> One of the initial proposals
> was an open, 1/4-wave stub tuned for the FM broadcast
> frequency, fed on a coaxial T-connector. This is, indeed, a
> common method to "trap" a particular frequency.
>
> I set forth that this wouldn't work, as the desired pass
> frequency was too near the 9th harmonic of the trap, which
> means it, too, would be attenuated.
Yes, that's true.
> I proposed that better success might be achieved with a
> shorted, 1/2-wave stub tuned for the 900 MHz receive
> frequency, which would be nearly invisible at the 900 MHz
> pass frequency, but provide 20+ dB of attenuation at most
> frequencies below about 450 MHz. I did this based on
> experience not only using such shorted traps, but also after
> much past experimentation with my Wavetek sweep generator.
>
> Two subsequent posts took issue with my suggestion. One, from
> a member claiming engineering credentials, suggested my trap
> would appear as a "dead short" on the operating frequency,
> and that a shorted quarter-wave trap was the correct method.
He's correct. A shorted half-wave would look like a short at the tee.
Impedance repeats every half wavelength. A shorted QUARTER wave at 900 MHz
would look like an open at the tee. Such a device is commonly used as a
means of lightning protection as it provides a DC short between center and
shield, in addition to providing attenuation of the even-order harmonics.
> Another post suggested that a shorted 3/4-wave trap was
> correct, based on recollection of an instructor's comment.
1/4 wave and 3/4 wave behave the same.
> I still have the sweep generator, but not a scope, so I put
> my MFJ 259B analyzer, a 50-ohm dummy load, and a 41-inch
> piece of RG-8M (1/2-wave cut for 2m) on a T connector and
> look at SWR and impedance.
41" of coax with a 66% velocity factor would be about 62" electrically.
That's about 3/4 wavelength at 2m, not 1/2 wave (don't math in my head here,
somebody check me). I'm not sure how you calculated 41" of coax to be a
half-wave on 2m. So, it appears to me you were really working with a 3/4
wave stub, not a half-wave stub. As such, all of your results were wrongly
interpreted.
If you want I'll cut a stub of whatever dimensions you want and give you
plots on the VNA.
You could try a quarter-wave shorted stub cut for 900 to see how that works.
I haven't done the math or swept such a device to see how it would look on
FM, but I would expect it to have substantial attenuation.
The best fix for your problem is to put a bandpass cavity on your 900
receiver. It should provide at least 60 dB of attenuation of the FM
broadcast band. What kind of filtering do you presently have on the 900 Rx?
--- Jeff WN3A
