On 8/12/2013 12:24 PM, Jim Thomson wrote:
Date: Sun, 11 Aug 2013 18:29:57 +0200
From: peter chadwick <g8on@fsmail.net>
To: Chris Wilson <chris@chriswilson.tv>, amps@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.
Add compensation. It remains the same across the HF bands and is often
required even with the 4 large glowbar resistors.
Contrary to some beliefs, water, distilled or otherwise is the most
effective "normal" cooling medium, but the dielectric constant does need
to be taken into consideration.
73
Roger (K8RI)
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
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