Conventions

By Jeff Schmidt N5MNW

If we didn’t have conventions to follow, there would certainly be (more) mayhem on the Ham Bands. We’d hear CW conversations and SSB “quacking” on popular FM repeater frequencies. These modes are referred to as “Non-Compatible” because they use radically different modulation and detection schemes. If you are listening to a SSB receiver and someone using FM starts a call on that frequency, you’ll soon hear real reasons for keeping the two modes separated! The 2-Meter band includes both FM and weak-signal (SSB/CW) operation. FM users of the band should not stray into the weak-signal or OSCAR (satellite) sub-bands.

Why even have different modes? Can’t we all just use FM and forget all this hoo-hah about SSB versus CW versus FM or RTTY or whatever? The answer is in the reasons the FCC lets us use these frequencies to start with- experimentation and developing skilled operators. FM works just fine for relatively short distances and high signal strengths. FM begins to suffer in long-haul or weak signal conditions largely due to the bandwidth involved (~16khz). Simplex VHF-UHF communications ranging hundreds of miles is observed using narrow-band modes (300hz-3khz)- SSB (single-sideband) or CW (Morse Code). You will find that all of the really serious VHF weak-signal operators use SSB and CW. For getting the most out of simplex operation, you'll want to check out the non-FM modes. For more info, check out the Roadrunner Microwave Group.

Specific FM frequencies for repeaters used in Texas have 20 kHz spacing. That means two repeaters in the same area will always be at least 20kHz apart. All repeater output frequencies below 147 MHz have the repeater input 600 kHz below the repeater output. All repeater output frequencies above 147 MHz have repeater inputs 600 kHz above the repeater input. An easy way to remember this is by setting 147 as the reference, with frequencies above it using a “plus” (+600) offset and below 147 using a “minus” (-600) offset. To operate your 2-Meter transceiver through a repeater, select the repeater output frequency and then select the transmit offset (+ or - 600 kHz). Most modern rigs will automatically select the correct offset for you as you scan across the band- it will switch over at 147Mhz.

More and more repeaters are beginning to use CTCSS or “PL” to access. This does not mean the repeater is “closed” or “private”. It is just one way to increase the number of repeaters “channel pairs” available while minimizing interference issues. It is not a perfect solution but does provide a margin of protection to co-channel users, especially during times of enhanced propagation. In my view, CTCSS is most effective in reducing or often eliminating symptoms of receiver overload. With CTCSS enabled, one will only hear signals with the matching PL tone. Many Austin repeaters are “carrier-activated”, requiring no tone to access, but have a tone on the output. If you activate CTCSS, you will give your ears a break from the 300 watt digital paging transmitters as you drive by.

Some repeaters have special features to enhance the capabilities and heighten our enjoyment. A telephone autopatch or voice recorder is a common accessory. Less common but very useful are links to other Ham bands, Shuttle audio or National Weather Service alerts. Remember that these things cost money and time to erect and maintain. The clubs or individuals who are responsible for proper operation may require dues or advanced permission. One also would expect a degree of expertise in those who use such a sophisticated system.

The ARRL suggests a band plan but not all states adopt the suggested plan. The Texas VHF-FM Society is our frequency coordinating body and the recommended bandplans are available at the VHF-FM Society website: http://www.txvhffm.org

The specific 2m and 70cm plans are:

http://www.txvhffm.org/bandplan/146plan.htm and http://www.txvhffm.org/bandplan/440plan.htm

So, there are plenty of things to learn about, even if you spend your whole Amateur career on VHF! Ask some questions, experiment and have fun.

N5MNW