This is easy enough to test for. Put the voltmeter or soundcard
on the receiver output. Feed the receiver with a steady source
like a signal generator, and then use the calibrated step
attenuator to check the linearity of the system. Very simple.
As Tom points out, the real problem with balloon/helicopter
method would be getting a steady source. You could minimize
the ground effects by adjusting the vertical height of the source
until you were in the peak of the antenna under test's elevation
pattern as the field strength should be least sensitive to source
perturbations at the point where the ground reflection and the
direct ray are adding in phase. You could probably minimize
the polarization "roll" by mounting the source antenna in a
gravity gradient configuration (imagine a dipole suspended
at the bottom of a vertical mast ). A fixed tower for the source
would be preferable, but then you either have to bring the
source tower to the test site, or the test antenna to a
dedicated range. The balloon/helicopter would make "in situ"
measurements convenient.
73 de Mike, W4EF.........................................
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Jim Lux" <jimlux@earthlink.net>; <TexasRF@aol.com>;
<jimjarvis@ieee.org>; <towertalk@contesting.com>
Sent: Tuesday, June 29, 2004 6:12 PM
Subject: Re: [TowerTalk] antenna FS measurements
> > 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
> problems.
>
> 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
> 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.
>
> 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
> about
> > 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
> time.
>
> 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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