FOUR METRES

I have an old Marconi PMR radio on 4 metres (once used in a fire engine). Sadly, it’s crystal controlled. I only have one channel fitted... 70.40 mHz. However, there is a small local net in Worthing on 4 metres and they know where to find me. Or where to avoid me!

The aerial I’m using is a homebrew half-wave dipole up on the chimney. The coverage is amazing in comparison with 2 metres. It’s a great shame that there’s not more interest in 4, and a shame that the rest of Europe don’t have the band.

VERTICAL DIPOLEThe other day, someone asked me how to make a vertical half wave dipole for four metres. That's a fair enough question. The thing is, how do you make a dipole?

The length is pretty easy... Half of four metres is two metres. This is the overall length of the dipole. Cut each element to one metre long and, when the aerial is assembled, trim each element by about half an inch and check the SWR. Trim again if the SWR is too high and check it again. Don't go cutting off six inches at a time! And remember that, if you cut an inch off one element, then do the same to the other. If you don't, the thing will not be fed in the centre and you'll never get the SWR right.

As a matter of interest, the formula is: 468 divided by the frequency. So, if we want a dipole for four metres, and we take the middle of the band to be around 70,40 mHz, then we divide 468 by 70.40. This gives us 6.6 feet. This is the overall length of the dipole, so each element will be 3.3 feet. Always make the elements longer than you need so you can trim them to get the SWR down to 1:1.

 

 

As to the construction of the thing, obviously the elements have to me made out of something strong enough to support itself. Aluminium tube is usually used but copper pipe will do. If the aerial is small enough, for example, a two metre dipole for the attic, then the old wire-type coat hangers come in very useful. Cut and straighten the wire and there's your dipole.

 

UK FOUR METRE BAND PLAN

UK Bandplan Usage
70.000 70.030 beacons 70.030 Personal Beacons
70.030 70.250 Telegraphy/SSB 70.150 MS Calling
70.185 Crossband activity centre
70.200 Telegraphy/SSB calling
70.250 70.300 ALL MODES 70.260 AM/FM Calling
70.300 70.500 NBFM CHANNELS, 12.5 kHz spacing 70.3000 RTTY/FAX
70.3125 Packet Radio
70.3250
70.4500 FM calling
70.4625 Packet Radio
70.4750
70.4875