I thought that the standard Bird 43 meter was 30uA at 1350 ohms, but
anyway... surely the problem you have Conrad is easily fixed with a
couple of external resistors... LZ2US's meter is too sensitive which means
that you can use a bit of ohm's law and a combination of shunt and
multiplier to acheive a meter that reads 30uA at 1350/1400ohms and
then just use it with the 2500C element... no need to adjust anything...
Better still you could pop down to M&B radio and buy him a Bird
43 meter for about £80 as a Christmas present ;-)
Mike G8TIC
>
> -----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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