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.


--
Jim Pennino

Remove .spam.sux to reply.

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.

JB wrote:
"Ian Jackson" wrote in message
...

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.

The whole point of wirewound is that I can make a T pad with 3 5 Watt
resistors.

They make "flange" attenuators but they cost a fortune in small quantities.

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.

wrote:


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.



What kind of 5 Watt resistors should I use?

In rec.radio.amateur.equipment dave wrote:
wrote:


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.



What kind of 5 Watt resistors should I use?

Just about anything you can get other than wire wound.

Mouser has metal oxide 5 W resistors for $0.49 quantity 1.

You don't really want to run them at full rating but you will likely
have to parallel two to get the values you need anyway unless you have
a cheap source of better than 5 W precision resistors.

You need two 14.01 Ohm resistors and one 82.24 Ohm resistor to build
the attenuator.

If you parallel two 27 Ohm for 13.5 Ohm, and two 150 ohm for 75 Ohm, you
wind up with an attenuation of 4.95 db and a VSWR of 1.04 for a 50 Ohm
system.



--
Jim Pennino

Remove .spam.sux to reply.

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

The whole point of wirewound is that I can make a T pad with 3 5 Watt
resistors.

They make "flange" attenuators but they cost a fortune in small
quantities.

Not really necessary for HF ops.

The only advantage to using individual resistors, is to reduce the stray
reactance. That goes way out the window with WW. Inexpensive solutions
often revolve around buying surplus 1/2 watt or larger axial Carbons and
arrange them in parallel to achieve the required values. This also helps to
disperse the heat generated. The parallel resistors are soldered close to
the body to Brass or Copper stock to improve dissipation. Also consider
that although tune up will require 100% duty cycle, CW is 50% duty cycle and
SSB is less. Check the Math in the Allied or ARRL book.

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.
--
Ian