Ferrite rod AM antenna doesn't work by itself.

[cool_kitty]

I pulled a nice big external AM ferrite bar antenna off a working stereo. 5.5" long.

it has 4 wires coming off of it. red must be the main wire, because checking that with an LCR meter connected to the other wires I get (uH)
blue, 310
black, 310
white, 372

so I connect the red and white wires to an air gap variable capacitor gang. 125, 250, and 250 pF. I can use any combination.

I put the antenna on a radio, right along where I know its ferrite bar antenna is.

the only thing that happens is when the VC is about midrange the signal gets nulled. not sharp at all, there is a wide range of VC that makes it happen.

it's not the wrong antenna orientation, because I had already chosen a weak signal with the radio oriented to pick it up.

why is it degrading reception instead of improving it?

[Pfraser]

Because some ferrite rod antennas concentrate the EM field too tightly. Others do indeed boost reception on nearby radios. I have noticed this myself, when trying out different radios as reception aids; similar to your method.

Explanation here.

You could experiment with a tapped coil wound on an empty toilet tissue cylinder or similar. Put up a wire aerial and try out your variable capacitor on the coil.

[Graham G3ZVT]

Interesting problem.

I've recently been experimenting with 7-8 turns with parallel VC wound into a kids hoola-hoop. It gives a phenomenal boost to an AM radio held close to it, but should I expect a boost and not a null? After all, a GDO or absorption-wavemeter would be expected to null the signal so what's the difference? The size of the coil I guess.

[G6Tanuki]

The effect the OP noticed - an unpowered tuned-circuit tuned to the same frequency as the wanted signal and coupled to a tuned-circuit receiving said signal and causing attenuation - was always a problem in closely-packed receiver coil-boxes; it was often known as 'suck-out', where a not-intended-to-be-active coil's inadvertent resonance caused the nearby active coils to be damped. It could suck out enough energy to stop a local oscillator.

The answer was to fit shorting-wafers to the bandswitch that prevented inadvertent resonances in unused coils.

Just putting another tuned-circuit alongside whose capture-area is the same as the receiver's tuned-circuit is not really a sensible approach; the two circuits will compete for the near-field RF energy!

You really need a 'secondary' tuned-circuit that has a much greater volume of space from which to grab RF energy, and so develop this into a high circulating current whose field the receiver's ferrite rod can couple to.

In times-past I had a -big loop- something like six feet in diameter, with 3 turns of 7/029 twin-and-earth mains cable [all cores wired in parallel] tuned by a 500pF variable; a cheap transistor-radio sitting on a shelf at the centre got significantly improved reception of the AFN stations broadcasting out of Germany.

It worked by having a greater 'capture area' than the receiver's ferrite rod could manage.

[Cruisin Marine]

It is absorbing the energy that the radio's antenna desperately wants.

If you place it further away (you will have to experiment to find out the spacing), I guess it will act like a lightly coupled band-pass filter coupling two tuned ccts resonant at the same frequency, and it should PEAK the signal instead of nulling it.

Originally Posted by cool_kitty

I pulled a nice big external AM ferrite bar antenna off a working stereo. 5.5" long.

it has 4 wires coming off of it. red must be the main wire, because checking that with an LCR meter connected to the other wires I get (uH)
blue, 310
black, 310
white, 372

so I connect the red and white wires to an air gap variable capacitor gang. 125, 250, and 250 pF. I can use any combination.

I put the antenna on a radio, right along where I know its ferrite bar antenna is.

the only thing that happens is when the VC is about midrange the signal gets nulled. not sharp at all, there is a wide range of VC that makes it happen.

it's not the wrong antenna orientation, because I had already chosen a weak signal with the radio oriented to pick it up.

why is it degrading reception instead of improving it?

[cool_kitty]

Originally Posted by Graham G3ZVT

Interesting problem.

I've recently been experimenting with 7-8 turns with parallel VC wound into a kids hoola-hoop. It gives a phenomenal boost to an AM radio held close to it, but should I expect a boost and not a null? After all, a GDO or absorption-wavemeter would be expected to null the signal so what's the difference? The size of the coil I guess.

pull out the top and bottom drawers of your dresser. wrap the wire around them a few times.

if you're really adventurous you can bolt a frame onto a door and have a big loop that rotates.

here's a picture of my setup. the gang on the left is 500 and 250 pF, the gang on the right is 125, 250, 250 pF. I can connect the VC's of a gang in parallel with jumper clips. I can connect the gangs in series or parallel. to the right is a variable L I pulled off an AM car radio. 4 different inductors ranging from 3 to 60 uH. I can connect any combination with jumper wires. on the left are on/off switches.

I put a length of 24 ga. wire inline with the antenna and wrapped it around the radio to couple them inductively. that's a technique I used with loop antennas. some loops were so strong it didn't even need to wrap around the radio, just the wire perpendicular to the radio's bar antenna was good enough. other times I noticed that too many wraps choked it off and reduced or eliminated signal instead if improving it. optimal seemed to be to use the strongest antenna possible and use 2 wraps on the radio.

I tried 1 wrap, 3 wraps, 5 wraps. no improvement, but what else I did was point the radio in a null direction to see if the antenna (pointed in a good direction) was having any effect at all. it was having a slight effect so I could determine the correct capacitance. the radio was tuned to 970 kHz, VC was about halfway from 125 pF (~75 pF) so that resonance freq should be roughly 910 kHz. allow for variables and they match up.

I left the VC where it was and wrapped the wire around a 1-3/8" cardboard tube 2 wraps, then 5, 10, 15, 20 wraps. no effect on the radio.

now what?

Originally Posted by Graham G3ZVT

Interesting problem.

I've recently been experimenting with 7-8 turns with parallel VC wound into a kids hoola-hoop. It gives a phenomenal boost to an AM radio held close to it, but should I expect a boost and not a null? After all, a GDO or absorption-wavemeter would be expected to null the signal so what's the difference? The size of the coil I guess.

it's ONLY the diameter of the coil. the number of turns affects L, has nothing to do with strength of the antenna.

I have also found that square and rectangular antennas are very much the same as round (circular) but far easier to make. as they become more rectangular they lose efficacy, less than a 3:4 ratio of sides is undesirable.

you can wrap 20 turns of wire about 2' in diameter by itself, hold together with tape, and experiment with shape. start off circular and compress it to oval, rectangular, etc. and see the difference it makes while holding the very wire that's collecting signal in your hands. that's a fun experiment.

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