Easy Link

A simple method of linking two VHF repeaters together.

Background: In the early 1990's the Bruce Amateur Radio Club and the Port Elgin Amateur Radio Club were looking at ways to link the VE3TIV repeater in Kincardine Ontario 146.61Mhz- with the VE3PER repeater in Port Elgin Ontario146.82Mhz -. At that time the main reason for linking was to have access to the IPARN network which was already connected to VE3PER. We considered using a 440 MHz link between the two repeaters and actually went as far as getting the UHF equipment and the special interface boards that were required to be part of the IPARN network. A short while later one of the principal promoters of the linking system, Jerry Dantzer VE3CAC, unfortunately took sick and was unable to take part in completing the setup. The subsequent passing of Jerry left the project in limbo. IPARN Canada (pronounced I-PARN) is a national non-profit organization for Canadian Amateur Radio operators supported by membership dues, and as a result the membership support influences the rate at which the network can expand and improve. IPARN started by building a satellite network interconnecting existing VHF/UHF terrestrial networks across Canada. This was accomplished using the Anik geo-stationary satellite. Deregulation in the satellite industry has allowed the service providers to increase their fees beyond the current ability of IPARN's membership dues. The directors of IPARN where forced to cancel the satellite service, and to search for alternative means to connect the LTN's (local terrestrial networks) back together. IPARN decided to add an additional side to the network using the Internet. The long term goal of IPARN is to increase membership to help provide for the return of the satellite portion of our network, providing a unique system for Canadian radio amateurs.

The Beginning:About three years ago yours truly became the repeater co-ordinator for the Kincardine repeater VE3TIV. One day while having a coffee with Glenn VE3LIZ and Michael VE3SLQ it was suggested that the Kincardine repeater should have an IRLP link. Thanks to a generous donation by Richard Shrader KD4YRO (The Bruce Amateur Radio Club has several U.S. Citizens as members) we proceeded to install the IRLP link in Kincardine. At the same time Terry VA3TJT undertook the task of setting up an IRLP link in Port Elgin. By mid 2002 both IRLP nodes were up and running.

The Evolution: With the IRLP nodes in full operation we began discussing various ways of linking the Kincardine and Port Elgin repeaters. One of the ideas that surfaced was the use of two cross connected VHF transceivers located somewhere between the two repeaters. In this approach the audio from one transceiver is fed to the transmitter in the other transceiver and vice-versa. Furthermore the receiver must key the opposite transmitter. (See Fig. 3 for details) On the surface we couldn't see any reason why this type of link wouldn't work. Two potential problems were considered. With both repeaters connected to the IRLP under normal circumstances a station trying to connect to the IRLP on one repeater might turn on both. During the initial setup of the Kincardine repeater we encountered a problem where sending the code to connect to an IRLP node activated the Phone Patch. Michael VE3SLQ suggested using a prefix of number one which would prevent the Phone patch from being activated. With this change already in place only one IRLP connection would be made.(e.g. for Kincardine to connect to the IRLP one plus the node number was required. To connect to the IRLP on the Port Elgin repeater only the node number was required) The other problem we visualised was the repeaters trying to connect to each other on the IRLP while the VHF link was in place. Michael again fixed the problem by making software changes which prevented this from happening. Thanks to Terry VA3TJT who volunteered to cover the cost of equipment we decided to go ahead with the project. At this point I contacted Paul VE3SY who had two GE Phoenix transceivers for sale. Paul generously programmed the two radios for the Repeater frequencies and the CTCSS tones that were already in place for the IRLP nodes. Note: The repeaters were required to generate a CTCSS tone when a signal was being received for IRLP operation. The reason for this was to prevent repeater identifiers etc. from being transmitted over the IRLP. In the linking system the receivers would only turn on when the tone transmitted by the repeater was present.

