Can I use wirewound resistors to build an HF attenuator?
 

I need to lose about 5 dB from a 15 Watt exciter. Thanks.

Can I use wirewound resistors to build an HF attenuator?
 

"dave" wrote in message
m...
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 I use wirewound resistors to build an HF attenuator?
 

Ralph Mowery wrote:
"dave" wrote in message
m...
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.

Can I use wirewound resistors to build an HF attenuator?
 

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

Can I use wirewound resistors to build an HF attenuator?
 

In rec.radio.amateur.equipment dave wrote:
Ralph Mowery wrote:
"dave" wrote in message
m...
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.

It becomes too frequency dependant to bother with.

Use a T attenuator.

See:

http://chemandy.com/calculators/t-at...calculator.htm

You will likely have to use series or parallel combinations of resistors
to get the values required anyway.

You can also play with using two attnuator sectoions to see if it makes
the resistor values (and wattage required) simpler.


--
Jim Pennino

Remove .spam.sux to reply.

Can I use wirewound resistors to build an HF attenuator?
 

["Followup-To:" header set to rec.radio.amateur.equipment.]
On 2009-08-18, dave wrote:
I need to lose about 5 dB from a 15 Watt exciter. Thanks.

Do you really need to lose some watts? Or do you just need to
reduce the voltage to whatever you are driving? Maybe a capacitive
voltage divider would do what you need.

Jim W6JVE
--

jhhaynes at earthlink dot net

Can I use wirewound resistors to build an HF attenuator?
 

"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.

Can I use wirewound resistors to build an HF attenuator?
 

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.
--
Ian

Can I use wirewound resistors to build an HF attenuator?
 

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

Can I use wirewound resistors to build an HF attenuator?
 

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 I use wirewound resistors to build an HF attenuator?
 

Ian Jackson wrote:
In message , dave
writes



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.

From your other post, I see that you are going to use metal oxide
resistors. In the circumstances, that's obviously the correct decision.

But your question did not ask about what were the best resistors to use
for the attenuator. It was "Can I use wirewound resistors to build an HF
attenuator?" I assumed that you knew that WW would be inductive, but
might have some reason for wanting to use them.

Ralph Mowery replied that the "Simple answer is no" etc. You responded
with "Can the inductive reactance be cancelled with capacitors? I'm
mainly concerned with 40, 30 and 20 meters" which again made me think
that you wanted to use the WW, if possible. I suggested that the effects
of the inductance might be reduced if you mounted the resistors hard
down against a ground plane. [This is a frequently-used technique for
preserving the characteristic impedance, especially at higher frequencies.]

JB then suggested "a bunch of chip resistors". I commented that you
would need quite a lot of them. It was only then that I actually
suggested that there might be a reason why you were asking about WW, and
that the application was 'only' for amateur radio. Your response was
"**** you". "How very strange", I thought!

Anyway, I'm glad you have got yourself sorted out. I hope - no, I know -
that the attenuator will definitely work as expected, with the advantage
that it will probably work way up into the VHF region.

That's way revisionist, but I am used to it. I apologize for the
vulgarity.

Can I use wirewound resistors to build an HF attenuator?
 

On Aug 18, 8:32*am, dave wrote:
I need to lose about 5 dB from a 15 Watt exciter. *Thanks.

You could build the attenuator so it takes the power down in steps
perhaps 2 1db attenuators followed by a 3db attenuator. I didnt run
then numbers but I think this is doable with 2 watt resistors.

Jimmie