Posted by
Bob McGraw - K4TAX on
Aug 26, 2018; 1:46am
URL: http://elecraft.85.s1.nabble.com/K3-TX-Calib-Dummy-Load-Recommendations-tp7644681p7644686.html
While one can measure a dummy load with an ohm meter, this is only the
DC resistance of the resistive element. It does not assure that it is
non-reactive as reactance is an AC component related to frequency.
Dummy loads should appear as 50 ohms at all frequencies within the range
where they are expected to be used. A good dummy load will be rated
at 50 ohms from DC to 500 MHz or higher.
A better way to verify a dummy load is to use an antenna analyzer such
as a MFJ-259, an Autec, or Comet CAA-500 or RigExpert AA-30, etc. Do
connect the dummy load direct to the instrument without a jumper cable
and sweep the range of frequencies desired. Any value measured other
than 50 ohms and with a SWR greater than 1.1:1 is deemed not
satisfactory for a dummy load. Another method is to use a return loss
bridge or a vector network analyzer.
Don't be mislead by the numbers printed on the label. I have two of the
"gallon can" oil filled types. They measure noticeably different values
than 50 ohms and get quite reactive above 30 MHz.
From this source you will find 100 watt Decibel Product dummy loads
DB4303B for $50 each.
http://www.nm3e.com/loadSampler.htm#LoadSampler<
http://www.nm3e.com/loadSampler.htm#LoadSampler>
And you will find Bird 100 watt loads, good from DC to 2.4GHz for $85.
And if you need one for that legal limit amp, they have a Bird 82C being
a 2500 watt water cooled load for $500.
Everyone should have a dummy load and an antenna bridge for their station.
73
Bob, K4TAX
On 8/25/2018 7:35 PM, Walter Underwood wrote:
> I built the Oak Hills Research RFL-100 dummy load kit. It is supposed to be good to
> 144 MHz, so it should be fine at 50. If you’ve never built a kit, this is a good one to
> start with. The kit is $50. I got the $5 BNC option, since my shack is 100% BNC.
>
>
http://www.ohr.com/rfl100.htm>
> Here are some photos of my build. Mine measured 49.9 Ω, which is within 0.2%.
> My Ohmmeter is accurate to +/-0.5%, so I’ll take that as a solid 50 Ω. Because the
> dummy load is built with 50 resistors, part tolerances will average out.
>
>
https://observer.wunderwood.org/2016/12/31/building-a-dummy-load/>
> wunder
> K6WRU
> Walter Underwood
> CM87wj
>
http://observer.wunderwood.org/ (my blog)
>
>> On Aug 25, 2018, at 4:49 PM, Mark Goldberg <
[hidden email]> wrote:
>>
>> A good one that you know is 50 ohms over the range of frequencies and power
>> you want to use it at. Unfortunately, any particular model may be good or
>> bad, depending on how it has been used or abused. Is there any way you can
>> get access to an analyzer to determine if the dummy load is good? I have a
>> Vector Network Analyzer, but few hams will have one of those.
>>
>> Also, if you really want to do a real calibration, you should have a
>> calibrated reference to check against. Lots of devices can be 5-10% off if
>> not calibrated, worse if abused or damaged. A Bird meter is only specified
>> to I think 5% of the full scale. So if it is a 200W full scale, it could be
>> 10W off. So 100W might read 110W or 90W. I've got a calibrated Spectrum
>> Analyzer and Signal Generator that I use to calibrate all my other stuff
>> against, but again, few Hams will have similar or spend to keep them in
>> calibration.
>>
>> In general, oil filled dummy loads with fins like the Birds shown lower on
>> this page are going to be good, but expensive:
>>
>>
https://www.birdrf.com/Products/Test%20and%20Measurement/Loads/Coaxial-Terminations.aspx>>
>> You can find similar ones used, hopefully not abused and not filled with
>> PCBs.
>>
>> I've got a similar one that must be decades old and it works well and is
>> pretty close to 50 ohms over the HF range. I have not opened it to see what
>> it is filled with!
>>
>> 73,
>>
>> Mark
>> W7MLG
>>
>>
>> On Sat, Aug 25, 2018 at 4:09 PM, Steve Lawrence via Elecraft <
>>
[hidden email]> wrote:
>>
>>> What are recommended dummy loads to consider for use when running the K3
>>> TX Calibration?
>>>
>>> Thanks - Steve WB6RSE
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