"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?


Wind up approx 40' of RG-174 into a small coil and tie wrap it. It will
handle 100W + at 50 MHz and have a great return loss.
Lots easier than fooling around with resistors of questionable
characteristics and heating problems.
Dale W4OP

Use a length of lossy coax for only 3 db. A lot cheaper. None of
the non-inductive wirewound or laser cut resistors will be truly non-
inductive at those frequencies. Realize that the values need not be
exact, because return loss will keep the SWR down. If you can't get
Carbon, Use the coax. You might also want to adjust Drive or ALC in
the amp.

The Collins 30L-1 used to come with a length of lossy coax for this
reason.

ac6tk
http://tekstuff.freespaces.com

"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?

Now there is a great idea and I have a 100' roll of RG-174 in the closet.
Sounds like a winner. It's a shame that we can't get parts like were
available 30 years ago. I'd like to build some old design tube receivers and
transmitters. I was born in 46 and just got a 1946 ARRL handbook off Ebay
with a lot of things that look interesting.

Thanks to everyone for their input on this matter.... and not one flame in
all of the answers, that's rare on the user groups anymore!
73 to all,
Ken, KC4IH


"Dale Parfitt" wrote in message
news:z4uAe.5729$Zy6.3230@trnddc04...

"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?


Wind up approx 40' of RG-174 into a small coil and tie wrap it. It will
handle 100W + at 50 MHz and have a great return loss.
Lots easier than fooling around with resistors of questionable
characteristics and heating problems.
Dale W4OP

At 50MHz, you should find that metal-oxide resistors will work fine.
You're just making a power-wasting pad, not a precision attenuator.
(Carbon comps wouldn't be appropriate for precision work, either...)
The low-value metal-oxides I've tested on RF component analyzers have
all been fine for non-precision applications out to 150MHz. Try, for
example, Vishay FP69 series 2W parts or TT/IRC GS-3 series.

A couple years ago, I went through my stock of carbon composition
resistors and found more than half of them to be out of tolerance at
DC. They don't age well at all, even while not dissipating power.

1/8-W SMT resistors are so cheap that you could just use
series-parallel combinations of them to get what you need, too. For
more serious power dissipation, Caddock (and some others) have power
film resistors in TO-220-like packages which are low enough inductance
to be quite useful up into VHF and maybe above.

Cheers,
Tom

K7ITM wrote:
At 50MHz, you should find that metal-oxide resistors will work fine.
You're just making a power-wasting pad, not a precision attenuator.
(Carbon comps wouldn't be appropriate for precision work, either...)
The low-value metal-oxides I've tested on RF component analyzers have
all been fine for non-precision applications out to 150MHz. Try, for
example, Vishay FP69 series 2W parts or TT/IRC GS-3 series.

A couple years ago, I went through my stock of carbon composition
resistors and found more than half of them to be out of tolerance at
DC. They don't age well at all, even while not dissipating power.

The other half of them would probably be out of tolerance too, after
having been soldered.

I just came across an attempt to use carbon comp resistors (obtained
with some difficulty) as RF impedance "standards" - completely hopeless.
Small wire-ended metal film resistors would have been just fine, because
they don't change value significantly when soldered.

Wire-ended metal film resistors can be quite good for RF, but the
parasitic inductance can vary. They are made by cutting a spiral track
into a basic resistive tube, and the pitch of the spiral is changed to
produce a range of resistor values. When the spiral becomes too fine,
the process steps up to a higher-resistance 'blank' and the whole cycle
starts again with about a one-turn spiral.

Thus the number of spiral turns can vary between about 1 and maybe 10.
You have to scrape the paint off to see. The inductance can then be
estimated using the standard formula based on diameter, number of turns
and length, and it varies from a few nH up to maybe 100nH.

The problem is that different manufacturers make the steps at different
resistance values, so resistors of the same DC value may have very
different values of parasitic inductance. If parasitic inductance is
critical, circuits that worked perfectly well with one make of resistor
may have problems with a different make (as Elecraft recently
discovered).

1/8-W SMT resistors are so cheap that you could just use
series-parallel combinations of them to get what you need, too. For
more serious power dissipation, Caddock (and some others) have power
film resistors in TO-220-like packages which are low enough inductance
to be quite useful up into VHF and maybe above.

Yes, easily. The only problem with the TO220 packages is the shunt
capacitance to the grounded tab - and the power dissipation is very low
if the tab is not grounded.



--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

I just found an old Narda 3db pad in my junk box. It checks good and is
rated at 5 watts. Since I am concerned with SSB only, I can cut the carrier
back on CW, I think the Narda will work with the SSB signal since it won't
be continuous duty and the pad is rated at 5 watts continuous. I'll give it
a try... I don't even remember having this thing!


"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?

On Mon, 11 Jul 2005 18:22:37 -0400, "KC4IH"
wrote:

I just found an old Narda 3db pad in my junk box. It checks good and is
rated at 5 watts. Since I am concerned with SSB only, I can cut the carrier
back on CW, I think the Narda will work with the SSB signal since it won't
be continuous duty and the pad is rated at 5 watts continuous. I'll give it
a try... I don't even remember having this thing!

Fifty feet of RG-174 would do it too.


"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?