Ian Jackson wrote:
In message , dave
writes
Ian Jackson wrote:
In message , JB
writes

"Ian Jackson" wrote in message
...
In message , dave
writes
Ralph Mowery wrote:
"dave" wrote in message
news:0vqdnaEyq4zYAhfXnZ2dnUVZ_hJi4p2d@earthlink. com...
I need to lose about 5 dB from a 15 Watt exciter. Thanks.
Simple answer is no.
While you can build one with wirewound resistors , the normal
formulars will not usually work at RF and you will have a lot of
inductance to deal with.
Even the socalled non-inductive resistors are only so at the audio
frequencies.


Can the inductive reactance be cancelled with capacitors? I'm mainly
concerned with 40, 30 and 20 meters.

If your highest frequency is only 14MHz, and you're not trying to
obtain
a sooper-dooper perfect match for your exciter, then I'd say that you
would certainly get away with using wirewound resistors.

To minimise the effects of the inductance, you could try mounting the
resistors 'hard down' against a ground plane. You might need a
sheet of
some insulating material to avoid any danger of shorting to it.

And presumably, to get rid of 5dB (nearly 3/4 of your original 15W)
you
intend to use paralleled-up resistors. This in itself will help
minimise
the inductance.
-- Ian

A bunch of chip resistors in parallel to ultimately make a T-atten.

Yebbut....
12W is a fair bit to dissipate with chip resistors (even a lot of
them). However, the question was if you could use WW resistors, not
'how to do it properly'. I presume there was a reason why this was
asked! After all, we ARE talking 'amateur' radio.

**** you.

Dave, that's a very strange response. Do you have a problem with me
suggesting that you probably had a reason for asking if WW resistors
could be used? I've already said that you will probably 'get away with
it' up to 14MHz, especially if you mount the resistors as close as
possible to a ground plane.

You may not realise that lot of 'engineering' is the art of 'getting
away with it' (for whatever reason). For example, the reason might be
economics, it might be practicality, it might be urgency, or it might be
availability. The list is endless.

I'm just putting myself in the situation where someone needs to lose 5dB
of RF drive, and (say) all he has immediately at hand is a selection of
WW resistors. Does he try them (knowing that they are not recommended
for RF work)? Or does he take advice from the experts who, without a
second thought, will probably say "No way!".

The point is, do you want to try the WW resistors, knowing full well
that they are not really the right thing to use, but will probably work
well enough for what you want? Or do you want to do things 'correctly',
knowing that what you have done will be almost perfect? You have
received various bits of advice (some conflicting). It's now for you to
decide what suits you best in the circumstances.

I know how to think. I have no resistors, period, except for the 1/4
Watt ones I use at work. Mr. Jim has pointed me in the right direction.

All "amateur" means in the context of Amateur Radio is that we do not
get paid for doing our thing. It doesn't imply that we are stupid or
incompetent.

I am fully cognizant of the possibility of meaningful reactance being
introduced by a coil of Nichrome, and was hoping that someone who deals
with passive components more regularly than I could point me in the
right direction, and that has happened.