Re: {Collins} Bird 82A

I disagree with your disagreement!  Respectfully, of course!

The primary purpose of oil within a dummy load is for thermal dissipation.  Otherwise, the resistor would have to be several times larger for the same power handling capabilities.  Yes, the Bird dummy loads are conservatively rated.  However, unless the coolant has a minimum thermal conductivity, then the overload margin is reduced.  With some oils, that reduction might even reach the point where the load might not even be able to meet the guaranteed level.

Although not Bird, one can look at the ratings for the Heath HN-31- series "Cantenna".  With proper "transformer" oil that load is rated at 1 kilowatt dissipation for 10-minutes and less than 1-minute with "mineral oil".  That is a 10:1 ratio in the capability of the dummy load.

The purpose of a dummy load is to safely dissipate the power generated by an r.f. source and that dissipation is by the conversion of the r.f. power into heat which, in turn, is transferred to the air.  I don't see where the dielectric constant enters into the equation.

 Glen, K9STH 

      From: scottjohnson1 <scottjohnson1@xxxxxxx>
 To: Glen Zook <gzook@xxxxxxxxx>; Richard Solomon <w1ksz@xxxxxxxxxxx>; Collins Reflector <collins@xxxxxxxxxxxxx> 
 Sent: Tuesday, October 10, 2017 2:20 PM
 Subject: Re: {Collins} Bird 82A
Glen I would respectfully disagree.  I think the most important attribute would be dielectric constant, which is all over the map with respect to various transformer oils.  I think thermal conductivity is secondary, especially with Bird loads, which are very conservatively rated.  I do an s11 sweep with my HP 8510C van when I add or change oil, and mineral oil gives the lowest reflectivity.  It also has a fairly low vapor pressure, so it doesn't expand the rubber bellows any appreciable amount.


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