Steve, When you mentioned "receivers" as plural, I inferred that the users could hear the repeater fine, but had difficulty in getting into the repeater- which is the classic case of deficient repeater sensitivity. I stand corrected.
I think your math is more optimistic than you realize. Typical losses at a repeater site are 1.5-2.0 dB loss in the duplexer and similar losses in the feedline. My preferred choice for any duplexer-to-antenna feedline is one that limits the attenuation of the higher frequency to no more than 1.0 dB. For example, a 100 foot feedline at 2m would call for 1/2" Heliax, and the same length on a 440 repeater would call for 7/8" Heliax. Here is where antenna selection is important, because a gain antenna with the appropriate pattern can compensate to some degree for a lossy feedline. There are a lot of poor-performing repeaters that have omnidirectional antennas, but are located at one end of the desired coverage area; an offset-pattern antenna is a better choice in this case. The 12-15 watts you believe is at your antenna may not seem like much, but it should perform better than you describe. For example, a Government VHF radio system that I took down several years ago had eight 100 watt MICOR radios feeding one transmit antenna through a network of hybrid-ferrite combiners. The power that left the final combiner was less than 12 watts. Despite this meager amount of transmit power, the radio system covered a huge area from its hilltop site. Regarding the Vertex VXR-5000 repeater, I found it to be too broadly tuned for my tastes- sort of a "barn door" front end. Also, the coaxial jumpers inside the cabinet were single-shielded RG-58, which is far too leaky. The internal connectors were nickel-plated BNC, which are prone to IM. I retrofitted the cables with double-shielded RG-400/U and silver-plated connectors. Finally, the integral controller does not include a station identifier, which is not a good thing. The controller died on my repeater within the first year of operation, and I installed a Pacific Research RI-310 in its place. Once I added the bandpass cavities and Angle Linear preamp, its performance has been pretty good. As to your final question, I am very pleased with MICOR Sensitron receivers. I have several such receivers in voter racks, both VHF and UHF, and I don't think you can get a better receiver. The Mastr II/Exec II receiver is also good. Please enlighten those who are following this thread with some info: What band is your repeater, length of feedline between the duplexer and antenna, and the make/model of the antenna. Others may have some ideas to pass along. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Stephen Liggett Sent: Wednesday, December 27, 2006 5:12 PM To: [email protected] Subject: [Repeater-Builder] Re: local club needs repeater amp Eric, You really can't comment much about our situation in regards to needing an amplifier unless you know more about the circumstances-- each repeater system is different. As I posted, our receivers FAR out- perform the one transmitter. You can be near full-quieting on receive, but get a choppy and low level transmit from the transmitter site. Lets do some math. Assume 1 dB loss from the duplexer, 1/2 dB loss from the isolator, and 1/2 dB loss from the feedline (all optimistic values). Given that 3dB is a loss of 1/2 of the power, our 25 watt transmitter gives about 12-15 watts at the top. Given location, etc, that is simply not enough. There are plenty of moderately-wide coverage repeaters, with a less than optimal transmitter site, that have greatly benefitted from amplifiers that can give 150-200 watts at the antenna. I am interested in the your other comment about the Vertex 5000 receiver. Its sensitivity measures out well, and there is never intermod or desensitization, even with a high quality preamp. But, since I am always looking for better receivers, what in particular is bad about the Vertex, and what receiver do you think is "best"? steve WA4BVO
