A Beginner's Guide to Four Metres by Ross G6GVI
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 practically no commercially-built Amateur equipment for the band. 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.
And finally, Four is known as "the friendly band": try it for yourself and find out why!
Getting on Four with ex-PMR gear
In the British Isles, 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, with 70.475, 70.425 and 70.400 being the most popular working channels. 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 cantered around 70.26MHz, but in Eire, FM is now becoming popular in this part of the band (due to their narrower allocation).
It should be remembered that mobile PMR transmitters were not intended for continuous transmissions of several minutes duration, so the long "over’s" 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.
Alternatively, using a small DC fan to provide forced-air cooling can be very effective. I know one GB2RS newsreader who manages 30-minute transmissions with his Ascom on high power!
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 you have a synthesized rig, there really is no excuse for not QSY-ing. If a lack of crystals mean that you can't find a clear channel, then please be considerate if you stay on 70.45: keep the over’s short, and listen carefully between them for others 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.
One plea at this point: please don't block the calling channel with your QSO. If are stations trying to call in.
For up-to-the-minute news of what's happening on the band, check out the forum and newsgroup for details. These can also be used to seek information, (such as how to alter the squelch setting on a particular set), and for sale or wanted items.
Building up activity
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 70.45 at every opportunity, and monitor this channel whenever you can for other calls. If you hear a station call on Four while you're busy in QSO on another band, call them back to tell them you've heard them, and ask them if they can stand by until you're free;
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. Some clubs have a 4m activity night once a week, or once a fortnight.
As an example, I saw a five-fold increase in 4m activity in the Bristol Channel area through 2003 (outstripping that of other VHF bands), most of which is down to the tireless efforts of two or three keen stations.
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 UK stations new to 4m FM were often astonished to hear strong signals from Ivan S51DI and his compatriots coming through from "the sunny side of the Alps"! Recently 4m allocations have also been made available to Amateurs in a number of European,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. Most of the SSB and CW activity takes place on 4m on Sunday mornings, especially on days when the RSGB run a contest event. The calendar for this year is available here. These contests on 4m are rather different to those on other VHF bands, with stations often exchanging conversation as well as the obligatory numbers, and since the events are all-mode, FM stations can join in too.
The 70MHz BandIn the UK the 70MHz (4 Metre) band is allocated with Secondary status. It is available on the basis of non-interference to other services outside of the UK. The power limit is 160W (22dBW) and permitted modes are morse, telephony, data, facsimile (fax) and radio teletype (r.t.t.y.).
Understanding the Band Plan
The philosophy behind band planning is that it assigns frequencies for certain activities in such a way that all current users can practice the various modes of amateur radio with a minimum of mutual interference provided they are using state-of-the-art equipment and communication techniques. The RSGB band plan is based on the International Amateur Radio Union (IARU) Region 1 band plan.
The left column designation of the band plan shows the frequency limits of individual ‘sub-bands’ or segments. The allocation of sub-bands enables the indicated category of users to employ any frequency within that sub-band provided that no appreciable energy falls outside that sub-band. Users must therefore take into account the bandwidth of their sidebands when selecting an operating frequency.
The centre column gives details of the maximum bandwidth/mode usage. The bandwidth determines the maximum spectral width ( -6 dB points) of all emissions recommended in a sub-band. The mode indicates the modulation methods ( e.g. telegraphy, telephony, machine generated mode) allowed in a segment. A machine generated mode (MGM) indicates those transmissions relying fully on computer processing, for example FSK441, JT6M, JT65, PSK31 or RTTY.
The usage column indicates the main usage of a sub-band or segment. It contains meeting/calling frequencies agreed upon for the convenience of the VHF/UHF/Microwaves amateurs practising specific modes of communication. These frequencies are not part of the adopted IARU Region 1 band plan and though in the normal amateur spirit other operators should take notice of these agreements, no right on reserved frequencies can be derived from a mention in the right-hand column.
This area of the band is allocated to beacon stations. In the UK it is permissible to operate unattended beacons and the frequency 70.030MHz is recommended for this purpose. The primary purpose of beacons is the checking of propagation conditions both for every day amateur use and for special propagation research projects.
Narrowband modes (in common with all v.h.f., u.h.f. and microwave band plans) are always found at the bottom of individual allocations. This is where you will find morse (c.w.), telephony (s.s.b.) and machine generated mode (m.g.m.) activity. Listen on and around 70.200MHz, the combined c.w. and s.s.b calling frequency.
This area of the band is allocated to All Modes with a maximum bandwidth of 12kHz. The 70MHz band is unique insofar that it still has an a.m. calling frequency on 70.260MHz.
This section of the band is allocated to All Modes channelised operation where both telephony and digital modes exist. These are narrowband f.m. (n.b.f.m.) channels with 12.5kHz spacing and in this sub-band area you’ll find f.m. telephony, packet radio, fax, r.t.t.y and internet gateways.
Incidentally although the UK usage column of this sub-band indicates that the majority of channels are
used by digital modes, internet gateways or emergency communication groups that does NOT mean you cannot use them for FM telephony. It is simply a case of listening on these channels to ascertain LOCALLY whether they are in use or not. If you hear no other traffic then you may conduct your contact on any
channel you wish to use.
This version is effective from 1st January 2006, based on that agreed at the 2005 IARU Region 1 Conference.
|Frequency (MHz)||max. BW||Usage|
|70.000-70.050||500Hz||Beacons, including: |
70.030 Personal beacons
|70.050-70.250||2.7kHz||Narrowband modes, including: |
70.085 PSK31 centre of activity
70.150 MS calling
70.185 Cross-band activity centre
70.200 SSB/CW calling
|70.250-70.294||12kHz||All modes, including: |
70.260 AM/FM calling
|70.294-70.500||12kHz||All modes. Channelised operations using 12.5 kHz spacing, including: |
70.300 RTTY/fax calling/working
70.350 Can be used by RAYNET
70.375 Can be used by RAYNET
70.3875 Internet voice gateway
70.400 Can be used by RAYNET
70.4125 Internet voice gateway
70.425 FM simplex - used by GB2RS news broadcast
70.450 FM calling
Amateur Service 70.0-70.5MHz, Secondary User status.
160 Watts PEP (22dBW) permitted.
Available on the basis of non-interference to other services (inside or outside the UK).
Note: The use of Amplitude Modulation (AM) is acceptable in the Telephony and All modes segments but users are asked to consider adjacent channel activity when selecting operating frequencies.
If you're intetrested in having a QSO on 4m FM, keep calling on 70.450, or take a listen for the GB2RS News on 70.425MHz at 10:30am and 9pm on Sundays, and join in the net which forms afterwards.
Or if you'd like to try a sked with me on 4m FM, then please email me firstname.lastname@example.org