[Elecraft] KX1 SWR limits: general discussion [+ bonus LED Logbook lamp tip]

[wayne burdick]

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