Repeaters for beginners: From FM stations to global networking

Repeaters: versatile networking from FM to digital

Unfortunately, an external antenna is required for most long distance connections in the VHF and UHF range. As an alternative, there are repeaters, which provides not only mobile operators but also fixed stations with an “extended horizon”. “Repeaters are boring!” you may be thinking. But radio communications via relay stations are far more diverse than some people interested in amateur radio might initially assume. We are talking about conventional FM relays, multimode and digital voice relay stations of the C4FM, D-Star and DMR systems, as well as the coupling of several relays and local or worldwide networking via Echolink, HAMNET and the Internet. The numerous repeater stations of the VFDB e. V., local association Leer, Z31, are a good example of the technical complexity that is now at work behind the current possibilities of the mobile radio network.

First repeater location, Leer water tower

Range extension through cleverly placed radio bridges

Repeater stations, also known as relays for short, extend the possible range of your own transmissions because they are installed on high buildings, towers, mountains and similarly exposed locations. A good location increases the number of potential radio partners. Relay stations are operated automatically, i.e. unmanned, and convert a single FM signal from an input frequency to another output frequency. As a rule, input and output are in the same amateur radio band. The relay input is the receiving frequency of the relay station; this is where the relay listens and the radio amateur transmits on this frequency. The relay transmits on the relay output frequency and the radio amateur receives the relay signal on this frequency. Only one person at a time can transmit via the relay. Weaker stations transmitting on the input at the same time will not be heard. Apart from the 10 m and 23 cm bands, FM relay stations are mainly operated in the 2 m band and the 70 cm band. The offset between relay input and relay output in Germany and the surrounding European countries is usually 600 kHz in the 2 m band and 7.6 MHz in the 70 cm band, but there are exceptions.

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The first 2 m FM relay DB0WO

Frequency shift explained simply

The frequency difference between relay input and relay output is called “shift”. The designation may vary from device to device, possibly “Shift”, “RPT” or “DUP”. The shift setting is usually possible as “Shift-” and “Shift+”. “Shift-” means that the transmit frequency of the radio is below the receive frequency. “Shift+” sets the transmit frequency above the receive frequency. The distance between the transmit and receive frequency is freely selectable on current radios, but in most cases is already correctly preset to 600 kHz or 7.6 MHz. In all cases, the transmit frequency of the radio amateur who wants to “work” via a relay is lower than the receive frequency. The “Shift-” setting is therefore correct.

Former electromechanical id generator

More efficient frequency utilization with repeaters

Current 2 m relay frequency lists show all new relay frequencies with a spacing of 12.5 kHz. This was not always the case: at one time, the relays generally had twice the frequency spacing of 25 kHz. At some point, the number of relays and the available frequency range were no longer sufficient. It was therefore obvious to increase the number of relays by reducing the channel spacing without increasing the frequency range used. It was agreed to arrange new relays in a 12.5 kHz spacing and to reduce the deviation from 4.5 kHz to 2.5 kHz in order to avoid interference between the relays. A smaller deviation reduces the bandwidth occupied by the signal. This also applies to the 70 cm band.

When allocating frequencies, efforts have always been made to keep repeaters with the same frequency and those with adjacent frequencies far apart. However, in the not uncommon case of good VHF propagation, users still send transmissions that are received and transmitted by several relay stations. Two or even more relays can then often be heard simultaneously on one frequency.

Antenna construction on the water tower

How operators avoid interference

One of the local peculiarities is the way in which relay operators deal with an excessively large deviation of participating stations. Most of them have reduced the FM relay's transmitting range so as not to interfere with neighboring relays in the 12.5 kHz system. Other operators are more consistent and technically exclude stations with significantly too large a deviation from relay operation. With current devices, the deviation can be limited via a switch or via the setting in a menu. With older radios, the maximum deviation can only be set internally in the device. In addition, it is absolutely pointless to use modulation frequencies higher than approx. 4.5 kHz, as the Repeater station suppresses such frequencies in any case!

2 m band filter from DB0WO

The operating technology via a relay station

If the relay is not used, the transmitter is switched off. But the receiver is constantly listening to the relay input. Due to the high position, it will hear a lot of signals, including many that are not intended for transmission by the relay. It therefore listens for a specific audio frequency that “opens” the relay. If it receives an FM carrier with the tone of 1750 Hz, the relay switches on the transmitter and transmits the following transmissions. If the participating person stops speaking and releases the PTT, the relay's transmitter also goes into “deep sleep” after a while, it switches off but continues to receive.

It can happen that permanent interference carriers are present at the relay input and these would then be constantly transmitted. To prevent this, some FM relays have a maximum talk time or keying time. This is usually two or three minutes. If this time frame is exceeded, the relay switches off the transmitter and any radio partners who continue to speak can no longer be heard via the relay. Excessively long monologues should therefore be avoided. After a forced switch-off, a relay may not respond to a tone call for some time. This is an intentional behavior to prevent misuse and playing with the unmanned relay station.

