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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Sun 03, 2019 12:17 am 
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Location: SoCal, 91387
neondave wrote:


I have tried to post a couple pics here, either it says to big or to small ???

Maybe I could email to a member for posting ? ? Thanks

simply go into Paint, and re-size to 800 pixels per side. If that doesn't work, PM your email addy.

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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Sun 03, 2019 11:09 am 
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Location: Saskatoon
There's a calculator on my website that you can use to calculate the inductance of any of these loop antennas. I didn't mention it earlier, because the calculator had a glitch in the code that caused it to display incorrect dimensions, causing a user not to trust it. There was also a slight but mostly insignificant inaccuracy in the calculated inductance value. I've been cleaning it up today, and it's working fine now. (I've cross checked it against other calculators and formulas that are known to be accurate, as well as some actual coils that I've measured.)
Here is the link:
http://electronbunker.ca/eb/InductanceCalcML.html
This is a do-everything calculator that handles many different types of coils including multi-layer coils. A flat coil can be considered to be a multi-layer coil with a single turn per layer, and the number of layers equal to the actual number of turns in the flat coil. Confusing?
Below is a screenshot, with the values that neondave gave for his coil, entered into the appropriate fields:
Attachment:
CalcInput.png
CalcInput.png [ 80.58 KiB | Viewed 395 times ]

For the flat spider coil, the actual number of turns goes into the "number of layers" field and 1 goes into the turns/layer field. (For a box coil like Macrohenry's, these would be reversed.)
These inputs are based on the most recent post where the number of turns has been decreased from the original 34 to 11.
Outside diameter is set to the distance between end points on the cross bar (length of the diagonal of the outer turn).
Because the antenna is 4-sided, the polygonal option is chosen from the Coil Shape menu, and 4 is entered into the No. of Sides field.
Radial Pitch is the spacing of the brads on the cross arm. (It automatically adjusts for the sqrt(2) factor.)
Wire size is self explanatory, but be sure to select AWG in the popup beside the conductor size field (unless you're using metric wire).
Everything else can be left blank.

With these inputs, the calculator gives an inductance value of 134uH which may be a bit low. If you want to use a standard 365pF variable cap and tune the AM band, then you'll need an inductance of about 247uH. Trying different numbers of turns in the calculator, 17 turns gives a value of 248uH. So I'd recommend going with at least 17 and maybe add a couple more just to be safe. It's easier to remove a turn than to add one afterwards.

If anyone tries the calculator and encounters any problems, please let me know so that I can fix them. There have been some issues in the past where it didn't work with some web browsers, mostly older ones. BTW, you need to have Javascript enabled in your browser, but not Java.
Also, if anyone is able to measure their existing loop antenna and compare it to the value from the calculator, I'd appreciate seeing the results. Note that because these loop antennas have fairly high self capacitance, that will affect the value measured with some types of inductance meters, and so it will differ from the value that the calculator gives. However, the difference shouldn't be more than a few percent with a reasonable quality LCR meter (and assuming that the calculator is working correctly, of course).


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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Sun 03, 2019 8:18 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 419
Location: Loveland,CO
fifties wrote:
neondave wrote:


I have tried to post a couple pics here, either it says to big or to small ???

Maybe I could email to a member for posting ? ? Thanks

simply go into Paint, and re-size to 800 pixels per side. If that doesn't work, PM your email addy.



Thanks. I haven't hooked the outer 'coupling turn' to a radio yet, but seems to work ok holding my pocket radio next to it.
The box is empty , its just a base support . 18 turns # 22


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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Sun 03, 2019 10:42 pm 
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Location: SoCal, 91387
That looks fine.
To test for it's efficiency though, set the Transistor radio to a station, hold it near the loop, then tune the loops capacitor until you hear a jump in the radio's volume. Next, try the radio and loop at stations on each end of the dial, to make sure the loop tunes the entire BCB.

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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Tue 05, 2019 3:11 am 
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Posts: 8142
Location: Beautiful Downtown Burbank CA
Here is an 18" antenna taken from a console.
Works great with a var cap, but a little bulky and ugly.
Image


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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Tue 05, 2019 6:06 am 
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Location: SoCal, 91387
Mike, get a piece of tinted plexiglass, cut it to shape, and put it over the coil.

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 Post subject: Re: AM flat loop antenna
PostPosted: Apr Sun 07, 2019 5:52 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 419
Location: Loveland,CO
fifties wrote:
That looks fine.
To test for it's efficiency though, set the Transistor radio to a station, hold it near the loop, then tune the loops capacitor until you hear a jump in the radio's volume. Next, try the radio and loop at stations on each end of the dial, to make sure the loop tunes the entire BCB.



Yes ,this tunes great with entire BCB. Thanks to all with help on this . I was only using a wire running along my house roof eave ( not the best local ), but my radios perform much better now. Here is a pic of the antenna on my Fairbanks M.


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 Post subject: Re: AM flat loop antenna
PostPosted: Apr Sun 07, 2019 9:17 pm 
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Location: SoCal, 91387
Perfect; looks right at home!

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 Post subject: Re: AM flat loop antenna
PostPosted: Apr Fri 12, 2019 6:55 am 
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Joined: Aug Fri 29, 2014 6:17 pm
Posts: 2575
Location: Vincennes Indiana
I used to make and sell one for 160m made of pvc, but that was back when the caps were plentiful and cheap. A friend took his to extreme, using litz wire for the vlf section, and caps and inductances added or subtracted to increase the tuning range.


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