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

To: amps@contesting.com
Subject: Re: [Amps] Dummy load. Can distilled water be used instead of
From: "Roger (K8RI)" <k8ri@rogerhalstead.com>
Date: Mon, 12 Aug 2013 15:15:39 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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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