Using repeater correctly: An overview of the rules of conduct

Opening (keying) a relay causes it to go on air and transmit the signal that arrives at its receiver. This is done by pressing a button on the microphone or handheld radio. It puts the radio on the air and at the same time transmits an opening or keying tone of 1750 Hz in the simplest case. After briefly pressing the button for about one to two seconds, it is released again. With some radios, it is necessary to press the PTT button at the same time. In the meantime, the newer multi-frequency dialing methods DTMF (Dual-Tone Multi-Frequency Signaling) and CTCSS (Continuous Tone-Coded Subaudio Squelch) have largely supplemented and replaced the conventional 1750 Hz tone.

If the relay is keyed, it answers with the callsign in Morse telegraphy (CW) and/or with a spoken identification. After the identification is over and no one is speaking, a general call can be made by pressing the PTT button again, for example. Here are some tips and important rules of conduct for users of relay stations, which are actually completely self-evident, but are unfortunately too often ignored:

  • Leave pauses befoire transmitting
  • Tell your Name and call sign
  • Let other calling stations in
  • Modesty, politeness, friendliness
  • Simply ignore interferers

These rules of conduct naturally apply to all relays. The last point in particular is often ignored. Once a jammer has “nested” at a relay and has been able to achieve some “jamming success” due to its field strength, it is almost impossible to get rid of persistent jammers. It is therefore particularly important not to react to jammers in any way. Only then will the disruption soon become boring and the nuisance will stop. However, if the jammer experiences that his actions have an effect and he himself becomes the topic of conversation for the stations talking on the relay or is even approached, the jammer feels confirmed in his behavior and increases his activities.

More than just FM - the variety of operating modes

An FM relay is also technically capable of transmitting other operating modes. It is therefore possible to release individual relay stations for special operating modes. For example, a relay may bring a lot of joy to fans of the SSTV (Slow Scan Television) operating mode. ATV relays require greater technical effort, both for the relay station itself and for the subscriber. ATV is the abbreviation for amateur radio television. A distinction is made between analog and digital ATV (DATV = digital ATV). Analog ATV relays usually convert an ATV signal with picture and sound to another band, often to the 23 and 13 cm bands.

The art of extended relay coupling

The coupling of two or more FM relays, if handled properly and with the necessary radio discipline, greatly increases the attractiveness of radio repeaters. It does not restrict local radio operation on the relays involved, as local operation on the relay always has priority. However, if several relays are linked, it can quickly become confusing for the user. One possible solution is coupling by request only. Here the relay user decides to which relay station a coupling or connection is desired. This is done using a two-digit DTMF tone sequence.

However, the Federal Network Agency (BNetzA) requires that every relay coupling can be deactivated remotely. It should be noted that a distinction must be made between several types of relay coupling: The oldest and simplest method uses a direct radio link between the relay stations. Currently, FM relays are linked to each other using their own servers via the Internet or, as in the case of the Nordwestlink, via HAMNET, the “Internet of Radio Amateurs”.

How to use Echolink for global connections

Worldwide contacts with a VHF base station, mobile radio or handheld radio? That's no problem within the area of an FM relay with Echolink connection! It is no different from any other radio contact via an FM relay, provided that a subscriber has connected to this FM relay somewhere via Echolink and has started a general call. They simply wait a few seconds for the slightly longer switching delay and respond to the CQ call as usual. The relay connected to Echolink will do everything to ensure that you are heard by the caller.

The situation is slightly different if you take action yourself and initiate a connection via Echolink using your radio and the FM relay in order to place a CQ call on another selected FM relay. To do this, your radio requires the DTMF function. DTMF is based on the fact that two precisely defined tones are sent simultaneously and decoded at the end of the transmission path in order to execute a function with this information.

Echolink assigns node numbers that uniquely identify each Echolink subscriber - these can be ordinary radio amateurs, a link or an FM relay. For example, if you type the corresponding sequence of digits into the DTMF keypad, the nearby FM relay or Echolink establishes a connection with the desired station. If the called amateur radio station or relay is not QRV, you will hear the message “Not found”. The node number has the same function as a telephone number when making a call. But how can you find out the node number of relays and users? On the Echolink homepage you will find a command list with the user commands under “Current Logins” and a list of the Echolink stations currently logged in. The node number is listed next to the callsign and a brief description. If a node number is unknown, the station is currently not “on the network”. Please note that every sysop is free to change command sequences and block commands. The “Status” menu item on the homepage takes you to another echolink list which, in addition to the node number, provides further information such as locator, entry frequency, details of the antenna and the link status of the stations. The search function offered at the top of the HTML page is interesting: if you enter your own locator field there, you will receive a list of all echolink stations for a radius of around 200 km.

Lighthouse Campen, location DB0ET

More use for lively communication

An outlook on the future of repeaters is appropriate. With all the perfection that the Repeater network has achieved, we should keep one development in mind: The technically sophisticated and constantly growing range of Repeater services should also lead to constant - and ideally growing - utilization. Otherwise, as paradoxical as it may sound, the increasing number of possible communication channels will lead to increasing silence. So - get on the microphone and make a real QSO again instead of just confirming the broadcast on Sundays.

Source reference:

Wöste, M., DL1DMW.: Relaisfunk für Einsteiger, special issue “amateurfunk2007”, p. 28-32. vth, Verlag für Technik und Handwerk, Baden-Baden.

Photos:

VFDB e. V., OV Leer, Z31

September 2024, Alfred Klüß, DF2BC

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