Quarter wave length cables are the thing to use to couple the cavities together at the antenna connection side of them.
The uhf cavity gets a cable that is a quarter wave length at the VHF frequency and the VHF cavity gets a cable that is a quarter wave length at the UHF frequency. These connect to a T connector at the antenna line. This is the same way that you connect TX and RX cavities of a duplexer to an antenna. The UHF cavity loop provides a short circuit at the VHF frequency but the quarter wave cable from it transforms the short to an open (high impedance) at the T connection so you get no attenuation of the VHF signal there. The VHF signal then passes to the VHF cavity as if the UHF cavity was not there. The same thing happens to the UHF signal going to the other cavity. Without the proper length cables between the cavities and the antenna T connector both UHF and VHF signals will be attenuated depending on the luck of the cable length. The quarter wave length cable is the electrical length. It you are not combining the UHF and VHF signals with cavities then a signal splitter should be used. Even a TV cable type splitter will work ok for this. Don't worry about it being 75 ohms rather than 50 ohms. Without a splitter one receiver can load the input of the other considerably (depending on the luck of cable lengths) if just a simple T is used to connect the antenna to the two receivers. 73 Gary K4FMX _____ From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Nate Duehr Sent: Tuesday, March 09, 2010 5:11 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Dual receivers on one antenna for RX only site Answers below.... On 3/9/2010 8:29 AM, Ross Johnson wrote: Can a dualband antenna VHF/UHF for RX ONLY be fed to two receivers one VHF, one UHF, without a quote "duplexer" using a T instead? Yes. Typically performance is better with mono-band antennas, since all multiband antennas are a trade off in their design, but a "T", or even splitting multiple times is certainly an option for any receive-only antenna system, with the caveat that there's loss at each "split". Pre-amplifiers can help a bit, but once an RF signal is lost, there's no "getting it back" by amplification. Here's the idea. This is a remote RX site. The idea is to run something like a beefed up X500 dualbander at tower top, then 7/8 hardline 100 feet down to the receivers. So far fine. Both receivers will have one or two bandpass cavities inline before the T. I assume when you say "before" the T you mean antenna -> split -> bandpass -> receiver. Yes, this is probably a good idea to keep the receiver from being hammered by other signals that are out-of-band, but not 100% necessary if this receive antenna is out in the middle of nowhere with no high-power transmitters nearby. The bandpass filtering is lossy too, of course. The higher the Q of the bandpass filter, the less the loss. (High Q bandpass cavities are typically MUCH larger than BpBr duplexer cans. At VHF they're enormous and take up a lot of space. Ceiling mounts are common.) remember also that you're really only adding the bandpass to design for what the receivers NEED to have filtered to perform at their best. If the receivers are something like the GE MASTR II or similar with a cavity helical filter front-end (bandpass filter) built-in, you don't NECESSARILY need more filtering in front of them. Just sayin'. Design your filters specifically for your receiver's ability to handle out-of-band or nearby signals and the signals that you expect to be present at the site. The filtering has nothing to do with the "multi-bandedness" of the antenna, etc. UNLESS your chosen receiver is particularly bad when say, a 1/4 KW 900 MHz transmitter is 2 feet away from the receive antenna, and your particular radio doesn't like that. (An example I saw once... even WITH filtering the amount of 900 MHz "energy" coming through the filters was enough to piss off a UHF receiver, being it was a 2x multiple of the UHF's front end and passed through without much loss. Would a duplexer be necessary in this case. Or could it be done with proper cable lengths and a T? A duplexer is a set of filters designed to pass a transmit frequency and filter it out of a receiver on a nearby frequency. Did you mean diplexer? I think that's what you're really meaning to ask. And the answer is no... you don't truly need a diplexer. ESPECIALLY if you're running separate bandpass filters on each receiver. Think about what a diplexer does... it passes lower frequencies to one port, and higher frequencies to another port... if you're already going to bandpass filter there's no need for it. As far as cable lengths go, I have no idea what you're asking. Cable lengths should have no effect on this system at all. Thanks for your time and for the probably obvious answer I'm not sure of. No worries, you're asking the right questions to learn what you need to know. We've all been there! (GRIN!) For more "thought exercise" on the topic of multi-band reception, pick something you know receives multiple bands, and think about it... Think about a scanner and a discone antenna. Technically inside the scanner, there's probably multiple "receivers" so to speak (not really, but bear with me... it'll receive on multiple bands, and what it's really doing is switching those receivers in and out for each band as necessary -- kinda... scanners really typically just have really broad receivers that are ultra-sensitive but tend toward not being very selective)... you just get the RF to the scanner, it'll hear it. Because it has a front-end with virtually zero filtering, It'll also get hammered by close-frequency transmitters and almost always suffer from "images" where strong out-of-band signals will mix in the scanner's IF and show up as frequencies you never thought had signals on them. (And don't.) The scanner nor the antenna "care" which band they're receiving. The RF just passes from the very wide-band antenna down the cable, where the receiver does what it can with the pile of signals that are constantly present. Other thoughts to think about: It is VERY common at busy sites where antenna space on a tower is at a premium to do things like require site tenants to share either a "community receive" antenna, and sometimes even a "community transmit" antenna. The receive antenna setup for a single band is simple... antenna -> perhaps a wide bandpass high-Q cavity -> perhaps a pre-amplifier to amplify only what's "left over" (the band desired) after that cavity -> a splitter, and multiple runs of cable to each tenant's receiver. And finally: If you're going to have TRANSMITTERS at the same site as these multiple receivers, this changes the ball-game (and the answers) significantly, because now you're going to suffer from desense from those and need to filter appropriately (duplexer, or other...). Hope this helps enlighten a bit... Nate WY0X
