SEITS LINKED REPEATER CONSTRUCTION

(Some information in this paper is slightly dated, but the main points and objectives are as valid as the day it was written. KE0BX,ed.) 

This paper is based on a earlier paper written prior to the construction of the Iowa City repeater. Upon reviewing it I am amazed how much has been learned since then! The result has been a considerable revision of the original paper. 

The following was developed from the experience of building two of the repeaters in the current repeater system and portions of two others. The knowledge here is based on several years experience not only in constructing repeaters, but more important, keeping them on the air! Some of what I say may be misinterpreted as being dogmatic. It is not, it is simply what myself and others have found that works reliably under the severe service conditions a linked repeater must operate under. 

The repeaters in the SEITS system vary wildly in construction and design. Just for example, four are Motorola MICOR mobile radios, one a GE Progline, one GE Master Exec II, one Motorola MAXAR and one homebrewed from much modified VHF Engineering strips! Controllers are three discreet CMOS logic, several RC-1000's and one custom 6502 processor. All share some common design specs. 

1.COOLING AND DUTY CYCLE.

De-rate your power supply by 50% if its an Astron. Then put a thermostatically controlled fan on the heat sink and duct some air into the case to cool the transformer (very important). Stock a spare LM723 regulator and spare pass transistors, you will need them! I have a 50 CU FT / MIN 12 volt fan on my Astron 35 amp supply. A thermostatic switch turns it on at 120 degrees F. 

An Astron supply will run very hot even derated 50%. One of mine is 35 amp supply delivering 25 amps output. Even with the cooling fan it still runs hot. Without a cooling fan at 25 amps the pass transistors will fail. No question about it. 

You will also need a BIG fan on the PA heat sink. Better yet two of them. I use 100 CU FT / MIN on a 120 watt amp detuned to 60 watts output. I use two thermostatic switches. ON at 110 degrees F. Amp power shut off at 150 degrees F. Amp is automatically bypassed when the power shuts off. 

Due to thermal resistance heat sink temperature is always going to be lower than the temperature of the die of the PA transistor(s). You absolutely must keep the PA heatsink as cool as you can. Take the room temperature into account. If in the summer your unairconditioned transmitter shack is 90 degrees your going to have a problem cooling the PA. This is one of the main reasons the PA must be derated by 50% or more if it is survive. 

On the link radio, the same as above applies, but 50 CU FT / minute will do for a 10 to 20 watt output radio. All fans are 12 volt and thermostatically controlled. Two types of Thermostatic sensors are used on the radios. In both cases the sensor is mounted as close as possible to the PA transistor(s) on the heat sink. 

The simplest sensor is a Klixon bimetallic thermostatic switch. These are available in various temperatures and feature simple positive on/off operation. The Klixon switches are available from various surplus dealers and new from Granger. Try to find one for 110 degrees F. (normally open) for the fan on sensor. The over temperature PA shut off sensor should be for 150 degrees F. (normally closed). 

Another fan control method uses a thermistor mounted on the PA final transistors mounting stud to detect the PA temperature. A control board available from SEITS uses a single LM334 quad comparator to control the fan, perform over temperature shut down and flash a "over temp" alarm LED. This method has the advantage that the thermistor can be mounted very close the PA transistor for more accurate control. 

Ideal power output for a two meter repeater is 50 watts into the duplexers on two meters. This seems to give the most balanced coverage with the short antenna heights we have on the SEITS system. The UHF links run 10 to 25 watts depending on the location. Remember that there can be 20 dB or more fades on 70 cm during the day, so you need considerable fade margin! 

2.AUDIO

On some of the repeaters the EQ was done mostly by removing excessive high frequency roll off in the audio section of the receiver. This was done in one case by simply removing bypass capacitors in the audio pre-amp section. In other radios a simple R/C network provided the required high frequency boost. These simple methods should be used where the roll off of highs and lows are not excessive. 

For difficult equalization tasks five band graphic equalizers are available for the 300Hz to 3,000Hz band are available from SEITS. These allow up to 13dB attenuation or boosting on each band. These equalizers are sued on several of the repeaters radios. With careful adjustment the result can be a receiver whose audio response curve is flat within 1dB across the band. 

