[NLRS] 2m Pager? noise

[Doug Reed]

If I remember right, if you want a nice tuneable notch, you will use a
cavity on the end of a coax stub near a quarter wavelength long. Pay
attention to the coax velocity factor when cutting the cable. And a
Tee connector in the coax going to the radio, usually a quarter to
half-wave away from the radio. The cavity at the end of the stub gives
a tuneable "open" where the cavity is resonant. At the other end of
the quarter wave it looks like a short at the Tee connector. The
better the coax, the better the "short" and the better the notch.

>From tuning duplexers, I can add that if the stub cable is longer or
shorter than a quarter wave, there will be a "hump" of lower insertion
loss either above or below the cavity frequency. You'd prefer to have
the cavity tuned for APRS to suck out that signal, with the lower loss
"hump" on the EME side of the notch. But you will have to play with
the cable length since I'm not sure if it needs to be longer or
shorter to put the "hump" on the lower side....

I'm going to guess that you probably don't need a hi-IP3 preamp since
the problem is overload of the dongle front end, not intermod. The
preamp gets you a lower noise floor and may add enough gain that you
can afford to lose a few dB in insertion loss from an inline bandpass
cavity. All you can do is play around and see what helps....

73, Doug Reed, N0NAS.






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