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 Post subject: LOOP ANTENNA QUESTION
PostPosted: Mar Thu 08, 2018 5:39 pm 
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Joined: Jul Wed 13, 2016 10:19 pm
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Location: Port Moody,British Columbia CANADA
Hi,

Does anyone know the reason why some loop antennas have one corner bent over? (See attached photo)

Early on , I thought that theses were accidentally bent during disassembly or reassembly of the radio, but now I believe it was done purposefully as part of some antenna adjustment.

Does it affect the inductance of the loop or what? Is it useful in the alignment of a radio, for instance, could it help in dial
tracking?

Thanks in advance.


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 Post subject: Re: LOOP ANTENNA QUESTION
PostPosted: Mar Thu 08, 2018 6:09 pm 
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Location: Dallas, TX
Yes, it effect the inductance. It is to trim the radio so that the stations line up on the low frequency end. The high frequency end is trimmed by using variable caps.

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 Post subject: Re: LOOP ANTENNA QUESTION
PostPosted: Mar Fri 09, 2018 3:59 pm 
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That is not a standard repair procedure. By the time that set was made, tuning capacitors, coils, and antennas had been fairly standardized in AM radios. There should not have been any reason to mutilate the antenna to get it to work normally. The fact that you see old glue on the corner of the loop that has been bent backwards strongly suggests that it did not leave the factory that way. What it suggests is that somebody replaced a part with a non-original one, or could not bring the set into alignment the way they wanted by normal means, so they hacked it. Either that, or the antenna kept getting caught on something so they folded that corner back to keep it out of the way.

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 Post subject: Re: LOOP ANTENNA QUESTION
PostPosted: Mar Fri 09, 2018 5:04 pm 
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Joined: Jun Sat 09, 2007 8:14 am
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Location: Florida
That loop does look as if the corner was just pulled loose and folded over somehow but I've seen more than one that had an inner turn or two pulled out to tweak the inductance. I have one that came out of a Zenith that was never serviced and the adjusted turn is sealed with the same wax as the rest of the loop.

RRM


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 Post subject: Re: LOOP ANTENNA QUESTION
PostPosted: Mar Fri 09, 2018 6:50 pm 
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Location: Santa Clara, CA 95051
Just curious, what's that large hole used for? Is there something that is adjusted through that hole? Clock motor?

Tomie


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 Post subject: Re: LOOP ANTENNA QUESTION
PostPosted: Mar Mon 12, 2018 3:54 pm 
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Joined: Jul Wed 13, 2016 10:19 pm
Posts: 39
Location: Port Moody,British Columbia CANADA
Tomie

This antenna came out of a 1965 Admiral clock radio Mode lYG790

The large hole is where the knob and shaft for the clock adjust .

I also feel this was a factory adjustment as i have seem a few radios with this that have otherwise never been tampered with.


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 Post subject: Re: LOOP ANTENNA QUESTION
PostPosted: Mar Tue 13, 2018 8:49 am 
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Location: Near Brandon, Iowa
Shockandawe wrote:
Tomie

This antenna came out of a 1965 Admiral clock radio Mode lYG790

The large hole is where the knob and shaft for the clock adjust .

I also feel this was a factory adjustment as i have seem a few radios with this that have otherwise never been tampered with.

The steel clock setting shaft passing through the plane of the antenna loop would have perturbed the loop's inductance a bit. Perhaps the folded-over section was an attempt to offset this.


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 Post subject: Re: LOOP ANTENNA QUESTION
PostPosted: Mar Tue 13, 2018 12:23 pm 
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Location: Annapolis, MD
Notimetolooz wrote:
Yes, it effect the inductance. It is to trim the radio so that the stations line up on the low frequency end. The high frequency end is trimmed by using variable caps.

This is a delayed-reaction nitpick:
Trimming the loop antenna does not affect the dial tracking (location of stations on the dial). That can only be done by adjusting the local oscillator and/or the IF frequency. Any adjustment on the loop (L or C) simply peaks up the response of the front end. (Adjust L at the low end, and C at the high end)

I'm curious if there is some significance to that "fold over" being centered around the shaft going through the hole.

Regardless, I have never seen a loop trimmed this way. Some show no evidence of trimming...others just a few wires moved.

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 Post subject: Re: LOOP ANTENNA QUESTION
PostPosted: Mar Tue 13, 2018 5:09 pm 
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Sure looks bent after the fact to me. I see glue on the coil and a dark shade where it may have been. Unless its brittle, bend it back and see how it affects the operation.

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 Post subject: Re: LOOP ANTENNA QUESTION
PostPosted: Mar Tue 13, 2018 5:46 pm 
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Do you all realize how "forensic" these kind of discussions are? Fascinating!

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 Post subject: Re: LOOP ANTENNA QUESTION
PostPosted: Mar Wed 14, 2018 4:13 pm 
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Joined: Jul Wed 13, 2016 10:19 pm
Posts: 39
Location: Port Moody,British Columbia CANADA
I was surprised at the response this post got, and at the same time surprised no one had asked this question before.


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 Post subject: Re: LOOP ANTENNA QUESTION
PostPosted: Mar Wed 14, 2018 5:12 pm 
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Location: S. Dartmouth MA 02748-1225 USA
Many "instructions" for alignment of the antenna stage include such terms as "rocking". This technique of front end tuning often causes shifting of station location on the dial. When I service receivers with no "L" adjustment for the loop antenna, mostly AA5's. I look for the manufactures solution and often seen as folded over turns, turns pulled into the center of the loop. If the receiver demonstrates an increase in reception at the low end of the BC band when a ferrite wand is brought near the the loop tuns are adjusted. Always the station location will shift. It then becomes a compromise as to the better reception or slightly "off" calibration...The same hold true of receivers with ferrite rod antennas. The latter is seen with a wax blob hoding the windings in place. Using the ferrite wand will determine if an adjustment will work. The wax is softened and the entire coil form is move. This is impractical with rods that have the winding distributed across the entire ferrite antenna rod. To improve a poor calibration a step further is taken by attempting an "L" adjustment of the oscillator, though not always as effective.

These low end receivers were manufactured at a breakneck pace. Pre-aligned and fixed tuned circuits were the norm. A little care will often prove very rewarding...

YMMV

Chas


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 Post subject: Re: LOOP ANTENNA QUESTION
PostPosted: Mar Fri 16, 2018 3:52 pm 
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Location: Cleona, PA
Once the normal alignment is done, antenna sections of tuning caps that have slotted outer plates can be peaked for every section of the band by slightly bending plates outward with a plastic tool, section by section starting at the high end of the band and working towards the low end. The section of rotor plate that is just at the stator is adjusted to peak that part of the band.

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