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 Post subject: AM LOOP Antenna
PostPosted: Jan Tue 21, 2014 5:29 am 
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As a follow-up to a successful H/J of this thread, viewtopic.php?f=2&t=234206&hilit=coax&start=40 :D
I am pleased to post pics of my home-brew loop.

The wood loops are from Jo Ann Fabric, called wood quilt hoops, and are 23 inches in diameter x 3/4 inch wide. I bought two, and glued them together to increase the width, to accommodate the wire winding.
http://www.joann.com/wood-quilt-hoop-23 ... 11016.html

The wire, originally lead-in wire from a Radio Shack SW antenna kit, is insulated stranded, approximately 20 gauge, and through experimentation, ended up being 12 turns, which allows tuning from below 540, to above 1700 Kilocycles.

The tuner came out of my junkbox, where it peacefully resided for a good 50 years, having been saved from a radio chassis. Measuring it with my RLC bridge determined it's maximum capacitance to be 525 pF, down to 23 pF fully open.

Since angling a loop helps to either increase or null a signal, I wanted this to be rotate-able, so I obtained a Lazy Susan type bearing, to interface between the two bases.
http://www.vxb.com/page/bearings/PROD/Kit18153

This thing works very well, and much better than the smaller Selec-A-Tenna, or Grundig AN-1500 commercial loops.


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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Tue 21, 2014 2:13 pm 
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That's a great antenna. Thanks for the links. One question: is the length of wire wrapped around the circle dependent on the capacitance of the tuner?

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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Tue 21, 2014 6:58 pm 
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The L (inductance) and C (capacitance) determine the resonant freq of the loops tank circuit . example if loop is 12 turns of 24 ga wire its inductance should be 212uh ( wound on a 24 inch form), then 23pf on low cap setting would give it a tune to freq of 2.28 mhz and 525pf setting would give a freq of 477 khz or .477mhz. find a coil calculator on web input wire gauge, number of turn s, form diameter and you will get inductance value, then use this ... one divided by 2x pi x square root of LxC. i just built a loop last week and it is nothing short of amazing what you will receive or improve, the simplest thing i ever built and the most bang for the buck if you want to do some bcb dxing this is a must build.


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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Tue 21, 2014 7:04 pm 
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Just ran "fifties" values and come up with 200.87 uh for coil and tuning range of .490 mhz to 2.342 mhz for 20 ga wire on a 23 inch form. ive heard these calculators are close but could be off a little but my building has shown them to work very well.


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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Tue 21, 2014 7:29 pm 
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aa5nut wrote:
then use this ... one divided by 2x pi x square root of LxC


Sorry for my ignorance, but what is the purpose for this calculation?

Thanks

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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Tue 21, 2014 8:31 pm 
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to find the resonant frequency you are tuned to with the inputs of inductance and capacitance.


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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Tue 21, 2014 8:49 pm 
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Well if I may humbly suggest; as I am not positive about the gauge of the wire, could be 20, 22, etc., the best thing, especially considering the ease of doing it, is to simply wind a number of turns, and then test the frequency range.

Mugginsjr, although the specific capacitance of the tuner would definitely affect the length of wire/number of turns for the loop, figure that a tuner from or for an AM radio would work with a pretty generic loop. I think the size of the loop determines the number of turns. I noticed that my smaller Grundig AN-200 has quite a few more turns, but I don't think it has as long a total length of wire as this larger one does.

When I was experimenting with mine, I initially wound 16 turns;
Image
and got these results;
16 turns = 540 - 1450 Kc.
15 turns = 560
14 turns = 580
17 turns = 520 - 1280

So I began un-winding, initially settled at 11 turns, but it just didn't tune quite low enough, so gave it an extra turn, and voila, range beyond either end of the BCB.

And yes, for the ease and low cost of this, it is definitely the most bang for the buck as a piece of DX equipment for the AM band.

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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Tue 21, 2014 10:22 pm 
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A ferrite rod antenna offers good results if you dont like a cumbersome loop. I have one on a Ray Jefferson SS multiband boat radio that has amazing directivity. It is one of the emergency radios here for when power goes out.

On many radios the loop is seperate or detachable so can be used with other sets.

Carl


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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Tue 21, 2014 11:15 pm 
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Thanks for the write up. Does the weight of the lazy susan and variable cap balance out the rig so that it is fairly stable or did you weight the base any other way?
Nice job!

Scott

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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Tue 21, 2014 11:21 pm 
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Thx for the kudos, Scott!

I used a fairly heavy type of wood to make the two bases out of, so it's stable.

The Lazy Susan assembly I believe is made out of aluminum, so it's very lightweight.

My only problem was in getting the loop to stabilize W/O tipping to one side on the upper base, as can be seen in the first photo.
In the second, you can see that it's evenly balanced.

The solution, actually after I had photographed it, was to use two star type lock-washers against each other, between the wood block and metal post.

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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Wed 22, 2014 7:26 am 
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Great Job Fifties. Wear it out!


