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trying to halve power to RF ammeter



 
 
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  #1  
Old April 19th 12, 03:41 PM posted to rec.radio.amateur.homebrew
Major Samwells
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Posts: 1
Default trying to halve power to RF ammeter

I'm trying to get some accurate power readings with a low power rig (5 W)
and an RF Ammeter. The trouble is that the ammeter doesn't read quite up to
the highest level of the transmitter and I need to reduce the power to the
meter by about half. I'm thinking a 3 dB attenuator, but how am I going to
find the oddball resistances and what about resistor wattage? Is there any
other way to reduce power to the meter? Need something simple, quick-- but
accurate. Thanks.

  #2  
Old April 19th 12, 11:13 PM posted to rec.radio.amateur.homebrew
Dave M[_3_]
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Posts: 30
Default trying to halve power to RF ammeter

Major Samwells wrote:
I'm trying to get some accurate power readings with a low power rig
(5 W) and an RF Ammeter. The trouble is that the ammeter doesn't
read quite up to the highest level of the transmitter and I need to
reduce the power to the meter by about half. I'm thinking a 3 dB
attenuator, but how am I going to find the oddball resistances and
what about resistor wattage? Is there any other way to reduce power
to the meter? Need something simple, quick-- but accurate. Thanks.



Use the calculator at
http://chemandy.com/calculators/matc...calculator.htm to
find resistor values. This calculator lets you adjust the resistor values
to values that you have, or can easily construct, and the recalculates the
attenuation with your new values.
Don't forget that you can use parallel/series combinations to get pretty
close to the ideal values. Mouser and Digikey stock a pretty wide variety
of resistor types and values. Be sure to check them out.

You'll need to know the impedances of your transmitter output and the
antenna in order to get correct R values.

Cheers,
--
Dave M
A woman has the last word in any argument. Anything a man says after
that is the beginning of a new argument.


  #3  
Old April 20th 12, 04:04 AM posted to rec.radio.amateur.homebrew
coffelt2
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Posts: 70
Default trying to halve power to RF ammeter


"Major Samwells" wrote in message
...
I'm trying to get some accurate power readings with a low power rig (5 W)
and an RF Ammeter. The trouble is that the ammeter doesn't read quite up
to the highest level of the transmitter and I need to reduce the power to
the meter by about half. I'm thinking a 3 dB attenuator, but how am I
going to find the oddball resistances and what about resistor wattage? Is
there any other way to reduce power to the meter? Need something simple,
quick-- but accurate. Thanks.



I'm not sure there are any simple answers to obtaining "accurate" power
readings
with an "inline" RF Ammeter. (due to variable input and output impedances
and response
of MOST RF Ammeters, and such). However it is relatively easy to increase
the
range of an RF Ammeter by shunting it (paralleling it) with a low
resistance, low
impedance resistor made from a short length of wire. (bailing wire, iron,
fence or
what have you)
The Ammeter will have a very low resistance to begin with, so a very
low resistance
homemade shunt wire resistor can be fairly short, and length determined by
quick
experimentation. Run RF through the meter and take a reading at almost (or
at) the
top end of the scale. Then temporarily jumper the meter with the homemade
shunt
and adjust it's length until the meter reads half scale (if you want to
double the meter's
range), and tighten things up. Done. At less than 5 watts you can do this
hot, with your
fingers. (assuming your final is not a tube type with plate voltage
appearing at the
antenna terminal... YIKES!!!!!
Results will definitely not be accurate, but should be repeatable for
most comparative purposes.

That's my story, and I'm sticking to it.

Old Chief Lynn, W7LTQ

  #4  
Old April 21st 12, 04:14 PM posted to rec.radio.amateur.homebrew
Ian Jackson[_2_]
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Posts: 389
Default trying to halve power to RF ammeter

In message , coffelt2
writes

"Major Samwells" wrote in message
...
I'm trying to get some accurate power readings with a low power rig
(5 W) and an RF Ammeter. The trouble is that the ammeter doesn't
read quite up to the highest level of the transmitter and I need to
reduce the power to the meter by about half. I'm thinking a 3 dB
attenuator, but how am I going to find the oddball resistances and
what about resistor wattage? Is there any other way to reduce power
to the meter? Need something simple, quick-- but accurate. Thanks.



I'm not sure there are any simple answers to obtaining "accurate"
power readings
with an "inline" RF Ammeter. (due to variable input and output
impedances and response
of MOST RF Ammeters, and such). However it is relatively easy to
increase the
range of an RF Ammeter by shunting it (paralleling it) with a low
resistance, low
impedance resistor made from a short length of wire. (bailing wire,
iron, fence or
what have you)
The Ammeter will have a very low resistance to begin with, so a
very low resistance
homemade shunt wire resistor can be fairly short, and length determined
by quick
experimentation. Run RF through the meter and take a reading at almost
(or at) the
top end of the scale. Then temporarily jumper the meter with the
homemade shunt
and adjust it's length until the meter reads half scale (if you want to
double the meter's
range), and tighten things up. Done. At less than 5 watts you can do
this hot, with your
fingers. (assuming your final is not a tube type with plate voltage
appearing at the
antenna terminal... YIKES!!!!!
Results will definitely not be accurate, but should be repeatable
for most comparative purposes.

That's my story, and I'm sticking to it.

Old Chief Lynn, W7LTQ


This is exactly how I would do it (in fact, I'm sure I did this about 50
years ago). As you say, a thermocouple RF ammeter will have a very low
resistance between the terminals - and there's no reason why you can't
measure it with a (probably digital) multimeter. You will then have a
good idea of the value of shunt you will need. You can then do the final
adjust-on-test 'make it read half' procedure.
--
Ian
  #5  
Old July 23rd 12, 05:26 AM
vu2nan vu2nan is offline
Member
 
First recorded activity by RadioBanter: Feb 2011
Location: Bangalore
Posts: 68
Default

Quote:
Originally Posted by Major Samwells View Post
I'm trying to get some accurate power readings with a low power rig (5 W)
and an RF Ammeter. The trouble is that the ammeter doesn't read quite up to
the highest level of the transmitter and I need to reduce the power to the
meter by about half. I'm thinking a 3 dB attenuator, but how am I going to
find the oddball resistances and what about resistor wattage? Is there any
other way to reduce power to the meter? Need something simple, quick-- but
accurate. Thanks.
The best way is to use a current transformer. You could make one using an RF Toroid. The primary would be a just a wire passing through the toroid (1 turn primary). The secondary would be 2 turns wound on the toroid and connected to the ammeter.
 




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