On 3/15/2015 4:47 PM, Brian Reay wrote:
On 15/03/15 16:40, Jerry Stuckle wrote:
On 3/15/2015 11:54 AM, Brian Reay wrote:


Well, I several of us often run full duplex 2m/70cm or even triplex
4m/2m/70cm nets locally and use a common 2m/70cm antenna without
problems- hence my comment that I've never experienced any issues.
(Being in the UK, we have access to 4m.) I've even run 2m/70cm crossband
duplex while mobile with a common antenna and with dual antennas on the
roof of a SUV. True, I've not used the harmonic of the 2m frequency but
the desense was never an issue. While mobile and using the single
antenna, I relied on the internal duplexer of the radio. When using two
antennas, there was no filter or duplexer added.

If you think about it from a practical stand point, dual band radio with
the ability to operate full duplex cross band would be pretty useless if
it was an issue.

I'm simply relating my experiences of actually using 2m and 70cm on a
common antenna with a duplexer. If your experiences differ, fine.





Brian,

Have you actually measured desense with good test equipment? If not,
you really don't know, do you?

I've run 2m/70cm crossband mobile also. However, I also haven't
measured the desense when trying to receive on the third harmonic. Have
you?

Additionally, when running crossband mobile, I've always gone from a
nearby handheld on 70cm to a 2m repeater. Never tried it the other way
around, and since the handheld is always (relatively) nearby, it always
has a strong signal into the mobile. So even if there were desense I
wouldn't see it.


I haven't measured the desense, I was relating practical experience as I
said. In the circumstances, they are probably more relevant in my
opinion*. Tom is free to take which ever advice he decides is relevant.


So you really don't know if you have any desense or not. You just
haven't noticed any. That does not mean it's not there.

* The desense would have a number of variables, including the tx power
(which may vary), tx and rx frequency (also variable). Plus the impact
of the dense on the receive performance is almost impossible to predict
in terms of who/what you could receive- by which I mean you could
generate a raft of numbers for loss of system sensitivity but how do you
translate that into practical link budgets, other than for known
stations? It would be virtually useless for a random contact.



73
Brian


Actually, knowing the characteristics of the transmitter and receiver,
as well as the characteristics of whatever filters are used, it's
possible to quite accurately determine desense. That allows you to
predict with pretty good accuracy how much signal it would take to
attain a certain signal level (i.e. 12db SINAD) in the receiver. But
that's the important part of ANY system.

And it's impossible to predict the results of ANY random contact without
knowing the details of the other station. But then that's true on any
frequency with any mode.

Something I did a fair amount of back in the mid 70's when I worked for
a worked for a 2-way shop and planned UHF repeater systems. I doubt
it's changed any today.

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