Rick, You made some very good points. I can offer two comments: For hookups within the repeater cabinet, use RG-400/U coax instead of RG-142/U. RG-400 has a stranded center conductor, while RG-142 has a solid steel center conductor that breaks easily when repeatedly flexed.
Regarding the Vertex UHF repeaters, I corrected an in-cabinet desense problem by replacing all three internal jumpers- which were unmarked gray-jacketed single-shield cables- with double-shielded RG-400 cable. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Rick Beatty Sent: Thursday, February 05, 2009 10:46 AM To: [email protected] Subject: Re: [Repeater-Builder] Re: Hamtronics Helical Resonator Preamp or Advanced Research Preamp Hi All -- Here is my take on the preamp vs no preamp situation -- and with Vertex repeaters -- First -- Preamps are not the solution to most of our troubles, in fact it has been my experience over the years that they are more trouble than they are helpful I agree with Skipp and others when it is said to get rid of the LMR-400 and replace it. For inside the box use 142b or 223, something with low loss but is manageable to use on the duplexer and interconnects. rg-214 is way over the top for most of this stuff. For antennas, don't use less than 1/2 heliax. Even '214 will get noisy if there is any flex or it is out in the weather a couple of seasons. No-Preamps, we as amateurs look a preamps as a panacea for many things, including the lack of perceived receiver sensitivity. But it is interesting to note a couple of things. One, a 0.4 uV receiver is as good as it is going to get, in most cases because of the fact you're setting in a high place and the MDS combined with the power out, 50 to 75 watts, is going to be about equal with a 4 pole antenna. Not rocket science, and you can actually run that test iif you're on a hill and have the proper equipment to do so. Secondly, preamps, even at 10 dB gain, really only add about 3 dB of signal and the rest is just moving the noise floor higher. And in some cases really create a need for more signal to open the repeater's receiver. If you're having issues with the receiver, it would be my recommendation to sit down and take a hard look at the equipment, duplexer, antenna, and cables. There are so many variables here in just those 4 items that it will take some time to optimize each of them to your needs. Coax, again I agree with all of those on here that LMR, 214, 213, RG-8 etc on a repeater is just not a good idea. Use hardline - Vertex, I have nothing bad to say about them other than my experience with 2 UHF machines. Both of them exhibit the same characteristics so I will just speak in general terms. What I found was that there was/is a lot of crosstalk in the repeater box itself and even though the duplexer was well tuned we could not get the isolation down on the system as a whole. The second problem I found was that the LO runs all the time! This not good; it interferes with others on the site and can cause strange mixes within the box that could desense your receiver, especially if there is a signal within the IF of the repeater, either high or low. How did we fix it? We went to a GE box, LOL! So, I guess in conclusion here, before spending your hard earned dollars, for preamps, helical resonators, and the like take good gander at the situation and try to break it down to a common denominator, with a clear picture as to what you're seeing. Do all of the things necessary for good repeater operation and then look again. I'll bet you just haven't found the right place to look yet. rick NU7Z On Thu, Feb 5, 2009 at 10:18 AM, skipp025 <[email protected] <mailto:[email protected]> > wrote: Hi Ralph, > Ralph <w7...@...> wrote: > I have never seen or used a Hamtronics preamp. I don't > know anything about their specs or how truthful they > are. Maybe some one out there in the great bits might > have an answer. Skipp? Eric? anyone?? I'll answer up to the anyone label... Through the years Hamtronics has offered up a number of quite different RF Preamplifier kits and assembled boards. Relative to the industry they are truthful and their products are a good dollar value. Even more valuable is the experience and knowledge many people receive for constructing and setting up their kits. The RF Preamplifier you're probably talking about is not a true helical filter design. Hamtronics no longer offers the HRA series with the on board Toko (brand) helical filter. Their current products (when I last looked) were broad-band and some with modest tuned circuits, which is not really a true helical layout. After completing a recent very large vhf receiver site distribution project... I'm now not so keen on using and depending on or trusting the classic (Toko) type Helical filter assemblies in front receiver pre-amplifiers at locations with strong adjacent signals. The shining star in this most recent project was the now famous GLB pre-amplifier. Please don't confuse my description of the small Toko helical filter assemblies with the helical filters built into many receiver front end circuits/layouts. However, both the performance of your receiver can be and is often directly related to both... but you often can't easily change the receiver (as-built) front end assembly. "You get what you get" built into the receiver as supplied by the manufacturer. After reading your posts and all the answers... I can write is how I personally would want to know more about the Vertex radio receiver front-end layout before I started making changes. Directly dependent on the receiver front- end layout and performance... would say a lot about what you can successfully park in front of it (the receiver). My personal suggestion is that you replace the LMR-400 coax with almost anything else... with relatively short VHF lengths RG-214 mil spec is probably a great choice if you don't have access to free-bee (gratis) hard line. As previously reported many times... I source a lot of repeater problems back to LMR-400 cable so I jerk it out of all our duplex (repeater)operations. The Internet Wifi guys like and swear by it a lot (because of the lower cost) but their operations are mostly half duplex (simplex). Half duplex radio operations don't appear to suffer the LMR-400 type grunge problem nearly as much as the many repeater (full-duplex) gremlins we've had to resolve. But I have seen a fair number of Wifi problems related to using LMR-400 but a lot of that sort of blame gets put off on the path being bad or co-channel interference. It's very smart of you to pay attention to the antenna beam-width related to both your elevation and location of the majority users. A smaller part about how much Transmit Power you use is modestly inter-related to your hardware and how hot you want the receiver side of things. Less TX Power Out is easier to deal with... but I do like a very solid TX signal on/at the user end. There are a gazillion pre-amp and filter options possible... for me it comes full circle back to the receiver performance and how it's laid out (constructed). Commercial mobile two-way radios can be decent repeater receivers and some can be "not so good". cheers, skipp
