REPEATERS

PART 1

In this series of articles I will attempt to increase your understanding of why we have repeaters, basically how they work, types of installations, the use of duplexers.

1) What is a Repeater ? 
A repeater is made up of a receiver and a transmitter and its primary function is to extend the range of hand helds and mobile stations. It can of course be accessed by base stations as well. Let us consider station A on one side of a hill and station B on the other side of the hill. These two stations can communicate via a repeater installed on top of the hill.

2) Does a repeater Transmit and Receive on the same frequency ?
No a repeater does not transmit and receive on the same frequency. In the 2m band the split is 600Khz. The repeater receives on 145,050Mhz and transmits on 145,650Mhz.
In the case of 70cm, the split is 1,6Mhz or 7,6Mhz. We use the 7,6Mhz split. The repeater receives on 431,050Mhz and transmits on 438,650Mhz. 

3) Which repeater frequencies are operated in the East London area ?
In the East London area you have a choice of four repeaters. There are three in the 2m band and one in the 70cm band. They are as follows :-
2m band.
Hospital / Youth repeater. 145,175 / 145,775
Town repeater. 145,050 / 145,650
Mt Kemp repeater. 145,125 / 145,725
70cm band.
Pick & Pay repeater. 431,050 / 438,650
When you use any of the above repeaters you transmit on the first frequency listed and listen on the second frequency listed.

4) What is the difference between SIMPLEX and DUPLEX operation ?
Simplex operation occurs when two stations, A & B communicate with each other on the same frequency. Let us presume they are using 145,500Mhz. When station A transmits on 145,500 station B listens. When station A has finished his over he hands over to station B who then transmits while station A listens. If they both transmit at the same time they will not hear one another.
Duplex operation occurs when two stations, A & B communicate with each other using different frequencies. Station A will be transmitting while listening to station B on a different frequency, while station B listens on station A's transmit frequency he transmits on the frequency that station A is listening on. As you have probably guessed, a repeater operates in the duplex mode. i.e. it receives and transmits at the same time.

The normal amateur does not have the necessary equipment to work duplex on the same band.
You can however work duplex as follows, try it some time.
Station A transmits on 2m, 145,500Mhz and listens on 70cm, 433,500Mhz.
Station B transmits on 433,500Mhz and listens on 145,500Mhz.
Incidentally 433,500Mhz is the national calling frequency on 70cm.

You could of course use any two amateur radio frequencies. ZS's could use 80m and 10m. ZR's could use 6m and 70cm. This mode of operation is similar to taking on a telephone. Your transmitter is live all the time you keep the PTT pressed and your receiver is receiving all the time the other station keeps his PTT pressed.

5) What do the following terms mean, when applied to repeaters :-
"Kerchunker filter / timer", "Hang timer" and "Time out timer".

A kerchunker timer is a timer that delays the action of the squelch on the transmitter PTT activation a second or so from when the squelch first opens. (More about transmitter PTT activation by the squelch circuit in a later article.) This delay requires the station keying his PTT switch to hold it in for the period of the kerchunker timer, hence encouraging him to identify himself. As you all know the kerchunker does not always identify himself. This timer also suppresses short bursts of noise received by the receiver, which would, in the absence of the timer / filter, trigger the PTT of the transmitter.
Another method used to keep kerchunkers and others from using a specific repeater is the use of a tone operated switch. The repeater will only be activated if a specific sequence of tones is received. We do not use this system on our repeaters. You say, "What about the tones which can be sent on the East Cape link down to Cape Town". Yes this the same sort of control, the difference being that these tones open and close the link at specific points along the link. They do not stop you from accessing the repeater nearest to you. Using the right tone sequence you can stop the transmission from going beyond the first repeater or any repeater in the chain. PLEASE remember to announce what you are going to do before you do it. I am sure you have all heard these announcements being done at some time or another.

A hang timer keeps the PTT to the transmitter active for a period determined by the setting of this timer. This timer produces what we all refer to as the tail. The purpose of this timer is to stop the repeater from falling out between overs when two or more stations are communicating through it.

A timeout timer begins timing when the squelch becomes active. When the pre-set period has been reached the repeater shuts down until the incoming signal shuts down.
Only our hospital repeater has a timeout timer. The period is about 15 min. Stop and think, a lot of your hand helds etc have timeout timers fitted. When these are activated after the pre-set period you have to release your PTT to be able to press it again to continue talking.

6) What is meant by Receiver Desensitization ?
When a transmitter at or near the repeater site transmits, its signal will overload the repeaters receiver. Desensitization is indicated when a weak incoming signal opens the receiver squelch and disappears when the interfering transmitter transmits. If the problem is caused by the repeater's own transmitter, the repeater can cycle on and off as long as the weak signal is being received. 
Desensitization also occurs at your home station.
If your packet station is operating and you transmit on 2m, the packet receiver is overloaded by the strong home signal and stops receiving. 
If you are working the BBS and the sysop transmits on another 2m frequency, that transmission will desensitize the BBS's receiver and you will not get into the BBS while he is talking on the other 2m frequency.

