System Engineering, and a few commments...
Compiled by Mike Morris WA6ILQ
Rule number 1: Always favor your system receiver. A system's coverage area is primarily determined by how well it receives stations in the field, not by how much transmitter power it has.
Rule number 2: This goes hand in hand with Rule number 1: Maximize your received signal. Maximize the isolation from receiver to transmitter. Concentrate on minimizing the insertion loss on the receive side of the system. Test for Effective Sensitivity frequently (at least twice a year) - this includes tests with the transmitter on (but keep in mind the effective sensitivity can be a dynamic value unless the environment is a broadcast site with a constant RF environment). Keep good records - a site logbook is important. If something changes between visits you need to find out what changed and why.
Rule number 3: If you are building a system on a budget, do the initial scrimping on the stuff inside the building. It's a lot easier and cheaper to update/upgrade later on.
Rule number 4: With repeaters, the best you can afford may be barely good enough.
Rule number 5: You can't "balance" a system unless you can define your user
demographics - i.e. who you are balancing it for. Are you catering to the base stations,
to the mobiles, or to the handhelds?
A repeater system that has the transmitter power to reach out to the base stations and mobiles that have 60-75 watts ERP could be accused of being an alligator system (all mouth and minimal ears) by a user with a 2 watt handheld that has a -3 dB rubber duck antenna. And if he's inside a car driving down the road then there's an additional -3 dB (at least) due to the steel car body... and then you subtract the mobile flutter...
On the other hand, a system that is designed to support hand-helds and has multiple voting receiver sites and a single centralized transmitter (like many paramedic systems) could be accused of being an elephant system (all ears and a small mouth).
Both the receiver and the transmitter depend on antennas - and both the antenna(s) and the feedline(s) are usually the most critical components in any amateur radio station, be it a home station, a mobile, a repeater, a remote base or a point-to-point link. Perversely they are usually are the most difficult to install, maintain, or troubleshoot - and it seems like all of the troubleshooting usually has to be done during extreme climate conditions like in freezing temperatures and/or high winds while on top of a tower. If you are installing a repeater that will be primary in any type of emergency services situations it does not make any sense to go "cheap" on the antenna system. Creative (legal) aquisition of good quality equipment, yes. Low or poor quality equipment, no... that decision will always come back and haunt you. Simply put, a good quality professional land mobile grade antenna will hear and talk better than a poor antenna and do so for a lot longer time. If you go with quality hardware and proper installation the first time you won't have to do it over and over (like my late father used to say about both hand and power tools: "buying quality only hurts once"). A good Phelps-Dodge, DB Products or Sinclair antenna connected to good Heliax™ feedline will last over 20 years of seriously hostile winters. How many Comets, Diamonds, Hustlers, homebrew antennas or replacement lengths of RG-series feedline will you buy and install in the same 20 years ?
And don't forget that most of the really good commercial 2-way sites are owned by people that have been burned - and badly - by ham radio groups in the past. Some sites require a bonded and/or certified tower climber to do the work. If that's the case you can expect to have to pay some big bucks - both an arrive-on-site fee and a hourly rate. Some tower monkeys charge by the vertical foot - doing something at 175 feet costs more than at 75 feet. So replacing your $100 Diamond antenna at 200 feet up the tower may cost upwards of $900, and in some cases upwards of $1100, and that's EACH TIME. For that much money you could have had a NEW DB Products, a Phelps-Dodge or a Sinclair complete with new Heliax the first time instead of a Diamond and cheap no-name RG-214 type cable... and no need to touch it or replace it for 20 years or more.
