PORT ELIZABETH AMATEUR RADIO SOCIETY

REPEATER TECHNICAL INFO

 

The Cape Repeater Linked Network consists almost exclusively of Storno Fixed repeater and link equipment.  The models used are the CQF614 VHF repeater, and the CQF662 UHF link.  This equipment is 24V positive earthed, and being construced using discrete components is easily repaired and maintained.  They are crystal-controlled units.

A purpose-designed DTMF control card was developed in the early 90's to allow remote control of each link or repeater by means of DTMF over the channel.  This is essential in the maintenance of such a large linked repeater network.  Interference could render the system useless, but with the remote control facility, individual sections can be isolated, which quickly indicates where any problems might be.  The control codes are public domain information, and are printed on the repeater network diagram.  Any radio amateur may switch any of the control points in or out - we do rely on common sense and courtesy as far as this is concerned, and only ask that the person identify themselves, and which point they are switching.

Repeater sites are all mains powered with battery backup, using regulated float chargers and lead acid batteries.  Most repeaters use two vertically separated antennae, the top for receive and the bottom for transmit.  Very few have the luxury of a duplexer.  The more densely RF-populated  areas have crystal band-pass filters in the receive line to aid selectivity.  Feedline is 0.5" foam dielectric heliax.  A homebrew bandpass/band reject filter removes unwanted RF components in the transmit line, and the crystal filter (or a similar homebrew filter) does the same job in the receive line.  

Links use 8 element Yagi's, and transmit at only 1W into the feedline.  However, where a link is run in duplex mode as a "hub", they use an omni-directional colinear antenna, and transmit at 5W.

Three sites double up as packet stations, which places a far higher spec on RF filtering to ensure the VHF repeater and packet equipment do not de-sense each other.  This is achieved by the use of extra pass-reject filters in each feedline.  

The interfacing design is simple yet versatile.  Looking at the schematic you will notice that each radio provides a known audio output level for a known standard deviation setting from the receiver.  It also has four audio inputs for the transmitter which are summed, and expect the same known audio level to produce the standard deviation on-air.  The keying is done similarly - four diode-isolated keying outputs, and one keying input.  These connections are wired onto the multiway plug in a standard pinout.  It follows therefore that by wiring an interconnecting cable in such a way that the audio output of one radio goes to one of the four inputs of each of the other radios on site, and similarly with the keying lines, up to five radios can be coupled together.  Because standard levels are used, equipment can be lined up in the workshop and taken to site, knowing that all levels will be correct.

The following information can be downloaded below:

Item Filename Size
Storno DTMF Control Card schematic p1 stdtmf6.pdf 24kb
Storno DTMF Control Card schematic p2 stdtmf6_dec6.pdf 25kb
Storno DTMF Control Card Parts List stdtmf6_bom.pdf 6kb
Storno DTMF Control Card PCB layout    
Storno CQM614 Control Card schematic storno_CQM614.pdf ?kb
Storno CQM614 Control Card Parts List    
Storno CQM614 Control Card PCB Layout    
Interconnection Cable Info stplugs_3way.pdf
stplugs_5way.pdf
8kb
11kb
Storno CQF614 VHF Repeater Lineup Instructions    
Storno CQF662 UHF Link Lineup Instructions    

 

THE SYSTEM:

The primary design spec was to develop a system that allows some diversity in its configuration. By that I mean that various repeaters can either be coupled together or isolated, and the backbone links can be broken at various points. The benefit of this is the number of possible linking scenarios for special events in the area, and the easy isolation of interference. However, the backbone links are run in a normally-connected state, so that should a repeater be coupled to it, the user can expect that the link is right through to the extreme ends. As it turns out, the whole system is normally connected, allowing easy comms from one end to the other, and sections are only isolated when conditions require. All linking was to be done at UHF, with immediate drop-off of the UHF signal when the repeater input is dropped. This means that stations from either end can break in to a conversation during the VHF repeater’s squelch tail. Also included is a receiver timeout timer, so that any permanent incoming signal into a repeater would cause that repeater receiver to disconnect itself, allowing the rest of the UHF backbone (and indeed, that repeater transmitter) still to function normally. This timer is reset only after the permanent signal is dropped away again.

