Isolation beteween the repeater receiver and other RF sources

Up one level
Back to Home Some Thoughts on Repeater Performance and the Isolation Between Receiver and Transmitter
Compiled, HTML'd and Maintained by Mike Morris WA6ILQ

Comments / critiques / suggestions on this page are welcome.

Repeater performance is defined by RF performance as well as audio performance. You can have the best sounding repeater on the planet, but if the users can't get into it or hear it then it's worthless. Audio performance is covered elsewhere on this web site. Good RF perfomance is a balance between receiver and transmitter... you want neither an "elephant system" (all ears and no mouth) or an "alligator system" (all mouth and no ears). It gets complicated when some users use use hilltop remote bases, some use 50 watt mobiles, some use 10 watt mobiles, some use flea-powered handhelds (and a rubber duck antenna is -3db to start with) and some users use flea-powered handhelds with a rubber duck inside a car (and the shielding of the metal car body costs you an additional -3 to -10 db). What set of users do you use as a reference for "balanced"? You have to consider the demographics of your users.

The two most important items that determines system performance is the antenna and the feedline. And perversely they are the most difficult items to fix. Some sites have a shared receiver antenna and you transmit off of your own antenna, at others you receive and transmit off of your own antenna with a duplexer. Do your homework! Pick the antenna for it's performance. Pick your feedline the same way! The antenna and feedline will define your "footprint" or coverage area much more than the transmitter power, and any shortcomings will shrink your footprint. And watch out for dBi numbers in antenna specifications. The term "dBi" was invented to be used in the microwave environment and when used in VHF and UHF are a way for the marketing people to gain a "free" 2.15 dB. To convert dBi to "real" dBs (more properly known as dBd) just subtract 2.15 dB. In fact, make it a point to Google and learn the difference betwen dB, dBd and dBi before you spend your money.

As to the feedline, don't forget that any losses in a duplexer based system have a negative affect on both transmit AND RECEIVE performance. In many cases a site's shared receive antenna is fed with inch-and-five-eights Heliax™. This is used for absolutely minimum loss. That feedline then feeds a quality preamp with the lowest noise figure available. Only then does it feed the window filter (if there is one) and the distribution amplifier.

If there is no shared receive antenna then you are on your own... You need to pick the best antenna for your own situation and the lowest loss feedline you can afford.

In many cases fixing a transmitter problem is easier than fixing a receiver problem - increase the power or the antenna gain, or reduce the feedline loss. In many cases the transmitter is a single unit, in others it is a lower power exciter (anything from 1/10 of a watt to 30 watts, usually 2 to 5 watts) followed by an external power amplifier, common values are 60, 90, 100, 120, 250, 330/350 and 500 watts.

Good receiver performance depends on being able to hear the users... and the receiver performance is divided into two major areas: sensitivity and selectivity. Sensitivity is obvious - how weak a signal can you hear clearly, and that is primarily affected by the receive antenna, a preamp (if there is one) and minimal feedline loss. Selectivity is a more difficult parameter to measure. A good analogy to selectivity is this: imagine trying to focus on only the person sitting next to you whispering during a LOUD rock concert - the more surrounding noise you can tune out the better you can hear the signal you are trying to hear.

To continue the rock concert analogy, it's not how well the receiver works on the bench, it's how well it works in real life, at the site. You may have good ears, but when the noise surrounds you just how well can you hear the person next to you? And if you can't accurately measure the before and after performance, you can't really tell if you've made any improvements, and if so, quantify how much. You need to be able to accurately measure the Effective Sensitivity of the receiver, and the measurement methodology must be repeatable.

Receiver performance in a duplex environment (doesn't matter if it's repeating or not) depends on isolation between the repeater receiver and not just the repeater transmitter but all other emitters within earshot. I used the term "emitters" instead of "transmitters" deliberately - you have to consider every RF source that might affect the receiver. Most folks think that you need some magic amount of isolation for good performance, and buy a duplexer with more than that magic number, and end up disappointed. In fact any more than "enough" (plus a reasonable amount of headroom) does nothing but drain your wallet.

