[aprssig] Re: Multiple Radio Desense

Jim Lux jimlux at earthlink.net
Tue Mar 15 16:50:37 CST 2005

----- Original Message -----
From: "Robert Bruninga" <bruninga at usna.edu>
To: <jimlux at earthlink.net>; <aprssig at lists.tapr.org>
Sent: Tuesday, March 15, 2005 1:50 PM
Subject: [aprssig] Re: Multiple Radio Desense

> >>> jimlux at earthlink.net 3/15/05 3:27:46 PM >>>
> >>Last week, I asked 3 AMSAT volunteers (OE1DMB, N6IZW,
> >>and ZL3RX to take some radio measurements for FM desense
> >>just to see if we were in the ballpark for multiple radio
> >>operations at one site....
> >
> >This is handy, except that you didn't tell us what kind of
> >radios, and what kind of antennas... and where they were
> >relative to the ground.
> 5 different radios were tested and though there was some
> diferences, we reported the range from the best to the worst.
> The antennas were dipoles in free space with an assumed
> 40 dB isolation (49 db for UHF).

Horizontal dipoles or vertical?  by isolation do you mean "free space path
loss" between isotropes 100 ft apart
Friis equation: Loss dB = 32.44 + 20 log (dist in km) + 20 log (freq in MHz)
works out to 45.3 for 144 MHz and 55 dB for 440 MHz.. if I add 5 dB to
account for dipoles at both ends, I get numbers fairly close to yours.

Or did you use something like Longley-Rice, which would take into account
the ground.

> >How did you define "desense" and "blockage"?
> Blockage was when the original signal was no longer heard.
> This was a voice or tone test, not a packet test.

Got it.. so the interfering signal was sufficiently strong to make the
receiver's wideband AGC push the desired signal down into the noise, or the
far out sidebands from the interferer (either transmitted, or the result of
reciprocal mixing from the receiver LO) were stronger than the desired
signal. Either one could be the case, depending on the particular receivers
and transmitters.  A good test would be to use a high quality signal
generator as the jammer.. then you'd know it was a receiver problem, not a
Tx spurious signal problem.

The "desired" signal was how strong compared to the noise floor (just
audible, full quieting, 60 dB over S9, etc.?)

I guess the real question is what was the J/S ratio.  If you figure that the
typical receiver has a receive sensitivity of a few microvolts (into 50
ohms) (0.68 uV for 10 dB (S+N)/N for a TH-D7A according to the 1999 QST
review.  that's -110 dBm.  I don't know if it's full quieting, but probably
in that ballpark.

You gave: 2 meters:             (the spread is worst-to-best radios)
25 KHz separation blocks at  1 mw to 100 mw
600KHz separation blocks at  about 100 mw to 1 W

So, if we knock off 40 dB for the path loss from Jammer to victim, we get at
25 kHz away, the Jammer level was -40dBm to -20dBm, or a J/S of some 70 to
90 dB over a fairly reasonable receive signal strength.  Since the same QST
review gave a close in dynamic range of 59 dB (25 kHz) and a farther out (10
MHz away) range of  78dB, your measurements are consistent.  Your desired
signal probably wasn't right at the lower limit, and the path loss on short
paths particularly close to the ground is notoriously hard to determine.
The ARRL lab probably also used a very clean intefering source when making
the measurement, so transmitter spurious emissions aren't part of the
picture, which it certainly is in a cosite interference situation.

> We just wanted to give a ballpark value of the general
> concept of blockage by inband operation between two
> radios..

And it's quite useful.

The whole problem of cochannel or adjacent channel interference is one
that's quite sticky.  I suspect that the average FM handheld doesn't have
all that hot a receiver or transmitter, from a spectral standpoint. The D7A
transmitter is only spec'd at -60 dBc spurious signals.  Putting out a watt
(+30 dBm), the Tx phase noise in the adjacent channel  is probably
around -30 to -40 dBm. Add that to your 40 dB path loss, and you're at
only -80 dBm, which is still 20-30 dB greater than your desired signal.
Putting in 10-20 dB of isolation from a beam and another few dB of loss from
the coax, as you described, would make this actually work, especially since
the antenna gain means you could run less Tx power and still make it to the
distant digi.

The QST report on the D700 shows the noise floor around -75 dBc (at 50W!)
with some spurs up to -70 dBc.  Since that's at 50W (+47 dBm), the spurs are
pretty strong, compared to even a booming desired signal into the victim

> Bob

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