[Elecraft] FW: K2/100 SWR accuracy

[Carl Strode]

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:crstrode at charter.net]
>     *Sent:* Wednesday, December 14, 2005 9:12 PM
>     *To:* w3fpr at arrl.net
>     *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
>>>
>>>
>>>    
>>>
>>
>>
>>  
>>