Using DTMF Squelch for Activation

Typically it's done in one of several ways:

1. Telephone callup.
2. Alphapager (text) e-mail to pagers.
3. Numeric message to numeric pagers.
4. DTMF squelched page over the PPFMA 146.97 repeater.

When the phone system is operating normally (i.e., the lines are not down, the 9-1-1 system is not overloaded, the normal system is not overloaded), then the first three methods will work. However, if for some reason the phone lines are inaccessible, the only other alternative is to notify our members via amateur radio. After all, that's what it's all about, isn't it?

This lesson will go into some detail on how PPARES uses the DTMF paging function of the 146.97 PPFMA repeater, and how you should use your DTMF-squelch equipped radio for notification.

What DTMF Squelch, or DTMF Paging, Is

Most radios built in the last 15-20 years come standard equipped with a TouchTone pad. When a digit is pressed, a set of two tones are generated. These tones can be used to do things like make a phone call via an autopatch, control repeater functions, or, as is the topic of this lesson, to control the squelch system of your radio.

In the early days of FM two-way radio, all that was necessary was a regular squelch. The normal squelch works by muting the speaker when no signal is present, so you don't have to listen to the continuous background noise. You put up with the noise on SSB or CW, but then you're possibly hunting for a weak signal. On 2m or 70cm FM, the repeaters put out a good signal level such that there should be no background noise when an unmodulated signal is present. When the signal goes away, we want our receiver to be quiet. So the squelch system mutes the speaker when the signal goes away. You can adjust your squelch control so it mutes the speaker when there is no signal, or when there are weak signals that you don't want to listen to, or you can tighten the squelch so it blocks all but the most powerful signals.

In time, as more and more signals showed up on 2m and 70cm, plus the interference from other radio systems, like pagers, mobile telephones, etc, radio operators had no other choice but to develop another level of squelching. One common squelch system is the CTCSS, or continuous tone controlled squelch system (generically known as a subaudible tone).

With this system activated, your speaker will remain muted until two things happen: (1) a signal is present strong enough to overcome the normal squelch, and (2) the signal must have a subaudible tone present. Without this subaudible tone present, the speaker remains muted. When the tone is transmitted by another station, your radio should become unmuted, and you'll hear everything going on until the signal goes away. Many ham radio repeaters now transmit a subaudible tone, along with the repeated signal. Many repeaters also require a subaudible tone to be transmitted to the repeater's input (from your radio), so it can discriminate against interference (without a subaudible tone) and your signal (with subaudible tone).

CTCSS is great for eliminating interference, and still allowing you to monitor a repeater to listen to all of the traffic on the repeater. But, you will still hear all of the traffic on the repeater. Let's say you only wanted to monitor for a callup from ARES®, though. You need a selective squelch system, and that's exactly what the DTMF paging does for you. It allows you to program a code into your radio's DTMF paging memory. This code, which must match the DTMF code being transmitted by the sending station, is the trigger to unmute your radio's speaker. So, besides the normal "signal present" squelch, we have now added a selective layer of squelch, where your speaker will remain muted until it hears the predetermined DTMF tone sequence.

Even though most radios are equipped with a TTP, it has been only recently, probably in the early to mid 1990's, that radios are coming equipped with a DTMF squelch, or DTMF paging, system. This is a feature that lends itself nicely to the callup procedure for PPARES.

For DTMF paging to work, a transmitting station must send a predetermined set of DTMF "digits", and the receiving station must have its DTMF squelch activated, and programmed with the same set of digits.

In the case of PPARES, the transmitting station is the 146.97 Pikes Peak FM Association's repeater, and the receiving station is your radio. Since the PPFMA repeater is already set up and configured to transmit the tones, we'll continue on by discussing your radio.

Programming Your Radio for DTMF Paging

First of all, you need to determine if your radio is capable of DTMF squelch, or DTMF paging. Look through your owner's manual for a section on DTMF paging or DTMF squelch. If you can't find such a section, chances are your radio does not have this function.

• Some of the PPARES members have identified the radios they own which do support this function.

If your radio does support this function, find the instructions for loading a DTMF sequence into your radio's memory which is associated with DTMF paging. Remember you will want to RECEIVE this sequence, not TRANSMIT it.

The DTMF sequence for PPARES is:     6 9 7

This should be an easy-to-remember sequence, since the repeater is the 1 4 6 . 9 7 repeater.

Once you have programmed the DTMF sequence into your radio's memory, you will also have to make sure the DTMF squelch is activated. On some radios, a small "DTMF" or "D" annunciator may appear on the radio's display when this is activated.

One way to make sure it's actually muting the speaker is to tune to a busy repeater. When someone is talking, activate the DTMF squelch. When you do that, you should no longer hear anyone talking on the repeater.

Now that you know it's muting properly, you now only need to test that the correct tones have been entered, and your radio will actually un-mute when the codes come in. PPARES conducts weekly tests five minutes before the weekly net. Tune your radio to 146.97 at about 6:50 pm on Tuesday nights. Activate the DTMF squelch. Then wait until 6:55 pm, when the test tones will be transmitted by the repeater. If your radio un-mutes properly, you should hear a couple of more tones, followed by a person stating that this has been a test of the DTMF paging.

Preparing for the Callup

Now that you've successfully programmed your radio, and you've tested it with a real live signal from the 146.97 repeater, let's put this into a regular practice.

How and when would you use this DTMF page feature? You'd use this feature when you wanted to continue to monitor for PPARES activations or Skywarn severe weather announcements, and didn't want to continue to listen to all of the other ragchewing going on.

The most common times when you'd turn on DTMF paging might be:

• Dinner or other meal
• Sleeping
• Your favorite TV show
• Other times when you don't want to be distracted, but do want to be notified of a callup

The key to its effectiveness, though, is your dedication to activating the DTMF paging feature instead of turning off the radio during these times. It also means you may have to adapt somewhat depending on where your normal operating position is, whether or not the radio is an HT or a base station, whether or not you have a portable power supply to power the HT all night (or you can afford a bunch of batteries). Small, low current regulated power supplies can be purchased from places like Radio Shack, probably Batteries Plus, or maybe OEM Parts Inc. Depending on the needs of your radio, you may be able to buy a small power supply for $10-$15, or even take this opportunity to build your first project!

Practice the programming operation, and get familiar with activating the DTMF squelch on your radio. Test it, then get into the habit of using it regularly!