[aprssig] Re: Multiple Radio Desense

Robert Bruninga bruninga at usna.edu
Tue Mar 15 17:02:08 CST 2005

>>> "Jim Lux" <jimlux at earthlink.net> 3/15/05 5:50:37 PM >>>
>> 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)
>if I add 5 dB to account for dipoles at both ends, I get numbers 
>fairly close to yours.

Yes we just assumed 40 dB Antenna isolation as a ball park 
benchmark.  Two physical antennas 100 feet aprart can have
values +/- 6 dB or more depending on everything around them.
THus, we did not intend to be rigiorous, but just take an
assumed 40 dB isolation...

>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.

Yes the tests were done with two signal generators
and a splitter.  Then extrapolated to "two antennas
about 100' apart" by adding 40 dB.

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

Starting at 12 dB SINAD and upwards it appeaerd to
be linear for 20 dB or so.  That is, increase the desired signal
by 10 dB and you have to increase the interfering signal
by 10 dB to get blockage...

>...  that's -110 dBm.  I don't know if it's full quieting, but probably
>in that ballpark.

Yes most of the tests were done at -110 dBm

A few years ago we did measure packet decode on the D7
and were amazed that it can decode packets well down to
-121 dBm or so...

Oh, and I agree with the rest of your post...  Bob

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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