A receiver preamp can help some systems, but realize that good preamps are all about the internal noise figure (NF) of the preamp and the amount of gain it provides. The lower the NF, the weaker the signal that will be heard. Regarding gain, most modern preamps have gain on the order of 14-18 dB in a single stage. That's enough to take a signal that's barely above the noise floor and make it very usable on a decent receiver. But what works on the workbench may not be what works at the site... it's all about effective sensitivity. If you're already hearing down to the noise floor without a preamp then adding one isn't going to make any improvement in sensitivity and in fact may create new problems. But if you do decide to add a preamp, don't attempt to use it to make up for a long, lossy feedline - once you've lost the signal all the preamp does is amplify what's left (the signal that is way down in the noise). Likewise don't attempt to use a preamp to make up for a poor receiver - fix or replace the receiver. There are ways to put a preamp up at the antenna (but only if it is a receive-only antenna). Many sites use shared "community receive" antennas that feed a preamp/multicoupler panel - at one 5,300 foot mountaintop site I visit semi-regularly there is a 120 foot tower with heavy crossmembers every 20 feet. The top level is receive only - there are dedicated antennas for VHF, 220 MHz, UHF, 800 MHz, 900 MHz and 1200 MHz with hot spare antennas already in place for VHF, UHF, 800 MHz and 900 MHz. The UHF antenna and it's spare is a broadband (403-512 MHz) Sinclair with an AngleLinear preamp mounted at the antenna base. It feeds a run of inch-and-five-eights Heliax that ends up connected to an AngleLinear UHF multicoupler panel. Each port of that multicoupler panel connects to the receiver in a UHF repeater (over 35 of them). Likewise there is a single broadband (132-174 MHz) high band Sinclair antenna (with another tower top preamp) feeding another run of inch-and-five-eights Heliax to a VHF AngleLinear multicoupler panel that feeds all of the high band receivers. In both cases the multicoupler panel has a power injector assembly that feeds DC up the the feedline to power the tower-top preamp.
In most repeaters the duplexer provides a certain amount of isolation between the receiver and the transmitter (some systems, like those that use two antennas, or even two sites, don't use duplexers). If the amount of isolation, however it is acquired, is greater than what is required (the excess is sometimes referred to as "headroom"), then the system design is adequate for the job (see the article Some thoughts on Repeater Receiver-to-Transmitter Isolation below). That situation is fine until they decide to add a preamp to help out the handheld users. Then they discover that the amount of isolation isn't enough. They forgot that you need (at least) the same amount of extra isolation ("headroom") as the amount of gain the preamp provides, since it raises the apparent noise floor as well as the signal of interest. In most cases you will have to fight with desense when you add a preamp (a top-quality preamp like an AngleLinear will help). Always have enough extra headroom in your receiver, transmitter and duplexer to handle either or both of a couple of situations: First, the site owner adds additional transmitters to the site, or second, that you want to add a preamp later on. If the duplexer is your primary provider of receiver-to-transmitter isolation do not scrimp on the duplexer. Next to a quality antenna and feedline the duplexer is the most critical part of a good repeater system. Long ago I gave up on four-cavity duplexers (two cavities on each side) on VHF/2m, 222 MHz and 440-470 MHz UHF, I use the six cavity pass/reject type exclusively. Duplexer tuning is very, very critical. A return loss bridge is preferred, a spectrum analyzer with a tracking generator is the second choice. And don't tune the duplexer on the ground, then transport it to the site over a bumpy four-wheel-drive road, and expect it to be as precisely tuned when you get there. Likewise don't trust the tuning of ANY duplexer once it's been shipped - always assume that the transportation method will shake things up. Always have the test gear with you at the site to verify final tuning after mounting it in the system rack.
On the transmit side, never forget that SWR is not the only measure of antenna performance. A low SWR only means that the transmitter is "seeing" a reasonably non-reactive load. That is, it is neither capacitive or inductive, it looks like a 50 ohm resistor (a dummy load). The SWR tells you nothing about what is really important, the antenna efficiency, the antenna noise level, its pattern (gain) and decoupling of the RF from the feedline. And don't forget that the feedline loss runs both directions and can dramatically affect the SWR reading !! Your transmitter sends power up the feedline (let's say it's 100 watts), and some gets lost going up (let's say that it's 20% and 80w gets there). The lightning-damaged antenna reflects some power back down (let's say it's 10%, or 8 watts). The 8 watts comes back down, and 20% gets lost, and you see 6.4 watts on the Bird wattmeter. So you see 100 going up and 6.4 coming back, and you think the SWR is a lot better than it really is at the antenna. Look at this web page on the topic: Power Antenna Manufacturing Inc. SWR Calculator (an offsite link). It removes the "masking effect" of the feedline loss.
Speaking of "gain", there are a lot of people that don't understand it. Some people actually think that by picking a higher gain antenna they can "regain the loss caused by the coax". Well, that's a total joke. Go read the Antenna Gain writeup by Marc Dekenah ZS6MGD at http://www.marcspages.co.uk/tech/antgain.htm.