Assembly and Test: On receiving the transceivers from Paul I modified both to generate a logic low signal when the received CTCSS tone was present. As it turned out there was a spare terminal, pin 5 on P910 of the radios, and the switched voltage was brought out through this pin. This voltage was connected to the PTT input of the opposite radio. Initially the audio output (filtered Vol/SQ Hi) was derived from pin 4 of P910and was connected directly to P911 Pin 4 the Microphone connector of the opposite radio.I then proceed to set up the radios at the Kincardine Airport which is about 2 miles North of the Kincardine repeater. The Airport Manager is Carol VE3IYY and her husband is Dave VE3DJL. Both transceivers were set to minimum power output which is about 7 watts. For the transceiver connected to the Kincardine repeater we used a rubber ducky antenna right on the back of the radio which was located inside the hanger at the airport. See Fig.1. The hanger itself has metal siding and roof. For the transceiver that was used to connect with Port Elgin which is about 20 miles away a Ringo Ranger antenna on the roof of the hanger was used. On our first test we had a pleasant surprise. The two repeaters were communicating with each other with a few minor problems. One problem was the de-sensing of the Kincardine receiver which we found we could eliminate by how we orientated the rubber duck antenna. The other problem was the signal was not full quieting. This problem was partially corrected when we installed a 4 element beam in place of the Ringo Ranger for the Port Elgin part of the link. However we still noticed that under some conditions the Port Elgin signal was still not full quieting. To correct this we installed a high gain six element beam which gave us full quieting almost all of the time.

Audio improvement: After several months of use it was apparent that the audio quality through the link while perfectly readable left a little to be desired. The audio frequency response was severely restricted. The audio which had been taken from P910 pin 4 (FILTERED VOL/SQ HI) was changed to P910 pin 3 which is the EXTERNAL SPEAKER HI. The audio level using the speaker volume control had to be adjusted very carefully so as not to overdrive the microphone input. The audio through the link as a result of this change was of excellent quality.

Purpose: One of the reasons for the VHF link was to provide communications between Port Elgin and Kincardine that is completely independent of the telephone system and did not rely on Hydro electric power. (Note that the Bruce Nuclear Plant is about half way between Kincardine and Port Elgin) Both repeaters had battery backup in place. The power supply we initially used for the linking radios was salvaged from the UHF radios we had from the original linking project. Battery backup, a 12 volt car battery, was then added to the linking radio setup. The battery was connected directly to the transceivers and the power supply was connected to the battery through a diode. This allowed uninterrupted operation during a power failure. The power supply was adjusted to charge the battery to a maximum voltage of 13.7 volts which keeps the battery fully charged and does not overcharge the battery.

Remote control: After several months of trouble free operation Michael VE3SLQ suggested that it would be nice if we could disable the link using the touch tone pad which is on most microphones. (the keypad generates DTMF tones) The reason for wanting the ability to disable the link is that the Bruce Amateur Radio Club is involved in several community activities during the year where VE3TIV is used quite heavily. By disabling the link we could allow the Port Elgin Repeater to continue operation without the constant chatter from Kincardine. Glen VE3LIZ donated a DTMF decoder that he had built and tested. This is the same decoder that is described in detail in recent editions of the ARRL Handbook for Radio Amateurs. All that remained was to build an interface to the Radios and logic circuitry which would drive a relay used to disable the PTT lines between the two transceivers. Not being a digital expert by any means I enlisted the help of my grandson Jeff, a recent graduate of the University of Waterloo, to design the logic circuitry. The circuitry used to disable the linking radios is very basic and a more sophisticated decoder is probable in the future. (See Fig. 4 for details) The audio from both receivers is fed to the 741 op amplifier using shielded lines to prevent RF pickup. A gain control is provided as the audio level as seen by the decoder board is critical for proper operation. The DTMF decoder in its simplest form can be described as having 16 outputs any one of which can be activated by its own DTMF tone. The outputs of the decoder were hard wired to the logic circuitry which in turn is followed by a single transistor relay driver. The sequence of operation is as follows. (1)When the output from pin 3 of the decoder goes Hi the logic circuitry is reset and the relay contacts in the PTT circuit are closed and the link system is operational. To disable the link tones one two and three must be received in that order which will activate the relay. Obviously any combination of tones can be used to actuate the switching relay however it should be pointed out if the repeaters have IRLP installed the initial tone should not be numeric (A,B,C,D * # can be used) to prevent false triggering of a node. During the development stages I encountered some problems with RF pickup and signal levels. In the end proper shielding and decoupling, just good design practices, cured the problem. Which goes to prove that taking shortcuts building the circuitry takes more time than doing it right in the first place.

The End Result: The linking system has now been in place for almost a year and has been relatively trouble free. It has also operated continuously through several prolonged power outages.

A picture of the Link Radio setup without DTMF Decoder FIG.1:LINK RADIO