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 Post subject: Loop antenna question
PostPosted: Jun Mon 28, 2021 9:50 pm 
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I am building a loop antenna for my Transistor, tube, and crystal sets.

I have a antenna base that is 5' in perimiter, insulated 28 awg magnet wire, and a transistor radio tuning capacitor.


So my question is how many turns of wire should be done in order to receive the am band?


Thanks, Henry


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 Post subject: Re: Loop antenna question
PostPosted: Jun Tue 29, 2021 1:11 am 
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https://on6wg.pagesperso-orange.fr/Loop ... ulator.htm


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 Post subject: Re: Loop antenna question
PostPosted: Jun Tue 29, 2021 2:14 am 
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http://kr1s.kearman.com/html/hooploop.html

http://makearadio.com/loops/loops2.php

Rich


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 Post subject: Re: Loop antenna question
PostPosted: Jun Tue 29, 2021 10:27 pm 
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Thanks, guys

wasn't able to figure out that calculator but from the links that were posted 5 loops isn't enough...


Any way I can measure inductance? I think 230 µH is what I am looking for.


Thanks


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 Post subject: Re: Loop antenna question
PostPosted: Jun Tue 29, 2021 10:36 pm 
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These L/C/R meters are pretty accurate at 240 uH (a typical value for an AM tuning coil or loop antenna). Not real accurate at 25 uH, but I find them very useful for homebrewing radios and loops.

https://www.amazon.com/Yosoo-Transistor-Capacitor-Inductance-Resistor/dp/B00ZOZUX6C/ref=asc_df_B00ZOZUX6C/?tag=hyprod-20&linkCode=df0&hvadid=216516247812&hvpos=&hvnetw=g&hvrand=2424752200314145361&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9032312&hvtargid=pla-351421597103&psc=1

Image

The number of turns depends on the loop diameter. I like to use 23 inch quilting hoops; some folks like hula hoops as antenna forms. I seem to recall my quilting loop antenna was about 14 or 15 turns.

Another trick is to use ribbon cable and stagger the soldering of the ends to make a helix.

http://earmark.net/gesr/loop/ribbonloop.htm

http://journeytoforever.org/edu_xtal/PLANS2.HTM

Rich


Last edited by Rich, W3HWJ on Jun Wed 30, 2021 12:49 am, edited 1 time in total.

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 Post subject: Re: Loop antenna question
PostPosted: Jun Tue 29, 2021 10:42 pm 
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Thanks, again


a lot of good info to digest. will report back, hopefully with good news!




Henry


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 Post subject: Re: Loop antenna question
PostPosted: Jun Wed 30, 2021 12:09 am 
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whfh99 wrote:
I am building a loop antenna for my Transistor, tube, and crystal sets.

I have a antenna base that is 5' in perimiter, insulated 28 awg magnet wire, and a transistor radio tuning capacitor.


So my question is how many turns of wire should be done in order to receive the am band?


Thanks, Henry


The circumference being 5 feet would make the diameter about 19 inches. For a 23 inch diameter, I found 12 turns of approximately 20 gauge wire, which allows tuning from below 540, to above 1700 Kilocycles.

You might try 12 turns, and add or remove one turn at a time, in order to get the correct range.

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 Post subject: Re: Loop antenna question
PostPosted: Jun Wed 30, 2021 12:28 am 
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May find that the plastic transistor radio tuning cap may not have enough mf to completely cover the BC band...

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 Post subject: Re: Loop antenna question
PostPosted: Jun Wed 30, 2021 12:50 am 
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I forgot some links in my last post. See edited version above.

Tnx

Rich


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 Post subject: Re: Loop antenna question
PostPosted: Jun Wed 30, 2021 1:02 am 
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whfh99 wrote:
Thanks, guys, wasn't able to figure out that calculator but from the links that were posted 5 loops isn't enough... Anyway I can measure inductance? I think 230 µH is what I am looking for. Thanks
If the loop frame is designed for expansion, turns could be added to achieve desired resonance, or close... chas

Rich,

Thanks much for the Yooso tip. I have been measuring inductance indirectly for years wasting a LOT of time doing so. No longer have the time to waste, Yooso is on my Amazon shopping list. I will order tomorrow... THANKS! chas

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 Post subject: Re: Loop antenna question
PostPosted: Jun Wed 30, 2021 9:52 pm 
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Here's my current loop situation-

Right now there are 10 loops of wire. Does spacing matter with these?

also here is my tuning cap, it's not the plastic-type. should I hook the two ends of the wire up to the two tabs?


