I have an RF body and a few HF slugs, what I dont have is a Bird
microammeter. I am trying to find data that will let me calculate the
power from just knowing the DC voltage output from the RF body.

Jimmie

I have an RF body and a few HF slugs, what I dont have is a Bird
microammeter. I am trying to find data that will let me calculate the
power from just knowing the DC voltage output from the RF body.

Jimmie

First of all you will have to terminate the slug DC output on a resistance equal
to that of the Bird meter, which is 1,440 ohm. If you do not do that, the slug
would not have the proper load and the DC voltage variation with RF power would
not match that of the Bird meter scale.

To calculate the DC voltage produced by the slug when you apply the full-scale
RF power, you shall multiply 1,440 times the meter full-scale current (30uA), so
obtaining 43.2 mV.

When you reduce the RF power, the DC voltage variation can be calculated
multiplying 1,440 times the current flowing through the meter, which is given by
the following expression:

DC current (in uA) = 30 * (-7.378E-12W^6 + 2.547E-09W^5 - 3.407E-07W^4 +
2.248E-05W^3
- 8.188E-04W^2 + 2.692E-02W - 2.749E-05)

where W = power (on a 0-100W scale)

Note: the above relationship is unverified. I found it somewhere (I do not
remeber where) and I never tried it.

However, I am not sure whether measuring DC voltage is the best way to proceed,
as the voltage values are very low.

You could actually build a meter equivalent to the Bird one. If you can find
(not easy!) a surplus meter that has:

- either a 30 uA full-scale current and an internal resistance not higher than
1,440 ohm
- or having a lower full-scale current (i.e. less than 30 uA) and an internal
resistance not much higher than 1,440 ohm

it would be easy to transform it into a Bird-like meter, by adding a resistor in
parallel (to be trimmed until you get 30uA at full scale) and then a resistor in
series (until you get 1,440 ohm).

At that point, profiting of fact that DC meters have a linear scale, you could
mark on your meter scale the power values by comparison with a real Bird meter
scale. I can help you in that regard, as some time ago I made a scan of a Bird
meter scale and I could send you the file via e-mail.

I did that for a Collins wattmeter (for which I had no DC meter) and the result
was perfect because the meter I found in the surplus had exactly the the same
size as the original Collins meter and I could then simply paste the new scale
(printed on paper from a scan of the original meter) on the original scale. In
that case however the job was facilitated by the fact that the Collins meter as
a significantly higher full-scale current than that of the Burd meter, and it
was then not too difficult to find a suitable meter.

Last resource? I am sure you know that on e-bay it is not difficult to find Bird
meters stating at 80$ or so.

73

Tony I0JX
Rome - Italy

73

Tony I0JX

Jimmie wrote:
"I am trying to find data that will let me calculate the power from just
knowing the DC voltage output from the RF body."

DC load is the microammeter itself. My instruction book for the Model 43
gives the replacement meter part no. as: 2080-002. I don`t find the
meter`s sensitivity or resistance. These might be determined from a good
meter. The meter is calibrated in watts so the scale is not a straight
line function of volts or amps. Meter characteristics for your
particular model? Bird may be willing to tell you. They have a number of
application notes and data sheets available for the asking. Their
address is given as:
Bird Electronic Corporation
30303 Aurora Road
Cleveland (Solon) Ohio 44139-2794

Best regards, Richard Harrison, KB5WZI