simple antenna questioin

[zachtheterrible]

hi all. i FINALLY got my little fm transmitter working flawlessly, so hopefully no more annoying transmitter questions. now i just gotta make a good antenna for it. This is a tiny transmitter, like the size of a thumb. What i wanna do is make the antenna short as possible but still work as best as it can. I don't care how, coil, capacitor, etc. as long as its small. Thanx 8)

 

[Nigel Goodwin]

hi all. i FINALLY got my little fm transmitter working flawlessly, so hopefully no more annoying transmitter questions. now i just gotta make a good antenna for it. This is a tiny transmitter, like the size of a thumb. What i wanna do is make the antenna short as possible but still work as best as it can. I don't care how, coil, capacitor, etc. as long as its small. Thanx 8)

Aerials are quite a complicated subject, and it's further compounded by the crude design of your transmitter. Aerials are usually made as either 50 ohm, 75 ohm, or 300 ohm (balanced). In your case you don't have a clue what the output impedance of the transmitter is, and it's probably not capable of providing enough power into it's optimum load anyway.

I would suggest you simply connect a few inches of wire, that's the way it's usually done on these little bug transmitters - it works quite well, and is fairly small.

 

[zachtheterrible]

Thanx nigel. Yeah, that wut ive done. I calculated it out once and the calculation said I should use 3 feet of wire or something like that. Does a few inches compared with 3 feet or so make much of a difference?

 

[Nigel Goodwin]

Thanx nigel. Yeah, that wut ive done. I calculated it out once and the calculation said I should use 3 feet of wire or something like that. Does a few inches compared with 3 feet or so make much of a difference?

A quarter wave whip is 19 inches at 145MHz (2M amateur band), but this is for 50 ohms output impedance, which you almost certainly don't have. The only way to do it for your bug circuit is to try different lengths and see which works the best - if a short one provides the range you need, why bother with a longer one?. But, like any aerial, there will be an optimum length, exceeding that will reduce range again.

 

[zachtheterrible]

isnt there a way of attaching a capacitor in front of the antenna to make it shorter or something along those lines? I am going to need all the range that i can get.

 

[Nigel Goodwin]

isnt there a way of attaching a capacitor in front of the antenna to make it shorter or something along those lines? I am going to need all the range that i can get.

All those sorts of schemes are simply tuning the aerial to match the transmitter, they don't improve the range, simply stop the mismatch blowing the transmitter.

As I've mentioned previously, as this isn't really a 'proper' transmitter (just a crude bug) you've no idea of it's output impedance, or even if it's capable of feeding into an impedance like that?. So calculations are not an option, you don't have any values to enter!. Just try different length wires to find the optimum length.

 

[zachtheterrible]

fair enough :lol: thx