The Marin Amateur Radio Society maintains a simulcast linked repeater system. It was built with the goal of providing broad coverage of Marin County’s challenging topography, while using only a single pair of input/output frequencies on the crowded 2 meter band. Though primarily designed to cover Marin County, its coverage extends far beyond county lines. It is maintained for emergency use (mountaintop sites have backup power), as well as for use in many public service events, and for general communications among club members and guests. The system is open to all licensed amateurs. Users are welcome to join the Marin Amateur Radio Society and/or donate to the Society’s repeater fund, but there is no obligation or expectation to do so.
Using the simulcast system is much like using any other repeater system, but to get the most out of the system, it will be helpful to know a few unique aspects of the system’s operation which are described in this document.
The club also maintains some more conventional standalone repeaters, and these are discussed near the bottom of this document.
Basics of the simulcast system
The system consists of three mountaintop input/output sites, each with a transmitter and receiver. There is one receive-only site, which allows hams to be heard on the northwest side of Mount Tam (the other transmitter sites can be heard by areas covered by this receive-only site.) Finally, there is a hub located on a peak with good line-of-sight propagation to all of the remote sites. All of the outputs share the same frequency, 146.700MHz. The inputs are minus offset, for an input frequency of 146.100MHz. This is the standard offset, and many modern radios handle it automatically.
Since all the transmitters share the same frequency, you cannot select which one you listen to. If you’re tuned to 146.7, you’re always listening to them all. But the receivers each listen for transmissions coded with a different CTCSS tone, AKA PL tone. So the tone you use when transmitting determines which repeater input site will process your transmission. It is up to you to select the PL tone that will work best from your location.
To use the system, you should program the following four channels into your radio, in addition to any other channels needed for other repeaters. Note that the only difference between these four channels is the PL tone (CTCSS tone); the transmit and receive frequencies are the same.
There is no tone on the output of the repeater system, so you must program your radio to send a tone when you transmit, without requiring a tone for receiving. See your radio’s manual for details. Some radio manuals and menus may use a term like “tone encode” or “encode only” for this type of squelch setup. If your radio has an option for “Tone squelch”, that’s probably NOT what you want for this repeater system.
Which Input to Use?
Mount Tamalpais is a very prominent and well-known peak, the highest in Marin County, with Mount Tamalpais State Park covering much of the mountain. Mount Barnabe is located near Samuel P. Taylor Park, in the more western portion of the county. Bahia is located on a water tower site on a hill north of Novato. Tam West is located on the southern side of the West peak of Mount Tam.
Some experimentation will be needed to determine the best input from a specific location, but Tam West is usually a good bet from places South or West of Mount Tamalpais, while the Tam input is often good from north of Tam through approximately San Rafael. In the Novato area, from approximately Gnoss Field up through the Sonoma County line, Bahia usually works well. From West Marin, Barnabe is often a good choice. From most areas outside of Marin County, Tam or Tam West will work best.
There are many areas of overlapping coverage where it is possible to get into more than one input. From some spots, the best choice will vary with weather conditions, since clouds and fog tend to scatter, absorb, and reflect VHF radio waves, sometimes in mysterious ways. Once in a while, one of the inputs will malfunction. So even if you’re using the system from a fixed base station, it’s not a bad idea to program all of the above channels into your radio.
Key up, Wait, Then Speak
When you transmit into the simulcast system, your signal is received by one of the receivers, relayed to the hub, sent back out to the three transmitter sites, and finally transmitted from all three sites on the 146.7 frequency. It takes some time for all the parts of this link to come up. You must pause for a half second or so after keying your microphone and before speaking, or else the first part of your transmission will be cut off. Many repeater systems have some delay before they come up, but this one may have a bit more than you’re used to.
Wait Between Transmissions
When someone stops transmitting, the repeater will transmit a quiet carrier for a short time, followed by a very brief courtesy tone, and then it will stop transmitting entirely.