Careful attention must be paid to band shaping of the transmit audio as well. On three machines a four pole active filter with the roll off point set to 4 KHZ is installed after the receiver detector. The same filter circuit is used on the transmitters after the active clipper and pre-emphasis circuit in a custom modulator patterned after commercial FM broadcast practice. 

The resulting circuit allows adjustment of modulation, clipping level and equalization independently. This allows precise adjustment of the repeater audio. All this is not as complex as it sounds. It just takes some test gear and experimenting to get things right. Note that all of us use direct FM, not PM to reduce transmitted audio distortion. 

3.SQUELCH

Think of the system now as a single two meter repeater with ten remote receivers. Each receiver has its own local interference problems. Added all together, they make for a very noisy system. Quite honestly, some of us are tired of listening to the constant kerchunking and long noisy "hangups" we have been experiencing. 

The most effective solution to the problem is the conversion to subaudible tone squelch (also called "PL"). With PL, the repeater is only keyed when it hears the correct sub audible tone on its input. It will ignore QRM, static and out of the area weak signals. The use of PL is universal on commercial repeaters to prevent interference. 

The only drawback to using PL on our repeaters is that not all 2 meter radios have a sub audible tone encoder. Almost all radios built in the last few years do have PL. All the owner has to do is program the correct tone frequency in and remember to turn it on! If the users radio lacks PL the problem is not serious at all since PL encoders are very simple to install in older radios and they are not expensive (more on this later). 

Most of the repeaters are using the so called "Universal" tone of 100 Hertz. Others use the traditional tone frequencies of the community they are located in. If you can not get into a SEITS member repeater as you have in the past, turn on your PL tone! If your not sure of the tone frequency, try 100 Hertz first. This is also a good trick to remember while traveling. Many repeaters have dual PL decoders. One is on the local tone frequency, the other on 100 Hz for travelers so they have access. 

Please understand that the repeaters on PL are still "open" re- peaters. This change is not being done to keep anyone from using them. It is only to correct the QRM problem we are experiencing. Everyone is still welcome to use these and all of the other system repeaters. The whole idea is to improve the system by making it more pleasant to monitor. Nor is this change to PL being forced on anyone. It is simply being done to cure the interference problems we are experiencing in our increasing crowded two meter band. 

In the future expect more PL installations on the SEITS repeaters. Our current plan is to use 100Hz (the universal tone) wherever possible. Remember, as I said before, this tone is being used increasingly across the country for access to two meter repeaters. Many "closed" repeaters have dual PL access tones. One for the local users and 100Hz for travelers. Thus a encoder set to 100Hz is a good investment. Here is a list of current repeaters and their tones. 

Where possible the SEITS repeaters use the Motorola MICOR bi-level squelch IC. This device is a complete noise operated squelch on one chip. Signals stronger then 1 uV have no squelch tail. Signals less then 1 uV have a tail. This reduces chopping considerably. Audio is switched by the IC's logic output driving reed relays or CD4053 analog gates. 

This IC is part number SCG709, Motorola part number 51-83977M16 WB0VHB stocks some of them. Local Motorola service shops have them at about $10.00 each. Having this device built in is one of the best reasons to buy a used Motorola MICOR radio. 

A interface board is available from SEITS that allows the MICOR bi-level squelch and the Comm Spec PL decoder board to be coupled together. It gives remarkably better squelch action by giving bi-level action on PL as well as carrier access squelch. This eliminates the annoying long squelch tail when operating PL. On weak or fluttering signals the Micor IC gives a long squelch tail to eliminate chopping of the repeaters C.O.R. 

4.ANTENNAS

If there are any flaws in the mechanical RF structure of the antenna (we're not talking about SWR here) you will hear noise in your receiver. This noise can range from a high level of desense hiss to crackling and popping in the receiver so severe that it makes operation of the repeater impossible. 

This noise is generated internally in the antenna when transmitter RF passes through diode junctions formed from corroded metal in the antenna. It can also be caused by mechanical movement of the antennas components. I can not stress this point enough: Just because a antenna works well on simplex, it does not mean it will work at all on a repeater! 