PART 2

In this second part of the series of articles we will examine the following terms :-
Inter-modulation, Circulators and Transmitter / Receiver isolation.

7) What is meant by Inter-modulation ?
Commonly know as intermod or IMD, inter-modulation occurs when two or more transmitters at the same location transmit at the same time. Their output frequencies either mix by addition or subtraction to cause the interfering signal which will desensitize a repeaters input. An example is as follows :-
On the same mountain top is a TV transmitter on 215,25Mhz, a commercial repeater on a frequency of 70,20Mhz and a 145,050 / 145,650Mhz amateur repeater. The intermod signal is produced as follows :- 
215,25 - 70,2 = 145,050. Slap bang on the repeaters input frequency.
This is a simple example. There are sometimes more complex computations, when for example the resultant intermod of two transmitters adds or subtracts from a third frequency to give the offending signal. Another example of intermod is as follows :-
On the same mountain is a FM radio station broadcasting on four different frequencies in the FM band. Also on the mountain is a repeater. Two of the FM transmitters transmit on 101,4 and 90,7 respectively. 101,4 - 90,7 = 10,7.
10,7Mhz is the first IF frequency used in many repeater receivers. This signal gets into the receiver and knocks out the input signal.
In addition to knocking out the input signal of a repeater intermod can also trigger the repeater and lock it in the transmit mode. 

6) What is a Circulator ?
A circulator, also know as an Isolator is a electro - magnetic device which has three ports.
Ports are connections to the device. Being a magnetic device it must be kept well away from other magnets and steel.
By nature of its construction the RF signal passes from port 1 to 2 and from 2 to 3.
Circulators may be round or rectangular as shown in figure 1.







FIGURE 1 

A transmitter is connected to port 1. A antenna is connected to port 2 . A 50 ohm dummy load is connected to port 3. Power from the transmitter flows into port 1 and out of port 2 to the antenna.
If the antenna is a perfect match. i.e. a perfect 50 ohms, no power will flow back to port 3. If the antenna were to be removed from the feed line, then all the transmitted power will be returned to the circulator and out of port 3 which being loaded with 50 ohms causes no problem for the transmitter. The load must however be able to handle all the transmitted power. Therefor if the transmitters output power is 20 Watts the load must be able to handle this power continuously. In this instance the circulator has protected the transmitter. If the circulator were not in circuit and the antenna were removed all the power would have been dumped back into the transmitter. As you all know this is not good for the transmitter and may even blow the final transistors, if the transmitter does not have SWR protection.

Remember inter-modulation. In the case of the 70,2Mhz repeater mixing with the TV transmitter and producing intermod on the 2m repeater. If a circulator were to be fitted on the 72Mhz transmitters output the problem would probably be cured.
The 70,2Mhz transmitter antenna picks up the TV signal and conveys it into the transmitter's output. Inter-modulation occurs and the intermod signal is then sent out on the feed line and transmitted as a new signal. If a circulator were fitted in the 70,2Mhz transmitter's feed line the signal from the TV transmitter would be fed into the dummy load connected to port 3. It would therefor not reach the transmitter's output amplifier and generate inter-modulation products.
A circulator can also be connected between a driver amplifier and a power amplifier. Any mismatch at the power amplifiers input will be fed into the load and not into the drive amplifiers output as SWR.

8) What is meant by Transmitter / Receiver Isolation ?
When a repeater's transmitter is triggered it subjects the receiver to severe overload. (Desensitization) For a 2m repeater whose transmitter puts out 10w and whose receiver has a squelch sensitivity of 0.1micro Volt, the required isolation between transmitter and receiver is 82dB. If the transmitters power is increased by 10dB, the isolation must also be increased by 10dB to 92dB. So how can we increase isolation ?

a) The transmitter and receiver can be separated by several kilometres and linked 
together by land lines or a UHF link. This however gives rise to a different coverage 
area for the TX and Rx antennas. This is not desirable. 
b) The transmitter and receiver antenna can be placed on the same mast, but at 
different heights. A separation of 12 metres will only give an isolation of about 60dB.
This leads to very high masts and long feeder cables on the transmitter and the 
receiver legs.
c) Use a combination of separate antennas for receiving and transmitting, with high 'Q' 
filters in each antenna feeder. Using high 'Q' filters in the feed lines also helps reduce 
the physical distance between antennas.
d) Use high 'Q' filters in a combiner system to allow the transmitter and receiver to 
share one antenna. Such a combination of high 'Q' filters is called a Duplexer.

