simple fm antenna

hey

I want to build a simple FM antenna (78-108MHz),
the catch is that I want it small enough to enter a matchbox.
I thought on two ways:
1. a simple wire that is folded right
2. a printed dipole on multylayer dielectric

can anyone help me with one of them or suggest another way?

thanks,

I would go for a loop antenna with a varactor tuning diode in series. Since you know what frequency the radio is tuned to, you would also know what voltage to give the varactor diode to resonate the loop.

For the FM receiver purpose you only need the first way. If you built an FM transmitter there will be many factors to consider such as matching impedance, choosing the antenna types - depend on the coverage area, etc.

hey biff44,
can you elaborate on the solution? how would the loop antenna look like and how will the circuit look like?

donny-skb,
I don't understand your answer, but I need only a receiver.

thanks all

If you need to design an antenna for FM receiver, it is not as complex as transmitter
You can make any antenna that you want. A very small peace of conductor is enough

I was thinking something like this:

The capacitor C1 tunes the loop to approximately midband. The varactor diode tunes the resonance +/- in frequency. I am assuming a balanced input to the FM chip.

It may be possible to get a broad enough bandwidth without the varactor, you will have to experiment around. You might have to de-Q the loop with a resistor across C1 to do that. Will not be as efficient though.

You could put the receiver electronics inside of the loop, for space saving during the layout.

hey biff44

thanks, but what are the two 10K resistors for?
what is each half loop length?

You make the loop as big as you can, which in a matchbox size will be well under the length required for resonance at 100 MHz, and use the capacitors to resonate it out.

The resistors are DC bias returns so that the varactor diode can have a tuning voltage impresses upon it.

thanks,
I am trying to do an ADS simulation without the varactor.

BTW, you do not want any ground plane directly underneath the loop!