Like many new hams, I have just one radio: an inexpensive 2m/70cm FM transceiver. It doesn't do SSB and it doesn't have a digital input. If I wire up a connection between the radio and my computer's sound card, are there any digital modes that I could reasonably take advantage of with this hardware?
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3$\begingroup$ It's worth noting that the "data" connection on most radios is really just an audio input (and output) which gets modulated like everything else from the microphone. The 9600 baud input bypasses preemphasis, but is still FM. Some D-STAR radios have an actual data input, but unless D-STAR is specifically being mentioned, this is probably not what people mean when they say "data port" in an FM radio context. $\endgroup$– Phil Frost - W8IIMar 15, 2016 at 11:26
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7 Answers
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I interpret your question to mean "what kind of cool digital stuff could I do with my inexpensive HT?" I can think of two digital activities that people commonly do with inexpensive handheld transceivers: APRS and Winlink 2000.
APRS is used to send and receive "tactical situation data". Most people use APRS to send GPS position reports from their cars, but it can also be used to send short person-to-person text messages, weather data, and all sorts of other things. (You can see what kind of information people are sending on APRS by pointing your browser at aprs.fi.) Winlink 2000 is a way to send and receive short emails over HF, VHF, or UHF: just the thing to help the larger community when disaster strikes. Both APRS and Winlink 2000 over VHF/UHF are based on AX.25 packet radio, which means that you would need some sort of Terminal Node Controller (TNC). The TNC is likely to cost more than your handheld, but as ham equipment goes they are considered relatively inexpensive. I use a Byonics Tiny Trak 4 for APRS; it's reasonably-priced and compact.
Check out APRS and/or Winlink 2000, and have fun!
answered Mar 14, 2016 at 22:35
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2$\begingroup$ There are 'software TNCs' available these days - there are programs that do all the APRS processing based on just audio input and output (using the computer sound card). This would be even cheaper than the HT, assuming that the OP has a computer (and he did post to this site, so that's pretty likely). $\endgroup$– Scott Earle ♦Mar 15, 2016 at 1:42
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We have been using traditional hf modes on a weekly 2 meter FM net for around 5 years. Participation varies from week to week. There is no weak signal advantage over a FM modulated signal. Our net is to encourage upgrading and to teach data modes. The local ARES group has recently started adding message and forms transfer using the fldigi suite to our ARES group to encourage wider spread adoption of these forms in our radio community for the same reason. SSTV, hell, PSK31, RTTY, and Olivia are among the most popular modes. We are now seeing activity on 10 meter data sub band locally. Give people a reason to upgrade and they will. Ham radio is more fun when it is more than just voice.
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If you can get audio between the radio and the computer, then there are a number of modes you can use.
Most common is AFSK, or audio frequency shift keying. This is some discrete number of audio tones, which are then FM modulated just as they would be if you whistled them into the microphone. The pitch of the tone encodes some digital bit or bits.
Usually, AFSK of a particular specification is used to encode the AX.25 protocol, which is then used for APRS and packet radio. Packet was common back in the day, but not so much today.
AFSK isn't an especially efficient modulation compared to designed-for-digital modulations like MSK. However, any FM radio can trivially be converted for AFSK operation (with an external computer), and you can find other hams using it. The same can not be said for most other modes.
It's also possible to take most digital modes meant to be used on HF SSB, and feed them into the audio input of an FM radio also. Apparently people do this with PSK31, although this is a horrible use of spectrum since FM-modulated-PSK31 spreads your transmitter power over a much wider bandwidth than PSK31 alone. There's also more noise power in this wider bandwidth, while the energy per bit transmitted is not increased. Consequently, the maximum error-free bitrate for PSK31-over-FM is much less than that of ordinary PSK31, or the theoretical maximum.
answered Mar 15, 2016 at 11:43
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Well, there is a difference in regards to "mode" and "encoding" and "modulation"
Example:
You can "encode" a PSK "mode" on an FM "modulation", as long as "the other end" is using the same to demodulate and decode, you can make a contact.
