Tuesday, May 2, 2017

More Adventures in Amateur Radio

Ham radio is interesting in that you can continuously tune a ham radio as opposed to channelized tuning on your typical AM/FM radios.  You can tune as closely as 1 Hertz at a time if you want.  A couple of the receive functions deliberately shift the received signal a few Hertz from the the transmit frequency for better reading of the signal.

Ham radio also has a number of different modes on which you can both receive and transmit within the legal limits allowed by the Federal Communications Commission.  Of course there are the usual Amplitude Modulation and Frequency Modulation (AM and FM).  But there is also Single Side-band Suppressed Carrier (SSB).   Continuous Wave (CW) uses Morse Code to transmit and receive information using very small band widths of as little as 500 Hz.  There is Radio Teletype, and a variety of digital modes. each seemingly the flavor of the month, though PACTOR and PSK31 have lasted.

Most voice is carried on what is known as SSB, where either the upper side-band, of the lower side-band is the only thing transmitted.  SSB is related to AM, in that the first SSB signals were generated as AM signals and then one of the side-bands and the carrier were filtered out.  An AM signal occupies as much as 6 KHz of bandwidth.  In the crowded ham bands, that is a lot of space.  Single Side-Band on the other hand occupies 2,4 KHz of space.  But for simplicity we can round that up to 3.0 KHz of bandwidth.  So, for any band width of the radio spectrum, you could have two SSB conversations going at the same time, or one AM conversation.

Besides occupying less bandwidth, the SSB transmissions uses the power available more efficiently.  The carrier frequency of an AM signal occupies as much as half of  the power consumed, but carries no information.  Only the two side-bands, each a mirror image of the other, carry useful information.  So, if one of the side bands and the carrier can be filtered out, the remaining side-band can carry the full power of output final amplifier of the transmitter.  If what you are trying to do is send a signal as far as it can go, with as much power as you can muster and have it carry the information as efficiently as possible, SSB or CW are your traditional modes.  The various digital modes require additional equipment,  But for basic operation at power levels of, say, 100 watts (yes, the power consumed by a single incandescent light bulb) CW and SSB are your workhorse modes.

Of course my transceiver, the Yaesu FT-450D has built in CW, SSB (LSB and USB) AM and FM modes. I am familiar with FM from my days as a Technician, and indeed I still enjoy getting on the FM repeaters and having a good "ragchew" with a new friend. FM is naturally quiet. But FM occupies 15 KHz, and so is confined to the 10m, 6m, 2m and above bands. Most of my contacts have been using either USB or LBS. But the AM world is also interesting. AM occupies little slices of each of the bands. Most activity on 80 meters, for instance. is in the vicinity of 3.885 MHz, or as the AMers call it, 3885 kilo cycles (kc). On the old tube radios that I remember, the bands were denoted in kc. It was not until I was in high school that the term Hertz replaced cycles referring to the frequency of waves. And while SSB may be transmitted at 100 watts, you have to turn down the power of an AM signal to perhaps 20 to 25 watts. But what you hear sounds more natural, less tinny, if you will. It sounds mellower. Most AMers use reconditioned "boat anchors," or old transmitters and receivers built in the 1930s through the 1950s. Names like Hallicrafters, National, Collins, and Heathkit, among others occupy the world of the AMers.  There is a different quality to audio that comes out of tubes and audio that comes out of solid state.  It is evocative of a slower paced time and it tends to be more local, though on 20 meters you can have world wide contacts.

As an interesting side note, because AM takes up 6 kc, you need to tune to 3 kc away from a band edge.  So, if your band edge is 4.000MHz, you need to tune 3 Kc down, or 3.997 MHz.  and on the other end, if the band edge is say at 3.800 MHz, you need to tune up 3, or 3.803 MHz.  For SSB, on the 80 meters band, typical custom calls for using the LSB.  So, again for the lower edge, you would tune up 3, or to 3.803 MHZ.  But you could tune all the way to the 4.000 MHz band edge because only the lower side band is transmitted.  The frequency you tune to is not the frequency of the side band, but the original, now suppressed carrier.  So, SSB also gives you 3 extra KHz of band width to play with.

If you can, get with a ham friend some time and listen to AM on the air, as well as SSB, and see if you don't notice the difference.

1 comment:

  1. KA8KRV here, and I was a ham back in the day when it was kilo cycles as well. They had just started to change over to hertz and it made it hard for awhile for some of us to keep it straight. I still find myself messing up. I miss the old days of the heathkit hw8 that I built and made contacts with all over the country, and mostly all over canada. Also down as far as Peru to a military station there, using my three watts, into a center fed dipole, called a double bazooka. It was made of two different diameters of cable and it resonated on several different freqs. and it worked quite well on cw, which I was limited to as a novice back then. I went on and got a technician ticket and then got my general a few years ago. I am not active on hf right now, but I just bought a qrp kit to put together and get my hand back into things. I do have a couple of handitalkies for fm repeaters. I was able to operate a few years ago from our cities local submarine that was from WWII, the silversides, during a contest weekend, to help the local club out. I had a blast, and now that I am officially retired early at 56, I have the time to get into the swing of things, so let the adventure begin, again.