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Some Thoughts on Repeater Performance and the Isolation Between Receiver and Transmitter By Mike Morris WA6ILQ

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 flea-powered handhelds, some use 10w mobiles, some use 50w mobiles, and others use hilltop remote bases.   What set of users do you use as a reference for "balanced"?

In many cases fixing a transmitter problem is easier than fixing a receiver problem.   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, but 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.

Receiver performance in a duplex environment depends on isolation between the repeater receiver and not just the repeater transmitter but all other emitters within earshot.   I use 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 little 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 next one over, and the one down the road), the RF-noisy tower strobe light blinker, even that intermod-creating rusty tower bolt.   Isolation starts at the antenna jack(s) of the equipment and includes everything - 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 (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. I've seen a pass cavity inserted into the line between an exciter and a PA deck.   If necessary you can add additional front end selectivity with an additional pass cavity in front of a receiver (at some cost of insertion loss), but 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 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 6db (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 where the desired signal was at maximum and the "grass" markedly 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.

Sometimes you come up against a brick wall of equipment performance:   If you are using a modern mobile radio as an exciter (meaning synthesized with a broad-tuned PA deck) you may find that the low level transmitted noise is the limiting factor.   Generally crystal-based exciters are cleaner than synthesized ones, and generally tube-based transmitter power amplifiers are cleaner than solid-state ones (and the receive situation is similar - broad-tuned or lumped-LC front ends have reduced selectivity and front end rejection).   For years Motorola specified 4-cavity duplexers on 90-100watt 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 sensitivity at the same sites.   Modern radios are designed to hear and talk over multiple megahertz of bandwidth, therefore the front end filtering and the interstage filtering is almost nonexistent.   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 desense or overload.   Or they burn up the transmitter.   Then they find an "elmer" and a few months later they are looking for a second-hand Micor or Mastr II station - i.e. something with a tight front end and a transmitter designed for 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 150-170 MHz receivers of the day covered roughly 1 MHz before you saw degredation, and maybe 1.5 if if you "stagger tuned" them. The transmitters covered 1.5 MHz, maybe 2 MHz if you stretched them. Radios on low band 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).

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).   I suggest you read the "effective sensitivity" articles on this and other web sites 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 half 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 out of his receiver - the notches are very narrow and the unit might as well not be there at all for any frequency except what 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 week after it was installed at a site.   The notchplexer was offering zero protection to the receiver from the UHF paging transmitter in the next building down the road, and he was certain that the paging transmitter was "dirty".   A quick look with some proper test equipment showed that the paging system was clean, and his duplexer was doing absolutely nothing to pretect his recever from a simple overload condition from a transmitter 20 MHz and three hundred feet (100m) away.   Adding a pass cavity to the receive line cured that problem, and a second one in the transmit line insured that his transmitter would not cause any problems.   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 ended up on the 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 150 feet (50m) 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 sensituve receiver.   Only after switching to heliax on both runs with it's 100% shield was that problem cured.

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...