settings.
73,
Don W3FPR
On 12/8/2011 6:28 PM, Eric Swartz - WA6HHQ, Elecraft wrote:
> Hi Ignacy,
>
> This is a common misconception. (One which I held until recently. :-) It
> turns out there is no advantage to placing the balun at the input of the
> L-Network tuner. Since one end of the balun is grounded by the input to
> the tuner, it is still presented with the same stresses under high SWR
> situations. Baluns at the input and output both drive balanced loads
> equally well.
>
> We've now put together a web page describing the impact of placing the
> balun at the input or at the output of a L-Network tuner. See:
>
>
http://www.elecraft.com/KAT500/input_versus_output_balun.htm>
>
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