[Top] [All Lists]

Re: [Amps] Amps Digest, Vol 110, Issue 2 (correction)

Subject: Re: [Amps] Amps Digest, Vol 110, Issue 2 (correction)
From: Jim Barber <>
Date: Fri, 03 Feb 2012 12:58:23 -0800
List-post: <">>
Sorry, make that 3200W instead of 3600W . I apparently can't multiply in 
my head any more.

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<>
>> Subject: [Amps] reducing the reactance of a wirewound resistor
>> To:
>> Message-ID:<>
>> 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
>> ------------------------------
> _______________________________________________
> Amps mailing list

Amps mailing list

Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>