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 Post subject: Loopstick antenna for midget Sears/Silvertone radio
PostPosted: May Tue 04, 2021 7:52 pm 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 105
Location: Seattle, WA
I've added a loopstick antenna to the back of my Sears/Silvertone 132.878; it previously had a short wire antenna with a tuned input filter which made the reception somewhat weak and noisy. The radio came to me already recapped so I didn't feel too bad about further modifying it, however this modification is fully reversible.

The ferrite antenna came from the handle of a scrapped portable Motorola radio (like the 55L1U), this antenna is a good match to the tuning capacitor of the Silvertone radio.

I mounted two standoffs to the two holes at the back of the chassis (which were presumably screw holes for holding the cardboard back?) and mounted the antenna with two plastic brackets. Then I soldered the leads of the ferrite antenna to the tuning capacitor. I also added a small trimmer, as the Silvertone radio tuning cap lacks the trimmer on the antenna side. The trimmer is soldered to a thin brass plate, which is then bolted into one of the holes of the tuning cap.
I left the original antenna coil and the antenna wire inside, this modification is reversible to the level where the only telltale signs would be the fresh solder joints on the tuning cap.


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 Post subject: Re: Loopstick antenna for midget Sears/Silvertone radio
PostPosted: May Tue 04, 2021 7:58 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 39577
Location: SoCal, 91387
Very good. So how well does it work now?

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 Post subject: Re: Loopstick antenna for midget Sears/Silvertone radio
PostPosted: May Tue 04, 2021 8:28 pm 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 105
Location: Seattle, WA
Previously I was only able to get clean reception on one local (~ 5mi, 5kW) station, everything else was noisy, including all the 50kW transmitters that are in my area within 50 miles. With this giant loopstick antenna the radio happily pulls in all these 50kW stations and the reception is very clean. The AGC kicks in on the stronger stations and I'm able to hear distant stations during the night (although still not hitting the noise floor). I would compare the RF sensitivity of this radio now to a decent 6 transistor superhet, so I'm pretty happy with the performance - one reason I made this modification is because although I liked the concept of this little radio very much, it was basically useless for any reasonable radio listening.

Downside though is that the radio develops self-oscillation / motorboating at the top of the band (starting at 1500kHz) especially when the trimmer is well aligned. I figured I could live with this problem for now, as the stations I'm listening to are at mid-band. I have plans to try to mitigate the self-oscillation in the future - one possible reason is that although the IF transformer in this particular radio is inside the chassis, it's still not in a can and hence it's probably radiating RF back to the loopstick antenna. The ferrite antenna is oriented in parallel with the IF transformer. Another reason could be that the screen grid of the converter is attached to the top of the IF coil and is not grounded, which provides a capacitive feedback path. I'll do some experimentation on trying to figure out the feedback path and mitigating it, once I'll have time.


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