Thanks


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 Post subject: Re: Loop antenna question
PostPosted: Jun Wed 30, 2021 9:57 pm 
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Yes, as much spacing as the frame will permit, 1/4" to 1/2" is optimum, close spacing increases the capacitance/turn and will lower the upper tuning limit.

Cap is still kinda small, but, connect the two tabs together for one connection and the frame as another. Probably will have to use a machine screw and a solder lug to connect to the frame...

GL Chas

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 Post subject: Re: Loop antenna question
PostPosted: Jul Thu 01, 2021 4:27 am 
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I just measured my loop. It's square, 20 inches on a side, so 80 inches, ~6.5 feet circumference (or perimeter.) IIRC it tunes the entire MF band with a varicap taken from a transistor radio. It has 18 turns.

In yours those metal screws could add some inductance. I have no way of knowing how much, but that's certainly not a low loss configuration. You can go to the dollar store and get a pack of plastic combs that can do away with that potential problem while giving you nice even spacing.

I'm surprised no one has mentioned Prof Coyle. I've never used it, but IIRC it can come up with the inductance for just about any coil configuration.


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 Post subject: Re: Loop antenna question
PostPosted: Jul Thu 01, 2021 4:58 pm 
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Rather than dealing with boards and nails, I think it is so easy to use a quilting/emroidery hoop. The 23" type are under $10, and cheaper if you find a sale or coupon. They come in smaller sizes also.

https://www.joann.com/wood-quilt-hoop-2 ... mpaign=amp

Image

You actually get two wooden hoops, nested. I wind my antenna on the smaller hoop, then tuck it inside the larger loop for a nice neat appearance. Also keeps the wire in place.

Image

Rich


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 Post subject: Re: Loop antenna question
PostPosted: Jul Fri 02, 2021 12:30 am 
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I am just going to get some Quitling hoops next time I'm in town, looks much easier.


Will report back


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 Post subject: Re: Loop antenna question
PostPosted: Jul Fri 02, 2021 1:09 am 
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I bought from Jo-Ann.com online. Here's my thread;
viewtopic.php?f=12&t=241661&p=2024254#p2024254

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 Post subject: Re: Loop antenna question
PostPosted: Jul Sat 03, 2021 7:30 pm 
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Bob Weaver has done some impressive work in developing coil inductance calculators. For round (solenoid) coils there's http://electronbunker.ca/eb/InductanceCalc.html and http://electronbunker.ca/eb/InductanceCalcRc.html for rectangular coils. These are suitable for small dimension coils and antenna loops. For the OP's 5' perimiter loop, using 28AWG wire with one wire diameter space between turns, the calculator finds that 13 turns will have about 250uH inductance:
Attachment:
AM Loop Ant Calc.jpg
AM Loop Ant Calc.jpg [ 208.42 KiB | Viewed 1405 times ]

John

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 Post subject: Re: Loop antenna question
PostPosted: Jul Sat 03, 2021 10:07 pm 
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Thanks, John,

With this info, I can experiment with my current loop a bit.

Will see how it turns out.


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 Post subject: Re: Loop antenna question
PostPosted: Jul Sun 04, 2021 8:38 am 
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If you use the rectangular coil calculator that John linked to, it should give quite accurate inductance values for your loop antenna. Since your loop is a square, just use the same values for side lengths a1 and a2, and b will be the width of the loop. For these loops, the nomenclature can be a bit confusing. The input value b in the calculator is referred to as coil length, but for very short coils such as loop antennas, we tend to think of it as loop width.

For these kinds of coils with relatively few turns, the spacing between turns has a significant effect on the inductance, so you'll want to measure it as accurately as possible. In fact, it's easier just to enter the coil length b rather than spacing. Just change the drop down menu selection from "Pitch" to "Coil Length" and then enter the b value.


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 Post subject: Re: Loop antenna question
PostPosted: Jul Sun 04, 2021 5:36 pm 
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Close spacing increases inductance, right?


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