[Elecraft] How Accurate of 50-Ohm Dummy Load for 50 Watt TX Gain Calibration?

[Don Wilhelm]

Phil and all,

There are two types of dummy loads - one is simply to provide a load to 
your transceiver or amplifier for a quick sanity check that the 
transceiver or amp is providing output and is capable of transmitting a 
signal.

The other type of dummy load is one used for calibration and/or 
measurement of the output power.

The first type does not have to be exactly 50 ohms non-reactive - it is 
only to protect the transceiver or amplifier, but the 2nd type needs to 
have a real 50 ohm non-reactive impedance if the results of the 
calibration or measurement is to be valid.

So it all depends on what you are using it for and the accuracy of the 
results you expect.  In other words, the dummy load that I switch my 
transceiver to when the antennas are not in use does not need to be 
accurate, but the dummy load that I use for the K3 TX gain calibration 
does need to the 50 ohms non-reactive over the frequency range that is 
to be calibrated (1.8kHz to 54MHz).

In the past, I have mentioned several ways of obtaining good and 
accurate 50 ohm non-reactive dummy loads.  Those range from the 75 watt 
dummy load available from Ridge Equipment, to those VHF/UHF dummy loads 
available at hamfests.  Add to that good dummy loads that can be 
homebrewed - Caddock 50 ohm thick film resistors mounted on an adequate 
heatsink, or several other sources for homebrew dummy loads in a quart 
can filled with mineral oil.

Yes, the accuracy of the 50 ohm non-reactive load will affect your 
results - get a good one if you want good results.

The K3Utility is an automated process which proceeds from band to band 
automatically.  There is no way to pause it to allow the dummy load to 
cool down.  If you want to use a dummy load of lesser wattage than the 
required transmitter output (50 watts), then you will have to do the TX 
gain Calibration manually from the K3 menu.

73,
Don W3FPR

On 3/27/2015 8:27 PM, Phil Hystad wrote:
> I have two dummy loads that might be useful for the 50-watt TX-gain calibration but neither of them are dead on with a 1:1 SWR and 50 ohms resistive load.  Actually, my smaller 150 watt dummy load is closest with a range of 42 ohms on the load to about 55 ohms on the high end as I step through the frequency ranges of my Antenna analyzer.  My 1500-watt dummy load is worse as the resistance goes up to 87 ohms Z (resistive plus reactive) with 54 MHz test signal (using MFJ 259 analyzer).
>
> My little Elecraft DL-1 20-watt dummy load (mini-module kit) is best as it is flat 50-ohms resistive across the entire HF spectrum.  I am thinking of using and following the advice of Wayne to let it cool down between bands.  But…
>
> If I want to let the dummy load cool down between bands then can this be done using the automated procedure with the K3 utility or do I have to run the manual procedure.  In other words, does the automated procedure allow me to automate the process on a band by band basis.  I have not tried it yet because I didn’t want to start something until I was ready to finish it (and I have not installed the latest firmware yet).
>
> Question:  How accurate should the dummy load be — is an SWR of 1.2:1 and under good enough?
>
> Question:  Does the K3 Utility allow me to perform the TX gain calibration on a band by band basis?
>
>