The basic UHF backbone system had its origins at Lady’s Slipper, with a single UHF link through to Grahamstown. This was later expanded with the addition of another link to East London. Temporary links for various local Motorsport events were fitted between PE and Knysna. These were later installed on a permanent basis, and the connection to George added. Unfortunately, the long-standing VHF back-to-back link from George to Cape Town via Jonaskop had since failed, so the two systems never overlapped. The George hams were no longer connected to Cape Town, but instead connected to PE and East London.

When further equipment became available, a link to Cradock was added. The then Garden Route Radio Club added the UHF links to Danabaai and Stilbaai, but with a one-hour maximum connect timer. In the meantime, the Cape Repeater Working Group was hard at work closing the link from George to Cape Town again, and this was eventually switched through in 1999. Teething problems were experienced, and initial audio quality was poor. This was resolved when a maintenance trip cleared up deviation setting problems.

TECHNICAL DETAILS:

Looking at the diagram, all points marked with an X and an associated 4 digit code are points that can be isolated or re-connected. The status of any controller card can be "queried" by sending only its 4 digit DTMF code. After any successful decoding, a reply tone is generated by the controller card. These reply tones indicate the status of the controller card connection - one continuous tone indicates connected status, a series of beep tones indicate disconnected status. To disconnect the point, the 4 digit DTMF code is sent plus a hash "#", and the connection is isolated. Similarly, to re-connect, the same 4 digit code is sent plus a star "*", and the connection is reinstated.

Any repeater isolated from the network continues to operate as a stand-alone repeater - it is not switched off completely, but simply not connected to the interconnecting cable at that site. Once disconnected, it can be reconnected from either side – i.e. from the UHF backbone, or using the local repeater channel.

The link at Lady’s Slipper forms a "hub" for the network, and any number of sites can link into it. This also allows for temporary repeaters to be linked into the network with great ease. Similar hubs exist at Mt Kemp, George, Umtata, Cradock and Colesberg, allowing more than one site to link into them without having to add more link equipment. This does require careful planning of "A" and "B" sites, to prevent transmitting and receiving on the same end of the band at any one specific high site. It also allows mobile UHF operation into these duplex UHF links. 

HF AUTOPATCH:

A very useful feature is the Motorola HF Rig fitted at Ladys’ Slipper. It is a crystal controlled MICOM Base Station rated at 120W. It is fitted with the PEARS 80m and 40m bulletin frequencies, as well as the popular SARL bulletin channels of 7066 and 7082. Also fitted is the national HAMNET channel of 7070kHz. The bulletin frequency channels of 7066, 7082 and 7070 kHz can be enabled as receive only for automatic HQ bulletin patching onto the VHF/UHF system, but can also be selected for two-way conversations by using channels 6,7 and 8 instead. The HF Patch has a 45 minute connect timer, to ensure that it can never be left engaged permanently. This feature makes it very easy for the PEARS Committee members to ensure they always have a quality simultaneous HF bulletin transmission. I am pleased to add that local discipline amongst users has prevented any unauthorised HF usage by ZR licencees via this feature.

The HF rig also acts as a 300 baud ZS0NTP Packet BBS user port on 3605 kHz when not being used for automatic HF Patching. The BBS sends a beacon every 10 minutes or so.

OTHER EXTRA’S:

In order to be even more versatile, the local PE Town repeater and the Uitenhage repeater have two options on their UHF link radios. Channel 1 connects them to Lady’s Slipper and the backbone network, while channel 2 connects them to each other via the PE UHF repeater on 438.650MHz. So these two repeaters can form an independent sub-network which locals can use to chat on, or for special occasions they can be linked to the wider society. The channel selection is done by means of DTMF, where a 1 or 2 is used instead of the "*" to connect. Disconnection is still done using the "#".

CONCLUSION:

A great team effort between the radio hams from Cape Town to East London has resulted in a system we are proud of. Technically sound and very versatile, it covers an extremely long distance, and with audio quality that can put many stand-alone repeaters to shame. When in our area, please feel welcome to try it out!

If you would like further details, please contact me.

My grateful thanks to TRAX Interconnect in Cape Town, for the kind sponsorship of top quality PCB’s used in the DTMF control cards. These were supplied at zero Ohms, because of the non-profit nature of Amateur Radio.  (Contact Frank Ellis on (021) 712 5011)

73

Chris
ZS2AAW

 

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