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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Wed 22, 2014 8:10 pm 
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aa5nut wrote:
The L (inductance) and C (capacitance) determine the resonant freq of the loops tank circuit . example if loop is 12 turns of 24 ga wire its inductance should be 212uh ( wound on a 24 inch form), then 23pf on low cap setting would give it a tune to freq of 2.28 mhz and 525pf setting would give a freq of 477 khz or .477mhz. find a coil calculator on web input wire gauge, number of turn s, form diameter and you will get inductance value, then use this ... one divided by 2x pi x square root of LxC. i just built a loop last week and it is nothing short of amazing what you will receive or improve, the simplest thing i ever built and the most bang for the buck if you want to do some bcb dxing this is a must build.


Excellent answer! Thank you for that explanation. Something is still not computing in my head, though. Also, I got a different induction from the website calculator. I think I must be applying the math wrong. Can you follow my logic to see where I made a wrong turn?

From: ( http://www.crystalradio.net/cal/indcal2.shtml )

Diameter of Antenna = 24 inches
Diameter of Wire = 24 gauge
Number of Turns = 12
Resulting Inductance: 6.9397590361445784 uH

That is different than the 212 uH that are in your calculations. But I'll use 212 in the calculations below:

Attachment:
inductance.png
inductance.png [ 12.56 KiB | Viewed 6829 times ]


Where did I go wrong?

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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Wed 22, 2014 9:36 pm 
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mugginsjr wrote:
. Something is still not computing in my head, though. Also, I got a different induction from the website calculator. I think I must be applying the math wrong. Can you follow my logic to see where I made a wrong turn?

From: ( http://www.crystalradio.net/cal/indcal2.shtml )

Diameter of Antenna = 24 inches
Diameter of Wire = 24 gauge
Number of Turns = 12
Resulting Inductance: 6.9397590361445784 uH

That is different than the 212 uH that are in your calculations.


Don't enter the wire gauge, enter its size in inches according to the chart.

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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Thu 23, 2014 2:41 pm 
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Bill M wrote:
Don't enter the wire gauge, enter its size in inches according to the chart.


Got it! Thanks Bill. The math works now. I'm still getting an inductance of 191, but that's much closer to 212.

I found another calculator that seems to have more info, including self-capacitance of the wire itself.

http://deepfriedneon.com/tesla_f_calchelix.html

Self-capacitance made me wonder if the wire capacitance plus the capacitance of the tuner in the antenna (as well as the tuner in the radio) don't all have an affect on the optimum number of turns. So I think Fifties hit the nail right on the head by ball-park-ing it with 16 turns and then narrowing it down until he found the sweet spot. I'll definitely be building one of these for my 1927 RCA console.

That's a great antenna Fifties!

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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Thu 23, 2014 4:50 pm 
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Fifties , put a diode in and hook it up to an IC
amp , you'll be delighted with the selectivity / low
parts ratio


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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Thu 23, 2014 8:52 pm 
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Davep, please give me a schematic for that (or a link to one); sounds very interesting!

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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Thu 23, 2014 10:19 pm 
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Looks like from the pics, just come off one of the variable cap ears with a 1n34 or your
choice and that can go directly to an op amp "s " input ( 1 watt kits you can google)
You will have to ground the other input for the amp to work.
You can try grounding the loop, although it may work without it.
I will draw it out later on.

Here's a big one josh8loop did
viewtopic.php?f=12&t=220415&hilit=loop+radio+loop+crystal+radio
Dave Schmarder http://makearadio.com/crystal/30.php


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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Sat 25, 2014 9:13 am 
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I connected the anode end of a Diode to the capacitor antenna gang lug, the cathode side going to a Rat Shack mini amplifier, with it's ground going to the capacitor frame, and logged 17 stations, about a third with usable volume! I couldn't believe it! Just a loop, tuning cap, Diode, and amp.

I tried several different types of Diodes, with varying results, but the IN64 was unquestionably the best.

This was really a trip! I had no idea whatsoever that a Diode detector could possibly pick up enough RF signal voltage without connection to a good ground, outside antenna, or some stationary metallic surface.

My reception wasn't just local, and included stations from San Diego and San Francisco.

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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Sat 25, 2014 2:34 pm 
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When you have this set up with diode, or even without, does the location of the radio in relationship to the antenna affect reception much. I know it needs to be in close proximity but curious about centered on the antenna or at the base vs top of antenna?

Scott

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 Post subject: Re: AM LOOP Antenna
PostPosted: Jan Sat 25, 2014 4:19 pm 
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Info most definitely, if you are using it with a radio with a built in loopstick.
You can rotate the radio in a direction and use the loop to null or to lock a signal.
Mostly I use it in that way since the problem is most often not in reception but separating multiple
signals.
There is a figure 8 pattern to the antenna magnetics. Moving one around you will see
the null and strong areas.
The diodes turn it into a receiver itself and will receive stations within the figure 8 pattern.

Fifties, log em over on lispo id like to see your stations 8)
You might try dropping various small caps between the diode and the amp to ground .
The drawbacks are there wont be a lot of signal strength like a random wire but that's the job of the amp.


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