Hi Fred. You wrote:
My KX1 has 30m and the ATU .... I've been using my MFJ-259B to
pre-tune the antenna (with the goal of having the inductor settings
marked so I don't have to take the 259 and try to explain that at the
security checkpoint in the airports!), and then count on the KX1 tuner
to handle the rest. So far, on 40m and 30m seem to come out OK. On
20, with the pre-sets I've been using, the best I can get is about 3:1
according to the KX1 after the ATU relays quit.... Any ideas on what
is an "acceptable" (in terms of actually being
able to work someone) SWR for the KX1?
The actual loss in radiated signal due to a 3:1 SWR can be as little as
a fraction of a dB, assuming the antenna is still an efficient radiator
(in the clear, good ground, low-loss feedline, etc.).
The real question is, how high can the SWR be without causing trouble
for the KX1's transmitter? We tested it into an "infinite" SWR (open or
short) for brief periods, and it's hard to break it as long as you
don't key down for a long time in this condition. So typical CW duty
cycles at 3:1 should never cause trouble.
Still, at very high SWRs you can end up with high RF voltages on and
inside the chassis, so it's best to aim for a lower SWR in general (3:1
or lower). If the KXAT1 can't get you there, it could be because the
impedance presented is outside the available range, OR because the
needed LC values fall directly between two of the available
combinations. The LC values we used in the KXAT1 are a tradeoff between
these two cases, and they favor longish, end-fed antennas of the
wire-in-a-tree sort, not short verticals.
Take the Maldol 20-meter whip, for example. The whip is very
narrow-banded and is cut for the SSB part of the band. The KXAT1 may
only tune it to around 1.7:1 or 2:1 given the relatively coarse
reactance step sizes on this band. By clipping an extra foot of stiff
wire to the end of the antenna, you can shuffle the deck a bit and give
the KXAT1 another chance--and the ATU may tune it to 1:1. In your case
(MP-1), you can try altering the length of the radiator a bit, or
slightly *detune* the coil with respect to the optimal value shown by
the antenna analyzer.
In all cases, a viable ground is of more importance than low SWR.
Suppose you hold the KX1 in your hand, connect the MP-1 to the rig and
hear lots of signals on 20 meters when the antenna is tuned to
resonance -- even with no ground radials. While this can work well on
receive, you'll be down some 8 to 15 dB on transmit vis-a-vis having
even one 1/4-wave trailing ground wire. Adding more than one radial
will usually improve transmit efficiency further, but that first one is
critical.
(Note that the ideal length of a trailing ground wire may be somewhat
shorter than 1/4-wave, say 12-15 feet on 20 meters. Also, even this
short ground wire will provide usable transmit efficiency on 30 and 40
meters as long as you have a fairly long radiator to go with it.
Performance on lower bands with a short ground wire will vary greatly
with ground conductivity.)
Of course, a wire tossed in a tree, even one that's only 10 to 18 feet
long, will almost always work better than a short whip. I did lots of
empirical studies of 10- to 30-foot wire antennas and concluded that
the best overall length is about 24 feet. This will typically provide a
2:1 or better match with the KXAT1 on all three bands (20, 30, 40 m).
In fact we optimized the LC values for this case. All tests were done
with *no* coax, by the way. You can connect the antenna and ground
directly to the KX1, as explained in the KXAT1 manual.
When you start working on that pile of vacation novels, don't forget
that you can use the built-in LED logbook lamp as a reading light. It
draws just 7 mA, and will run circles around incandescent "Itty-Bitty
Book Lights" in terms of battery life. Plus, our reading light comes
with a transceiver ;)
Have fun on your travels with the little rig.
73,
Wayne
N6KR
---
http://www.elecraft.com
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