Antennas to avoid and why: 

• RINGOS: The worst! Not only are they low in gain and have considerable RF on the feed line, they generate RF noise you will not believe. Unless you weld them together, they turn into one giant noise diode. Plus your duplexers will not like the RF on the outside of the feed line. The worst problems I have ever had with a repeater were caused by a Ringo. You have been warned.....
• ISOPOLES: They suffer from low gain (some say none), The design does eliminate feed line RF currents. The main problem is Isopoles suffer from rain static! You have to hear one on a repeater to believe the noise they make. You can't even use your repeater in heavy dew.
• COLINEARS: Station Masters and other antennas consisting of a vertical array of dipoles connected by phasing lines. The problem is the phasing lines. They slowly degrade and get water in them no matter how much tape you put on. When the phase angle changes in the harness, your RF can go ANYWHERE but where you want it. 

The real problem is they still test ok, your signal coverage just slowly goes sour. Normally the degradation of coverage is so slow that no one notices till its a real problem. Then since the antenna LOOKS OK and the SWR is low, the dummies want to keep the blasted thing on the air. Ask about the problems a colinear caused the 28/88 repeater in the QC sometime........

5. LINK ANTENNA

One more note on the link antenna. Don't try to use the popular Cushcraft 70 cm Yagi. This thing is a bad joke. It has a aluminum UHF connector (it corrodes) and a gamma match no one has ever gotten to tune properly. If money is a problem, build a corner reflector out of the handbook. They have considerably more gain and are easy to tune. 

6. FEED LINE

On two meters the water won't completely put you off the air. At least it hasn't completely knocked out my repeater yet. I'm the one who keeps seeing water drip out of his repeater feedline. UHF is another story. Moisture in a 70cm feedline will put you off the air as sure as if you put the radio on a dummy load. 

You have two alternatives to 9913 that work very well. One is the new Times LMR400 coax. This coax uses a closed cell foam insulation that will not absorb moisture. It also has loss as low as the air dielectric 9913. The LMR series coax has 100% shielding using a metalized mylar shield along with the conventional tinned braid. These new coaxes are just as good as the much more expensive heliax. Both have low loss and 100% shielding, perfect for repeaters. 

Be sure if you are buying surplus heliax that you test it for loss before purchasing and that you be sure there are affordable (that's the hard part) connectors available for it. If you do buy heliax make sure you get the foam dielectric type. Most commercial applications using long runs of heliax use the air dielectric type. This puts you right back with the same moisture problems as the 9913 if you don't have a air dryer system on the feedline. 

Cable TV 70 ohm hardline works very well. There are many repeaters on the air using it with no problems including the systems central UHF hub repeater. Don't worry about the impedance being 75 ohms, the mismatch is not a problem. On the hub repeater the SWR is 2:1 just as predicted on the Smith Chart. 

There are some expensive non-water proof 75 to 50 ohm baluns on the market for CATV hardline. A 2:1 SWR causes absolutely no problems at all. Don't waste your money on them. If they get water in them, they will cause far more problems than a 2:1 SWR ever will. Please keep in mind that SWR is far less of a problem than feedline loss or not having a feedline at all! 

If you do decide to go the CATV coax route try to get the new style 5/8" vinyl jacketed direct burial cable. If you can't get it, the older unjacketed 1/2 cable works well too. You have to be very careful installing CATV coax. Its shield is brittle drawn solid aluminum tubing. The inner conductor is copper plated aluminum as well. 

Aluminum coax cannot tolerate being bent or flexed. The radius of all bends should be kept greater than two feet. It must be handled very carefully. Short jumpers of LMR400 or RG-213 should be used to make connections to the duplexers and the antenna. 

Conventional N connectors can be modified to work with 1/2" CATV coax. They take some work to be used. First the collet nut must be reamed out to the O.D. of the coax. Then a split ring has to be made to retain the connector on the coax after the nut is tightened. I make the ring by cutting a 1/8" section of the aluminum jacket off the coax and splitting it. 

The modified connector's body can be sealed by a conventional rubber "O" ring available at any plumbing supply shop. Do not attempt to seal the connector (or any other connector) with silicon seal. It contains acidic acid that will corrode the interior of the connector. 

Connectors for the larger 5/8" coax can be made from modified copper tubing compression fittings and "N" connectors. This requires some lathe work. Full details will follow in another paper. 

