[Test-Equipment] Dummy load

[Brian Clarke]

URGENT URGENT URGENT


When you applied your antenna analyser, how much oil was 
in the dummy load? The oil affects the capacitance between 
the resistor and the ground funnel. Without a complete 
inundation with oil, the impedance measured will not be 
accurate - and is no indication of the impedance vs frequency 
when it is full of oil.

Go easy on the mineral oil. Many mineral oils contain soaps 
and other compounds that are fine for the original purpose. 
But, mineral oils can form unusual compounds with the 
carborundum and change its resistive characteristics 
permanently. Better to stick with a product designed for the 
job. 

Perhaps ask Bayley Engineering. Perhaps ask the radio 
and electronics engineering department at your nearest 
university. The people selling mineral oil are unlikely to 
have tech info relevant to your application. Their aim is to 
increase sales - not the life of your dummy load.

When the load is sealed up correctly, there should be 
no smell.

73 de Brian, VK2GCE.

 Rasputin said:
I connected the dummy load to my MFJ antenna analyzer. 
It measured 50 ohmsin the lower frequencies and wiggled 
(+ & -) a few ohms in the UHF. Later, I found a 
calibration sticker that said the samething, so I'm confident 
my resistor is undamaged.

There is a faint odor of oil about the thing, and a very faint
film of oil residue around the filler screw. I'm going to drain
the load, then refill with mineral oil which is easier to find
than transformer oil, and will probably have less smell. If 
possible, I'll replace the gasket on the filler screw. Then I'll
place it on newpaper and watch for leaks.