In reality you have to isolate your receiver from all of the other RF sources at the site that your receiver can hear - the local oscillators, the transmitters in the building (and the one next door, and the one across the road, and the one down the road), the RF-noisy tower strobe light blinker, even that intermod-creating rusty tower bolt or the internal hidden crack in the antenna. Isolation starts at the receiver antenna jack(s) of the equipment and includes everything - a tunable front end is preferred over a fixed tuned 5 MHz wide front end. Then the cables... double shielded RG400 is preferred... the best duplexer in the world won't help if the jumper cable between it and the receiver was made from thirty year old Radio Shack CB coax with 75% shield. Actually, all it takes is one nickel-plated connector, or even one good quality connector that was improperly installed.

Isolation is inherent in the design of the radio equipment, for example minimizing the off channel energy (noise) created by a transmitter exciter (sometimes called "grass" after the pattern it makes on a spectrum analyzer screen), and while it's better if it is designed in, sometimes it can be added on. As one example, I've seen a pass cavity inserted into the line between an exciter and a PA deck (the transmitter consisted of a low power GM300, which is adjustable from 1 to 10 watts (and set at 5 watts), followed by a 2 watt to 130w continuous duty amplifier that was set to 100 watts). The loops in the pass cavity were set to a -3db insertion loss (which results in a sharp peak). Sometimes you can add additional front end selectivity with an additional pass cavity in front of a receiver (at some cost of insertion loss)... Sometimes the need for increased add-on isolation can be reduced by using careful tuning (with a spectrum analyzer) of both the transmitter and the receiver. Yes, the receiver also. I say this because I watched a gentleman fine-tune a Motrac receiver local oscillator / multiplier chain with a spectrum analyzer on the injection point of the mixer, and he was able to reduce a mix in the receiver front end by about 6 to 9 dB (there were nice smooth peaks on the test set meter at each test point, but on the spectrum analyzer there was a very narrow peak on each adjustment where the desired signal was at maximum and the "grass" remarkedly dropped in level). On the receiver side, you can take advantage of the fact that a repeater is a single channel device and you can ignore the tuning instructions that tell you to stagger tune the front end and multiplier chain, and instead precision tune the receiver for one frequency. And MANY factory built new repeaters arrive with stagger-tuned front ends.

An increase in receiver isolation can be achieved by first looking at the receiver front end by sweeping it with a service monitor. In most cases you will see that the further you move the transmitter away (in frequency) the better. The 6 meter repeaters in some areas have 1 MHz offsets where others have 0.5 or 0.6 MHz offsets, and those that have 1 MHz have it easier. Many others have found that the 5 MHz offset on UHF makes it much easier to duplex than the 0.6 MHz on 2 meters. And crossband is even easier, but do the math. Don't try to listen to 443.425 MHz and talk on a 147.81 MHz transmitter at the same time.

Sometimes you come up against a brick wall of equipment performance: If you are using a modern mobile radio as a transmitter (or as an exciter), meaning synthesized with a broad-tuned PA deck you may find that the broad-tuned or lumped-LC receiver front ends have reduced selectivity and front end rejection. That's a polite way of saying that the super-wide front ends designed for ham radio mobile environment (sometimes referred to as wide eneough for DC to Daylight) are a total joke when it comes to trying run full duplex. You need some serious selectivity to run duplex. Then you have the low level transmitted noise from the receiver and the PA deck - that is another limiting factor. Take this as a hint: for years crystal-based exciters were cleaner than synthesized ones, and tube-based transmitter power amplifiers are cleaner than solid-state ones. For years Motorola specified 4-cavity duplexers on 90-100 watt tube based high band and UHF repeaters and had to switch to 6-cavity designs to get equal performance with solid state repeaters of the same power level and same sensitivity at the same sites (and sometimes they had to add an additional cavity to the 6-cavity duplexer to make the Micor work as well as the old tube repeater they were removing).