Before you sign a site agreement, pay your money, and go to the trouble of installing a complete system you will want to measure the noise floor on your frequency at the site. Just borrow an appropriate already-installed antenna for fifteen minutes or so (with permission) and make the measurement (i.e. if you are installing a VHF system use a VHF antenna). If site A has a 0.8 µV noise floor (due to the broadcast and paging systems there) and site B has a 0.1 µV noise floor take a guess as to which site will hear better (and I have seen sites that have noise floors that are above 3 µv on the frequency of interest). And fifteen minutes may not be enough - you want to get all the offenders, and that may take an hour or two, or even longer. Some signals bay be from systems that only run Monday through Friday. There are spectrum analyzers with digital memory and some can do additive recordings - they can assemble the worst-case scenario, and I have seen one of those parked at a site for two weeks. But there are workarounds: on a site visit one weekend I saw a tripod with a video camera (with a 6 hour tape in it), with the camera tight-focused on a spectrum analyzer... the person volunteered the info that he had borrowed a good but abandoned-in-place antenna for the analyzer, had a motor home with a TV and DVD player outside, and that he had a enough tape to get over 96 straight hours... said he was going to sleep for 5 hours and 55 minutes at a time (he claimed he had two alarm clocks, just to make sure), change tapes, or read books, or whatever he could find to pass the time... He volunteered that when he got home he was going to do frame grabs with his computer, superimpose them and build up a worst-case analyzer image. ANd he was going to make three trips over a month to try and get all the signals.
Once you have a site interference profile mapped, you may chose to install a UHF system at a particular site if the noise floor on VHF is intolerable (or vice versa). And don't overlook 220 MHz, as the noise floor on many sites can be way below that of 2m or UHF. And the atmospheric noise at 220 is lower than 2m or UHF as well. Remember that with repeaters it all depends on how well you hear (see the article on measuring effective sensitivity below). Given the limits of antenna power rating, feedline and the local geography / topography, increasing the talk range is easy - how much amplifier, duplexer and antenna can you afford?
When you go attacking a desense problem, don't assume that any on-channel receiver noise is a function of energy from the transmitter. That's often not the case. The transmitter can be absolutely clean, the cavities and / or duplexer tuned perfectly, but nevertheless, if there is some corroded metal joint in the antenna induction field, it will generate broadband noise, some of which inevitably will be on channel noise. You can play with your transmitter, duplexer and cables forever and it won't mitigate this problem. It may take crawling all over the tower with a sniffer to find it. Basically, you find the biggest culprit, solve that, and try again, and keep doing this till you're fed up, happy with it, too tired, or give up.
There are several publications that you should look at if you are going to be doing any serious antenna systems work at a site. One is the site owner/manager's requirements, others are below on this page in each section. Another is the Motorola "R56"manual (however it's way too expensive for the the average ham to buy). Another is here: GE/MA-Com/Tyco Installation Manual LBI-39185C titled "Specifications, Guidelines, and Practices, Tower Requirements and General Specifications".
Some people build their own repeater antennas from scratch, copying a commercial design. Others rebuild surplus commercial antennas, others just buy something new. It's worth getting to know someone who knows how to bend and weld aluminum rod and tubing, and someone that can build a phasing harness... The antenna systems page at this web site has drawings of many good commercial antennas and their harnesses.
The Metric System - learn it!
The U.S. has been slow to adopt the Metric system of measurement, yet in ham radio circles we use metric terminology daily ("80 meters". "2 meters", "23cm" and more). However whenever I'm building an antenna, or a phasing harness, I'm doing conversions back and forth all the time. I often thought that it would be very nice to have a dual-marked shop ruler and tape measure (both Metric and English graduations). Well, after some searching I found that Stanley Tools of New Britain, CT has a model 30-575 (7.5 meter/25 foot) steel tape, and there's a model 34-827 PVC coated fiberglass 30 meter/100 foot for doing long runs. Most of the big "super home centers" don't seem to carry metric tapes with any consistency, but if several radio club members can get together on a single order, you might try one of the smaller, family-owned hardware stores to see if they can special order some of the Stanley tape measures - or the metric replacement blades - that are shown in Stanley's catalog. It can make your next antenna project go much more quickly and with more confidence that everything has been measured correctly.
Update July 2011: Harbor Freight has a 50 meter / 163 foot tape for under $12. It's metric on one side and feet on the other. However they caution you, on a sticker on the housing, that temperature and humidity can cause the tape itself to stretch and shrink as much as a full inch causing that much inaccuracy in the full length. But getting a 150+ foot measuring tape for under $12 is a darn good deal. Just don't use it for making a duplexer harness or making a dipole array - that's what steel measuring tapes are for.
The author can be contacted at: his-callsign // at // repeater-builder // dot // com.
This web page split from the main page 12-Nov-2011.
Layout and hand coded HTML © Copyright 1995 and date of last update by Mike Morris WA6ILQ
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.