[Elecraft] K2 Antenna Tuner Question

[George, W5YR]

----- Original Message -----
From: "Julian (G4ILO)" <g4ilo@qsl.net>
To: "George, W5YR" <w5yr@att.net>; "Elecraft List"
<elecraft@mailman.qth.net>
Sent: Friday, January 17, 2003 12:46 PM
Subject: Re: [Elecraft] K2 Antenna Tuner Question


> George,
>
> Perhaps what you don't realise is that the KAT2 can be turned off
from the
> menu so that it is effectively out of circuit.

Actually, Julian, I am quite familiar with the KAT2 circuit and its
use. I have done a lot of circuit modelling around the KAT2 component
values to investigate the suitability of the tuner for various
feedpoint impedances.

The point of the suggested
> exercise was to see how well the SG-239 matches the antenna compared
with
> the KAT2, just for curiosity, since the 239 appeared to be quite
lossy when
> installed at the feedpoint.

Thank you for making that clear - I failed to understand how the units
were to be hooked up. You are entirely correct that by removing the
KAT2, one could evaluate the SGC in its stead. I haven't looked into
the SGC schematic so I am unfamiliar with its component values,
relative to the KAT2.
>
> Personally, I suspect that any benefit in using an "optimum feedline
> length" (presumably with the aim of presenting a matchable SWR at
the TX
> end)

Actually a "matchable line input impedance at the TX end . . ."

might be negated by the extra losses incurred by using a longer than
> necessary length of co-ax. I have some charts on my web page that
> demonstrate just how high co-axial feeder losses can be when the
antenna is
> a long way from 50 ohms. I claim no credit for these charts by the
way:
> they were produced using an Excel spreadsheet that was mentioned in
an
> article in RadCom about a year back.

The TLDetails program is outstanding in its capability for doing
t-line analysis and modelling. I believe that AC6LA is the author - it
is part of the XLZIZL spreadsheet system that he offers. It is also
available as a free-standing executable program. Check his website for
downloads.

Actually, our group has given up on coax for any field antenna
operations where multiband performance is required. As you point out,
regardless of how well the K2 can match the line, if 50% of the power
or more goes to heat up the coax, it is academic. And that is not an
unrealistic figure, even with RG-58 or 8X, with some antennas on some
bands that we have modelled and later tested.

On the other hand, either 300 ohm TV twinlead - for the lightest
weight applications - or regular 450-ohm ladderline show remarkably
small loss even at SWR>10 so one can feel free to "fiddle" with the
line length (as long as things remain reasonable!) to facilitate
matching a difficult antenna setup.

Our general approach is to use EZNEC 3.0 to model the antenna and
estimate its driving point impedance on the bands of interest. The we
use the TLDetails program to transform the antenna load Z to the line
input Z. It is here that we can evaluate the effect of different line
types, lengths, etc.

The Reg Edwards' (G4FGQ) program "LTUNER" designs the optimum or exact
L tuner for a given load impedance to provide a specified input
impedance, typically 50+j0 ohms. Those component values can be
compared against those afforded by the tuner involved to estimate the
liklihood of the tuner performing the match.

The effects of not using the correct values can be evaluated by a
program that I have written which determines the input impedance of an
arbitrary L or PI network presented with a given load. It is designed
for rapid component value changes so that one can quickly arrive at
the best compromise between what the tuner can do and what it should
do to most nearly match the load. If you would like, I will be happy
to email you an executable of it. It is called NDEXPL3.EXE . . . Reg's
program is available from his website with which I assume you are
familiar.

The KAT1 is the hard nut to crack. With its limited number of L and C
combinations and the values of the components, its impedance matching
range is much smaller than that of the KAT2 both in terms of resistive
and reactive components.

Interesting business!

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489  IC-765 #2349 IC-756 PRO  #2121 IC-756 PRO2 #3235