[Elecraft] KX-1 antenna question

[Terry Conboy]

At 07:13 AM 2005-07-28, Lloyd Lachow wrote:
>Posted for Eric N0HHS KX1 #670 <eric.ward at verizon.net>:
>
>So I have settled on a 26-28' wire with a 16' counterpoise.  I found
>that a 24' wire working against a 16' 'poise would not always load up on
>40m--it needs to be at an obtuse angle to the plane of the counterpoise
>to tune well on 40m with the internal tuner.
>
>In contrast, the 26-28' piece will tune to <2:1 on 40m even if strung
>carelessly over doors, picture frames, or whatever else is available (e.g. in
>a hotel room with windows that don't open).
>
>20 and 30m seem to tune fine with the 24'/16' ant, even if 
>suboptimally deployed.
>
>Question: Assuming both tune up about as well, do you think the 24' 
>ant with a longer (33') counterpoise will radiate better on 40m than 
>a 26-28' ant with a 16' 'poise?  I realize there are many 
>uncontrolled variables involved, but maybe it is simpler than I think?

The feedpoint capacitive reactance with 27-foot antenna wire and a 
single 16-foot counterpoise wire is quite high at 7 MHz, on the order 
of 680 ohms, depending on proximity to ground, etc.

You might want to try adding a second 16 foot counterpoise wire, 
separated from the first by 45 degrees or more.  This causes the 
reactance at the feedpoint to drop to around 450 ohms, which should 
make for an easier match (and less loss in the KX1 tuner).

In my EZNEC model, with the two 16-foot counterpoise wires 2 feet 
above average ground, the overall efficiency rises by about 1 dB, as 
well.  This configuration appears to produce relatively good 
efficiency on 20 & 30m, too.

Using a single 24 foot counterpoise wire might be a workable 
compromise.  The 40m reactance is around 420 ohms.  However, the 20m 
impedance rises from 225+j523 to 289+j977 when going from a 16 foot 
to 24 foot counterpoise, so some experimentation might be required to 
see how the KX1 handles this.

A single 33-foot counterpoise wire is about 1/2 wl on 20m, so it 
might make matching there difficult due to the resulting high 
impedance.  The overall losses increase by around 3 dB, too.  On 40m, 
my model also shows slightly lower efficiency (-1 dB or so).  This 
might seem odd, but since the counterpoise wire is longer than the 
"antenna" wire and it carries at least as much current, there is a 
tendency to have more radiation from the counterpoise than the 
"antenna" proper.  Since the counterpoise is probably closer to 
ground, etc., the losses may be somewhat higher and the pattern degraded.

(Note that using two 33-foot counterpoise wires, 180 degrees apart is 
a different story on 40m.  This is a common "elevated radial" 
configuration and the efficiency is much better since the radiation 
from the two wires tends to cancel in the far field.  Of course, the 
impedance would still be high on 20m.)

73,
Terry N6RY