ideas for stopping the squeal on broadcast AM?

[st25r]

I built this ferrite loop rod antenna with an AM tuning capacitor. It has 3 Chinese 110mm ferrite rods shrink wrapped together I used litz wire with the exception of the induction loop, I used 16 guage stranded copper wire (3 turns). I will say that I soldered in a jack to create an external antenna jack which this is plugged into. I'm having some high pitched squeals which reminds me of shortwave. What should I do to decrease the hf squeal? It does work pretty well for dxing, which makes me think I hooked it up correctly. I'm picking up 760 WJR from Detroit and 77wabc from NY, I believe. The closer stations still have the squeal (WSM 650 Nashville) even when they are loud and clear. My goal eventually for this antenna is to put it outside and connect coax cable to the loop to run it inside to the tuning capacitor along with another coax cable to the pickup coil, although I would have to get a rotor for it and waterproof it somehow, but for now, I'd like to stop the squeal.







There are 4 wires from the loop stick. I believe two go to the oscillator and the other goes to the built in loopstick pickup coil.


This is where I connected the external antenna:

 

[majoco]

I believe you have over-complicated your loop. There is no need for a coupling loop to you radio - especially to the oscillator - or joining your external coil to the internal one. Just have your external tuned winding near to the ferrite rod in the radio - tune the radio to the required station then peak the signal on your external tuning capacitor. If you were connecting to a radio without an internal antenna - then you would have a coupling loop connected to the antenna socket - but still no connection to the main tuned coil.

 

[Boombox]

Your radio (obviously) has an internal loopstick. You would probably get better results just coupling the big external loop to the internal loop inductively, i.e. setting it near the radio.

Most external loops are coupled to portable MW radios inductively. I.e., set the external loop near the radio, and peak the external loop to the MW frequency you're trying to listen to.

Portables generally aren't designed for external loops to be directly wired to the PCB or internal components unless they have an actual external antenna jack -- and even then, it doesn't mean you're going to get better reception that way. I have a GE Superadio with external antenna terminals. I've never used them. I set my external loop antenna near the radio. Works great that way. Even inside the house. Although my home has minimal RFI.

 

[st25r]

I believe you have over-complicated your loop. There is no need for a coupling loop to you radio - especially to the oscillator - or joining your external coil to the internal one. Just have your external tuned winding near to the ferrite rod in the radio - tune the radio to the required station then peak the signal on your external tuning capacitor. If you were connecting to a radio without an internal antenna - then you would have a coupling loop connected to the antenna socket - but still no connection to the main tuned coil.

Hi majoco, I would like to have a connection for an external AM antenna to have the freedom to put the antenna in a different place without moving the radio. The radio unfortunately does not have an external option for AM. I used the Service manual (download pdf for easier viewing) to have an idea of where to solder it.

This is an example of the stations it can pull in recorded 4-5 hours after sunset

. I was near the north Florida/ GA line when I recorded this. The high squeal isn't really noticeable on the video but I would also like some ideas to filter it out. A snubber cap of some sorts across the antenna connection or an adjustment that could be done by ear, if possible? Thank you for your input, I appreciate it!

 

[WA8ZTZ]

The squealing is caused by adjacent channel heterodyning. A portable just does not have the necessary selectivity.
Selectivity is expensive. Portables generally are not expensive. You probably did not notice as much if any squeal before
connecting the loop antenna, but now with the loop connected you are pulling in more stations and the radio is having
a problem trying to separate them.

 

[Boombox]

I use a large external loop with small walkman style radios. No such squeal. I've even DXed the MW with walkmen using a large box loop. No squeal.

The issue is somehow related to your direct connection of the external antenna to the radio. Before doing any other wiring experiments I would disconnect the cable connecting the antenna to the radio. Try using the antenna by just sitting it near or on top of the radio. Let the radio and the external antenna couple inductively. See how that works. A multi-ferrite bar external loop should have plenty enough gain to work well sitting next to the radio.

 

[WA8ZTZ]

One other thing comes to mind... be careful that you do not overload the portable with too much signal.
Try inductively coupling the loop to the radio as others have suggested... see what happens... let us know.

 

[st25r]

The squealing is caused by adjacent channel heterodyning. A portable just does not have the necessary selectivity.
Selectivity is expensive. Portables generally are not expensive. You probably did not notice as much if any squeal before
connecting the loop antenna, but now with the loop connected you are pulling in more stations and the radio is having
a problem trying to separate them. Any recommendations for an AM radio that has excellent selectivity that won't break the bank? It doesn't have to be portable or brand new.

Thank you for your explanation of why the squealing is happening, WA8ZTZ. I believe from the magnetic loop antenna video I watched, he said that more turns of the inductive loop would pull in more stations, but it would be harder to tune. I have three turns of insulated 16 gauge stranded copper wire now, but I can try to take a turn off to see if that improves sound or use try litz wire over the coil as well.

One other thing comes to mind... be careful that you do not overload the portable with too much signal.
Try inductively coupling the loop to the radio as others have suggested... see what happens... let us know.

I made an rf gain control with diode protection with a 1K pot. I didn't think to put this inline when I made the video. I'll have to try that on a night I can't sleep!

 

[st25r]

I use a large external loop with small walkman style radios. No such squeal. I've even DXed the MW with walkmen using a large box loop. No squeal.

The issue is somehow related to your direct connection of the external antenna to the radio. Before doing any other wiring experiments I would disconnect the cable connecting the antenna to the radio. Try using the antenna by just sitting it near or on top of the radio. Let the radio and the external antenna couple inductively. See how that works. A multi-ferrite bar external loop should have plenty enough gain to work well sitting next to the radio.

Hi Boombox, I've tried that and it works well; however, the tube that the ferrites are in have to be placed directly on top of the radio. I'm always worried that it will fall off (along with the tuning capacitor just dangling), not to mention having to rotate the entire radio for best reception.

 

[Boombox]

Hi Boombox, I've tried that and it works well; however, the tube that the ferrites are in have to be placed directly on top of the radio. I'm always worried that it will fall off (along with the tuning capacitor just dangling), not to mention having to rotate the entire radio for best reception.

Maybe you could find some plastic (maybe at a hobby shop or crafts section of a store) and build a small stand?

Also, if the external loop is powerful enough, sometimes you don't have to rotate the entire radio, just rotate the loop. I do this with my Superadios, and a few of the portables still work that way, too. The internal loopstick still picks up the boosted signal, and any messing with the bearing of reception is minimal.

 

[WA8ZTZ]

Hi Boombox, I've tried that and it works well; however, the tube that the ferrites are in have to be placed directly on top of the radio. I'm always worried that it will fall off (along with the tuning capacitor just dangling), not to mention having to rotate the entire radio for best reception.

Come up with a way to attach the ferrites and tuning capacitor to the top of the radio. Then go to the craft store and get a cake decorating turntable (it looks like a lazy susan), place the radio and loop assembly on the turntable... now all you have to do is spin the turntable... works great.