"singing" Track (rf)

sparky230

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Hi all, Have a problem, Posted this in couple of amateur radio forums, but thought try here
If I key up a radio on 70cms (433mhz) the track "sings"
I have 2 loops 150ft long, one rail on each loop is a complete circle, the other is cut into block sections

Thoughts?
 
70cms.. So highly likely you have one (or more) track lengths that are close to multiples of wavelength? - Or half a wavelength.

My guess, you have either a 'dirty' joint, or even droppers off the track to something else, with less than perfect joints..
In a previous existence, we could hear Voice of America on the barred-wire fence between Daventry transmitter site, and the golf course. Really eerie on a foggy morning / night! - slight rust where the 'barbs' were wrapped round the wire were the cause. Acted as a diode, more so if damp, hence louder when it was foggy.
 
walked round today with a 70cms handheld, found 2 fratcured solder joints on a cutdown 5ft section, just about 2 full waves on 70cm, resoldered
would sing on 70cm, 23cm but not 2m, 2 duff joints must have been acting like a diode reciver
 
Soooo, what precisely is a third harmonic? :worried:
 
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