KX-1/MP-1 SWR limits

[k6dgw]

My KX1 has 30m and the ATU.  Works way more than great, if there is such
a term..  We're signed up for a trip thru the Canal in the spring
(reason #1 for the KX1), and I have acquired what I think is a homebrew
version of the MP-1 or sort of.  N6QYS loaned me his real MP-1, and it
looks pretty much the same and works (electrically) the same.

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.

Questions:

1.  Any ideas on what is an "acceptable" (in terms of actually being
able to work someone) SWR for the KX1?

2.  As with all antennas, I can tune the antenna for a variety of
resistances and reactances.  I've been shooting for an impedance
magnitude of around 50 ohms.  Possibly some combination of R and X is
more optimum?

It's getting a bit chilly here in N. Cal. to spend time on the deck
using trial and error (MN folks get one, and only one, joke apiece
regarding "chilly") and I was hoping for some accumulated knowlege that
will save me from the cold.

73,

Fred K6DGW
Auburn CA CM98lw

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[Bob Nielsen]

The $25 KD1JV "Tenna-Dipper" <http://www.4sqrp.com> is a lot smaller
than a MFJ-259 and works great for tuning the MP-1.  I took mine to
France along with my K2 last spring.  Airport security didn't say a
thing, although they ran the bag with the K2 through some sort of
detector.

Bob, N7XY

On Mon, Nov 22, 2004 at 05:37:13PM -0800, Fred Jensen wrote:

> My KX1 has 30m and the ATU.  Works way more than great, if there is such
> a term..  We're signed up for a trip thru the Canal in the spring
> (reason #1 for the KX1), and I have acquired what I think is a homebrew
> version of the MP-1 or sort of.  N6QYS loaned me his real MP-1, and it
> looks pretty much the same and works (electrically) the same.
>
> 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.
>
> Questions:
>
> 1.  Any ideas on what is an "acceptable" (in terms of actually being
> able to work someone) SWR for the KX1?
>
> 2.  As with all antennas, I can tune the antenna for a variety of
> resistances and reactances.  I've been shooting for an impedance
> magnitude of around 50 ohms.  Possibly some combination of R and X is
> more optimum?
>
> It's getting a bit chilly here in N. Cal. to spend time on the deck
> using trial and error (MN folks get one, and only one, joke apiece
> regarding "chilly") and I was hoping for some accumulated knowlege that
> will save me from the cold.
>
> 73,
>
> Fred K6DGW
> Auburn CA CM98lw
>
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--
Bob Nielsen, N7XY                          n7xy (at) n7xy.net
Bainbridge Island, WA                      http://www.n7xy.net
 
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[Vic K2VCO]

Fred Jensen wrote:

> 2.  As with all antennas, I can tune the antenna for a variety of
> resistances and reactances.  I've been shooting for an impedance
> magnitude of around 50 ohms.  Possibly some combination of R and X is
> more optimum?

The ideal is a resistance of 50 ohms and reactance of zero.  However, if this is
unobtainable, try to zero out the reactance and let the tuner take care of the
resistance.

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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[Leigh L. Klotz Jr WA5ZNU]

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.

Leigh WA5ZNU.
On Mon, 22 Nov 2004 7:34 pm, Vic Rosenthal wrote:

> Fred Jensen wrote:
>
>> 2.  As with all antennas, I can tune the antenna for a variety of
>> resistances and reactances.  I've been shooting for an impedance
>> magnitude of around 50 ohms.  Possibly some combination of R and X is
>> more optimum?
>
> The ideal is a resistance of 50 ohms and reactance of zero.  However,
> if this is unobtainable, try to zero out the reactance and let the
> tuner take care of the resistance.
>
> --
> 73,
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco

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[Don Wilhelm-3]

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.
>

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[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

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