You can do automated measurements with S-Meter readings if you use the
attenuator to calibrate your S-Meter. This will give correct relative spacing
between cal points in dB. If a signal source of known output is used, absolute
calibration is possible.
In TRX-Meter and in my program, a lookup table is used to calibrate S-meter
readings against the standard... this lookup table is then used to provide
corrected dB readings (actual dBm if absolute calibration is done) . Steps
between calibration points are interpolated. The process is a little tedious,
but only has to be done once... then automated runs can be made.
Larry N8LP
----- Original Message -----
From: Jim Lux
To: Tom Rauch ; TexasRF@aol.com ; jimjarvis@ieee.org ; towertalk@contesting.com
Sent: Tuesday, June 29, 2004 7:46 PM
Subject: Re: [TowerTalk] antenna FS measurements
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <TexasRF@aol.com>; <jimlux@earthlink.net>; <jimjarvis@ieee.org>;
<towertalk@contesting.com>
Sent: Tuesday, June 29, 2004 4:11 PM
Subject: Re: [TowerTalk] antenna FS measurements
>
> Turning the AGC off and using some soundcard device to read
> level is not a good method.
Why not? Presumably the gain of the radio will be stable (in the short
run). The sound card gain likewise. The sampling rate of the sound card
does change a bit with temperature and time, but it's down in the 100s of
ppm range on sound cards that I've measured statistics for (motherboards
with onboard sound devices in the open air). The temperature inside the PC
case stays fairly stable, so gain and rate for the sound card is probably
pretty good. I haven't characterized the gain stability, because in my
application, only frequency and phase was important.
>
> MFJ sells a surface mount step attenuator that is accurate
> within a small fraction of a dB per step. Of course I'd
> check it first. I have three or four, and they are within
> .05dB per step.
That makes it hard to do an automated measurement. Any sort of measurement
procedure which depends on lots of manual operations is going to tend to
reduce the total number of measurements made, so you lose the good
statistics. Off hand, I'd trust the measurements from a sound card, or from
a DVM measuring the audio or IF output more than manually entered switch
flipping. (If you're measuring the IF level, I would have a question about
the linearity of the detector). Sure, manual methods can make good
measurements, but over the long run, for instance, I'd trust that automatic
network analyzer more than the slotted line and voltmeter. Likewise for
power measurements. Carefully done attenuator substitution measurements to
the same detected level are metrologically good, but tedious, and probably
inappropriate for a measurement campaign that should have hundreds of
measurements. If you don't trust the linearity of the measurement system,
then use the attenuators to calibrate the measurement system. If you don't
trust the repeatability of the measurement system, then using attenuators
isn't any better than the other technique, because the repeatability is poor
regardless.
Out of curiosity: How stable are those MFJ attenuators (over aging and
temperature.. I assume that the connector repeatability is in the 0.5 dB
range, but I assume you'd not be mating and unmating them during a test)?
I'd expect fairly good over aging, but not so hot over temperature
(depending on the tempco of the resistors and other components).
Jim, W6RMK
>
> 73 Tom
>
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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