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Chinese duplexers
On Tue, 30 Aug 2011 16:29:03 +0000 (UTC), "Geoffrey S. Mendelson"
wrote: Jeff Liebermann wrote: Take two radios, two j-poles and a lot of coax and make a repeater. Retch. Why? Don't you make experimental stuff? I don't like J-pole antennas. I've also built too many repeaters in the past to know that they can't just be thrown together from available parts. One of my friends learned that expensive lesson with his 2m repeater. He started with two ham transceivers, went to two Maxtrac radios, and finally to a GE MSTR II. Duplexers went from 4 cavity junk, to a 6 cavity monster. I did not say it was a permanent repeater, it's actually a test bed for some repeater/internet linking systems we are trying out. Sounds interesting. Plan on everything costing at least 4 times what's predicted, and for things to take perhaps 4 times as long. Unplanned repairs and "tweaks" should double the required time. Budget is zero, but I can swing a roll of rg-6 and maybe a $60 duplexer if I don't buy anything else (radio/computer related) that month. Initial budget can be zero. Just plan on a replacement budget equal to the price of a proper repeater. How's this for an expensive mistake? http://802.11junk.com/jeffl/K6BJ-MSF5000/ I've burned about $800 on this mess and it's still not on the air. Wanna guess what went wrong? (Hint: it involves the internal duplexer). http://awapps.commscope.com/products/bsa/_calculators/qhisolation.asp Hint1: You'll be close with vertical antenna isolation. Explain, do you mean just the distance if they are both vertical, or the distance AND one is vertical and the other horizontal? Assuming vertically polarized monopole antennas, vertical isolation is from tip of one antenna to the tip of the other antenna. Horizontal isolation is just the horizontal spacing. If they're seperated both vertically and horizontally, then there's a problem. You only need to be offset horizontally slightly, for the antennas to couple. I ran into this problem when I tried to put two vertically seperated antennas on opposite sides of a tower. The isolation was better then they were directly in line with each other, on the same side of the tower. Incidentally, mounting one antenna upside down offers additional distance and improved isolation. Hint2: Some radios have quite a bit of synthesizer noise, much of which will be on your repeater receive frequency. This is why some repeaters still use crystal oscillators instead of synthesizers. Also almost irrelevant. The radios wll be what I have, HTX-202s or what I can scrounge (TBD). Retch 2.0. Radio Shock HTX-202 handhelds tend to stick on the air with low battery voltage. The receivers are also easily overloaded. We have the remaining few HTX-202 radios used for APRS WX packet. I would love to replace them, but nobody (including me) want's to spend the time. I'd love to get my hands on 2 channel Maxtracs or similar radios out of taxis, but the people sitting on piles of them are much more interesting in refurbishing and enhancing them than selling them cheaply or giving them away. Patience. On Jan 1, 2013, everything in the US is suppose to go narrow band. There should be tons of cheap radios available on eBay and from public safety outlets. I'd be very happy with ones that receive only or have blown finals that I can get 2w or so out of. Make sure you get the ones with the 16 pin accessory connector. Many of the Maxtrac's on eBay are actually cobbled together Radius or M120 radios, with odd boards and strange firmware. Caveat emptor. I know, but I don't have a "pair" yet. I'm not even sure there are any available or a test one. So why be specific when I don't have that information yet, and why wait for it to ask a technical question, which will have the same answer no matter which frequencies I use? Careful. Some repeater frequencies are a problem. For example, 146.760 is on the 41st harmonic of the common 3.579545MHz clock osc frequency. No problem at the repeater, but it will drive the users nuts. If co-located with other radios, be sure to do an intermodulation study. http://www3.telus.net/PassiveRF/ We found out too late that a mix of some of our transmit frequencies lands on one of our inputs. You can't improve things by simply adding more cavities. All you'll do is add more loss: Bigger cavities = higher Q and therefore closer frequency spacing. More cavities = deeper notch and therefore more isolation. And that's the answer I need. You might find it easier to build a UHF repeater. With 5MHz spacing, the cheap mobile duplexers will work with low power (25w) radios. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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