On 15/03/15 13:33, Jerry Stuckle wrote:
On 3/15/2015 5:50 AM, Brian Reay wrote:
Tom W3TDH wrote:


Would you please advise if the third harmonic issue is likely to be
serious enough to require the use of a low pass filter between the two
meter transceiver and the Diplexer/Band Pass Filter when using separate transmitters?


It depends on your tx.

I've never required filters nor heard of their being required.

Remember, you would need to be listening on the 3rd Rx harmonic, or 'close'
to it for the rx to suffer and the harmonic should get several 10s of dB
down relative to tell 2m carrier.


Brian,

Yes, it will be several 10s of dB down - but that can still be
sufficient to be heard in (and potentially desense) a near-by receiver.
It doesn't take a lot of signal if it's within the receiver's passband.

But Tom's diplexer should handle this easily.


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.