7. CONNECTORS

Now please don't tell me how so and so uses them and they work fine. I can show you just as many cases where they don't work worth a damn and I can make the measurements to prove it. If your UHF connectors are not causing a problem now, they will sometime soon. 

USE N SERIES CONNECTORS or BNC's wherever possible. N connectors are constant impedance, low loss and water proof. You can get them cheap as cut offs as hamfests as most hams are scared of them. If a antenna does not have a N connector, don't use it! You will not be able to seal a UHF connector to keep the water out of the coax. It is impossible because the moisture infiltrates in the form of vapor. N connectors have a built in air tight gaskets and are perfectly vapor tight. Use them! 

8.DUPLEXERS

Wacom duplexers act as shunt elements not series. The shunt design does not have as deep a notch as the series. It does have a broader notch and this is the important part! A 30 dB notch on a series cavity is no good if it is narrower then the side band noise from the transmitter. 

In other words, solid state transmitters can radiate considerable wide band noise that requires a wider notch to completely eliminate from the wide pass band of a FM receiver. The Wacom shunt design also has a very simple to tune series capacitance in the link coupling circuit that sets the important pass band of the cavity. That is, its easy to get the cavity to pass the de-sired signal that's 600 kHZ from the notch. They use a unique variable ca-pacitor that tunes by varying its dielectric constant! 

My only complaint with my Wacom duplexers is that they do use UHF connectors. I do check the connectors and clean them periodically. If you start hearing popping and crackling on your repeater, check the UHF connectors. 

9. SHIELDING AND ISOLATION

• A. The duplexers are properly tuned and have double shielded cables with tight clean connections. 
• B. The feedline is double shielded coax (9913) or heliax. For long runs nothing beats Heliax.
• C. There is good RF decoupling on the antenna and there are no RF currents flowing on the shield (another reason not to use a Ringo).

HOW THE CURRENT SYSTEM WORKS

To the two meter repeater, the link radio appears to be a second receiver. This second receiver has its own COR that can key the two meter PTT bus. Audio from the link radio goes through a logic switch (more on this later) and right into the two meter machines audio mixer. Think of the link radio as being simply strapped in parallel with the two meter machine. 

The two meter machines receivers COS (carrier operated squelch) keys the link radios PTT directly. Thus to the hub repeater, the signals it hears repeated on UHF from the input to the two meter machines IS EXACTLY THE SAME AS ANY NORMAL SIGNAL ON UHF FROM ANY NORMAL HAM STATION. 

Thus the whole system is transparent to the users. Other then the slight key up delay on the link, link operation appears the same as normal repeater operation. 

Since the link receiver keys the two meter machines PTT, it has to have its own COR. There are some simple but important differences between this COR and a normal repeater COR. 

• A. A simple logic function allows any COS and audio from the link to be automatically overridden by local transmissions coming in on the two meter receiver. Thus locals always have priority over link signals.
• B. The COR also has the remote control function built into it so the link may be disabled in case a system wide problem develops.
• C. Link receivers use digital tone squelch not carrier squelch. Motorola DPL (Digital Private Line) is used. This system uses a simple serial code transmitted by sub audible tone instead of the more common PL (Private Line, Motorola's name for continuous tone squelch). This system eliminates interference and squelch tail from the UHF receiver.

This signal shuts off the audio from the link receiver before the UHF repeater's carrier drops. Thus the noise burst caused by the link receivers normal squelch delay time is not heard on the linked two meter repeater. On out system this "turn off" signal comes about 2 seconds after the UHF repeater ceases to hear a signal. 

This delay or "repeater tail" supplies the delay for the UHF link "reset beep." Note that the link receiver COR has no squelch tail time delay! The tail for the UHF input to the two meter repeater is supplied by the tail of the UHF repeater. 

BEEPS

Several of the system repeaters use two beeps. The first lower frequency beep indicates that the wind bag timer on the two meter machine has reset. The second beep is supplied by the link receiver COR. It is a higher frequency and indicates that the hub UHF repeaters timer has reset. The hub does have a time out of ten minutes! So users are encouraged to wait for both beeps. 

The beeps also tell users the origin of signals heard. A single high beep indicates the signal heard is over the link. A double beep is a local signal. SEITS strongly encourages at least the link reset beep be on all system repeaters to ensure the hub is not timed out. 

