At 11:45 PM -0400 7/28/99, Amps Digest wrote:
>
To: <amps@contesting.com>
>Date: Wed, 28 Jul 1999 15:47:11 -0400
>From: "Gilmer, Mike" <mgilmer@gnlp.com>
>Subject: RE: [AMPS] Good Meter vs CRT was Hot wire....
>
>Seems a lot like a high performance car - unless they catch you going
> >65mph (or whatever), which plenty of cars can do, the cops can have
>little to say. 100hp or 400hp doesn't matter.
>
>However, I've read that the police often get (lame) excuses of a
>speeder's poorly-calibrated speedometer thrown out - it's the "real"
>speed not the one on your dashboard that matters. Real speed is,
>usually, the police's version of speed since only their stuff is ever
>(albeit arguably) calibrated. So don't deliberately mis-adjust a
>wattmeter and then expect it to be a valid excuse, hi.
The way I ensure that my speedometer is close to mr policemans, is to
use the Kustom SR10 K band (24 GHz) moving car radar set I picked up
(hamfest). It allows you to read either the patrol car speed (mine in
this case) and the oncoming doppler signal (caused by oncoming
traffic plus the patrol car platform speed, subtracted from it). My
old speedometer seems to be within one MPH of the radar set display.
I haven't used tuning forks to check it though. Not that I speed,
mind you (in a VW), but I do enjoy 'tagging for tailights".....driver
of the 944 Porsche running by last week during rush hour about had a
stroke. I saw a friend commuting behind me one day, so I 'cleared the
corridor' for them. 3 Lands of traffiic converged into two. It only
takes a short burst to freak out a determined speeder with their
countermeasures (receiver).
Back to the amplifier topic, does anyone else use real directional
couplers? Bird makes them, but the inherent accuracy of the movement,
the diode curve, the ballistics, etc, does leave some things to be
desired.
When I want to make an absolute measurement I use a Werlatone C1460
directional coupler, a ferrite design , 0.02-250 MHz, 2 kW CW rating,
which is accurate (measured with a network analyzer to the nearest
0.1 dB), that gives a precise fraction of forward and reflected power
at two BNC spigots, that feed a small 50 Ohm load. I can either feed
a scope directly, or a peak reading meter. By the way, the Hp8900C
meter is a nice peak reading wattmeter, but I don't know if it works
with voice as well. It works with pulsed RF fine. I think the lower
freq response may preclude it below 100 MHz or so.
When I was rebuilding an SB220, the coupler helped me detemine that
the Bird was fine and that the Heathkit was doing all it could to
make 1300 Watts PEP output at 21 MHz.
Anyway, use a homemade capacitive divider if you must, but then you
have to deal with the vectoral sum of the forward and reflected
voltage, the VSWR. If you are driving a good 50 Ohm then the reading
is accurate enough to determine power from the voltage. I have swept
some homeade dividers that use doorknob caps and found that they have
resonances at <10 MHz that make them worthless above 40 meters. Even
the Jennings probe, which cost $$ and is very accurate, has a big
resonance at about 20 MHz above which all bets are off. I use them
all the time for 2-3 MHz high voltage RF though.
Hp made a vacuum cap divider for their 410 voltmeter years ago, the
Hp 11040A i think it was. It had the funny socket for the Hp vacuum
tube probe to mate to it, instead of a generic connector. I wonder
if anyone has modified one of these to pick off HF RF voltage?
John
K5PRO
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