-----Original Message-----
From: Conrad G0RUZ <g0ruz@btinternet.com>
To: AMPS <amps@contesting.com>
To: <amps@contesting.com>
Date: 16 September 2002 23:40
Subject: [Amps] Bird thrulines
>A friend of mine in LZ has a Bird thruline and a 2500C element. He only has
>a 10uA meter movement and wondered if his meter could be scaled to 1) Read
>lower power levels due to increased meter sensitivity b) read higher power
>levels because the increased meter resistance would lower the current
>through the detector diode.
>
>I do not know the answer to this never having dissected a plug in element
>and never having burnt one out. I suspect that the normal damage is to the
>internal resistor and the diode due to excessive voltage but I am not sure.
The thruline plugin is basically just a mechanical variant on a transmission
line coupler. There's a length of pickup line terminated in a resistor at
one end, feeding a series diode at the other. The rectified output goes
through a feedthrough capacitor, then through a series pot (10k I think),
which is used for calibration. The pickup line varies in detail with
frequency range, but I think the principle holds. In vhf and uhf ones I've
been in the diodes have been the same - I forget the part number, but it's a
uhf germanium type. I got acceptable results replacing it with a low barrier
shottky on one that was blown. The Bird meter is 30uA, 1400 ohms. Assume the
pot in the plug-in is 5k and you should be able to work out reasonable
predictions of what readings you can get with different meters.
Steve
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