Communication specialists (They advertise in QST) builds a DPL board that can be adapted to the various popular mobile radios. Contact them directly for details. For information on DPL theory, contact WB0VHB. 

IDers

"Silent" Link IDer's have been discussed. In this plan, all the repeat-ers and Links would have crystal controlled ID'ers. Notch filters using switched capacitor filters would notch out the IDs coming in from the link. Only the local ID would be heard since it would be injected into the mixer after the notch filter. 

LINK RADIOS

The new RADIUS is a much better radio that can be programed externally by means of a special serial link to a MS/DOS computer. It has DPL built in (and PL as well) and a receiver with excellent selectivity. If cost is no object, go for a new RADIUS (contact WB0VHB for info on RADIUS). 

There is a special interface board available from SEITS that simplifies the connection of the Maxar to the remainder of the repeater. The board supplies the transmit and receive audio interface as well as the squelch circuitry for both PL(or DPL) and carrier operation. 

CONTROL CODES

The current plan is discontinue control over two meters and do all repeater control over packet radio. This would allow any sysop anywhere on the system to monitor or control any of the repeaters functions. The method of implementing this is still under discussion. One method would be to use the TAPR METCON ONE remote control board. The other would be a custom packet radio remote control system designed by WA0RJT. 

NOAA ALARM

An additional minute of the alert message can be heard by sending the touch tone code XX back over the system. Use of this code to bring up the weather broadcast by casual users is discouraged. Its primary purpose is to provide information during storm watches. 

AUTOPATCH

DOCUMENTATION

RESOURCES

• WB0VHB: Documentation on various Motorola and GE radios. Information on PL and DPL.
• WA0AUQ: RF and audio custom work on repeaters. Large stock of parts, GASFET Pre-amps, antennas, etc.

SPECS

• RESET BEEPS: Each repeater should have some audio indication of end of signal from both the two meter and 70cm receivers. The beep should be delayed long enough to allow breakins.
• AUDIO: Flat 300Hz to 3,000Hz plus/minus one dB on both the two meter and 70cm. system.
• DISTORTION: (THD) Less than 5%
• DESENSE: Less than 2dB white noise on the two meter receiver with the two meter transmitter operating at normal power output.
• SQUELCH: PL access on all two meter repeaters (all the time!). 
• UHF FADE MARGIN: At least 20dB over the signal required to produce full quieting at the receiver at the other end of the circuit.

CURRENT PROBLEMS

2. No way to identify source of two meters signals across the link 

3. QRM from non PL repeaters 

4. Lack of fade margin on some of the 70cm links. 

5. No redundancy at the UHF hub. 

SUGGESTIONS (IDEAs)

I am going to make provision on the new hub repeater for the super sonic PL. Looks like it will be a big project...... 

2. It is possible to tie solution one to problem two. We could start with the concept of a tone panel at the hub repeater. Each incoming 70cm signal would have its own PL. The "branches" would be a little harder as they would have to have their own mini tone panels so they could work with multiple tones as well. The central PL tone board would in turn encode the outgoing UHF audio with some form of ID to identify the incoming repeater. 

A related question is the choice of tone frequencies. We should be using the tones assigned by the repeater council according to our geographical location. The problem being "the old radios" with non-programmable PL. In the end perhaps the ideal solution would be the two decoder system advocated by the ARRL. That is one tone on the coordinated frequency for use by the local population and the other on 100Hz for travelers and those with fixed PL radios. This is something that should be considered. 

3. Lack of fade margin is a tough one. This winter has been bad due to antenna icing. Summer will be the same story due to the good old hot humid day. Main problem will be the link between Muscatine and East Moline. Dwight needs a better antenna too. Burlington is marginal now. Summer will be bad. 

4. I simply need to build a complete second hub repeater. I just bought two 45 watt UHF Mitreks. Till I get a 100 watt radio that is more appropriate for repeater service, I will start work on one of the radios that I have. I have started the new audio boards. Most likely the controller will be pretty simple like always. Barry tells me he has a 68HC11 controller that can be operated over packet radio. That sounds like the answer to me. 

Comments or submissions for publication can be sent to muldoon@seits.org.