Many thanks to all who replied on this question. I adjusted the trimmer cap
for minimum reflected power on 28 MHz, and it definitely was out of
adjustment.
I finally located my 1975 edition of W6SAI's "Radio Handbook". He discusses
the bridge circuit in detail and then goes on to describe the
"reflectometer" circuit, which is the scheme used in the Ameritron. In the
final paragraph, he explains how to adjust the capacitor in question. It's
interesting that in his reflectometer the toroid goes over a piece of coax,
and also used a cylindrical can over the toroid as a faraday shield. I
suppose those precautions would make the sensor more frequency independent?
In the Ameritron, the toroid simply surrounds a single unshielded conductor
running from the T/R relay to the RF output connector. I note quite a
difference in power readings between the internal wattmeter and my external
wattmeter on the higher bands.
K8AC
----- Original Message -----
From: "Floyd Sense" <sentek@sprintmail.com>
To: <amps@contesting.com>
Sent: Thursday, February 06, 2003 7:53 PM
Subject: [Amps] Question on Ameritron amp wattmeter circuit
>>snip<<
> The question is: what is the function of the caps connected between the
> center tap and the primary conductor? I assume that the adjustment has
> something to do with the calibration of the forward/reflected power
meters,
> but don't know how it comes into play. There's a pot to ground connection
> in each of the legs to calibrate the meters with. Perhaps this variable
cap
> is used to correct the power reading at higher frequencies?
>
>>snip<<
> K8AC
>
>
|