Have to be careful when using "non inductive" style resistors such as
listed here,
for RF service. As was pointed out, watch for the spiral grooved film
resistors. But many of the others that say "non inductive" such as the
Caddock use a serpentine conductor that bends back and forth, each turn
cancelling the fields from the previous. Unfortunately, it doesn't
appear low inductance at higher frequencies. Another style is quoted to
use Aryton Perry winding. Again, it cancels out the inductance, but
remains somewhat inductive, enough for inductive reactance at higher
freq. I think these resistors are probably fine to several MHz, but not
to 30 or to VHF. The film on substrate type are better. And the best are
the bulk carbon style, with old-fashioned Globars (rods) being among the
best RF resistors.
When in doubt, use an impedance meter, network analyzer or component LRC
meter to check these parts before assuming they will be prefect Rf
resistors. You might be in for a surprise.
73
John
K5PRO
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of mikea
> Sent: Thursday, September 03, 2009 6:38 PM
> To: 'Amplifier Reflector'
> Subject: Re: [Amps] Carbon Resistor Substitution
>
> On Thu, Sep 03, 2009 at 05:51:05PM +0300, alexeban wrote:
>> You have to be careful!
>> The film resistors are formed by cutting a spiral groove in the metal film
>> to lengthen the path and obtain higher resistance. It looks something akin
>> to a wire wound resistor actually.
>> I think Vishay has a family of non inductive resistors, especially made
> for
>> RF work but I can't remember the power ratings.
>> Alex 4Z5KS
>
>>From the May-July 2007 Mouser catalog:
>
> Vishay/Sfernice thick film power resistors page 527
> LTO 30 and LTO 50: 30 and 50 watt TO-220 package
> LTO 100: 100 watt TO-247 package
> Affordable
>
> Ohmite TAP series heat-sinkable low inductance and capacitance page 526
> TAP 600: 600 watts, air or liquid-cooled heatsink
> TAP 1000: 1000 watts, requires a liquid-cooled heatsink
> Very Pricey
>
> ARCOL thin and thick film non-inductive power resistors page 516
> FPA series: 100 and 250 watt
> Somewhat pricey
>
> Caddock MP series non-inductive power resistors Page 517
> 15, 16, 18, 20, 25, 30, 36, 60, and 100 watt
> TO-126, -220, and -247 packages
> Affordable
>
> Caddock MP725 series non-inductive power resistors, page 518
> 25W, "D-pack" package.
> Affordable
>
> All look interesting. If I had to make a high-power dummy load, I'd look
> at putting a number of LTO-100 in series-parallel.
>
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