I worked with the cans quite a bit this weekend, and with a simple test rig consisting of a transmitter, low-power wattmeter, and a dummy load, I was able to find good sharp nulls in the cans at the desired frequencies. In this case, I just had one can at a time teed into the line from the tx to the wattmeter.
The depth of the notches was not so impressive, however....I put two notches in the rx line and one in the tx line and had a big improvement with my 2-antenna system, but still I can find places where the rx antenna can be placed which cause overload and self-keying of the repeater. I fear I'm still really far from a single-antenna level of isolation. I clipped this from one of Skipp's notes: "You can add lengths of coax in series with the Tee Connectors to the top of the cavity port (loop), which extend the notch depth (to say 25 to 30 plus dB, the direct original notch depth is only 15 to 18 dB ... from memory)" .....this sounds like it may be the next place to go, but how to determine the proper length of these series cables...just trial and error? It gets pretty old soldering on PL-259s and pulling them back off to shorten the cable by 1/4" to try again. Would a "stub" teed into the cavity do the same thing? This way, I could clip off a bit at a time to try and find the best null. Oh, and BTW, the hybrid ring deal turned out to be a real bust- only thing I could get to pass thru the ring was at about 139 MHz.....pretty narrow, but it was not sensitive to the notch cavity setting at all! Maybe I had my velocity factor wrong- we're using this neat Belden Ethernet coax, double braid/double foil with solid center conductor. Guy told me it's Vf was 0.8, but he couldn't prove it. Thanks again for the helpful input. Bob -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Coy Hilton Sent: Sunday, August 14, 2005 1:42 PM To: [email protected] Subject: [Repeater-Builder] Re: Duplexer Configuration Question --- In [email protected], "Bob A" <[EMAIL PROTECTED]> wrote: > Thanks Skipp and Paul for input so far- I suspected that this would be a > bit marginal. However, we're only running 40 watts, and the rx > selectivity seems pretty good. It's an older Motorola, and it has 4 > small cavity filters at the input of the rx inside the box. Running 600 > kHz split with 2 antennas now, and I can find spots for the rx antenna > in which the system works OK for local communications. > > To Skipp's question, the old cavities have tee connectors attached > direct to the ports, and they are all SO-239. > > With cascaded hybrid ring pass filters on the rx, and just a notch can > teed into the tx side set to rx frequency, maybe it'll go. Paul, when > you say "capping" the other port, do you mean shorting it or leaving > open, or with stub or what? Capping, in this case, normally means removing the loop and put a covering over the opening. Also, to get the deepest, sharpest notch use the lowest insertion loss setting on the can. This will make a great Notch cavity. By the way, the can in this cinfiguration is a tunable open quarter wave stub. By coaxial theory, that means, what ever frequency that the "stub" is tuned for looks to be a short at that frequency...oh the same stub, if shorted, would appear to be open at the same frequency. > > And, if these old cans are single port, no shunt Ls or Cs, does this > mean they will notch as-is or will they need external help? > > -----Original Message----- > From: [email protected] > [mailto:[EMAIL PROTECTED] On Behalf Of Paul Kelley > Sent: Friday, August 12, 2005 7:28 PM > To: [email protected] > Subject: Re: [Repeater-Builder] Duplexer Configuration Question > > > > No piston trimmer that I can see, unless it's inside the can across > > the loop. I was thinking of cracking one open to find out. Anyway, > > though, it would have to be adjustable to peak up the cavity, so why > > would they hide it in there? > > No, they wouldn't be hidden inside. Just checking. > > > On the ring duplexer, it supposedly just inverts the > > notch of the can into an equally sharp bandpass....by making a steep > > cut away from the rx frequency, it should hold off the tx to some > > extent....right? > > Well, it'll keep the transmitter carrier out of the > receiver, at least to a degree. But you also need to knock > down the noise generated by the transmitter on the receive > frequency. If this is to be a 600 kHz split system, you > probably need to attenuate that noise 60 dB or so... > perhaps less, probably more, depending on the particular > transmitter, receiver, etc. With cavities only on the RX > side you'll have no protection from the noise and a LOT of > desense. > > > As I said, I have one additional can now, with two ports in/out, so > > I'll throw that one in the tx lead to make a 3-can setup. If this > > sounds all wet, please let me in on the right answer. > > If you mean just inserting a pass cavity into the TX lead, > it won't give you nearly enough protection from the noise > on your RX freq. I'd suggest making another hybrid ring > for the TX side with this cavity used as a notch cavity. > Most pass cavities work fine as a notch by capping one > connector and just using the other. A hybrid ring duplexer > with two cavities on the RX and one on the TX just might > do it, although it'll be a bit marginal on the TX side. As > I understand it, you'll need to construct 3 rings, one for > each cavity. > > Hopefully someone with real experience with the hybrid ring > will jump in here and help you out! > > Paul, N1BUG > > > > > > > > > Yahoo! Groups Links > > > > > > > -- > No virus found in this incoming message. > Checked by AVG Anti-Virus. > Version: 7.0.338 / Virus Database: 267.10.8/71 - Release Date: 8/12/2005 > > > -- > No virus found in this outgoing message. > Checked by AVG Anti-Virus. > Version: 7.0.338 / Virus Database: 267.10.8/71 - Release Date: 8/12/2005 Yahoo! Groups Links -- No virus found in this incoming message. Checked by AVG Anti-Virus. Version: 7.0.338 / Virus Database: 267.10.8/71 - Release Date: 8/12/2005 -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.338 / Virus Database: 267.10.8/71 - Release Date: 8/12/2005 Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
