[Amps] Dummy load. Can distilled water be used instead of

[Jim Thomson]

Date: Sun, 11 Aug 2013 18:29:57 +0200
From: peter chadwick <g8on at fsmail.net>
To: Chris Wilson <chris at chriswilson.tv>, amps at contesting.com
Subject: Re: [Amps] Dummy load. Can distilled water be used instead of
oil ?

One problem is that water has a dielectric constant of about 78, as opposed to around 3 for things like liquid paraffin. The resistors, assuming that you have thin wall tube, will be in a transmission line of about 66 ohms. This will drop by the square root of the dielectric constant, so each resistor will be in a transmission line of around 7.5 ohms. I figure that by the time you get to 28MHz, you're going to see a noticeable SWR and even more so at 50MHz.

I once had a 50 ohm resistor in a suitable copper tube to make 50 ohms, but discovered that water screwed it up at the top of the HF range.
In my apprentice days, we had water cooled 50 watt resistors as loads for 30kW HF transmitters, but the dimensions of the container were adjusted to get the SWR down. Quite a few gallons per minute were used, but at 30kW PEP, it took less than 5 seconds  to burn the resistor if the water flow failed.

73

Peter G3RZP

##  good point.  My new globars are glass bodied, so water or oil is not an issue.   A compensation cap MIGHT work at the top end of the HF range.   Water will transfer heat better than oil, BUT water boils at a lower point.
I could modify the 7 gal load  with a pair of .75 inch garden hose fittings, and  just pump  TAP water through to the bottom of the load, via a tube running down the inside of the load,  then out the top of load, then  into the sink drain. 
Another method is to circulate distilled  water  from the load externally through a small 4 gpm pump, small air cooled radiator, then back to the load, in a closed loop.  Another version would be to circulate oil externally  through the 
same pump and air cooled radiator, same deal, closed loop.    I dont like the idea of an external oil leak though.     Tap water  through the 7 gal load then into drain would be the ticket.  Then if brief tests are done, the 7 gals will suffice.
If not, then the tap is turned on, and tap water fed through to bottom of load, then out the top to sink drain.   That way I have the option of supplementing the 7 gals. I don’t think the tap would have to be cranked up very much either.

Jim  VE7RF