Modern amateur and commercial radios are designed to hear and talk over multiple megahertz of bandwidth, therefore the front end filtering and the interstage filtering is almost nonexistent, meaning that they are prone to in-band desense. Time after time I have seen people start out building a repeater by taking two synthesized mobile radios and connecting one up as a receiver and the other as a transmitter, and wonder why they have receiver desense or overload. Or they burn up the transmitter (google the term "duty cycle"). Then they find an "elmer" and a few months later they are looking for a second-hand MICOR, MASTR II or MSF5000 station - i.e. something with a tight (and tunable) front end and a tuned exciter, and designed for single frequency continuous duty from the outset. It's interesting, but 1960s-1970s tube based radios were inherently tighter and much cleaner, therefore requiring less isolation in the antenna system. The VHF receivers of the day covered roughly 1 MHz before you saw degredation, and maybe 1.5 MHz if you "stagger tuned" them. The transmitters covered 1.5 MHz, maybe 2 MHz if you stretched them. If you peak-tuned them on one channel (like for a repeater) you could get by with a minimal duplexer. Radios on low band (10 meters, 6 meters or the commercial frequencies in between) covered a similar percentage... if you had two low band channels more than 275-300 kHz apart you probably had two separate radios (base or mobile), feedline and antennas.

At some point you can't get any better because of other parameters outside the radio chassis - maybe the noise floor at your site is higher than the effective sensitivity of the receiver (and as said on the linked page above, the effective sensitivity is NOT the bench sensitivity). The noise floor on VHF is generally worse than UHF, and 220 is frequently better than UHF. I suggest you read the "effective sensitivity" articles because improving that number is the single best thing you can do to improve the ability of the repeater to hear your users.

Or maybe it's the cheap duplexer. Mobile duplexers are tempting for the newbie repeater builder since they are one-half to one-third the price of a "real" duplexer, and he's happy until he discovers that what he bought is nothing but notches (sometimes called a "notchplexer"). Yes, it notches the transmitter out of the receiver, and the receiver out of the transmitter, but offers nothing in keeping any other radio at the site (or the sites down the road) out of his receiver - the notches are very narrow and the unit might as well not be there at all for any frequency except the one frequency that the notch covers. I saw a dramatic example of this many years ago when a friend's UHF two-mobile-radios-and-a-mobile-duplexer repeater was receiving interference a few days after it was installed at a hilltop site. The notchplexer was offering zero protection to the receiver from the UHF paging transmitter (330 watts into a 10 db antenna) in the next building down the road, and he was certain that the paging transmitter was "dirty". A quick look with "elmer's" spectrum analyzer showed that the paging system was clean, and his duplexer was doing absolutely nothing to protect his recever from a simple front end overload condition from a transmitter 20 MHz and six hundred feet (180m) away. Adding a pass cavity to the receive line cured that problem, and a second one in the paging transmitter feedline insured that his transmitter would not cause any problems to any one else (it helps when "elmer" knows the paging system technician and the owner of the building that the paging transmitter was in). The new repeater owner learned a lesson about purchasing the wrong equipment for the job. A few weeks later a real 6-cavity pass-notch duplexer ended up on the new repeater, and the mobile duplexer was shelved (and later ended up in a portable repeater system).

Or maybe it's that the feedlines are not enough for the site: one six meter repeater I worked on was limited by the fact that the transmit and receive antenna feedlines were coupling into each other... Two parallel runs of over 100 feet (about 30m) of 97% braided shield coax tie-wrapped to each other and to the tower leg means that there is 150 feet of 3% holes facing each other... made a nice lossy coupler for a 100w transmitter into a sensitive receiver 500 KHz away. Only after switching to heliax on both runs with it's 100% shield was that problem cured (and proper heliax clamps to mount it to the tower).

