# Ferrite bar antenna for car radio?

Discussion in 'Electronic Design' started by [email protected], Sep 14, 2005.

1. ### John WoodgateGuest

I read in sci.electronics.design that wrote (in
20 turns does look too many for an 80-turn primary. If you have a
standard 93 ohm input impedance in the radio, the load on the primary is
93 x (80/20)^2 = 1488 ohms. If your 365 pF cap tunes the primary to 540
kHz (I don't remember the lower bound of the AM band in US, but it's
around that frequency), it has an impedance of:

1/(2pi x 540.10^3 x 365. 10^-12) = 807 ohms,

so your circuit Q is only 1488/807 = 1.84.

Try 5 turns, would round the ground end of your coil. You have grounded
the frame of the variable cap to the case of the radio, haven't you?

2. ### Guest

Try 5 turns, would round the ground end of your coil. You have grounded
5 turns reduces the signal and noise level quite a bit. The volume
level is much lower. 20 turns works better.

I tried gounding the capacitor/inductor to the chassis with not much
effect. I did notice a noise reduction if I connect the chassis to a
electrical ground.

I thought the impedance ratio of a transformer was the square of the
turns ratio, so that 20 turns relative to 80 turns would be a ratio of
16:1 ?

-Bill

3. ### John WoodgateGuest

I read in sci.electronics.design that wrote (in
Yes. OK, designing by remote control with no input data to speak of was
never very accurate. What you've shown is that the optimum is probably
somewhere between 5 and 20, but it could be 25.

4. ### Guest

Yes. OK, designing by remote control with no input data to speak of was
Yes, the secondary winding seems to be optimum around 25% of the
primary. But I noticed the phase connections also makes a difference. I
wound another air core coil using 1 inch PVC pipe, 6 inches length, and
about 300 turns of #24 copper wire and 75 turns on the secondary for a
1:16 impedance ratio. But it wouldn't tune very well until I reversed
the secondary connections to the radio input and ground. Now it works
fairly well. One side of the primary coil/capacitor is connected to the

Any idea why the phase connections of the secondary output effects the
operation so much?

-Bill

5. ### John WoodgateGuest

I read in sci.electronics.design that wrote (in
It depends where on the primary coil you wound the secondary. The right
place is at the end of the primary that is connected to the grounded
frame of the tuning capacitor. Then the end of the secondary nearest the
grounded end of the primary should also be grounded. Any other
arrangement adds capacitances to the circuit which are not wanted and
will upset the working in various ways, some difficult to explain.

6. ### Guest

The right place is at the end of the primary that is connected to the grounded
Yes, that's the way it's connected. The secondary is close to one end,
about 1 inch from the end.

I'm using a couple lithium ion batteries from a laptop power pack for a
7.5 volt supply, so the whole thing is self contained with no external
wires. I disconnected the dial light to conserve power and it draws
only about 80mA at low volume and works well down to 6 volts.

The only problem I have now is a slight buzzing in the output on weak
stations, which I suspect is digital noise from the front panel display
circuit. If I hold the radio in my hand, (hands on chassis), the noise
goes away and reception is good..

I suppose there is no solution to the grounding problem other than
connecting the chassis to some electrical ground?

But, I was trying to avoid that and make the whole thing portable so it
could be set on a table and moved around without any external wires.

-Bill