Four Metres

By Don G3JNJ

Have you tried 4m FM? This band covers 70.0MHz – 70.50MHz and makes an ideal band for local rag-chews. If your present equipment doesn’t cover these frequencies then you might like to try out 4m at very little outlay. The illustrated rig is available for the sum of £15. These are ex-commercial sets and come with microphone only. You need to supply a loud speaker and 12v DC and of course an antenna. The type of antenna is a matter of choice. I use a home-made vertical dipole (shown below). This consists of two lengths of ?” (15mm) aluminium tubing. A four-turn coil of the feeder is used to make an RF choke. The antenna is mounted at gutter-level and works quite well from my QTH. You could of course make an alternative antenna such as a Ground Plane.

Initially I suggest that you cut the length of each element to 40” (1016mm) and ‘trim’ the antenna to resonance by reducing the length of each half of the dipole. In my case I arrived at a length of 39½” (1003mm). Using the MFJ 249B Analyser the antenna was resonant at 70.4MHz with 1.1:1 VSWR and Z = 48O. If you prefer to make a Ground Plane antenna then start with the same dimensions and ‘trim’ to resonance. The three or four radials should be sloped down at 45º - 60º to give a 50O match to the feeder. The GP I tried used ¼” (6.5mm) solid rod for the radials and it gave similar measurement results as the dipole. I decided upon the dipole for aesthetic reasons.

The initial interest in this band was sparked by the Southgate ARC with the hope that it would encourage amateurs to use the band and they chose 70.4MHz as the frequency to be used. The rig shown is xtal-controlled and comes with a pair of xtals for that frequency. It has a power output of around 17W and draws approximately 4A on transmit.

This is a cheap way of ‘testing the water’ so why not give it a try?


half wave dipole		quarter wave groundplane