see below
On 11/30/2011 1:44 AM, Jim Thomson wrote:
>
>
> --------------------------------------------------
> From: "herzog" <herzog@frontiernet.net>
> Sent: Tuesday, November 29, 2011 4:08 PM
> To: "Jim Thomson" <jim.thom@telus.net>
> Subject: dummy load
>
>> My hb 50 ohm dummy load consists of 4 x 2.00 ohm globars in
>> parallel [type SP, glass bodied
>> and suitable for oil immersion]. Each resistor is 1" diam x 12"
>> long. [275 W CCS in air]
>> Each resistor resides inside it's own 2.875" OD x sched 40
>> Aluminum tube.
>> All 4 x thick walled AL tubes are heliarc welded.
>>
>> I heard some place that instead of xfmr oil, that distilled water
>> can be used instead?
>> The same glass bodied type SP resistors, [50 ohm, high wattage] are
>> used in water cooled loads,
>> the type that has the garden hose on one end, etc.
>>
>> Seems to me that distilled water will conduct heat a lot better than
>> xfmr oil. The question is,
>> will the use of distilled water in a 7 gallon metal container, with
>> resistors immersed present any
>> swr problems ? If I remember correctly, swr was high on heath
>> cantenna loads if no oil used at all,
>> and dropped to dead flat once the oil was poured in. There was also
>> a similar 50 ohm load in the
>> old arrl books, same resistor as I'm using but it was a single 50 ohm
>> unit, and not 4 x 200 ohms in parallel.
>>
>> I would prefer not to use xfmr oil if at all possible. It's a
>> little tougher to get a hold of these days..and not cheap.
>> My main concern however is something leaks. Water on the concrete
>> floor in the shop is one thing, but 7 x
>> gals of xfmr oil is a disaster. Can you add rust inhibitor's or
>> glycol to the distilled water ??
>> Do I even have to use distilled water, or will clean tap water work ?
>>
>> A 50/50 mix of water/glycol will result in a 265 deg F boiling
>> point..BUT the heat transfer is no
>> where as good. I'm using a new 7 x gal paint can for this project,
>> the 4 x resistors and the welded AL
>> tube assy, hang from the top lid.
>>
>> Thanks... Jim VE7RF
>>
>> =====================
>> K2LB SAYS:
>> i tried 50 ohm globar's like yours,placed in tap water tank.
>> SWR was way off, on low side resistance.
>> I removed the resistor, and swr then high side resistance.
>> even shown with a dc ohmmeter!
>> So i went to cafeteria and got a packet of tablesalt, stirred it in...
>> 2 pachets gave 50 ohms for 5kw transmitter for 20 minutes, until the
>> water boiled, making erratic swr.
>> the radiation from the open tank aolso caused us to buy a commercial
>> shielded load, but we could carry on testing for a couple weeks.
>
> ## were u using glass bodied type SP globars ?? I have the old
> type CX globars..and they can't be immersed in oil/water...air cooled
> only. IF u stuff NON glass bodied resistors in oil/water...will
> get absorbed into the
> resistor itself [carbon]..and it's resistance will change. MY
> small 90 watt glass bodied globars are 50 ohms in air...and also in
> oil. The 4 x 200 ohm units ARE glass bodied..and read 50 ohms in
> air. I have not tried em in oil/water.
>
yes, i guess; i recall...didn't know there was a variety in 19656...
as i inferred, i found the load was still there, even with no resistor,
just the copper straps that were supporting the resistor..
> ## I know abt the salt water loads. Also, the bird/altronic/cd
> water cooled loads use plane jane tap water..... and I could go that
> route....but its a waste of water. Toilet flushs..and then i need a
> flow meter/shut down the amp/open off key line, etc.
>
yes, we used one, with hose running...until a tech forgot to connect the
interlock, and blew the res...
ps we learned that the globar loads would cause intermodulation and
harmonics. we had a 60 db spec at 10 kw output, and so finally got
corning class film, oil cooled, and then blower air cooled for the test
line.
It was a southern new jersey company whose name i can't recall,
redmond or impulse or RED something?
> ### I'm gonna try an experiment with a heath cantenna.... and take
> the lid off the old cantenna/resistor.... and stuff it into a new
> paint can....with distilled water..and then blast it with rf..and time
> how long it takes to heat up, any swr change, etc. I need something
> to handle 10-15 kw pep... and also 10-15 kw, 3 -sec cxr's [ long
> enough to take steady state readings. [ plate/grid, etc. ]
>
we finally sold the transmitter to the navy by running it into their
antenna, and having their reception site verify that it met specs; even
though their dummy load also generated crap.
WHAT TEST TAKES TOO LONG TO REQUIRE COOLING??
> tnx.. Jim
>
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