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Ferrite bar antenna for car radio?

I read in sci.electronics.design that snipped-for-privacy@att.net wrote (in ) about 'Ferrite bar antenna for car radio?', on Sat, 17 Sep 2005:

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?

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate
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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

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wrongaddress

I read in sci.electronics.design that snipped-for-privacy@att.net wrote (in ) about 'Ferrite bar antenna for car radio?', on Mon, 19 Sep 2005:

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.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

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 radio chassis.

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

-Bill

Reply to
wrongaddress

I read in sci.electronics.design that snipped-for-privacy@att.net wrote (in ) about 'Ferrite bar antenna for car radio?', on Sun, 25 Sep 2005:

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.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

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

Reply to
wrongaddress

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