[Elecraft] KX-1/MP-1 SWR limits

[W3FPR - Don Wilhelm]

Folks,

Of course Lee is correct, but the range of the KXAT1 should also match a low 
impedance - and short antennas do produce a low impedance even if the 
reactance is close to zero.

Whether you need a 'unun' (unbalanced 'balun') at the antenna or not depends 
a lot on the length of the coax to the antennas is and the operating 
frequency.  The coax losses increase with feedline length and also increase 
with frequency.  With a short coax run, such as that normally encountered in 
a portable situation, the losses should be quite low, and the only question 
is the matching range of the KXAT1.

An antenna having a low impedance at the feedpoint will translate into a 
high impedance with a quarter wavelength of coax attached - so figure 
whether you need a balun such as the Elecraf BL1 (1:4 balun) or a 1:1 balun 
(alternately unun) depending on the antenna feedpoint impedance AND the 
feedline length.  In antenna situations, look at your feedline length in 
terms of how many quarter wavelengths it is - if it is an even number it 
will repeat the antenna impedance, but if it is an odd number of 1/4 waves, 
it will transform a low impedance to a high impedance and vice-versa - if 
the length is somewhere in between, it will still act as an impedance 
transformer, but you will have to do a bit more involved calculation to 
determine the impedance presented to the KXAT1 output.

73,
Don W3FPR

----- Original Message ----- 

> Alan's Balun from WB6ZQZ will help low impedance antennas such as 
> electrically short verticals, and is an easy build.  Placed at the antenna 
> base and hooked to coax with a BNC connector, it will reduce SWR on the 
> coax feed, and ease the matching job of the KXAT1 at the rig end of the 
> coax.
>
> Elecraft's balun has a step-up transformer to go to high-impedance 
> antennas, but Alan's Balun has a tapped step-down transformer to match to 
> a Pac-12 or MP-1.  A plus is that once you find the right tap, you can use 
> the Tenna Dipper and actually hope for something closer to 50R 0X.
>
> I built one and it was pretty fun.
>