Don,
My resistors arrived yesterday.
Today I hacked up a piece of aluminum for a heat sink - about 3/16"
thick, 1-1/2", wide and 3" long.
Drilled four little holes and bolted the resistors on.
Two resistors wired in parallel for 25 ohms, two resistors wired in
series for 100 ohms.
A phono jack was attached to each network.
I used a short piece for coax with a PL-259 on one end and a phono plug
on the other end to connect the K2 antenna output to the dummy load.
It worked like a charm!
The C1 null adjustments I had made using a good quality voltmeter were
about 1.4:1 with 25 ohms, and 2.0:1 with 100 ohms. Tweaking C1 got both
values to read the same (1.7:1), and adjusting the REF pot brought the
SWR indication right on the money at 2:1.
This technique works very very well. Thanks a million!
Carl
WA7CS
Here is a photo of the SWR tuning dummy load
http://webpages.charter.net/crstrode/Photos/dummy.jpg
Don Wilhelm wrote:
Carl,
If you want good resistors for dummy loads, try the power thick film
resistors. I have had good luck with the Ohmite 35 watt jobs, but
they do not have a 50 ohm value - 51 ohms is available and close to
what you need. The Caddock brand is good too, but I have found that
only the 50 ohm units are reliably non-reactive - yes they are
non-inductive, but I have found ones that are capacitive, and that is
just as bad. The Caddock are available from Mouser and I would
recommend the 30 watt resistors - you can series parallel connect 4 of
them for a 120 watt load if you want that capacity, otherwise one will
handle 30 watts for lower power. The Mouser number is 684-MP930-50,
currently $3.55 for one or $2.85 each if you order 10. I use 2 in
parallel for my 25 ohm load and 2 in parallel for my 100 ohm load.
Checks with several antenna analyzers indicate that they can be used
up to 30 MHz, but are really on the money at 7 thru 10 MHz, and I do
most of my calibration at either 40 or 30 meters.
These resistors do have to be mounted on a heat sink, but that is easy
since they are TO-220 packages and heat sinks from defunct computers
are plentiful - just drill and tap a hole and mount them with short
leads to a BNC or SO239 chassis mount connector. Use short leads for
best high frequency performance - Hint, if you use 4 of them, mount
them like spokes of a wagon wheel so all connections are in the center.
73,
Don W3FPR
-----Original Message-----
*From:* Carl Strode [mailto:[EMAIL PROTECTED]
*Sent:* Wednesday, December 14, 2005 9:12 PM
*To:* [EMAIL PROTECTED]
*Subject:* Re: [Elecraft] FW: K2/100 SWR accuracy
Don,
Thanks for your very interesting comment.
Believe it or not, I was rummaging about in my junk box last night
looking for some big fat resistors to do just what you have suggested!
However, I'm gonna probably need to surf Ebay a bit to find some
good ones. It seems that the only candidates I could fine in my
shack were either way to small (power dissipation), or were of the
wire wound variety.
Now, I am charging full speed ahead with the knowledge that my
proposed method has actually been used by an expert - You!
Thanks again for the advice.
73
Carl
WA7CS
Don Wilhelm wrote:
Carl,
You may want to re-calibrate your KPA100 wattmeter (and also check the
calibration of your other meters). If you have a good non-reactive dummy
load, it is really easy, but check your dummy load with an antenna analyzer
first to be certain that it will do th ejob properly at the frequency where
you will do the calibration.
Wattmeter calibration is a subject for extended discussion - just be aware
that no matter what the advertizement implies, many wattmeters can be off by
a great deal. Look at the specs and you will normally see something like
+/-10% of full scale - what that means is on a 200 watt scale, the reading
can be off by as much as 40 watts!!! and that applies to the entire scale
too, not just the upper end. Most are not really that bad, but some are,
and SWR can be proportionally incorrect.
I calibrate the forward power in a wattmeter by driving a non-reactive 50
ohm dummy load and measuring the RF Voltage across that load - my calculator
then tells me what the K2 display should be showing for forward power -
adjust until it agrees with the actual power calculated. I would be willing
to wager that the resulting setting will be as accurate as the best external
wattmeter you can find on the ham market (Bird included).
After setting the forward power, I calibrate SWR on the KPA100, KAT2 and
KAT100 wattmeters with my pair of 2:1 SWR dummy loads - I have a
non-reactive 25 ohm dummy load and a 100 ohm dummy load. I switch between
them and to check the null capacitor adjustment first, the SWR reading
should be the same for both loads - if not, I adjust it slightly until it is
the same - then I set the REF pot so the K2 displays the SWR as 2.0. This
method works fine, and I believe it was first suggested by Don Brown. The
settings for the KPA100 and KAT100 will not be veery far away from th
einitial setting suggested in the manual, but for some reason that I have
yet to explain, every KAT2 requires that the REF pot be set much higher than
the FWD pot to achieve a 2:1 display with the SWR=2 loads.
Since the KPA100 reduces power when an SWR>2 is encountered, it is
informative to know when that is likely to happen and take steps to avoid
it.
73,
Don W3FPR
-----Original Message-----
My K2/100, SN 3777 works great. No complaints
However, I am a bit leery of the indicated SWR.
The indicated SWR seems to tell me that my antenna match is better in
nearly all cases than several other SWR and power meters I have
laying around.
When SWR is measured using a MFJ 962 tuner, a Daiwa 101N meter, a Bird
43, Autek RF-1, or a MFJ 249 analyzer, the indicated SWR is always higher
than that shown on the K2.
For example, pressing TUNE on the K2 may indicate a 1.0:1, but the other
meters all consistently indicate a value significantly higher - lets
say 1.4:1
If I happily ignore the other SWR measurement tools, and only use the K2
SWR indications, everything still works great, I make lotsa contacts and
all is well.
However, at the home QTH, I do have a fairly well matched antenna, and I
do know that it is resonant in the band segments that I work.
Taking he K2 into the field is another question altogether. At this
point, I don't trust the K2 SWR readings and have been dragging along
other SWR meters when loading a temporary or unknown antenna.
Please advise.
Carl
WA7CS
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