Another limiting factor might a missing shield on an exciter, a PA deck, or on a receiver board... remember, the RF design engineers added those shields for a reason, not for cosmetic looks.... and someone may have lost a shield before you got the radio and you don't know it's missing... (this is the voice of experience from the first MICOR station I worked on)

Or it might be one defective jumper... I saw a situation where the receive side of a six-cavity highband duplexer fed a preamp which fed a receiver. The preamp was physically adjacent to the receiver. The jumpers from the duplexer to the repeater cabinet was new, good looking coax but when you pulled back the jacket it had a loose weave braid. The system was desensed and the owner could not figure out where the grunge was coming from ("But I've got all new cabling"). Replacing the jumper from the duplexer to the preamp with Superflex (100% shielding) fixed 90% of the problem, replacing the one from the preamp to the receiver fixed the rest, but we replaced the jumper from the exciter to the power amplifier and the one from the amplifier to the circulator and from the circulator to the pass cavity and from the pass cavity to the duplexer on general principles. And the connectors have to be installed properly on the cable, especially on the superflex. The PL-259 connector is frequently found on older duplexers and cavities, and was never designed for superflex, but Tony King W4ZT developed an innovative way to make it work, and did a photo article on installing PL-259 connectors on FSJ1-50A 1/4" Superflex Heliax.

A comment by Eric Lemmon WB6FLY (SK) on the repeater-builder mailing list:

Be very cautious about buying any coaxial cable that has the word "TYPE" or "LIKE" printed on its side, even if the maker claims that it is "military specification" cable. That one word can allow the maker to market a totally inferior product to unsuspecting buyers. Genuine, RG-214/U coaxial cable made to the MIL-C-17 specification (and it will be labeled as such) has the double copper shields made from silver-plated wire. Several companies manufacture an RG-214 "TYPE" cable that is very similar, but without the silver plating on the shield wire. As you would expect, it's a lot cheaper than the genuine MIL-specification RG-214/U cable. Such junk cable may also have less dense braid coverage than the genuine cable.

And while Eric specifically mentions RG-214, the problem exists on many, many types of cable. I've seen both "RG-400 TYPE" and "RG-174 LIKE" cable out there. The RG-174 TYPE had a very loose shield - the gentleman standing next to me commented that it resembled his window screening.

I don't know if the defective jumpers mentioned above were "TYPE" or "LIKE" cable (it's been over two decades), but they could have been.

Sometimes the isolation problems are just not possible to overcome due to the equipment in use. In the USA the UHF amateur band is 420-450 MHz, with repeaters from 440-450 MHz... and many local band plans have some spectrum between 420 and 440 MHz allocated to point-to-point links (usually 420-421 or 420-422 on the low end and 438-440 or 439-440 on the high end). The Motorola Mitrek is an older crystal controlled radio and is popular as a UHF repeater and as a link radio - but some design unintentional oversights can severely limit the performance (and the Mitrek is not alone in having design oversights).... when you duplex a 406-420 MHz range Mitrek mobile (which will get up to 430-431 MHz without a problem) you will find that you have spurs on the transmitter. The spurs happen because the receiver local oscillator multipliers just happen to couple into the transmitter multipliers due to a poor choice of multiplying factors, too-close physical spacing in the original design (i.e. across-the-PC-board coupling), and the fact that the original design had the receiver multipliers powered down while it was transmitting... (note that the 450-512 MHz models will do the same thing but most of the time the recevier multiplier frequencies are separated far enough from the transmitter to not cause a problem)...

Another limiting factor can be when the transmitter antenna couples RF energy into the tower itself, and the tower couples it back into the receiver antenna. This is more visible on 10 meters and 6 meters, but can affect systems as high as UHF. Grounding the feedline to the tower can help... Feedline manufacturers such as Andrew typically recommend a ground strap at the top and bottom of the support structure, plus physical clamping every so often, and an additional grounding kit where the feedline enters the building. Good practice suggests additional ground kits at specific intervals on really high towers. The basic idea is not RF but lightning: the grounding kits keep the feedline and tower at the same potential so there isn't any arcing between them during lightning strikes. With kiloamps of current you can get kilovolts difference with very small resistance differences between the tower leg and the feedline - and you will have those differences between them due to the differing electrical resistance of steel versus copper. In addition I have seen feedlines stripped from the tower by the intense magnetic fields caused by lightning hits... don't forget that current through a conductor (the feedline shield) generates a magnetic field, and the parallel current through a steel tower leg (or an adjacent feedeline) generates a like magnetic field, and like magnetic fields repel each other... and kiloamps of current create intense magnetic fields.

