John,
I used CommShop to come up with the minimum required duplexer isolation
for a 2m transmitter putting out 50 watts, and a receiver of 0.35 uV
sensitivity 600 kHz away. The answer was 87.2 dB.
Bear in mind that computer programs like CommShop that calculate
duplexer isolation have to make many assumptions, including the response
characteristics of a typical receiver and the spurious harmonics of a
typical transmitter. For example, a MICOR Sensitron receiver is far
superior to the majority of synthesized mobile radio receivers, and a
tube-based GE MASTR PA is much quieter than the majority of solid-state
PAs. Nevertheless, I don't think the TPRD1454 duplexer is a good
choice.
For a few hundred bucks more, Telewave will sell you the TPRD1456
duplexer, which has six cavities and better than 95 dB of isolation. I
just bought a TPRD1456 duplexer directly from Telewave for a new 2m
repeater I'm putting up, and I measured it with my network analyzer
right after it arrived. The insertion loss was -2.1 to -2.3 dB, and the
isolation was -104 to -108 dB. These are very good numbers, and will
ensure that my 100 watt MICOR repeater will have zero desense.
Most duplexer manufacturers will agree that a duplexer with six 5"
cavities will almost always out-perform a duplexer with four 8"
cavities. However, when you have a Decibel Products or RFS/Celwave
duplexer with four 8" copper cavities that are silver-plated, you're
talking high performance and big bucks! The Telewave cavities are
similar in quality and performance to the now-defunct WACOM products.
Bottom Line: Go for a minimum of 90 dB of isolation.
73, Eric Lemmon WB6FLY
johnmichaelwelton wrote:
>
> Need to get several 2M duplexers for a hospital project. The Telewave
> TPRD1454 (4 cavity, 5 in) provide 77dB isolation (~$750). I'll be
> using 40-50 watt Mitrek's into DB220 or DB224 antennas. Is the 77dB
> going to be a headache = should I spend an extra 1.5 to 2K per
> duplexer for better isolation or will Telewaves work reasonably well?
> I need 4 so price does matter.
>
> thanks,
>
> John/N4SJW
>
>
> Yahoo! Groups Links
>
>
>
>
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