Over the last couple of days, I've had the 70MHz FM set on when I've been in the shack. I've probably said, but the set is an converted Philips FM1000 series set.
A few days ago, I called CQ and Dave, G4AQK from Swindon called me and asked if I knew there was a lot of hum on my transmission. I didn't but, switching on the monitor receivers confirmed the issue. I soon found that if I touched the case, the problem went away. I tried various earthing points without any success. Caught up with Rob, G4XUT a bit later and he suggested that it might have been an RF feedback issue.
Bearing in mind Rob's suggestion (which I was quietly a little sceptical about, as the issue seemed to be independent of power level) I grabbed a ferrite ring and put it on the power lead, close to the rig. That solved the problem - so I forgot about it.
A few days ago, I called CQ on 70.45MHz FM again and thought I'd check the transmission. Despite the ferrite ring, the hum was back! Tried another ferrite on the microphone lead, but that didn't do any good! Then the penny droppped!
The outside temperature was around -12C (unseasonally cold in these parts). I think there was ice on the antenna (probably in it too!) and this was affecting the SWR. This morning, outside temperatures had risen and thawed a lot of the snow and ice. The hum has gone! So I'm sure that's what causes the problem.
Meanwhile, this morning, I heard Chris, G4CCC from near Reading calling CQ on 70.450MHz. Called him back and was surprised to hear him say that three people had come back to his CQ! Two of the others were in Surrey and barely audible up here in the frozen wastes of Oxfordshire - but it's great to hear more activity on 70Mhz.
Got a 70MHz FM set? Why not hook it up and try and make a contact or two?