Back to antennas... I've seen interfeedline coupling when a receive antenna is mounted above the transmit antenna and the transmitter RF power couples into the outer shield of the receive antenna feedline. Grounding the shield at multiple places helps (especially if you can do it at a voltage peak). So when you order the grounding kits mentioned above you may want to order an extra one or two... This appears more on low band than on high band or UHF, but it does happen. I've also seen it on another low band repeater when they added an amplifer and the transmitter RF coupled down the receive feedline shell and overloaded the receiver. On ANY decent system (no matter if it's low band, high band, UHF or 900 MHz) you need 100% shielding on your feedline(s), and you really want to keep the attenuation in the receiver feedline under 1-2 dB from antenna though the filtering to the receiver front end. If the noise floor at the site is low enough, a RF preamplifier may be appropriate, but cheap preamps can be another can of worms. And if they overload, they can create more problems than they solve.

Sometimes it's just outside of your control. For example, the signal strength difference due to trees having leaves (spring to middle of summer) and having none (middle of winter) can be as little as 2 dB or as much as 10 dB on UHF and even higher on 900 or 1200 MHz. And that's going both ways - the UHF users have 10 dB less arriving at the repeater receiver, and have 10 dB less arriving at the users mobile or handheld receiver. The foliage absorption factor was a big surprise to public safety agencies that were conned into switching from 150 MHz or from 30-50 MHz to 800 MHz or 900 MHz... It's all about received signal level and noise figure and you really need a working grasp of the concept...

Some systems have adequate duplexers until they add a preamp to the system receiver. The problem turns out to be that they put the system in several years ago, with an adequate duplexer that had several dB of extra isolation ("headroom"). Then more radios were added to the site. Then they added the preamp - and the amount of isolation isn't enough. They forgot that you need the same amount of extra isolation for 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 with a huge dynamic range like an AngleLinear will help. And adding a pass cavity in front of the preamp will frequently help... a -3 dB cavity in front of a +15 dB preamp still nets you +12 dB. And Anglelinear makes both regular and high level preamps for a reason. I've had to replace regular preamps with high level replacements at several sites.

Or you can go split site (the physical design of the MASTR PRO and MASTR II makes that very easy)... One six meter system near here uses a 200-300 milliwatt 420 MHz "cross-link" (with home-made 6 dB beam antennas on each end) between the two sites (which are about a mile apart). They took a 406-420 MHz MASTR II mobile radio with a dead power amp (really cheap on eBay) and jumpered the exciter to the transmitter output filter. They then swapped the 6m receiver into it (making a 6m to low-power 420 MHz repeater), then put the 420 MHz receiver board into the 6 meter base station radio, then duplexed both of them. Note that to do it right a split site repeater needs two controllers since it is actually two repeaters back-to-back. The receiver-to-link-transmitter site has the first controller, the link-receiver-to-transmitter site has the second controller. Split site repeaters are not limited to 10 meters or 6 meters, one local 2 meter system started out as a split site, with about 600 feet of cable between the receive and transmit sites, and after the club gained enough members they did a funds drive and bought a real duplexer, feedline and antenna. Once they were installed the repeater became a single site, and the "real" antenna gave them much better coverage than the same site had provided before. Later on they found a duplicate receiver and built a 2-channel voter and the old second site became a diversity receiver site - when it came online the received mobile flutter dropped by 50-70%.

In summary, isolation is acquired from multiple sources, all of which add up - a tuned front end instead of a lumped LC front end, cavities on the receiver or transmitter (or both), a 6-cavity duplexer instead of a 4-cavity, and even proper antenna placement on the tower (careful positioning can get you an extra 10 dB for free - just put the receive antenna in a voltage null of the transmit antenna - and nulls exist both vertically and horizontally). Each piece of the puzzle contributes some isolation by itself and when added together makes up the total. When the total exceeds what you need you have a properly performing (and therefore useable and useful) system. Many people think that the more duplexer isolation you have the better. And that's true - up to a point.

The web page Vertical and Horizontal Antenna Separation Charts provided by Kevin Custer W3KKC may be of interest.

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.

Back to the top of the page
Up one level
Back to Home

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.