[NLRS] Aurora Rumor

[Doug Reed]

Hi Jim.

I'll go along with what Jerry said, you can build the RF power
reference from the W1GHZ link or you can look for a "cheap" HP435 or
similar power meter that already includes the 50MHz reference output.
I seem to recall seeing an email from someone recently offering HP435
meters only, no power heads, for under $50 delivered. But if you look
around a bit you can probably find all the parts required to build the
W1GHZ circuit for under $10, if not completely free from the junk box.
The original article was in April 2007 QST.
<http://www.w1ghz.org/small_proj/RF_power_ref.zip>

The W1GHZ circuit starts with a 50MHz crystal clock oscillator module,
adds a 5 volt regulator, a couple diodes to clip the RF output, and
then filters it to give a sinewave and adds attenuation to get it to a
known fixed level, around -10dBm.  You really don't care that it be
-10.00dBm, it can be anything as long as it is a KNOWN output level.
But keep at least 6dB in the output attenuator to improve the
impedance matching of the output.

Since all we care about is having a simple and stable RF source, you
can use whatever crystal oscillator module you have on hand and just
change the low pass filter to match. 50MHz matches what is in the
HP435 and similar power meters I've seen, but nothing says you can't
use any frequency you happen to have.

If you look, you should find usable oscillator modules on just about
any old computer motherboard or video card you have laying around. Or
you can take a look through the bins at AxMan Surplus. I saw them on
Ebay last week also, 2 pounds (several hundred) of assorted
frequencies around 38-40MHz for $17 delivered.

Obviously, if we do have a well calibrated power meter and source
available at Aurora, you could test your HP431 and head. If you build
yourself a RF reference you'll be able to calibrate it and use it as a
test reference in the future.

For your weak signal sources, that is going to require a spec-an or a
sig-gen that has known calibration. That calibration might be a bit
more difficult to believe, if you can find one. I'd be very skeptical
if anyone tells you their equipment is calibrated to 0.1dB accuracy.

As Jerry suggested, if you have a signal generator for the band of
interest and you believe its calibration, you can put enough
attenuation between the weak signal source and your receiver to get a
weak signal with a known *LOW* S-meter reading, or preferably a
voltmeter reading from one of the AGC test points. Then adjust the
Sig-gen to match. That is probably as good as you're going to get for
ham-grade calibration.

I'd really suggest using a voltmeter or DVM to read the AGC voltage,
not the S-meter. I'd also suggest always adjusting from the same
"direction" when making the readings. Start low, raise the signal
level to your mark, then note the reading. Then start well above that
mark and lower the signal until you hit the same mark. If the two
readings are different, try it a few more times and decide if it is
slop in the mechanism or just take an average.

73, Doug Reed, N0NAS.

On 3/11/13, Jim Froemke <jim.k0mhc at earthlink.net> wrote:
> 2.      "Approximate" calibration of my current, un calibrated RF power
> meter (HP431C) - this need not be exact but I'd like to be in the ball park.
>3.      Measurement of my current, un calibrated weak signal source power
>outputs on several microwave bands - not total broadband power output.