[Elecraft] KXAT1 with dipole

[Ron D'Eau Claire]

Dan wrote:

I was out in the woods of Oregon today (up by the coast range town of 
Jewell) with my KX 1 and a few different antennas. I used a long wire 
for a while, but then for the heck of it I set up a 40M inverted vee 
with my portable mast. The KXAT1 had no trouble with the long wire 
(and counterpoises) bringing it right down to 1:1, but no such luck 
with the dipole ( cut to resonate on 40, and fed w/RG 174).I've used 
this antenna a bunch w/ the K2 and it's pretty much a 1:1, but when 
using the KX1 atu I got around 9:5 SWR! 

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You should not have had any trouble with a center fed dipole, Dan. The issue
with the KX1 ATU, as with any ATU, is impedance extremes. Because of the
limited number of inductors and capacitors that could be fitted into the
tiny space available in the KX1, it has a more limited tuning range than any
of the other Elecraft tuners. 

The end of a dipole presents a very high impedance, possibly in the
thousands of ohms. At the other extreme, a very short antenna will present a
very low impedance, often less than an ohm. Actually, end-fed short antennas
often tune up more easily because they are working against ground and
typically Hams have lousy RF grounds <G>. Even with several "radials" the
ground impedance will likely be in the tens or, more likely, the hundreds of
ohms. That's in series with the antenna so what the tuner is matching is a
total impedance of perhaps a hundred ohms or more. The fraction of an ohm
that the antenna represents is just part of the total. It's also why such
antennas have a very, very low efficiency: often less than a few percent. 

In your case, I'd suspect you had an unlucky length of feed line. Unless the
SWR is truly 1:1 on that feeder, something that is almost never achieved in
practice, the feed line acts like a random impedance transformer. I suspect
you got unlucky and the combination of the effect of surrounding objects,
antenna length, feeder impedance and feeder length all added up to an
impedance the KX1 ATU couldn't handle. Adding a few feet of feed line will
usually fix that. 

Another possibility is a short or open, as you suggested. 

Ron AC7AC