True, but the broadcast band notch (not the 900 band) stub from a section of coax will have a fairly low Q... translated to mean it might be a popcorn fart (not nearly enough). Plus you want to use (if possible) a notch cavity that has a dc shunt to ground.
Minumum you should use a section of quality hard line, which if you were not able to find locally I would probably send you some free although I don't believe it would be the best solution. If you were local I would lend you a cavity or two to test with. You might ask around your local area. cheers, s. > "Paul Plack" <[EMAIL PROTECTED]> wrote: > Skipp, > > I appreciate the value of the high-Q VHF notch can, but that's a mighty big and expensive first-line troubleshooting tool compared to a 2.5-inch pieve of coax and a tee. > > But, yes, agree with all. > > 73, > Paul, AE4KR > > ----- Original Message ----- > From: skipp025 > To: [email protected] > Sent: Sunday, January 13, 2008 9:33 AM > Subject: [Repeater-Builder] Re: Revisiting Shorted 1/2-wave Traps > > > Other things to think about... > > Probably the more practical "bang for your buck" help would > be a very High-Q notch network/cavity placed in the 900 MHz > antenna system. The better High-Q network option is most > often found to be larger diameter cavities and/or a > combination of more than one cavity. > > A proper notch cavity (single port - internal probe/loop) > would/should also have a much desired dc return to ground. > > A high-q vhf notch cavity placed into the 900MHz antenna > system to "suck-out/reduce" most (in the real world not > all) of the broadcast station energy/power. > > Coaxial stubs are a nice idea but not so easily applied into > 900MHz antenna system without introduction of unwanted impedance > bumps. So nothing less than rigid line is desired and the > resultant Q might (using anything other than a very high-q > cavity) not be enough for your specific problem. > > Your friend should go back to square-one to try and confirm > the problem source is exclusive to the receiver antenna port. > Many people are surprised to find the reported gremlin is > traced back to multiple sources/paths within the repeater > system. Large amounts of local rf-power seeks all inbound > paths... every wire, every conductive path you can picture > and many non-electronic conductors you can't imagine would > be part of the big picture. > > ... and some of those sources are not so easily remedied. > > cheers, > skipp > > > "Paul Plack" <pwplack@> wrote: > > > > I'm posting this with all due respect to those who disagreed with an > earlier post, and in the hopes of discovering any error I might be > perpetuating. > > > > A few weeks ago, a member of the group was asking for help with > interference on the input of a 900-MHz ham repeater from a co-located > FM broadcast station. Among the possible remedies discussed were > coaxial stub filters on the receiver's transmission line. 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. (These traps are VERY wide when fed on a T-connector, > and work at all odd harmonics of the fundamental.) The open 1/4-wave > coax trap, sometimes called a "suck-out trap," is best suited to a > case in which the reject frequency is at least double the desired pass > frequency, to avoid attenuation of the operating frequency itself. > > > > 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. No supporting theory > was offered. > > > > Another post suggested that a shorted 3/4-wave trap was correct, > based on recollection of an instructor's comment. > > > > I've built and used several of these 1/2-wave traps, but it's been a > few years, and I didn't want to dispute these comments until I'd gone > back and made some actual measurements. I'd drop the matter, but this > is too useful a technique to have it discredited unfairly. > > > > 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. > > > > Here are the resulting measurements of resistance, reactance, and SWR: > > > > 146.15 MHz (Shorted 1/2-wave): R=47, X=2, SWR=1.0 (Virtually > unchanged from the dummy load alone) > > 73.08 MHz (Shorted 1/4-wave): R=23, X=20, SWR=2.4 (Z= about 31 ohms) > > 73.08 MHz (Open 1/4-wave): R=25, X=26, SWR=2.7 (Z= about 38 ohms) > > 146.15 MHz (Open 1/2-wave): R=3, X=1, SWR=12.0 (Z= about 3 ohms) > > > > 152.8 MHz (Open 1/2-wave): R=2, X=8, SWR=21.1 (Z= about 9 ohms. > Note: This was the SWR peak, higher in frequency than the "shorted" > frequency in part because the braid was folded back, instead of > connected to the tip of the center conductor.) > > > > The readings at 146.15 MHz, coax shorted, were nearly identical with > my 2m ground plane attached in place of the dummy load. > > > > Note that the only arrangement which looks like a "dead short" is > the open 1/2-wave stub. > > > > The bandwidth of the shorted 1/2-wave trap on the dummy load was > about 12 MHz for and SWR of 1.2 or less on 2m. Note that this trap > would be plenty wide for use on the antenna side of a duplexer. (The > corresponding 900 MHz version would theoretically be 45+ MHz wide > given the same Q.) It also puts the feedline at DC ground, and serves > as a crude high-pass filter below its fundamental frequency. > > > > It was explained to me by a cavity guru who first showed me this > trick that the reflected energy in the shorted section returns to the > T-connector at near-equal amplitude, and in phase, with the original > signal. If the coax was lossless and the connectors perfect, the > impedance of the stub at the pass frequency would be infinite, making > it truly invisible in the system. > > > > In short, (no pun intended,) these measurements look just like what > I've seen for years on my sweep gen. If you can demonstrate where I'm > wrong here, based on actual data, please elaborate. If you're not > sure, please cut one yourself and measure it. > > > > 73, > > Paul, AE4KR > > >
