Date: Mon, 12 Aug 2013 15:15:39 -0400
From: "Roger (K8RI)" <k8ri@rogerhalstead.com>
To: amps@contesting.com
Subject: Re: [Amps] Dummy load. Can distilled water be used instead of
Message-ID: <5209345B.8060304@rogerhalstead.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
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
### I just thought of something. IF tap water is used, wont the tap water
short out the input connection ? One could have as much as 700-1 kv peak
sitting across the input to the load. Distilled water would probably be
fine, even circulated through an external rad-fan setup. But if tap was used,
and fed into the load, and out the other end, into a drain, I would think that
tap water would conduct the .7 – 1 kv ?? In my case, the way the load
is already constructed, it would be pretty tough to keep water away from the
7-16 din connector. Each of the 1 inch diam x 12 inch long resistors sits
inside a 2.875 inch OD x sched 40 AL pipe. All 4 x al pipes are also
12 inch long..and all 4 are heliarc welded together. The base ends of the
resistors are bonded to the bottoms of the AL pipes. The top ends of the
resistors are floating via insulators from the tops of the al pipes. The tops
of the
4 x resistors are strapped to the hot side of the 7-16 din. Tops of the 4 x
al pipes are bonded to the cold side of the 7-16 din. The V breakdown of
these chassis 7-16 dins is aprx 2900 v peak..in air. I may have just painted
myself
into a corner. I just can’t see this working with tap water. OK, I just
checked a clean glass of tap water with my B+K 875B....and its 14.5 K ohms,
with the leads .25 inch apart. Increases to 17.5 k ohms with leads 2 inchs
apart.
The hot ends of the resistors are aprx .72 inchs away from the tops of the AL
pipes. But I only have .25 inch between hot side of 7-16 din pin and top
lid. Flash, just tried distilled water,and its 60K with .25 inch
spacing....and
increases to to 200 k with 1.25 inch spacing. I have a few inchs between
top of resistors and 7-16 din. I could put any outlet fitting on the side of
the 7 gal can.... just above the resistors, that way circulated water never
gets
near the 7-16 din. I just might have a solution.
Jim VE7RF
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