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What's a repeater and how does it work?

Repeater Info - N7KP Articles

Since there has been a lot of posts on a couple of other forums, "on the air" and in person questions about repeaters, I am starting a thread about repeaters from a more technical point of explanation for the newer hams. (and some experienced hams that just have not had a reason to learn repeater stuff yet)
I think this thread will eventually go to both answering very technical and very superficially to fill most peoples questions and needs.
So, off we go...
Essentially a "repeater" can be just a receiver connected to just a transmitter.
As you all may have guessed there REALLY needs to be more than that.
But in the basic sense, that is where you start.
Received audio (like that coming out the speaker) is routed to the mic input of the transmitter.
Next you must have some circuit that tells the transmitter to turn on.
On receivers for repeaters there is a circuit called "carrier operated squelch" (COS), carrier operated repeat (COR), and by several other names.
So, that "I hear a signal" circuit is connected to the transmitter "push to talk" (PTT) circuit.
Now we have receive audio connected to the transmit audio, and a circuit to tell the transmitter when to transmit.
ANY signal heard on the input freq is re-transmitted on the output freq.
So, usually there is a tone decoder added to the receiver, and usually a controller that is between the RX and TX that allows for using "touch tones" to turn the system on and off, and to provide automated identification.
Up to this part, making a repeater is "fairly" easy.
But, here is where the challenges can start.
(WARNING, Technical part)
Your receivers can "hear" signals that are VERY weak. That "weakness" is measured in microvolts or in decibels compared to a milliwatt (dBm).
A newer ham mobile will hear a usable signal around -120 dBm which is also approximately .25 microvolts. (a quarter of a microvolt).
You transmit at say 40 watts, which is around +48 dBm.
Here is the biggest challenge, when you want a receiver to "hear" a weak signal, like a handheld trying to talk thru the repeater, with a signal arriving at the receiver of say -100 dBm, AND you want the transmitter to re-broadcast that signal at say +43 dBm there is a "difference" of 143 dB. (-100 added to the +43)
What happens is the RX can "hear" the person on the handheld until the repeater transmitter comes on, then the TX "overpowers" the RX.
This is called receiver "desense".
To avoid desense, you have to use either a duplexer or separate the reciever from the transmitter and separate the antennas by LOTS of distance.
An example here is on the two meter band, our repeaters are on a 600khz offset.
That difference between the RX freq and the TX freq provides several dB of isolation.
But a good rule of thumb for 600 khz offset is that you need around 110 dB of isolation to make a common receiver work with a common transmitter.

Ok, how do we get 110 dB of isolation?
Either separate the RX, TX and their antennas, or get the right kind of duplexer.
At two meters that separation would take thousands of feet horizonatally, or hundreds vertically.
So, using a duplexer or at least "cavity filters" AND separation is required to make things work.
If you use a receiver that is not quite as "sensitive" ( say it hears at -110 dB instead of -120) and you use a 10 watt repeater transmit (+40 dB instead of +48) you have just saved yourself 18 dB of isolation needed.
So, instead of needing around 110 dB, you might only need 92 dB.
Which equates to a smaller and less expensive duplexer of combination of filters and antenna separation.
Some folks even just "suffer" a few dB of desense, and their systems work for all but the weakest stations.
As long as people understand what is going on, and put together a system that works for them, all is OK.
At UHF, the repeater offset is 5 mhz (about 8 times the offset on two meters of 600 khz)
So, the isolation needed to make a workable repeater is more like 75 to 80 dB.

