What's Up Doc?


3 days ago by WA1TCC

This is a high level review of repeaters and their use. Repeaters have been around for quite a while but their use is becoming increasingly popular especially with the use of inexpensive HT transceivers.
Repeaters work in two manners – simplex and duplex. Simplex uses one frequency and repeats traffic using a delay in transmission. It is used to boost a weak signal but makes for difficult communications due to the delays. Duplex works by using a separate input frequency or band and then sends out the input on the repeater frequency. Most repeaters are limited to same band coverage but there are a few that repeat from one band to another.

Repeaters are set up in a location that has good coverage from their antenna for the signal. Many times they are set up on commercial antenna masts or water tanks as repeaters are frequently used in public service and these locations provide a high altitude for the antenna. Since most repeaters are VHF and UHF they need line of site for propagation of the signal and the higher the tower the better the coverage.

Simplifying the repeater description:

The repeater is a radio transceiver. It is set up for an incoming frequency which the user sets on their transmitter. The repeaters usually use various controls to control access to the repeater. The most common is a PL tone which is published with the repeater information if it is a public repeater. This is added to the user’s transmitter and is sent before a transmission. (See you Transceiver documentation for setup instructions) If the repeater reads the signal and recognizes the tone it then begins to process the signal. There is a very slight delay then the transmitter on the repeater is activated and the signal is repeated on the output frequency.

This raises the question of how can the repeater receive a signal and transmit at the same time if it is using the same antenna or one on the same mast? Without going into the complete design of a repeater the answer is filtering. Even if the output is going to a second antenna the RF on a close frequency will interfere with the reception of the repeater. Very precise bandwidth filters limit the RF feedback and allow the reception on a limited clear bandwidth. This is why you get a repeater that is a repeater or have some serious filters to build.

So lets talk about coverage.

VHF and UHF is mostly line of sight transmission. Getting on a hill is always better then being in a hole. Putting the repeater antenna high in the air increase the line of sight but no matter what the earth is round and that impacts the signal. The process of an initial signal is from the user to the repeater and then from the repeater out to the receiver. First step is, if the receiver can receive my signal? I have to put out enough signal to get to the repeater and carry enough signal to be above the background noise – something that is reduced as we go up in frequency. The repeater has to be able to read the authorization command, in most cases the PL tone, and the tone has to be of the right frequency. The repeater is now listening to the signal. It will repeat what it hears. This is where quieting comes in. The signal must be above the noise level and be readable to the receiver in the repeater. All the features of any receiver will be there and provide as much clarity to the signal but if it is not above the noise level the signal out of the repeater will not be quieted. This means you will hear broken voice and low level of voice as well as background static. To fix the non quieting issue, the transmitter has to change the location, increase the power, raise the antenna or increase the forward gain of the antenna or all of the above. It is a repeater and can only put out slightly better than it receives.

What about increasing the repeater power?

Can’t increasing the power help? Yes it will but only as a coverage fill-in. A power increase will not impact the coverage area but putting out more power will fill in the dead spots in the footprint of the repeater. This does not mean the range of the repeater will increase significantly. There will be a slight improvement in the coverage pattern for border line areas but line of sight is still a limiting factor with some extra bounce with increased power off of objects. This helps fill in dead zones in the pattern. So if the coverage pattern of a repeater is plotted and there are dead spots due to terrain or buildings an increase in power can help. It won’t help to get into the next county. Look to linking of repeaters for that issue. More on linking later.

How do I use a repeater?

Once you are setup and have the repeater programmed properly – listen!. And listen some more. Users in the area will have their own way of using the repeater. Do not call CQ. Standard procedure is to call directly to the station you are looking to QSO with or to announce your call sign stating you are monitoring. There will be other stations monitoring also and if they want to talk they will reach out to you. When keying up to the repeater – wait before talking. Most have a built in delay and the first part of your transmission will be cutoff. Many times this is the call prefix. If you get a call in that manner you can repeat the suffix and ask for phonetics on the prefix. Phonetics slow the user down so you should be able to read their transmission.

Repeaters reset after a few minutes so keep your side of the QSO short otherwise you will be dropped. If you have a lengthy conversation like an announcement, drop and pick up the repeater by stopping transmission and restarting. Some people say "resetting" but just stopping and starting is usually more efficient. Wait between transmissions in case someone has to use the repeater for an emergency and needs to break in or in case someone wants to join your conversation. The bands are open to all licensed Amateurs so encourage someone else to jump into you discussion.

Know the repeater’s use.

Some repeaters are for short duration calls and others are available for rag chewing. The local club’s repeater may be seldom used so you can enjoy the QSO with a friend but Florida’s SARS repeaters on 70 CM are for short term QSOs. Most of all remember it is a shared resource and use appropriately and join some nets!

Our local area simplex and repeater frequencies: CITY FREQUENCY PL/OFFSET COMMENTS Tallahassee 147.030 PL94.8 – +600 Tallahassee ARES district primary repeater Lee Analog 145.190 PL 123 – -600 Lee Repeater Madison DMR 442.000 447.000 CC white (1) Slot 1 & 2 Lee Analog 145.110 PL 123 Lee Repeater South Side Any 146.520 Simplex/no PL National calling channel Local 146.550 Simplex/no PL Madison and Suwannee ARES simplex frequency Local Simplex VHF Digital Local Simplex UHF Digital