I can see doing those things as a learning experiment, but if the goal
is an inexpensive load you can get NOS Florida RF 250-watt flange mount
resistors from Henry Radio on eBay for $24.95, or 800-watt for $59.95 .
(plus shipping, of course)
I just bought (4) 800-watt, 200-ohm units from Henry for $189.95 and
then a length of 10-inch-wide extruded heat sink from John Ango for $22
. Pretty inexpensive for a 3600-watt dummy load good to 500mhz with a
max VSWR of around 1.3:1 . (contiunous duty around 900 watts with no
fan, or add a fan and shroud for a few dollars - or a bigger sink)
My .02, adjust for local currency as appropriate...
Jim Barber, N7CXI
On 2/3/2012 12:37 PM, Dave wrote:
> You can get some inductance reduction, by wrapping the resistor with
> copper foil, soldering it so it makes one huge shorted turn, somewhat
> reducing the inductance. But it often compromises the cooling of the
> resistor.
>
> Passing a close fitting copper tube through the middle (if it's the type
> wound on a ceramic tube) that too creates a shorted turn effect, to some
> extent. Plus, you can pass coolant through it.
>
> Make sure with either, if there is HV involved on the resistor, that the
> insulation is OK. The enamel on some is poreous, leading to some
> surprises.
>
> Of course, the frequency range of this sort of thing is limited,
> especialy at LF. And at VHF, parasitic cpacitance to the foil or tube
> limits it's usefulness.
>
> The few times I've tried, it seems to work best on larger wire wound
> resistors.
>
> Dave G0WBX.
>
>
>
>> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Fri, 03 Feb 2012 09:23:20 +0100
>> From: Angel Vilaseca<avilaseca@bluewin.ch>
>> Subject: [Amps] reducing the reactance of a wirewound resistor
>> To: amps@contesting.com
>> Message-ID:<4F2B9978.40901@bluewin.ch>
>> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>>
>> Hi all
>>
>> I have a nice large 50 ohms wirewound resistor, which I would like to
>> use as a dummy load. Problem is, it is inductive of course.
>>
>> To cancel the inductive reactance I was thinking of winding some
>> enameled wire all along over the resistor, with as many turns as the
>> resistive wire, solder the enameled wire to the resistor terminal at
>> one end and feed the RF at the other two separate terminals (wire and
>> resistor) at the other end.
>>
>> The idea is that the inductive reactance of the resistor winding and
>> the enameled wire winding will cancel each other. With some care and
>> measuring gear the remaining reactance could be very low. The assembly
>> would then behave almost as a pure resistance.
>>
>> Of course, care should be taken not to reach too high temperatures,
>> because the enameled wire would then be damaged.
>>
>> Also, the added winding should be wound in the appropriate diraction (
>> CW or CCW depending of the winding direction of the resistive wire)
>> otherwise the two inductances would add instead ao canceling each
>> other.
>>
>> Has anyone alredy tried this?
>>
>> 73
>>
>> Angel Vilaseca HB9SLV
>>
>>
>> ------------------------------
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