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 Post subject: Loop Antenna & Different Radios
PostPosted: Sep Sat 15, 2018 3:44 pm 
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Joined: Sep Mon 03, 2018 2:24 am
Posts: 652
My Midwest B17 responds extremely well with loop antenna vs a long wire antenna where as my Wards 62-402 does not have much difference in-between a 30 foot wire or the loop with the wire antenna being a little better. Both radios have about same sensitivity. Why?


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 Post subject: Re: Loop Antenna & Different Radios
PostPosted: Sep Sat 15, 2018 4:13 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 27288
Location: Annapolis, MD
Impedance matching.

There are two fundamental ways of getting a signal into the front end:
-Antenna transformer with a tuned secondary.
-Tuned loop (including the more modern ferrite core loop

Many of the early sets had a simple transformer input. What is interesting is that some used a low-impedance primary (step-up transformer), and others used a high-impedance primary (step-down). I started to research this, but could find very little on why to choose one over the other.
If you buy a modern "universal" antenna coil, it will normally have a high-impedance primary.

There were also early sets with loops. But the loop totally dominates in later designs, particularly the ubiquitous AA5s

The ideal way to setup a set with an external loop is to replace the antenna coil with the external loop. The downside of this is that the loop has to be matched to the existing tuning cap.

The simpler method is to use a "pickup coil" that is part of the external loop. The external loop is then just another transformer. Ideally, you would match the impedances between loop and the radio inout, but this is not always possible.

What is your experience with resonance and transformer equations, and complex algebra?

_________________
-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Loop Antenna & Different Radios
PostPosted: Sep Sat 15, 2018 4:26 pm 
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Joined: Sep Mon 03, 2018 2:24 am
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pixellany wrote:
What is your experience with resonance and transformer equations, and complex algebra?

I had algebra in HS. Experience with algebra? About zero. I am keeping the Midwest & selling the Wards, so it does not matter as one console is enough. I was just curious.


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 Post subject: Re: Loop Antenna & Different Radios
PostPosted: Sep Sat 15, 2018 4:57 pm 
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Location: S. Dartmouth MA 02748-1225 USA
Try placing the loops tuning condenser in series rather than parallel for the Wards.

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 Post subject: Re: Loop Antenna & Different Radios
PostPosted: Sep Sat 15, 2018 7:13 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 27288
Location: Annapolis, MD
allied333 wrote:
pixellany wrote:
What is your experience with resonance and transformer equations, and complex algebra?

I had algebra in HS. Experience with algebra? About zero. I am keeping the Midwest & selling the Wards, so it does not matter as one console is enough. I was just curious.

Just wanted to know whether we should go down the math road.....;)

Understanding the differences between radio designs can be a bit daunting....some of it was trial and error in the test labs.

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-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Loop Antenna & Different Radios
PostPosted: Sep Tue 18, 2018 2:33 am 
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Joined: Sep Mon 03, 2018 2:24 am
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Chas wrote:
Try placing the loops tuning condenser in series rather than parallel for the Wards.

How would I wire that?


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 Post subject: Re: Loop Antenna & Different Radios
PostPosted: Sep Tue 18, 2018 4:54 am 
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Joined: Nov Fri 10, 2006 12:24 am
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Location: Thornhill, Ontario, Canada
I've used both re-purposed AA5 loop antennas and ferrites.
With the AA5 loop: tune the main loop with a parallel 375 or 500 pf tuning capacitor and connect the 2 turn tap via 100 pf to the radio antenna input. Ground connected, too.
With the ferrite: tune the main winding as above. Connect the secondary to the radio also via 100 pf, antenna and ground.
In both cases, tune the radio, then peak the antenna... a two dialler. Loop and ferrite tuning is quite Hi-Q, easier with a slow motion drive. Signal (by AVC voltage) in my basement is about the same as 20 feet of wire in the ceiling, but the loop/ferrite seems to have a lower noise level.
Results vary, depending on the radio's RF coil, my sense is it's better with the lower impedance antenna inputs.
Cheers,
Roger

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Roger Jones,
Thornhill, Ontario
Ontario Vintage Radio Assoc. http://www.ovra.ca


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