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Re: [TowerTalk] antenna FS measurements

To: "Jim Lux" <>, <>,<>, <>
Subject: Re: [TowerTalk] antenna FS measurements
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Tue, 29 Jun 2004 21:12:38 -0400
List-post: <>
> 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

The word "presuming" about sums it all up. It's a HUGE chain
of gain stages including everything from RF through IF and
audio stages and right up to the A/D conversion. This method
would depend on that entire system to be LINEAR, not just
gain stable. Gain stable is probably one of the less glaring

An expensive spectrum analyzer intentionally designed to be
accurate is working pretty well if it is within 2dB over a
wide range of input levels. We can safely bet a receiver
designed to operate with AGC won't be near that good.

> > MFJ sells a surface mount step attenuator that is
> > within a small fraction of a dB per step. Of course I'd
> > check it first. I have three or four, and they are
> > .05dB per step.
> That makes it hard to do an automated measurement.

I doubt anyone will do an automated measurement anyway. It
would be tough to obtain a stable source (it has to have a
pattern to the RX antenna direction that is steady, and that
includes polarization). Worse yet, it is subject to ground
effects on it's pattern.

> 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.

Having worked with receivers and even DVM's most of my life,
I wouldn't.

(If you're measuring the IF level, I would have a question
> the linearity of the detector).  Sure, manual methods can
make good

If you are measuring the very same IF level through an
unknown detector and several additional AF stages including
a sound card, I'd have a question about linearity also.

> measurements, but over the long run, for instance, I'd
trust that automatic
> network analyzer more than the slotted line and voltmeter.
Likewise for

The typical amateur receiver is not designed, constructed,
or corrected as well as a $40,000 network analyzer. It's
more like the slotted line and voltmeter.

> power measurements. Carefully done attenuator substitution
measurements to
> the same detected level are metrologically good, but
tedious, and probably

That about sums it up also. Reseting level to the same point
removes all errors except attenuator calibration errors and
gain drift errors. It's a good method.

> 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

I've never seen cheap HF connectors vary 0.5dB no matter how
many times they are disconnected and reconnected. Geeze, if
you transmitted through the thing at 1500 watts it would
probably glow red. Are you sure you don't mean .05dB? Or did
someone forget to solder something?

As for temperature, I don't worry about temperature. My room
stays around 78 degrees as long as I pay my electric bill on

Flying something around the antenna may be interesting and
challenging, an interesting hobby even unto itself, but you
really need to look at the entire system. With the thousands
of dollars and hundreds of hours required to set up the
system, I'd budget a little more time and money to calibrate
with a known good attenuator.

73 Tom


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