Simple AM radio - how to modify it so it can work without ferrite rod antenna?

Question

I am a beginner in electronics and I'm currently the most interested in RF-related topics. I did build simple AM receiver usind good old TA7642 and, after adding small 9V amp, the radio broadcast can be heard across entire house. It works well.

But now I want to modify the circuit so it can use different antennas and coils, so I can change receiver frequency range, RF signal strength etc.

This is the antenna input cirucit part. That's basically default TA7642 setting:

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Now tests with different coils and antennas:

1. Ferrite rod with 2 coils separated magnetically - WORKS.

2. Ferrite rod with 1 coil, but with additional output in the 1/3 coils length - WORKS. It's interestig that RF signal is taken directly from the coil, without separating it magnetically.

3. Just a coil (not ferrite), capacitor and external antenna - NOT WORKING. Even though that's the most basic design actually. 3rd case is definetly wrong, I don't receive anything desired but very distorted... FM stations. Capacitor rotating doesn't change anything, adding or removing external antenna doesn't help, too.

Why doesn't it work? How to fix this?

I have a few air coils without ferrite or three outputs. I also have external antennas: one for SW on the balcony, another one (long-wire) for LW. But I cannot use them as the only 1 & 2 designs do work.

Can you help me, please? Thanks!

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BTW: If I attach external SW or long-wire antenna to circuits 1 or 2, I can receive tons of shortwave radio stations, but rotating capacitor doesn't allow me to tune in to any station. I understad it as follows: the external antenna gives so strong signal that it overrides what ferrite rod gets. Because of high capacitance of variable capacitor (20 - 500 pF) and/or high inductance of ferrite coil (probably 400 - 500 uH) such radio is totally not selective, so I cannot tune in to anything. Is this explanation true?

Answer 1

Your coil in parallel with the variable capacitor works as a bandpass filter. Filtering just the station you want to hear. The frequency selected depends upon the capacitor and the coil. The formula for the frequency is f= 1/(2piCL) where C is the capacitor's capacitance, L is the coil's inductance and f is the frequency in hertzs.

When you are using a ferrite rod in a coil it increases the inductance of the coil.

So just the coil without the ferrite rod has a much lower inductance and thus the frequency tuned is higher by the formula. Thats the reason you are receiving FM stations because they have a much higher frequency than AM stations.

For a normal AM air core coil you can wind copper wire 100 times around a toilet paper roll very tightly. It should give you enough inductance to tune in AM stations.

You can build a better air core coil to tune in the AM band using simple calculators as http://www.daycounter.com/Calculators/LC-Resonance-Calculator.phtml and http://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml

Answer 2

On the whole I personally found crystal radios (which is not what you are doing but similar on the antenna side) a more frustrating than satisfying part of "traditional basic electronics" - could be lousy radio exposure in the valleys between mountains, could be just me.

I did manage to get one working (for the one nearby AM station) with a "basket-weave" coil, a long-wire and a water-pipe ground. A supposed FM crystal-set design failed miserably for me even on the high-power station on top of a mountain in clear view. I've been too irritated with it to try taking it closer. That station will come in on a regular set with no antenna at all (badly) and quite well with even a paper clip stuck into one of the antenna wire clamps.

I found "toilet paper tube" coils had measured inductance values that were quite a bit too low - you might want to try oatmeal canisters - there's quite a bit of decent info on oatmeal canister crystal sets, the larger diameter seems to be beneficial in air-core coils. By the time I had read that far I was infected with the basket-weave coil idea if I was going to bother re-winding the coils, and it was not too hard to do with a board and some toothpicks.

Answer 3

Lucas has provided a decent answer for your first question regarding circuit #3 (why doesn't a small air-core coil work).
Your explanation for the second question (why are tons of shortwave stations received but are not tuneable for circuits #1 and #2) is likely true, and it is also an impressive feat of sleuthing for a "beginner in electronics".
You might try a further experiment to further verify your explanation of plentiful shortwave with the following circuit:

^{simulate this circuit – Schematic created using CircuitLab} The audio output of the TA7642 has been omitted from this schematic, as you have done. This is a broad-band receiver that accepts all radio signals in from your long wire antenna. You should try this arrangement only as a short-term experiment because the antenna can easily conduct transients or static or even distant lightning strikes into the TA7642 that can destroy its input stage. This circuit accepts a broad spectrum of radio signals, with a preference for the strongest one(s). This is only roughly true, because the antenna itself delivers some radio frequencies more efficiently than others, and TA7642 has an upper limit to frequencies, above which it becomes increasingly less sensitive.
Since it eliminates your ferrite rod and variable tuning capacitor, if it gives a similar result as your circuit #2 (with external antenna attached) your explanation of why short-wave signals dominate is supported.