[Amps] Power metering in linears

[craxd]

One other thing I forgot to mention about that toroid. Check to see if 
that may be going into resonance at 80 meters or so with a dip meter. 
You mentioned that there was hardly any output on 80 but was some on 160 
and then from 40 on to 10 I take it? If it is going into resonance on a 
needed frequency, you may have to change the turns on the transformer by 
a few turns. This will move the resonant frequency to somewhere you 
won't be using. Most I've seen had about 26-28 turns but seems like I 
remember one with about 40. There's a different process for checking 
toroids with a dip meter than there is with an air coil. This is due to 
the self-shielding action of ferrite cores. Anyhow, you might want to 
check that. If it's not, I'd say it's in the material of the ferrite itself.

Will Matney



>>>>Don't think any diodes in the circuit are linear from 160 to 10.


Diodes are not linear.  But, that is not the problem.  The non-linearity does not depend upon frequency but is controlled by the rectification process.  Non-linearity can be calibrated out easily with a meter scale.  The change in levels due to frequency are caused by the frequency response of the toroid device.  Both the core material and the number of turns can be adjusted.  It is quite easy to determine what the bandwidth is by feeding the sensor with a wideband signal generator and measuring output of the sensor versus frequency.  If adjusting the number of turns and the core does not give you the range you need, you can make frequency dependent compensation on the output of the sensor (for example, small value bypass caps on the output line will not charge the diode at the lower frequency and be useful if the voltage output of the sensor dropped off with frequency).  The right combination of these three factors should allow you to construct a sensor that covers the range you need.  This empirical approach is perfect for the amateur since it requires no math, no knowledge of physics and very little expenditure of money.  You should be able to become an expert in this area with less than a 6 pack and one evening of bench work.

Colin  K7FM


-----Original Message-----
From: craxd <craxd1 at ezwv.com>
Sent: Oct 7, 2004 11:13 AM
To: robrk at echomatrix.net, "amps at contesting.com" <amps at contesting.com>
Subject: Re: [Amps] Power metering in linears

Rob,

You are correct on this. Most use a 1N270 or so, but I'm not sure what 
would be needed for that wide of a range.

Will Matney


robrk at echomatrix.net wrote:


>>Don't think any diodes in the circuit are linear from 160 to 10.
>>
>>  
>>
>  
>
>>>>Tom,
>>>>
>>>>Stripline type RF sensors are narrow-banded and generally won't work
>>>>with that type of band spread. The toroidal transformer should work if
>>>>the core is rated for the complete frequency range like 3-30 MHz. If
>>>>not, you may have to add a switch to kick in another potentiometer just
>>>>for 80 meters and maybe 160 meters. To get the meter to null is another
>>>>thing and has to be done to read forward and reflected power. You may
>>>>just try different toroids first before doing any modifications.
>>>>
>>>>Will Matney
>>>>
>>>>
>>>>Hello technicians,
>>>>
>>>>I was adding a power meter to a new homemade linear. I used the
>>>>directional coupler of the "Digital PEP Wattmeter and SWR calculator" from
>>>>an old ARRL handbook (1987). I did not get a too deep null on calibrating,
>>>>so the directivity was only 20 dB. As I only wanted to monitor the output
>>>>of a linear with that coupler, the directivity was sufficient.
>>>>
>>>>The problem: The output voltage for a given power is the same from 40-10m,
>>>>but nearly null on 80m and different on 160m.
>>>>
>>>>Then I tried the design from the linears in the W6SAI handbook. They use
>>>>Amidon 50-6 toroids. The output voltage ón 160 is about 1/2 of the voltage
>>>>on 10m.
>>>>
>>>>Has anyone successfully built a directional coupler with constant
>>>>frequency behaviour from 160-10m?
>>>>
>>>>I would like to have proven designs.
>>>>
>>>>Thanks!
>>>>
>>>>Tom, DJ5RE
>>>>
>>>>
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>>>>
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>>
>>
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