In message ,
writes
In rec.radio.amateur.equipment 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.

A 5db T attenuator with 15 W in has the maximum dissipation in the shunt
resistor at about 5 W.

The input series resistor dissipates about 4 W and the output series
resistor dissipates a little over 1 W.

I see no reason to go to wirewound resistors at these power levels.

Oh, I quite agree. I wouldn't really choose to use WW myself, but I
think they would work in this application. It's just that I usually try
first to answer the question as asked, and, if necessary, start
embellishing things from then on!
http://www.youtube.com/watch?v=k7vMvlRio5Y&feature=related
--
Ian

In rec.radio.amateur.equipment Ian Jackson wrote:
In message ,
writes
In rec.radio.amateur.equipment 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.

A 5db T attenuator with 15 W in has the maximum dissipation in the shunt
resistor at about 5 W.

The input series resistor dissipates about 4 W and the output series
resistor dissipates a little over 1 W.

I see no reason to go to wirewound resistors at these power levels.

Oh, I quite agree. I wouldn't really choose to use WW myself, but I
think they would work in this application. It's just that I usually try
first to answer the question as asked, and, if necessary, start
embellishing things from then on!
http://www.youtube.com/watch?v=k7vMvlRio5Y&feature=related

In general I agree with that, but if I ask a question based on a shaky
or incorrect premise, i.e. "special" resistors would be required for
a 15 W attenuator, I'd prefer to be told my premise was foobar rather
than waste my time chasing after something less than desirable.


--
Jim Pennino

Remove .spam.sux to reply.

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.

A 5db T attenuator with 15 W in has the maximum dissipation in the shunt
resistor at about 5 W.

The input series resistor dissipates about 4 W and the output series
resistor dissipates a little over 1 W.

I see no reason to go to wirewound resistors at these power levels.

Oh, I quite agree. I wouldn't really choose to use WW myself, but I
think they would work in this application. It's just that I usually try
first to answer the question as asked, and, if necessary, start
embellishing things from then on!
http://www.youtube.com/watch?v=k7vMvlRio5Y&feature=related
--
Ian

He obviously has a source of WW on hand. I hate to place an order
when I find stuff in the junk box.

So now the question is:
Will it be a significant problem? If he really knew what the inductance
values were, then it would be a simple matter to make sure it is
insignificant or at least minimal compared to the resistive component.
If the resultant SWR is low enough, the 5 dB return loss will help.
After that, the question would be "How will the amp act with
whatever input load is there?

Some resistors have a few laser cut turns that reverse themselves to
cancel the inductance. Some are a Carbon pack. A true WW
is either made by several turns of resistance wire over carbon or
ceramic for larger power ratings. Those have way too many turns.
It was mentioned that these even have a significant inductance at
audio frequencies. Now I ask you, is 5k inductive reactance
significant on a 50 ohm resistor?

You can always build one and see, if the exciter SWR is under
2:1, the amp input would see that too and you might be OK!

The other part of this is: How much power gain is the amp if
you need to kill 5 dB. Another thought is to incorporate
the inductance of the resistors to design a bandpass filter.
With many solid state exciters, some of the spurious outputs
don't reduce when you turn down the power.

http://www.repeater-builder.com/tech...ndbook-4th.pdf
All might find it helpful.
A more legible html version:
http://mikeyancey.com/files/Other%20...ion/title.html

All food for thought.