On 4/13/07, Dan <[EMAIL PROTECTED]> wrote:
I find this very interesting. This repeater ran on Belden 9913 for 7 years with no problems. Same duplexer, same power, same antenna.
Makes sense. Read on. I'll share my understanding of the phenomena. Talk to an RF pro if you want more info. (GRIN) I'm not a Doctor, but I play one on TV. Please tell me why LMR cable is not good for duplex ops? It is no
better or worse than 1/2 hardline.
Not true. I'll preface these comments with a note that there's still some debate about this amongst the group, but... Many of us have personal stories to share of LMR causing problems in repeater (duplexed) service. It's good cable for simplex (non-repeater) service, low loss, flexible, yadda yadda...but even the pros are starting to publish articles about its noise problems. If you've never done this or seen it... metal rubbing on metal in the near-field of high power transmitters can act as a non-linear diode at RF (someone with an RF engineering background can share the real deal about this if they like), and create broad-banded RF noise. Diode mixers... like a lot of older radios, if you get that concept. If your repeater is at a high RF site with broadcasters, etc... key the transmitter sometime without the receiver active (you do have a switch to do this so you can check for desense easily, right?) and then listen on an HT you're holding while you rub your car keys on a tower leg. You'll hear a mess of crackling and crud. This same thing is why people go ballistic when some doofus hangs a new antenna on a tower and leaves all their cables banging around, loose mounts and hardware, no grounding kits on their hardline or proper hardline hangers to keep it still on a tower leg, etc... in other words, a NON-professional installation. Because all that metal banging around up there is bound to create passive IM and crud, just like your car keys, and hit SOMEONE's receiver, even if it's not yours. Okay, now let's look at the construction of LMR-type cables. Inside you have a braid over a piece of foil with lots of little "contacts" between the two. If the cable moves at ALL -- the little "rubbing points" do exactly what your car keys did on the tower leg... and even if the effect is TINY, it's inside your cable where your receiver is listening, and it's driven by the high RF power you're pumping into the same cable. There are now thousands of little places inside the cable where you're transmitting RF up the hose and receiving down the hose where things can move and scrape and touch. That's the general concept anyway -- someone smarter than I can explain at a more reasonable hour of the day, since it's 1AM here... and I'm still bored on this conference call. LOL! Anyway, if you MUST use LMR cable, make every conceivable effort possible to tie it down very very well so it can't move at all. You do NOT want it moving at all. Meanwhile, you're right -- when comparing numbers and specs, LMR and smaller sized hardlines look somewhat similar. It's not about the numbers, it's about the construction. Hardline is much better, for duplexed services like repeaters, because the outer conductor is solid, and there's nothing to move around in there. And for a run of 50ft using
hardline is just not an option.
I assume this is a financial comment more than a technical one. It's painful, but be happy you only have 50'. Many of us have much bigger runs of bigger hardline to get up really big towers. We're lucky here, mountain-top towers are typically pretty short, but a friend has a repeater in the flatlands of the midwest and he's up 300' with a UHF machine. Even 7/8" hardline sucks at those distances. He got the chance to "borrow" an unused 1 1/2" hardline feed going to the same platform he's on, and I think that made him very happy. It also makes an N connector look really small, in comparison. :-) I have a fiberglass 3db Maxrad UHF
antenna on the tower also and am going to switch to this to see if the antenna is the problem. way too many folks disregard the fact that some of us cant afford to buy a DB-404 or something like that and have to use other options for antennas.
Many of us have either "been there done that" on cheap antennas, or have been yelled at by those who have. Our club certainly had our days of using whatever antenna was handy, but nowadays we either do a fund-raiser for a good antenna, beg for commercial cast-offs/abandoned gear, or just don't put the system up. Some antennas truly aren't worth the headaches. I truly believe that you'll replace the cheap antenna three times and make six site trips troubleshooting it, in the ten years or so you own a good Sinclair or DB. The time and money to replace the cheapies does add up, kinda like buying a cheap car an paying more in maintenance on it over the years. Everyone does what they have to, though -- and if you find that an antenna works, duplexes well and doesn't add noise, and it's a cheapie, consider yourself VERY lucky and start saving up for the next one, or an upgrade. There's no harm in a privately owned system of doing incremental upgrades, and club members are surprisingly willing to help out in most clubs when there's a real goal to be met... people do help more when there's a goal if dues and/or the budget simply won't allow you any options. Just be honest and share the situation on-air without complaint... just say you'd like to fix it, here's the problem, and it'll take about X number of dollars, and that the club is saving up to do it. You'd be surprised what might happen. Thanks Chuck for all the
ideas...I will be checking into all of this over the next few days.
Have fun hunting... noise problems are a pain. Yours sounds "findable"... at least. We have a weak signal on a VHF repeater input that's been driving me mad for three years now... right on channel, broadbanded crud that doubles but doesn't capture over HT users who are 20 miles away (to give a feel for how weak it really is). We hunt and look and haven't found the source yet. Can't even hear it at ground level. A total pain in the ***... , or is that ****. Dan/NØFPE Have fun, that's the important part. Everyone always wants newer/better toys for their hamshack, even if the hamshack is a repeater on top of a mountain far away... (GRIN). Nate WY0X
