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Author Topic: Is there hope for this repeater?  (Read 2042 times)

Posts: 78

« on: July 21, 2001, 06:50:33 PM »

The management of the TV station where I work was kind enough to let us put up a couple of ham radio repeaters on their new tower.  It's a sweet deal--full emergency power and the gear sits in an air conditioned cabinet on an elevator-accessible platform 500 feet up!  We moved our club UHF machine there and have had great success, even with a short fiberglass UHF/VHF colinear (a "real" antenna will go up later).

Space was offered to the local ARES/Skywarn group for their 2M machine (a perfect match for a weather-conscious broadcasting operation).  Last week we installed the 145.39 (input 144.79) repeater, sharing the antenna with the UHF machine via a diplexer, but the VHF receiver was deluged with RF crud.  Tightening connectors helped, but even with PL the squelch had to be closed so much as to render the repeater ineffective.

The problem (and why didn't we see this one coming)?  The repeater is sitting 400 feet below an antenna for TV channel 4, the outer edge of which is 72 MHz (you do the math)!  We've got a narrow passband receiver filter yet to be installed, and theoretically the second harmonic of the transmitter is supposed to be something like 80 dB down, but even so, is there hope for this repeater?

Put on your thinking cap and let me know your thoughts, suggestions, recipes, etc.  Let's rule out split sites, though.  Hurry, because it's really thundering out there and the Skywarn folks are getting anxious! (HI)


Posts: 21830

« Reply #1 on: July 23, 2001, 12:35:16 PM »

Sure, there's hope.  There are working amateur repeaters on the World Trade Center in NYC, which is home to all the VHF-UHF TV and many of the FM BC transmitters serving the NY Metro Area, and they coexist.  Same goes for many similar situations around the country, if not the world.

There are so many variables in a duplex repeater that diagnosing this issue remotely is impossible.  I'd probably try, among other things:

-Different duplexer designs.  Band reject duplexers are certainly more effective in this environment than band pass ones, and can provide deeper notches.

-Making absolutely sure that _every_ single coaxial cable in the system, from antenna to receiver and transmitter, and all the way through, are 100% shielded types: Hardline, Heliax, Coaxitube, etc.  No flexible lines with braid!!!

-Making absolutely sure that the antenna used has no clamped connections and does not use a "UHF" connector (these provide only 60dB or so isolation -- not enough).  Antennas used in this kind of environment should be all-soldered copper or brass construction with no compression fittings or clamped connections; Aluminum should be totally avoided unless any connection to it is fully brazed (Heliarced), no clamps used; and the feedpoint connector should be Type N as a minimum (N type provides 120dB isloation at 144 MHz).

-In many cases, common mode interference (such as from a 50kW BC transmitter only 400' away!) completely bypasses the duplexer and other normal "isolating" elements, and just conducts along ground surfaces right into the receiver.  In such cases, you can add one hundred cavities to the duplexer, and it won't help.  The RF must be stopped using stubs as traps tuned to the interfering frequency, and the stubs must literally tune the "ground," not the differential signal mode occurring between the center conductor and ground of the transmission system.  Speciality common mode traps are commercially available or may be built.

-Obviously, the transmitter used for the amateur repeater must be scrupulously "clean," and devoid of spurious products...standard commercial equipment is probably insufficient and will require additional filtering.  It's too easy to mix with a 50kW BC VHF transmitter located that close by...and every single output from the amateur transmitter will mix with that signal and create new intermodulation products, many of which cannot be effectively filtered by the duplexer.

-And of course, the amateur repeater receiver will probably need to be doubly or triply shielded, with tight RF gasketing around all enclosures, and very effective RFI feedthrough filters (not just feedthrough capacitors!) on all DC, audio and control lines entering the RX chassis.

What I'd do is bring a VHF-UHF spectrum analyzer with a really good front end and lots of selectable attenuation right to the repeater site and use it, in lieu of the amateur (2m) receiver, as the repeater "receiver" for a while to determine what's going on.  Make adjustments and improvements using the spectrum analyzer as the RX, and when satisfied that the input frequency is looking pretty "clean," then re-connect the actual amateur receiver and go for it!

73 de Steve WB2WIK/6

Posts: 161

« Reply #2 on: September 21, 2001, 04:13:57 PM »

And then there are circulators, which pass RF coming out of the transmitter but block RF trying to get back in. That's to help with transmitter intermod, obviously not useful for your problems with receiver desense.
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