This repeater system has a time-out timer. If you talk for more than the limit (approximately 90 seconds), you will be cut off. The timer resets only when the system stops transmitting entirely after the courtesy tone. If you jump in quickly, without waiting for the courtesy tone and allowing the system to completely stop transmitting, then you’ll only get what’s left of the previous operator’s time limit.
When it’s your turn to talk, normal practice is to let the repeater completely stop transmitting after the previous person’s transmission, then key up, wait for the repeater to come up, and speak. This handoff takes some time, but it lets each person transmit for the full allotted time, and it leaves a bit of a break in between transmissions for someone to interrupt if needed.
An interruption is always welcome if the interrupting party has an emergency and the parties already on the repeater are conversing about some routine topic. In this case, you don’t need to let the machine go all the way down, just state your call sign and the words “emergency traffic”, as soon as someone stops transmitting.
An interruption is sometimes OK in situations that are less than emergencies, but not always — normal social etiquette applies. If you want to break into a conversation, the best way is to briefly throw your call sign out right after someone stops transmitting, but before the repeater shuts down. Then wait for one of the other parties in the existing conversation to acknowledge you.
Frequency Control through GPS
The simulcast system can only work properly if all three transmitters are at precisely the same frequency. If the frequencies varied by a few parts per million, heterodynes would cause awful squeals and noises. The transmitters are able to keep their frequencies precisely locked through the use of GPS disciplined oscillators which use precise timing signals from satellites to keep them all exactly on the 146.700 frequency.
How to interpret Morse IDs to select an input
When you are listening to the system, you will periodically hear various Morse code signals. The repeater callsign, K6GWE, is transmitted from all three sites simultaneously to comply with legal requirements. In addition, each of the three separate transmitters periodically transmits its own individual short unofficial ID. These individual Morse transmitter IDs are the only transmissions on this system that are not simulcast from all transmitters — each is sent only from the one corresponding transmitter.
|System as a whole
If you don’t know Morse code, it would be nice to learn it, but that’s a subject for another day. You can still teach yourself to distinguish between these signals without learning code. Practice by listening to the audio files linked in the table above. “K6GWE” is far longer than any of the three transmitter site IDs. The transmitter site IDs consist of different numbers of letters. Even if you can’t understand the letters, if you can count how many letters you hear, you’ll know whether it was “B”, “RN”, or “TAM”. Also, listen to those mp3 files and notice how the very last bit of sound from each of them is different.
Why is this useful? Since the “B”, “RN”, and “TAM” signals each come only from their own site, you can use the received signal quality as a rough gauge of the quality of the path between the site and your location. So you can get a pretty good idea of which PL tone is likely to work best, before you try them all and ask for comparative signal reports.
Alternate Standalone K6GWE Repeaters
In addition to the simulcast system, the Marin Amateur Radio Society operates other VHF repeaters on 2 meters and 70cm. These other repeaters typically receive much less traffic than the simulcast system. If you want to engage in a long “rag chew”, it would be courteous to move to one of these repeaters if possible, if not to a simplex frequency. These repeaters are also maintained for backup use in case the simulcast system goes down.
|Tam Standalone 2m
For those who hate entering frequencies and tones
Below you will find a link to a .csv file containing all of the frequencies and tones for the repeaters discussed in this document. You may import it into the free open source software CHIRP, and from there, upload it to your radio. CHIRP supports most computers and operating systems (Windows, Mac, Linux) and supports a large number of radio makes and models.
One way to use it in CHIRP is to open the file in CHIRP, and then, in a separate tab, download an image of your radio. Copy and paste the channels from this .csv file into the image of your radio, and then upload that image back to your radio. You may decide you want to choose different names for the channels, depending on what you find meaningful and easy to remember, and depending on how many characters your radio can use for a channel name.
For whatever it’s worth, the .csv file can also be opened and edited in most spreadsheet programs.