[TowerTalk] antenna FS measurements

[Jim Jarvis]

I'd suspect the short term stability of the MJF attenuators
is reasonably good.  Most things ARE, for short intervals.

The big source of error is likely switches in the attenuator.  

I agree with Tom, however, that the best method, given what
we typically have to work with, is a variable attenuator, and
driving to ONE S-meter reading.  That removes the receiver 
variables, as far as we can see them.

Our discussion, I would submit, is taking place within the 
10%-1% window.  Or perhaps the 20% to 5% window.  I've limited
what I've done to a select few directions....

Dead front, +/- 35 degrees of front, dead rear,
+/- 15 degrees of rear.. and +/- 90 degrees.

Usually good enough to know if it's close to reasonable.  Having
greater resolution of a 10-20% number is not meaningful, except to
get a view of the overall pattern, given that absolute amplitude isn't
right.  

N2EA
jimjarvis at ieee.org 

-----Original Message-----
From: Tom Rauch [mailto:w8ji at contesting.com]
Sent: Wednesday, June 30, 2004 1:13 AM
To: Jim Lux; TexasRF at aol.com; jimjarvis at ieee.org;
towertalk at contesting.com
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