I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it would
be an easy matter to find them with enough current rating to do the job but
can't find anything larger than 1 watt carbon (not film, which are somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

"KC4IH" wrote in message
...
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it
would
be an easy matter to find them with enough current rating to do the job
but
can't find anything larger than 1 watt carbon (not film, which are
somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

By winding the wire so the inductance cancels out you get a noninductive
resistor. They are really noninductive at low frequencies. Once you get
above the audio range they become inductive. I doubt they would work for
your case.

KC4IH wrote:
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it would
be an easy matter to find them with enough current rating to do the job but
can't find anything larger than 1 watt carbon (not film, which are somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

Either there are two stacked windings, wound with opposite rotations,
or the winding is a zig zag, one turn one way, followed by one turn
the other way.

KC4IH wrote:
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it would
be an easy matter to find them with enough current rating to do the job but
can't find anything larger than 1 watt carbon (not film, which are somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?



There used to be great big resistors -- essentially carbon composition
-- that were truely noninductive. I suspect they're still available.
My 1988 handbook has a UHF dummy load design that features a Carborundum
part number 889SP500K. With coolant it's good for more than 1500 watts,
so it may be oversized for you...

You can still find the occasional 2W or 5W carbon comp resistor -- if
you're working at HF you can easily parallel these to get the resistance
that you need.

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Thanks Ralph, that is what I was thinking but when they are advertised as
non-inductive you would think "non-inductive" wouldn't you. I guess I'll
have to dig around in some old junk boxes or parallel several 1 watters to
get them to work.
I appreciate your information.
Ken

"Ralph Mowery" wrote in message
nk.net...

"KC4IH" wrote in message
...
I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it
would
be an easy matter to find them with enough current rating to do the job
but
can't find anything larger than 1 watt carbon (not film, which are
somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

By winding the wire so the inductance cancels out you get a noninductive
resistor. They are really noninductive at low frequencies. Once you get
above the audio range they become inductive. I doubt they would work for
your case.

Tim Wescott wrote:
KC4IH wrote:

I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my
6 meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought
it would be an easy matter to find them with enough current rating to
do the job but can't find anything larger than 1 watt carbon (not
film, which are somewhat inductive ) resistors. I can find want is
called non-inductive wire wound resistors. I can't understand how a
wire wound resistor can be non-inductive. Could someone explain this?



There used to be great big resistors -- essentially carbon composition
-- that were truely noninductive. I suspect they're still available. My
1988 handbook has a UHF dummy load design that features a Carborundum
part number 889SP500K. With coolant it's good for more than 1500 watts,
so it may be oversized for you...

You can still find the occasional 2W or 5W carbon comp resistor -- if
you're working at HF you can easily parallel these to get the resistance
that you need.

These are a smaller version:
http://www.ohmite.com/catalog/pdf/a_series.pdf

KC4IH wrote:

Thanks Ralph, that is what I was thinking but when they are advertised as
non-inductive you would think "non-inductive" wouldn't you. I guess I'll
have to dig around in some old junk boxes or parallel several 1 watters to
get them to work.
I appreciate your information.
Ken

Crazy idea but I'll throw it out here anyway.

We know that carbon film resistors have some worriable inductance at 6
meters. If a guy were to take a group of say 1-watters to make up the
power rating, could you count on the consistency of the outer markings
to the extent that you could group them somehow in parallel/inverted so
that they cancel each other's inductance in this application?

-Bill

In article ,
KC4IH wrote:

Thanks Ralph, that is what I was thinking but when they are advertised as
non-inductive you would think "non-inductive" wouldn't you. I guess I'll
have to dig around in some old junk boxes or parallel several 1 watters to
get them to work.
I appreciate your information.

Check out the Caddock and Vishay noninductive metal-foil resistors.
As I understand it they use a serpentine-path foil pattern, which
cancels out the inductance quite nicely. Some of them are advertised
as having an inductance equivalent to an equal-length piece of wire.

They're available in packages such as TO-220, and can be attached
directly to a heatsink (no insulator required).

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Sun, 10 Jul 2005 21:21:34 -0400, "KC4IH"
wrote:

I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it would
be an easy matter to find them with enough current rating to do the job but
can't find anything larger than 1 watt carbon (not film, which are somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

Look up for bifilar winding.

If your exciter is reasonably clean so that you do not think about
proper termination also for the harmonics, you just need an attenuator
for a single frequency, which should simplify things a bit.

Even if a "non-inductive" resistor has some inductance at 50 MHz, it
should not be too hard to compensate for the inductance and turn the
system into a very lossy series resonant circuit, in which the
impedance is dominated by the resistance in the area around the
resonance. An other alternative would be to put a capacitor in
parallel with the resistor to form a parallel resonant circuit at 50
MHz and the actual resistance would heavily damp the resonance. If the
inductance is sufficiently low, you will end up with capacitances that
are practical (i.e. not affected too much by stray capacitances).

This kind of approach should work at a single frequency(band), but if
multiple bands are needed or if the harmonics also needs to be
correctly matched, this approach does not work.

Paul OH3LWR

On Sun, 10 Jul 2005 21:21:34 -0400, "KC4IH"
wrote:


I need to build a 3db pad, 50 ohm, 10 watt, to reduce the drive to my 6
meter amp. I need 2 X 300 ohms and 1 X 18 Ohm resistors and thought it would
be an easy matter to find them with enough current rating to do the job but
can't find anything larger than 1 watt carbon (not film, which are somewhat
inductive ) resistors. I can find want is called non-inductive wire wound
resistors. I can't understand how a wire wound resistor can be
non-inductive. Could someone explain this?

I found a graph that I copied off the web sometime back showing the
reactance of various R vs freq for metal film resistors. The 10-1000
ohm range of resistors show as being dead flat at 50 MHz compared to
DC...and well beyond. Above and below values tend to go nuts with
frequency, the high 100k values in particular.

I don't remember where I found this graph but based on it I'd think you
have a good chance of making those particular values work using garden
variety ten-cent 1-watt resistors...thinking along the lines that
manufacturing of plain old carbon film resistors would follow the same
trend.

I think its worth a try.

-Bill