TheRadioBoard

OK, here is the scenario ... I want to make a wavetrap to use with a radio that uses a ferrite antenna ( no outside antenna connections ).

On the one hand, I see wavetraps that crystal radio users have that are effectively a coil and capacitor tuned to the frequency that needs to go-away, and that tank is placed near the radio to absorb the field.

On the other hand, I see devices like the Select-A-Tenna that are basically a coil with a capacitor that is placed near a radio to BOOST the signal so the radio can more effectively receive a weak station.

How can one thing work both ways ?? What the heck am I missing ??

Good question, NightFlyer.

In a crystal set, the wave trap is inductively coupled to the tuning tank. Signal from the tuning tank to the trap is reflected back into the tuning tank. BUT, the signal is now 180 degrees out-of phase with the original signal to the tank. Hence, the cancellation or QRM trapping.

It's important to note that, in a crystal set, the external antenna feeds the signal directly to the tuning tank.

In a booster circuit, the coil in the booster itself receives the incoming signal. The signal is then re-radiated or fed to the radio's built-in 'antenna' coil, again with a 180 degree phase shift. So, IF the radio's antenna coil is picking up substantial signal, THEN the two signals WILL cancel in wave trap action. But, typically, booster circuits are built with large coils for enhanced signal pickup -- much greater than the pickup by the radio's antenna coil alone. If the radio's antenna coil does pick up some signal, it will be canceled by the booster's much larger signal, and this results in a net signal boost to the radio.

This is not so easily done. You could inductively couple a ferrite rod wave trap to your radio's antenna coil. But you need for the trap to have zero pickup of the outside signal, while the radio's antenna coil picks up all of the outside signal. I'm not sure how you'd do that. Another possibility is to install a parallel trap in the radio's internal circuitry. But that risk causing all sorts of tracking problems.

Thanks for coming back Mike ...

So, the trap is actually "reflecting" the signal already in the tuning tank ... just out of phase ... and therefore "cancelling" it. The trap isn't actually "sucking" ( technical term ) the signal out of the ether around the tuning coil.

So, for my application, I would need a ferrite "trap" coil that is similar to the ferrite antenna coil in the tuning circuit of the radio I want to add a trap to ... and then the two ferrites would need to be set such that the signal from the one ferrite is 180 degrees out of phase with the other ... and then the signal in one would couple to the other and cancel it out ...

Should I wind the "trap" ferrite in a counterclockwise ( contra ?? ) direction as opposed to the clockwise direction the antenna ferrite is wound ? Would that achieve the phase shift ?

I am also wondering if I could couple the two ferrites with a coupling loop from one to the other ...

Many things to try ...

Tony

That's right -- cancelling, not sucking or absorbing . . .

The trap coil can be wound in either direction. The phase is altered when the signal is induced from antenna coil to trap coil and from trap coil back to antenna coil. There's a 90 degree phase shift in each step, or a total of 180 degrees.

But again, the tricky part is getting all the received signal to the antenna coil and no received signal to the trap coil.

Link coupling the two coils is worth a try. However, the phase shift through such a link might be 360 degrees (no cancelling). And again, if your radio's antenna coil is part of the tuned circuit, there's a risk of upsetting the tracking.

Good luck! -Mike-

I don`t remember exactly where I read this and name of person (on of the Russian ham forums). If he is correct this will work only with a superhet. If your RF amplifier and oscillator LC circuits are missaligned, placing the other contour next to it will compensate this and you till get louder signal.

I've just completed one successful experiment. I soldered a 240uH pi wound choke across a 360pF mylar film variable cap and placed the choke right against the ferrite antenna of my radio, at the middle of coil. I was able to trap out a very strong station pretty severely. But only when the station was center tuned. I could not trap out an untuned background station that was weakly spanning the spectrum of the radio. I suspect that there needed to be a strong enough signal in the ferrite antenna's field to successfully couple across to the trap choke, and then back again. So this approach is probably not going to work for my TRF radios ...

Thanks Mike ... I think I better understand what is going on now.

Cheers,

Tony

..

Problem Solved !!!!

See: http://theradioboard.com/rb/viewtopic.php?p=36359#36359

Tony

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I have an in-line wave trap connected a short distance away from my TRF set, in the antenna line. It works as both a wave trap or a signal booster, depending on how I tune it.

As I tune it to either peak or trap a particular station, there is a peak and a null, very close together. For boosting signals, it is effective mainly on the lower BCB frequencies. It works fairly well as a trap on the higher frequencies. I use it to boost signals at the bottom of the band, where the set is less sensitive than the higher frequencies. On the higher frequencies I use it to trap out the local torches when trying to pick up a faint DX station on a nearby frequency.

I've also seen the same thing, dezzertrat. It was explained to me that the null+peak is a by-product of overcoupling. Less coupling and the notch will be sharper/deeper and the adjacent peak will go away.

It sure comes in handy for DXing though. Some of my best DX has been achieved using the overcoupled state!

-Bill

How does one change the coupling with an inline trap? Would changing the number of turns on the coil in the antenna line do it?

Yes, that would do it.