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@windstream.net> wrote:
From: Lloyd Korb <k8dio@windstream.net>
Subject: [Amps] Drake W4 Wattmeter Question
To: amps@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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|