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 Post subject: Building a replacement loop antenna
PostPosted: Apr Tue 27, 2021 5:17 pm 
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Joined: Mar Mon 16, 2009 12:57 am
Posts: 234
Location: ottawa ON
The directional loop antenna is missing for this radio (Canadian General Electric JK-70) but I would like to try and make a replacement based on the circuit attached. I'm assuming that coil of wire indicated is more for the shortwave bands but any suggestion as to the number of turns and wire gauge to start with for the AM band? Other similar radios having these, the panel is likely about 8 by 4 inches square.


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Loop antenna CGE.pdf [97.86 KiB]
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 Post subject: Re: Building a replacement loop antenna
PostPosted: Apr Tue 27, 2021 5:24 pm 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
I think without having the original antenna, we would just be guessing!

Image

Rich


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 Post subject: Re: Building a replacement loop antenna
PostPosted: Apr Tue 27, 2021 11:17 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4577
Location: Lexington, KY USA
The schematic fragment linked above does not show enough of the circuit to tell anything about what you might need.

Is there a separate antenna coil used in conjunction with the loop for any band using the loop?

What test equipment do you have for working on this? RF signal generator? RF voltmeter? GDO?

Ted


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 Post subject: Re: Building a replacement loop antenna
PostPosted: Apr Wed 28, 2021 12:22 am 
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About the only way you can even guess is to measure the capacitance present across the loop wires, with no coil or loop in the circuit, with the bandswitch in the AM position and the tuning capacitor set at the low end stop of the AM broadcast band. Then you can calculate the inductance required to resonate at that frequency and capacitance, and construct a loop accordingly.

It would be a whole lot easier to find a used part which was the correct one for your make and model as they are set specific and not interchangeable.

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 Post subject: Re: Building a replacement loop antenna
PostPosted: Apr Wed 28, 2021 2:16 am 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
A complete schematic is found at RadioMuseum:

https://www.radiomuseum.org/r/canadian_g_jk70jk_7.html

Unfortunately, this is a complex muli-band, pushbutton-tuned, radio. Tracing the antenna circuit through the switching is not a simple task. The RM scan is not really clear enough to understand the rotary switches.

Perhaps info for the corresponding RCA International versions, the RCA A-22 and A-31 might be helpful.

Were there USA built versions as well?

There are other examples of this set out there; perhaps someone who has one can help.

With a very clear schematic and plenty of time, the switching might be traced out. It would be best to have a chassis at hand for frequent reality checks.

Ted

Edited: The RCA versions cited were Canadian, as well.


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 Post subject: Re: Building a replacement loop antenna
PostPosted: Apr Wed 28, 2021 2:33 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4577
Location: Lexington, KY USA
Attachment:
Screenshot from 2021-04-27 21-25-57.png
Screenshot from 2021-04-27 21-25-57.png [ 346.15 KiB | Viewed 558 times ]

The RCA versions have clearer schematics at Pacific TV:

http://pacifictv.ca/schematics/rcaa22data.pdf

I may save trying to figure out the RCA schematic for a bit later.

It is definitely possible to create a new loop antenna that works well, but it may take quite a bit of time effort and expertise. Having some basic test gear would help.

Ted


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 Post subject: Re: Building a replacement loop antenna
PostPosted: Apr Wed 28, 2021 1:14 pm 
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Joined: Mar Mon 16, 2009 12:57 am
Posts: 234
Location: ottawa ON
A fellow collector has offered to loan me his (very similar model), the antenna is along the lines of a beam-a-scope which I was looking for a few years ago, to fix up a GE console. That radio used a drum-type antenna, while this one is more a paddle-type. I have permission to take it apart and draw a circuit, when I do iIwill post a sketch and then try to duplicate.
i don't have any of the test equipment that has been suggested, other than a sig gen, but being able to disassemble one will be a great starting point I think. Thanks to all for feedback.


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 Post subject: Re: Building a replacement loop antenna
PostPosted: Apr Wed 28, 2021 4:49 pm 
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Location: S. Dartmouth MA 02748-1225 USA
Croydonboy wrote:
A fellow collector has offered to loan me his (very similar model), the antenna is along the lines of a beam-a-scope which I was looking for a few years ago, to fix up a GE console. That radio used a drum-type antenna, while this one is more a paddle-type. I have permission to take it apart and draw a circuit, when I do iIwill post a sketch and then try to duplicate.
i don't have any of the test equipment that has been suggested, other than a sig gen, but being able to disassemble one will be a great starting point I think. Thanks to all for feedback.
During the creation of a loop frame design, allow for winding wire such that the length/turns can be altered easily. Getting the correct inductance though a tedious tuning procedure, can be made far easier if the inductance is easy to alter.

For example the flat, basket weave types often have several turns of which are pulled inward and glued with wax. others the loop windings are stuck together with a coating that allows the wire to be pulled off., another allows an entire corner to be folded. it is also possible to use a brass or tin sheet to raise/lower the inductance, though that method is lossy. If this is an antenna for multiple bands concentrate on the BC band first... chas

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 Post subject: Re: Building a replacement loop antenna
PostPosted: Apr Wed 28, 2021 11:03 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4577
Location: Lexington, KY USA
Try the other guy's antenna on your chassis, and be sure it works correctly before trying to duplicate it.

I don't doubt that several models used the same antenna circuit values. However, one that is similar in principal, but different in detail, won't be a good model.

If you can get the antenna circuit to track properly when aligning the front end, you are go to go.

It will likely be useful to have a way to easily change the inductance while setting -up. A large shorted loop of 12ga copper wire should work to reduce the inductance. Probably a really big toriod core of powdered iron or ferrite, or a few flyback transformef cores would serve to increase it. Use it like a "tuning wand".

If you have a VTVM or high impedance DMM, it is easy enough to make an RF probe. With the signal generator, this will let you determine the resonant frequency and get the inductance close to what you need.

In any case, any nearby metal affects the tuning, so you may need to mount things in the cabinet to take measurements and do the final adjustment.

I did try again to figure out the circuit using the clearer RCA schematic. I think the external loop is the only inductor for the broadcast band antenna tuning. That would mean that it needs to resonate at the bottom of the MW, band when tuned with a capacitance equal to the tuning capacitor, trimmer, and strays, taken together. Perhaps someone else will verify this.

Ted


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 Post subject: Re: Building a replacement loop antenna
PostPosted: Jun Thu 03, 2021 7:37 pm 
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Joined: Mar Mon 16, 2009 12:57 am
Posts: 234
Location: ottawa ON
A beam-a-scope was loaned to me and I aligned the radio using it. It came from an identical radio model (CGE JK-70) so I took it apart and made a circuit diagram (attached). Made one using the same wire as best as I could determine, making a flat coil about 3" x 8" on duct tape to hold it together. Then sandwiched it between two light project boards. Worked great, had to tweak alignment a little but other works fine until I find an original.


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