Certain "modes" will work more efficiently with certain modulations, but it is nowhere written that you MUST use a particular modulation for any modes (with some restrictions and exceptions)
If you start using (the example given) PSK on top of an FM modulated signal, you might not get a lot of responses, as there not be a lot of people listening for that mode using that modulation. But let that not stop you ! Experimenting like this can be fun; but for first trials I would urge you to "line up the other end" to ensure you have someone listening, decoding and responding to your signal.
HTH, Edwin.
answered Mar 16, 2016 at 8:23
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There are digital modes specifically designed for FM.
VaraFM
8PSK
As examples.
Further - all modes with fewer tones will make signal stronger.
I have done analysis and practical tests of signal strength. Both analysis and practical tests confirmed the improvement of signal strength.
DominoEX, Thor, MFSK with few tones worked magic.
So in essence - there is no advantage of S/N but it works by improving signal strength.
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Using narrow-band digital modes on FM actually wastes bandwidth. PSK31 sent over a 12.5 kHz-wide FM channel will take up the entire channel's RF spectrum. A modem such as MT63-2KL (available in fldigi) is far faster than PSK31 (you can send files over it, although it is still a lot slower than dial-up Internet), and unlike with SSB modulation, will take up exactly the same RF bandwidth as PSK31 when sent over FM.
If you're using a wire link between the computers and radios (not acoustic coupling) and have relatively strong signals, you can probably use even faster modes such as the high-speed and multicarrier PSK modes.
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Nearly all common modes will work this way, however because of the way FM is generated and received, you get absolutely no advantage over voice, so it's only useful if you specifically want to transmit data for some reason.
Because of the much narrower envelope, a CW signal has between 9 and 12dB advantage over an SSB signal at the same power. Using PC audio to modulate an SSB signal as most digital modes do offers an additional 6dB more than CW, as they can be fantastically narrow. SSB is 2-4kHz wide on average, a clean CW signal should be less than 40Hz wide, and a properly generated JT9 signal may be less than 10Hz wide.
This means that a 100 watt CW signal will be about equivalent to a 1,000 watt SSB signal, and a JT9 signal will be equal to 2,000 Watts or more on SSB.
With FM, the envelope does not change width regardless of input, so all modes are equal.
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1$\begingroup$ Nearly all common modes will work if the other end is using the same configuration — I think that's worth clarifying. $\endgroup$– Kevin Reid AG6YO ♦Mar 14, 2016 at 20:56
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2$\begingroup$ I'm not sure what to make of the last sentence. There are many constant-envelope modulations and they are not all equal. $\endgroup$ Mar 14, 2016 at 21:06
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$\begingroup$ @PhilFrost, I meant that anything you send via FM on a standard transceiver, presuming there is no clipping or other undesired behavior, will be the same bandwidth, whether it's a whistle or a slow scan image. There is no advantage to PSK31 over voice as there is on SSB. $\endgroup$ Mar 15, 2016 at 3:03
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$\begingroup$ Perhaps not, but bandwidth is not the only factor in how much useful communication range you get for some fixed transmitter power. Compare CW to olivia, for example. Olivia occupies much more bandwidth than CW, yet olivia can work reliably at such low SNR it's nearly impossible to detect by ear. $\endgroup$ Mar 15, 2016 at 11:23
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$\begingroup$ MFSK cannot be accurately compared to voice in that way when both are used on a standard FM radio. MFSK's instantaneous bandwidth is only slightly wider than a CW signal, it is a single narrow tone at any given moment. The rapid tone frequency shift increases the overall bandwidth to a small degree, but the multiple tones provide noise and fade immunity. All three of these benefits are lost when doing MFSK over FM, due to constant bandwidth and capture effect in the receiver. $\endgroup$ Mar 15, 2016 at 14:57