In general, the higher the freq, the wider our repeater offsets (at 900 mhz it is 25 mhz, at 6 meters it is 500khz)
And the higher the freq, the smaller (physically) a duplexer will be for a given isolation value.
Wider offset equals less isolation needed.
Six meter cavities are about 6 ft tall, 440 cavities are about 6 inches.
So, just what is a duplexer?
Well, first lets talk about cavity filters. (that is what we make duplexers out of)
In a broad sense there are really two main types of cavity filters.
Notch filters (they STOP a narrow range of frequencies from passing thru)
Pass filters (they only ALLOW a narrow range of frequencies to pass thru)
Duplexers are assembled by using the above two types of filters to get the isolation we desire for a particular system. More cavities in a row, means more isolation, but more loss also.
For a true portable system that is NOT going to be permanently installed near other repeaters or radio systems, a simple notch duplexer of 4 or 6 cavities is the smallest and might be appropriate.
The idea being to only NOTCH the RX freq out of the TX antenna path, and to NOTCH the Tx energy from the RX antenna path. (getting that 100 ish dB of isolation for two meters or around 75 dB for 440 mhz)
On the far other side of the "spectrum" of repeaters, for a two meter repeater at somewhere like Peavine or Mt. Rose, you need both notch and pass to be ensured that not only do you Notch out Rx from TX, but you ONLY pass your two frequencies (input and output) out to the rest of the world.
Because they may not know, they beg, borrow or buy a simple notch duplexer and use it at a high RF site. THIS WILL CAUSE INTERFERENCE.
Your transmit energy (whatever TX power in watts that you transmit) also has SIDEBAND energy and wideband noise near your frequecy.
The best way to reduce that energy is to filter it with a pass filter.
Unfortunately, notch duplexers are smaller AND CHEAPER. And because of the way they "work", they have less loss.
Any thing you transmit thru (including your antenna coax) has some loss.
On cavity filters, and other antenna system items, this is called insertion loss.
On a notch dupe for two meters, the insertion loss might only be 1 dB. On a good true Pass/Notch (also called Pass/Reject) the loss is more likely to be 3 dB.
Three dB is HALF. (meaning 40 watts into the dupes yields 20 watts out)
So, often the new repeater owner wants to get the least expensive, and less loss duplexer. He may like the idea that 40 watts in gets 33 or so out, instead of loosing half.
This makes sense when you only have part of the info.
But, just as we need to keep OUR transmitter from desensing OUR receiver, we need to keep our tranmitter from desensing ( or otherwise interfering with ) other radio systems near us.
If you are wanting to build a two meter repeater and have it at someones home or business or out in the middle of no where, notch dupes can be fine.
And MANY people even get by at mountain tops with notch dupes.
But there IS interference. Maybe not often or bad enuf for anyone to find out the source, but believe me it is there.
We have interference between Peavine (tx 147.21) and Ophir (Rx 146.01 / Tx 146.61)
and they are over 20 miles apart.
Think what it could be with them a few dozen yards apart.
Fortunately, the inteference is not that strong, or that often.

There are many other things to consider when wanting to build (or buy and set up) a repeater.
Quality of the components, quality of feedlines, antennas, tranmitter isolators, grounding, and on and on.
But the last point I will make today is frequency coordination.
Our system of repeaters relies on coordinating the use of the repeater "pairs".
It really doesn't work for anyone to just buy crystals for a pair of repeater freqs and put a system up. (or if it is a programmable repeater)
You want to do the research to make sure (as best as possible) that the pair you want to use is not in use near enuf for them to be bothered by your operation and vice versa.
Next, you want to avoid being physically very close to an "adjacent" repeater pair.
Example, you would not want to put a repeater on a building in downtown Reno on 145.21 and have another repeater on 145.23 nearby.
People wanting to "work" the 145.21 (mobile tx being 144.61) will desense the 144.63 receiver of the other system when they transmit near it.
And when near the 145.23 repeater, they may not be able to hear the 145.21 repeater when both are transmitting.
Coordinating repeaters is not all that tough, its just another thing to go thru the exercise to do.
This should be enuf info for people to ask questions on other things, and there are several folks on here that have LOTS of experience with this stuff.
(Not just me)
We are VERY fortunate to have lots of curious people in our area, so there are MANY that have tried things and are willing to share what worked and what didn't.

I forgot to mention this...
So, that "last" last point, was the "second" to last, last point.
Working on this stuff CAN require expensive test equipment.
If you are creative, and really understand what you want to accomplish (or ask how),
you can tune up most of this stuff with more common test equipment.
Yeah, a nice service monitor with Spectrum Analyzer with Tracking Generator, or Vector Network analyzer is nice and FASTER (sometimes better) you can tune up duplexers with a simple signal generator and programmable receiver.
It just will take longer.
Thats part of the fun.
If you wanna' play with repeaters, have at it.
Please do a little research and ask questions to keep from going down a path that won't work (and will ultimately frustrate you). ( YOU WILL NOT MAKE A REPEATER WORK TRANMITTING 100 WATTS WITH JUST SEPARATE ANTENNAS)
And it will be less likely that you would cause anyone any interference.
Somewhere around 5 or 10 watts, and an old Mastr II or Micor receiver that hears around -105, with separate antennas is where you can kind of start to make a simple system work with strong signals.
Easier on UHF than VHF.
Its a hobby, play.


-S. Kometz N7KP