As I am decorating the living room and putting up cornice around the top of the walls I was thinking of hiding the unsightly T style FM aerial behind the cornice, and cutting a shallow channel down the wall for the lead in cable. Then I got to thinking, in order to make it possible to move the HiFi about the room, about fitting an aerial into the "HiFi" cabinet similar to the ones seen in some of the old FM radios from the late 50's and 60's.
Which would work better, aluminium foil fitted as 2 plates on the top and sides, or a wire similar to the T aerial. Or maybe even a loop of foil or wire.
What dimensions would you recommend for either.
Thanks
Mike
I'm assuming you are building a half wave dipole.
So, 100MHz - the centre of Band II - is 3 metres. Thus 1.5 metres is a half wave.
Take 95% of this (to allow for the speed of signals in the aerial elements being slower than the speed of light) and that's your answer!
1.5 x 0.95 = 1.425 metres or 4' 8.1" in old money ...
Allow ~½" gap in the centre between the two elements.
When all else fails, read the instructions
I would guess two 'fat' elements made of foil, of the sort pasted into cabinet interiors, would need to be quite a bit shorter than than a classic dipole.... perhaps someone more savvy than me knows how to work this out?
Steve
Thanks guys, would a fat element be better than a thin element, and is it best to feed this via a 75 ohm cable to the coax socket on the radio or 300 ohm flat cable to the 2 terminals for a dipole.
What should I adjust the length to if a fat element is better?
Thanks
Mike
Where space only allows for a truncated aerial, I believe 'fat' will give better results. A good idea would be to copy what you find in other sets ... these often use balanced feeder and a tapped coil across the foil elements, presumably for matching and/or base loading.
Steve
It will be closer to 75 ohm, the 300 ohm connection is for a folded dipole.
Cheers,
Thanks
Mike
... would a fat element be better than a thin element ...?
Yes. The length to width (or diameter) ratio determines the bandwidth. For FM you need a bandwidth of 10% of the centre frequency, so fatter is best.
The length remains the same (95% of wavelength/2) in either case and the centre impedance is 75Ω.
When all else fails, read the instructions
Thanks Terry.
Mike
I have finished the decorating of the living room. So got around to sorting out the FM aerial for the Hifi tuner.
I glued 2 pages of A3 paper end to end and then glued kitchen foil to one side and wrapped the spare foil around one end. Then I cut it to roughly 75cm long and cut it in half down the middle and glued to the inside of the cabinet.
It is not brilliant but does pick up most of the stronger stations including my FM transmitter, with a little background noise on some of the weaker ones, but better than nothing and I wanted to be able to move the cabinet around if needed, so could not fit a permanently wired in loft aerial.
Mike
