Monday, 31 January 2011

What's so special about 4m?

The fact that it is not in widespread use around the world (especially not in the USA and Japan) means that there is very little commercially-built Amateur equipment for the band (with the exception of the Anytone AT-5189 mobile radio and the Wouxun KG-699E Handheld). Consequently, everyone using it uses either home-made or modified equipment. This means that 4m operators generally have more of an interest in the experimental aspects of the hobby than some of their counterparts on other VHF bands.

4m is also particularly good for mobile operating: the fading is not as severe as on 2m or 70cm, and the aerial efficiency is better than on 6m.
In Ireland, a considerable amount of de-commissioned VHF low band (68-88MHz) PMR equipment has found its way into the hands of Amateurs and been converted for use on 4m. Since Autumn 2002, over one thousand Ascom SE550s and many Philips FM1000 series sets have been "recycled" in this way.

Although modern PMR gear uses synthesized oscillators, there are still a number of crystal-controlled sets still in use - just two or three channels is really all that is required to be effective on 4m FM.
70.450MHz is now widely used as the FM calling channel in the UK. More recent PMR gear has 12.5kHz channels, and these are starting to be used for FM operation too.

In some areas, there is still a little AM operation centred around 70.26MHz, but FM is now becoming popular in this part of the band.

It should be remembered that mobile PMR transmitters were not intended for continuous transmissions of several minutes duration, so the long "overs" used by some stations may cause the sets to over-heat! Some sets have a built-in time-out which cuts off the transmission after a certain period. It is also worth considering modifying the transmitter to reduce the output power, or to have a switchable low-power setting to reduce over-heating.

It is not uncommon for PMR equipment to have a pre-set squelch circuit (rather than an externally variable control) and for this to be set at a level well above the receiver noise-floor. This may cause difficulty on the Amateur band, where signal levels are often weaker than in the PMR environment. It is thus possible for a receiver with a badly-adjusted squelch to miss signals which would be easily workable with the squelch defeated. A station using one of these transceivers would therefore miss answers to their "CQ" calls, and may unwittingly block the calling channel, or cause QRM to an existing QSO. It is therefore important to adjust the pre-set squelch to a suitable level, or use a squelch-defeat switch to listen for weaker signals.
Most stations on FM use simple vertical aerials, either a quarter-wave or half-wave.
Here are a few suggestions on ways in which you may be able to increase 4m activity in your area (they worked for me):

Put out CQ calls on 70.450 at every opportunity, and monitor this channel whenever you can for other calls. Mention your activity on 4m in QSOs on other bands, and when you meet up with a new station who may be in range, ask them if they have any 4m capability. I sometimes make obvious use of 4m as a "talkback" channel when in a net on 2m;

If you can find another local 4m enthusiast, try to set up a regular sked, and encourage other stations you work to join in..

As well as being a super "local natter" band, 4m can sometimes exhibit enhanced propagation: tropospheric ducting is not as common as on 2m or 70cm, but Sporadic-E can often be heard during the summer months. This used to be a nuisance, since it brought in loads of QRM from broadcast stations in Eastern Europe. However, in recent years, most of the broadcasters have moved into Band II, allowing Amateur operation to take over.

Slovenia was the first to appear on Four (back in 1996), and recently 4m allocations have also been made available to Amateurs in a number of European nations so we can expect to hear a lot more DX on Four from now on.

As with the other VHF bands, greater range can be achieved by switching from omnidirectional vertical aerials to horizontal beams. Some stations seem to think that all FM operation must be vertically polarised, but there's no reason why this should be so! Further improvements in range can be achieved by using narrowband modes, such as SSB, CW, PSK, etc.

I hope that this was of interest to you.

If anybody would like more information on 4 meter operation please feel free to email me on the following email address: 

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