Source for instrument tuning loads

[rich01]

I'm looking for a source for loads (like dummy loads) but for calibrating SWR and other meters. I've been using my own set of home made loads to show proper settings for SWR of 1:1, 2:1 3:1 short and open etc. They work up to about 150 mhz. However they are considerately less accurate as I approach 500 Mhz. I've found plenty of commercially made RF dummy loads that work well up to the GHz range but they are all made for higher power RF testing and all are for 50 ohm only loads.

I wonder if anyone has a source for precision loads for test equipment calibration hams would use with their analyzers and other meter calibrations.

Thanks .. Rich

[Bob]

Hi Rich,
For standard resistors in the VHF/UHF range, the shunt capacitance is very critical. Even a few tenths of a pf make a difference. For a good 50 ohm load it is better to use two 100 ohm resistors in parallel. In this way the stray lead inductance and the shunt capacitance tend to cancel and the performance is much better than a single 50 ohm resistor. That's the way the cal resistor for the AIM is made.

This principle might be extended to get 100 and 150 ohm resistors by putting the equivalent "50 ohm" loads in series.
Calibrate your AIM using the good 50 ohm load that came with it and then evaluate the larger resistors.

--73/Bob