[Amps] Drake W4 Wattmeter Question

[Mel]

The "What-meter" question".
 
I am going to going to give you what is IMHO the answer to your question: Why the meter reading are different with frequency?
 
However, before I answer the question, there has to be a major presumption here; that is, that you are measuring the power output into a non-radiating 50 ohm load. If, you are use an antenna system and seeing different outputs then, other variables come into play.
 
The short answer, D1 and/or D2 do not have a uniform response (they are not linear) so, the devices is showing greater rectification at lower frequencies than at higher ones. Also, for the reading that you are noting, the meter seems to be out of calibration; the specs say it is accurate to 5% + 2 watts of the 200 watt scale. With 100 watts into the meter, reading of 107 to 93 watts could be expected.
 
Because this is a 1970's vintage meter, the 1N295 diodes may have changed in resistance, the adjustment may have moved or, the value of the fixed resistors might have changed. There is an adjustment procedure n the manual, it calls for a VTVM with a RF voltage probe; you do have them, right?
 
I often come across "What-meter" who's calibration is suspect, this is what I do: I have a zero - to - 8 amp RF-Ammeter and a Heath Cantenna; I put the RF (thermocouple) Ammeter in series with the meter to be calibrated and, I fire the transmitter up and read the meter. Step two involves a little math; Current squared times the load (Ohms, Law) tell me how many watts are flowing. Step three is to crank the calibration of the meter in question to show the proper reading.
 
If *I* had an extra W-4 meter, I would hunt up some matching diodes and install them at the junctions of D1-R5 and, D2-R5 in a voltage doubler configuration and, I would add a couple of small electrolytic capacitors to the junction of C5-D1 and C4-D2. The purpose of this is to allow the capture of the audio peaks on transmit.
 
In closing, I hope that I have answered your question but, I have a question to ask you: Why don't you just go out and but the approiate 0-30 Mhz slug for your Bird 43 wattmeter?
 


--- On Sun, 12/21/08, Lloyd Korb <k8dio at windstream.net> wrote:

From: Lloyd Korb <k8dio at windstream.net>
Subject: [Amps] Drake W4 Wattmeter Question
To: amps at contesting.com
Date: Sunday, December 21, 2008, 9:09 AM

I have a question on my Drake W4 wattmeter.  Since I don't have a high power
wattmeter I picked up the W4 on eBay to use on my AL-1200.  I have found
that the calibration is off starting on 15 meters.  I did some comparisons,
at the 100 watt level on my TS-2000, with my Bird with a 100A element and my
Drake WV4 VHF wattmeter. I didn't go below 15 meters as the WV4 is only
good
down to 20 MHz. The Bird 100A is good from 25 - 60 MHz. This is what I
found:

 

Freq            Drake W4       Bird 43       Drake WV4

50 MHz          70W            100W            95W

28 MHz          75W              95W            90W

21 MHz          80W              91W            90W

14 MHz          85W

  7 MHz          90W

 

I took a look at the W4 schematic and couldn't find anything, obvious, that
would make the meter so frequency sensitive.  Any ideas would be
appreciated.

 

73,  Lloyd   K8DIO

 

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