PART 3

In this third part of the series of articles I will cover the duplexer used in the repeater world.

9) What is a Duplexer ?
A duplexer is made up of a series of cavity filters and is used to provide the necessary isolation between the transmitter and the receiver when a common aerial system is used. They can be made up of six cavities or as few as four, depending on the configuration used.
At present our town repeater has a six cavity duplexer. The hospital and Mount Kemp repeaters have four cavity repeaters.
New cavities are very expensive to buy. We rely on obtaining second hand cavities and re-vamping them to work on our bands. This is why there are different configurations at our repeaters. A typical six cavity duplexer is shown in figure 19.

9) What is a Cavity Resonator ?

Figure 15 (picture of a cut away cavity)

Take a look at Figure 15. This is a cutaway view of a typical cavity. All important parts are labeled. A cavity consists of :-
An outer casing, a center conductor and one or two coupling loops. The outer casing can be either round, square or rectangular in shape. They are mostly round, because the round materials are more commonly available. They most commonly made from brass, copper or aluminum. The brass and copper cavities can also be silver plated. I have seen cavity resonators being use in a valve RF amplifier that are gold plated.

Without going into the heavy mathematical calculations as to the size of the outer casing, its length is very nearly a quarter wave at the lowest operating frequency plus its diameter. There is of course a limit to the band it will cover because of the way the center conductor is constructed and adjusted. The center conductor consists of a fixed section into which a moveable section telescopes. The center conductor is set to a quarter wavelength of the operating frequency by varying its length. This is how it is tuned to a different frequency. There are two methods used for adjusting the center rod :-

a) The center conductor is connected to a threaded rod which passed through a
threaded section at the top of the cavity. By turning the rod the center conductor is
raised or lowered. Its is then locked by a lock nut at the top of the cavity.
b) The center conductor is connected to a smooth rod which passes through a section
at the top where it can be coarsely adjusted by pulling it out or pushing it in. 
Fine adjustment is done using a threaded portion which is clamped to the rod with
a grub screw. This threaded section screws into the fixed section at the top of the
cavity body.

The coupling loops provide a means of inputting and outputting signal to the cavity. They are connected to either N-Type, PL259 or BNC connectors on the outside. The loops can also be rotated from the outside. This way the input and output impedance can be adjusted to match the input and output impedance of the transmitter, receiver and antenna system. The impedance being 50 ohms in all cases. The cavity resonator is by design a very high 'Q' device. A filter with a high 'Q' has very high loss of signal on either side of its resonant frequency.

Take a look at figure 14. On the LHS is the frequency response of a single cavity. Take note of the attenuation at 1Mhz up and 1Mhz down from the operating frequency fo. 
Note that the attenuation is only 20dB. Now take a look at the graph on the RHS which is of two cavities in series. Notice that the attenuation at the 1Mhz points has increased to 45dB. If a third cavity were to be fitted the attenuation would be further increased. There is a limit to the number of cavities that can be connected in series or parallel because of insertion loss. Insertion loss is the amount by which the series of cavities decreases the level of the signal passing through the system. In our cavities we try and curtail the loss to about 1,5dB.
In another article planned for the future I will explain the term dB when applied to Losses and Gains in systems.
A cavity with an input and an output functions as a series resonant, band pass circuit. It is connected in series with the transmission line. The signal goes into it and then out again.
The town repeater has three series cavities in each transmission line. Transmit and Receive. Resulting in a six cavity duplexer.
Connecting a capacitor between the input and output connectors of the cavity produces an anti-resonant notch below the resonant frequency, fo. 
Connecting an inductor between the input and output connectors of the cavity produces an anti-resonant notch above the resonant frequency, fo. 
The values of the capacitor and the inductor determine the distance between the notch and the resonant frequency, fo. Figure 23 shows a diagram of a six cavity diplexer. Note the capacitors and inductances across the cavity resonators.



This fact is put to good use in our duplexers. The cavities in the transmitter's path have the capacitor therefor the notch is below the transmit frequency. 
The repeater transmits on 145,650. The cavities in the receiver's line have the inductor therefor the notch is above the receive frequency. Repeater receives on 145,050.
Figure 21 on the next page shows a typical response curves for a single cavity with a capacitor. 
Note that the notch is only down to -35dB.
Figure 22 on the next page shows the response curves for both the transmit path (pass 146,94Mhz, notch 146,34Mhz.) 
and the receive path (pass 146,34Mhz, notch 146,94Mhz.)
A cavity with only an input functions as a quarter wave stub. It is connected across the transmission line. The hospital and Mount Kemp repeaters have two cavities across each line. Transmit and Receive. Resulting in a four cavity duplexer.
When cavities are placed across a transmission line the impedance of the line changes so tuning stubs are inserted to tune out the mismatch caused by the parallel arrangement. 
The response curves for quarter wave stub cavities is the same as for series resonant cavities.



