David,
If your use of the dummy load is only to provide a load to the
transmitter for things like a quick "does it work" check, then precision
is not necessary. Anything that presents an SWR of 1.5 or less would be
adequate.
OTOH, if you want to do things like the TX Gain Calibration of the K3 or
KX2/3, or do measurements with a 'scope (or other measuring device)
across the dummy load, then better precision is required if one expects
the results to be meaningful and accurate.
In other words, if your expectation of measurement accuracy is a range
of +/-50%, then using a load with a 1.5 SWR is acceptable. If your
expectation requires an error of less than 10%, then more precision is
required. That goes for not only dummy loads, but any other measurement
instrument.
Parallel non-reactive resistors in a can of mineral oil can be used
effectively up to 200 watts in a quart can, while up to the legal limit
can be obtained in a gallon can of oil (Heathkit Cantenna style). Those
can be easily homebrewed and the amount of precision depends on the
resistors used and the care with construction.
73,
Don W3FPR
On 7/23/2019 6:43 AM, CUTTER DAVID via Elecraft wrote:
I often wonder just how useful a precision dummy load really is in the amateur
service. I picked up home brew dummy load at the club the other day. It
measured 57ohms dc and comprises 4 wire-wound resistors attached in a cluster
across the end of the coax. After lots of disparaging remarks I put it on the
VNA and it didn't look at all bad out to 30MHz: nothing over 1.5:1 and no
spikes or dips to indicate resonances. I'll have to do it again and make a
print-out for posterity.
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