[VHFcontesting] Duplexer Isolation

Jeff DePolo jd0 at broadsci.com
Thu Dec 6 13:31:21 EST 2012

> What is the correct way of calculating duplexer isolation for 
> a given output
> power and receive sensitivity and/or IP3?

Are you asking about a duplexer as would typically be used for a two-way
radio repeater, or what hams often refer to as a "duplexer" but in reality
is a cross-band coupler (i.e. allowing VHF and UHF to share a common
transmission line)?  The way the question was posed I first thought it was
the former, but then considering this is the VHFcontesting list, I
second-guessed myself and am thinking maybe it's the latter?

If the former, it's not a simple question to answer.  First of all, there is
not just one isolation requirement -- at the very least there are two
requirements.  The first, and often the most critical, is the supression of
transmitter noise falling on the receive frequency.  The amount of noise
supression required is a function of a number of variables including
transmit-receive offset and the transmitter's noise power spectral density
at that offset, receiver noise figure, ambient site noise level, etc.  The
second parameter is how much carrier supression is needed to prevent
overloading the receiver.  This is a function of the transmit-receive
spacing with regard to front-end filtering in the receiver, dynamic range,
etc.  Noise supression and carrier supression requirements are often not the
same.  That's a really simplified answer in the interest of brevity...there
are other factors to consider.  Ask this question on the repeater-builder
list if you want more details, or email direct.

If the latter, it depends largely on the equipment being used and how well
it tolerates strong out-of-band signals.  A typical crossband couplers might
afford 40 dB of isolation between the high-pass and low-pass ports, which
alone may or may not be adequate depending on what you're doing with it.  If
the crossband coupler were being used at a multi-op station, or for
full-duplex satellite work, or some other situation where there was a high
power Tx operating on one band while a very sensitive Rx with little front
end filtering is trying to hear on another band, the coupler alone may not
afford sufficient isolation.  Or if it is two Tx's operating simultaneously,
there has to be sufficient isolation to prevent IM, and the amount of
isolation required is greatly dependant on the PA designs, LPF or BPF
filters built into the PA, etc.  Without knowing the intended use, and what
kind of equipment is connected to each port and its performance, there's no
real straightforward answer to the question.

					--- Jeff WN3A

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