Figures 14, 15, 19, 21, 22 and 23 were taken from the ARRL Antenna Handbook.

PART 4

In this last part of the series of articles I will cover the configurations used in the repeaters used around us.

In the earlier articles I omitted to mention why we use a single antenna in repeater installations. If separate receive and transmit antennas were used, the distance they would have to be placed apart, so that receiver desensitising does not take place is large. This gives a different foot print to the two antenna. Using the same antenna gives a equal footprint to the received and transmitted signal. The foot print is the area which is targeted by the antenna. Image switching on a torch on a dark night. The torch would be the antenna transmitting the signal, in this case it is light. Shine the torch away from you. The lit up area where the beam strikes the ground or a side of a hill etc would be the foot print. The torch would of course be equal to a beam antenna with the transmitted wave (light) going out in one direction with a certain amount of "spill" on the sides. An Omni-direction antenna could be compared to a lot of torches arranged is such a way that the light is sent out equally in all directions from a central point.

The simplest repeater set up is shown in diagram 1. It consists of a receiver, a transmitter, a carrier operated relay, a diplexer and a common antenna. The audio from the receiver is connected directly to the audio input of the transmitter. A carrier operated relay circuit is connected to the receiver's squelch circuit. When the receiver receives a signal its squelch triggers the carrier operated relay.  This relay then keys the transmitter and causes it to transmit. Audio from the receiver modulates the transmitter and so the received audio is rebroadcast by the transmitter. A diplexer is used to couple the receiver and the transmitter to a single antenna.
Looking at diagram 1 you can see that as soon as a station triggers the repeater the carrier operated relay causes the transmitter to transmit. This is all very well while the station triggering the repeater holds his PTT in. The second he releases his transceiver's PTT the repeaters transmitter shuts down. When the replying station presses his PTT the transmitter fires up again and the process is repeated. To stop the transmitter from powering on and off a timer is fitted in the keying line as shown in the diagram below. When the carrier operated relay is triggered by the squelch of the receiver, it in turn turns on the timer, which switches the transmitter to transmit. It will keep the transmitter transmitting all the time the carrier operated relay is operated. When the calling station releases his PTT the timer keeps the transmitter transmitting. The replying station them presses his PTT, the transmitter is already on, so no switching off and on of the transmitter occurs. This timer is what gives the repeater its tail.

The above works well, except when the "Kerchunker" gets busy. A quick press on your PTT will cause the timer to be triggered. Its output will trigger the transmitter, which will then transmit. A kerchunker timer delays the action of the output of the Carrier Operated Relay for a second or so from when the squelch first opens. This delay requires the station keying his PTT switch to hold it in for the period of the kerchunker timer. This timer also suppresses short bursts of noise received by the receiver, which would, in the absence of the timer / filter, trigger the PTT of the transmitter. A timeout timer begins timing when the squelch triggers the Carrier Operated Relay.  When the pre-set period has been reached the repeater shuts down until the incoming signal shuts down.
When this timer is activated after the pre-set period, you have to release your PTT to be able to press it again to continue talking.
All these timers are contained on the Timer Board.

On the next page is a block diagram of a typical repeater system which has links to another repeaters several kilometres away. 
Looking at the diagram you will notice that each TX/RX set has its own antenna and diplexer. Each section TX1/RX1 and TX2/RX2 can be operated as a stand alone repeater when the outputs from the DTMF boards are switched off by the appropriate codes being sent into the repeater in question.  Audio and PTT lines to there own transmitter are not affected. For simplicity the timers have not been shown. Believe me they are there in the system. 

For simplicity only one set of lines is shown on the diagram from each TX/RX set to the Summing Unit "S". Each set of lines consists equally of Audio and Keying lines. The Summing Unit "S", contains summing and splitting circuitry for the audio circuits as well as a diode matrix for the keying (PTT) paths. Let us assume that set TX1/RX1 (UHF link No. 1) receives a signal. RX1 triggers its own transmitter and also the transmitter of TX2/RX2 via the Summing Unit "S". Audio is also routed to TX2 via the Summing Unit S. A contact between the person triggering set 1 and anyone else tuned to the output of set 2 or set 1 will result. The Mt. Kemp repeater has got one VHF set and one UHF set. The VHF set is for area coverage, input and output, while the UHF set links with Grahamstown, Butterworth and East London (Hospital repeater). The station listening on the output of the repeater to the station coming in on set 1 must wait until the tail timers have timed out before he presses his PTT to talk back the other person.

On conclusion, I wish to thank Jim, ZS2JM for supplying the information on the above setup. There are lots of different ways of configuring repeaters.  I have stuck to the method used in our repeaters.

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