[Elecraft] K2 Antenna Tuner Question

[Julian (G4ILO)]

An interesting thought indeed. I've been waiting for an excuse to write
about this...

My antenna, for reasons you can read about on my website if you're
interested, is a loop of wire run round the periphery of my loft. Since
this is a new English house, it's a small loft, which will come as no
surprise to anyone who has visited this country. Ever since I moved here, I
have fed this antenna with an SGC SG-239 autocoupler at the feedpoint, in
the center of one side. The loop is not resonant at any amateur frequency,
so its SWR varies from quite high to unmeasurable (i.e. K2 CAL S reads
9.9.)

After I found out how great the mismatch losses can be using even a couple
of metres of co-ax when the SWR is very high, I felt that matching the
antenna at the feedpoint was the only practical solution. (Since the radio
shack is also my home office and has to look tidy, running a length of open
wire feeder up the wall and into the loft is not an option.)

I have been quite happy with the performance of this antenna for over a
year, but there was a niggling annoyance that I was not making use of the
excellent KAT-2 that I took the trouble of building. So, a couple of weeks
ago, I swapped the SG-239 for a 4:1 balun and connected this via about 2.5
metres of RG-213 to the K2 in the room below. I expected to be
disappointed, and for this to work not as well as the previous arrangement.
Boy, was I in for a surprise!

When I switched on, signals on 20m seemed to be about couple of S-points
stronger than before. In the evening, the background noise level is so high
that the first S-meter bar is always illuminated, which it never was
before. All the bands from 20m - 10m seem better than before. When I went
on the air for a quick test, I could hardly get off again for people
"tail-ending." I wasn't convinced that there was any improvement on the
lower bands, but I really have no expectation of being able to work below
20m with such a small antenna. Well, I have just come off the air after
making several QSOs on 30m, the first time I have tried it, and my QRP
signal got reports of 579 and 589. It's nice to see SWRs of 1.0:1 or 1.2:1.
Often, on 10m, the best the '239 would produce was 2.5:1.

I leave it to the reader to decide whether the SG-239 is a piece of junk or
the KAT-2 is simply magic. Where was all that RF going? All I can say is
that, more than 3 years on, K2 #392 still manages to delight me. And I
continue to be astonished at what you can work with 5 watts and a loft
antenna.

73 de
--
Julian, G4ILO. (RSGB, ARRL, G-QRP, K2 #392)
G4ILO's Shack: http://www.qsl.net/g4ilo

Ron D'Eau Claire wrote:

An interesting thought. So if the ATU achieved a match to the
non-antenna, what was the efficiency of the ATU at that setting? To
provide a match, it has to absorb all of the r-f power being put out by
the rig, since the "non-antenna" clearly is not...

So, an ATU efficiency of Zero percent? (Great smokin' toroids, Batman!
All that r-f is "heat" in the ATU!)=20

[snip]