On Sep 2, 2009, at 10:37 PM, tahrens301 wrote: > Nate - I only tried the 'ham quality' antenna because I knew > it would be a better match than the DB224. It was easy > to change, standing on a 6' ladder! Just wanted to see if > a poor swr would induce the desense. > > There are no other communications systems within miles of > my location, so who knows. Perhaps the metal building is > the problem. >
Got it now. I was somehow under the impression you were at a busy commercial site with other transmitters and things. Honestly it probably rules out some stuff if you're not. You didn't mention your frequencies. Another minor gotcha is always if you pick a frequency pair that just happens to have bad mathematical frequency relationships to your IF frequency, etc. Kevin has some interesting stories about the MASTR II and things he's found out about UHF ones over the years, regarding this... and there's been discussions in the past about high and low-side injection crystal frequencies when we move these ex-commercial repeaters designed for use higher or lower in the bands into the far reaches of the Amateur bands.... > A side question, dealing with separation. > > Obviously, when you are using a split site, vertical separation > makes you a lot more $ than horizontal does. > > But, in this type of situation, where you are a single antenna with > a duplexer, what real difference does vertical or horizontal > separation from the station make? If I'm horizontal, & could > turn the whole system on it's side (including the antenna system), > then it would be vertical. Ahhh, you're missing what they're talking about. Vertical antennas "push" a majority of the signal toward the horizon, usually in a donut shaped pattern. When you're directly under or over a vertical antenna, the amount of signal you'll "be illuminated with" from that transmitter/antenna combo is much lower than when you're off to one side of it. In your case, what they're concerned about is shielding... if your receiver isn't shielded well, and your feedline isn't top-notch, since your repeater is 100' horizontally from your antenna, you're in your own transmitter's "illumination" pattern, and signals are stronger that you're trying to keep OUT of your receiver, than say if you were directly under your antenna in a box at the bottom of the tower. Anything "leaks" RF into your receiver... there's going to be "more" RF out to the sides of your antenna than there is to contend with directly underneath it. > The straws that I'm grasping are getting smaller!! Here's a relatively simple test, and also not super expensive if you don't already have the gear... can you put a good quality 50-ohm dummy load up at the end of your feed line where the antenna is at, instead of the antenna temporarily. If you transmit into THAT and you have desense, something is leaking your TX BADLY back into your RX, and odds are it's in the building... not out on the tower. With only a dummy load out there to "radiate", you'd eliminate RF getting back into your "shack", just like a non-duplexed station might suffer from things being "RF hot" if you had a KW amplifier on HF and your antenna was only a few feet away (bad idea for RF exposure but just using it as an example) and without proper grounding, things like a CW key or an old Astatic microphone might "bite" you when TXing. If the desense goes away -- suspect the antenna or the feedline anywhere along the path. It would show that the RF from the antenna is getting Into something. If you're feeling overwhelmed, stop, take a break... new ideas come often when you're not actively "thinking" about the problem. Another good habit to get into is to write every test scenario down... if nothing else, you can hunt down other local repeater "people", especially those with lots of experience like the commercial 2-way folks who are often also hams, and for a beverage and maybe the cost of lunch, they can look over what you've tried and offer suggestions... heck, maybe if you're lucky they can show up with fancy test gear you could NEVER get your hands on at a reasonable price in a million years. And of course, if you can get their EYEBALLS on it, the problem might become obvious. Example... a new repeater operator in this area is using some hardline that looks like it was whacked every 3 feet with a ball-peen hammer. His repeater works reasonably well, and I'm just bloody amazed it works AT ALL after seeing this junk hardline someone gave him that he knew in THEORY should be better than LMR 400 or the like... but in PRACTICE he didn't know what giant dents in hardline do to the stuff. That hardline is probably EATING all of his power before it ever reaches the radiator/antenna, and it probably looks like a WONDERFUL 1:1 match on a wattmeter. But return loss INCLUDES feedline losses, by definition... and it's a two-way thing.... if you're losing X dB going up the cable, even if the antenna is reflecting ALL of it.... with enough feedline loss you won't see anything returned to the transmitter end, in a completely fictional "worst-case" scenario. Oh, and the test for that is... put the wattmeter on the OTHER end of the cable, at the tower top, with that same quality 50-ohm dummy load beyond it as a termination, and read your forward (and hopefully no reflected!) power there. If you take the manufactuer's number for feedline loss and subtract it from the transmitter's power you already read down in the shack, the math had better add up. If it doesn't your feedline is toast. If you find that local "friend" with the right test gear, he can "sweep" that cable and know for sure (say with an Aniritsu "SiteMaster" type meter) that it's good if you put a tiny little high- quality calibrated dummy load in your pocket and put it on the far end of the cable. Toys like that are "priceless" for the commercial folks, 'cause they do hundreds of cable runs a year. But with a little common sense you can set up to do the same basic tests... Hopefully all of that is really just to say: Hang in there... you'll find it, even if it takes some help from someone who's "been there, done that" seen it before... Being a super stubborn German doesn't hurt either, but that's me. No silly radio is going to beat me. BUT... Passive IM at a busy transmitter site, definitely has before. I was helping someone else who also was more experienced than I am, and I learned how to go to "plan B" with both split antennas and a duplexer AND a split CTCSS tone, just to get the **** thing working that year, too... always tricks up someone's sleeve to learn! That part's the fun part for me, but I still feel WHIPPED by that damn problem. The repeater's STILL in that mode, and works fine for the users that way... but that site is an utter RF mixing disaster... probably always will be. Beaten, but not really... just in my head. :-) -- Nate Duehr, WY0X [email protected] facebook.com/denverpilot twitter.com/denverpilot
