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
========================================
Message Received: Aug 11 2013, 03:40 PM
From: "Chris Wilson" <chris@chriswilson.tv>
To: amps@contesting.com
Cc:
Subject: Re: [Amps] Dummy load. Can distilled water be used instead of oil ?
> I built a dummy load consisting of 4 x 200 ohm globars in parallel.
> Each resistor is 12 inchs long x 1 inch OD.
> Each resistor resides in its own 3 inch od x 12 inch long AL pipe.
> All 4 x AL pipes are heliarc welded together.
> The entire mess will sit in a 7 gal new paint can. A 5 gal can +
> top part of a 2nd can were brazed together.
> A 7-16 DIN was installed dead center in the top lid....along with a
> pressure relief tube, off to one side.
> Instead of xfmr oil, can distilled water be used instead ??
> It will only be used on 160-10m..and maybe 6m.
> If something goes terribly wrong, Id rather have water all over the
> basement floor vs oil. Distilled water is
> an insulator. Heck, Bird + CD have water loads that use tap
> water. AG6K depicts a hb water load...with a
> single 500W resistor inside some pvc tube and a garden hose fitting
> on one end....and coax connector on the
> other end.
> I realize water boils at 212 F. Has anybody tried distilled water
> in a 1 gal dummy load ? Is it still 50 ohms ?
> Jim VE7RF
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
11/08/2013 15:38
My 10kW Bird 8738 water cooled load has tap water which I believe si
in direct contact with the resistive element, and is still working
fine so I don't see why not.
--
Best Regards,
Chris Wilson.
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