How to build a loop antenna?

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I need to know how to calculate the number of turns and the diameter of a loop antenna to receive AM broadcast of 525–1705 kHz.

Can a loop antenna be used to receive shortwave frequencies above 2.3 MHz?
 

ka3jjz

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Yes they certainly can work above 2.3 Mhz but you start to loose directional nulling of a signal. However you can use a loop on HF to avoid noise sources. This is due to the fact that skywave propagation starts to be the dominant mode on HF, while that's not necessarily true on lower frequencies

Our Loops wiki has many homebrew links, viz.


Mike
 

Boombox

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For MW, loop about 110 ft. of wire around a plastic milk crate, secure a 365pf variable capacitor to the milk crate with zip ties, and alligator clip each end of the loop / wire to the variable capacitor. You may have to cut the wire down by a loop or so to make the loop track 540-1700.

I built one of these in 2011 and it's worked like a champ on MW ever since.
 
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For MW, loop about 110 ft. of wire around a plastic milk crate, secure a 365pf variable capacitor to the milk crate with zip ties, and alligator clip each end of the loop / wire to the variable capacitor. You may have to cut the wire down by a loop or so to make the loop track 540-1700.

I built one of these in 2011 and it's worked like a champ on MW ever since.
So, as per the formula (300/frequency = wavelength), if I need to build a whip antenna to receive frequency range of 525–1705 kHz, I will need a antenna of a size that can be determined by doing the math below.

525+1705 = 2230/2 = 1115 kHz = 1.11 MHz (focal point of MW range)

300/1.11 = 270 m

So, does that mean that I will need a loop antenna of 270m diameter or a loop of any diameter with a wire of 270m length to receive the AM broadcast?
 

Boombox

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So, as per the formula (300/frequency = wavelength), if I need to build a whip antenna to receive frequency range of 525–1705 kHz, I will need a antenna of a size that can be determined by doing the math below.

525+1705 = 2230/2 = 1115 kHz = 1.11 MHz (focal point of MW range)

300/1.11 = 270 m

So, does that mean that I will need a loop antenna of 270m diameter or a loop of any diameter with a wire of 270m length to receive the AM broadcast?
Whip antennas and longwire antennas are a different animal than a coiled, loop antenna, which is technically an antenna that is also a coil, or inductor.

Forget the formulas and just get ahold of approximately 110 ft. of wire and wind it around a plastic milk crate. Get a 365 pf varicap, alligator clip the ends of the 'coil' to the varicap, and if it doesn't tune the high end of the band, cut some wire to fit. I've done it. Got the plan online in 2011. It works.

If you were going to put out a longwire for MW, then yeah, you're probably looking at a 270 meter / 1000 ft wire antenna.

Loops are different.
 

WA8ZTZ

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Forget the formulas...

Yup, good old cut and try... you will drive yourself nuts with the formulas.
Part of the fun is tuning the thing to make it work where you want it to.

FWIW, built a loop here a while ago on a 12"x15" rectangular wooden form.
Wound on 100' of 20AWG insulated stranded copper wire and connected a
365uuf variable capacitor across it. It tuned from 300 to 1340 which included
frequencies of interest to me at the time.
 

WA8ZTZ

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Yup, good old cut and try... you will drive yourself nuts with the formulas.
Part of the fun is tuning the thing to make it work where you want it to.

FWIW, built a loop here a while ago on a 12"x15" rectangular wooden form.
Wound on 100' of 20AWG insulated stranded copper wire and connected a
365uuf variable capacitor across it. It tuned from 300 to 1340 which included
frequencies of interest to me at the time.
edit... that was with both gangs of the variable capacitor paralleled; it will tune higher with only one gang connected
 
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Yup, good old cut and try... you will drive yourself nuts with the formulas.
Part of the fun is tuning the thing to make it work where you want it to.

FWIW, built a loop here a while ago on a 12"x15" rectangular wooden form.
Wound on 100' of 20AWG insulated stranded copper wire and connected a
365uuf variable capacitor across it. It tuned from 300 to 1340 which included
frequencies of interest to me at the time.
But without the formula you can't build one as per the required frequency range. Do you know the formula or any online calculator?

I came across a few calculators but can't get the size and turns using frequency range.
 

majoco

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Like everyone says - cut and try! Here's my loop and capacitor from the junk box. I was trying to get down to the LF NDB band but it was unsuccessful - there are multiple loops on that frame connected in series and the ends are soldered to those tags. The outer turns are 1metre per side. I have found by other experiments that the easiest way to get the turns right is to wind on too many, connect the capacitor and then check the range - remove turns one at a time and check again...and again...
Don't forget a one or two turns coupling loop.
 

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rivardj

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But without the formula you can't build one as per the required frequency range. Do you know the formula or any online calculator?

I came across a few calculators but can't get the size and turns using frequency range.
@dave3825 gave you links in post #2 of this thread that have the formulas you need.
 

Boombox

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But without the formula you can't build one as per the required frequency range. Do you know the formula or any online calculator?

I came across a few calculators but can't get the size and turns using frequency range.
For MW loops like the one I described, no formula is needed. Any formula was already worked out by the MW DXer who put the milk crate loop plans online. My loop covers 1700-520 and has worked well since 2011 when I built it.
 

krokus

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Like everyone says - cut and try! Here's my loop and capacitor from the junk box. I was trying to get down to the LF NDB band but it was unsuccessful - there are multiple loops on that frame connected in series and the ends are soldered to those tags. The outer turns are 1metre per side. I have found by other experiments that the easiest way to get the turns right is to wind on too many, connect the capacitor and then check the range - remove turns one at a time and check again...and again...
Don't forget a one or two turns coupling loop.
Did you try other experiments, with more loops and/or different capacitors?
 

Boombox

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Did you try other experiments, with more loops and/or different capacitors?
I'm not Majoco, obviously, but in the late 1980's I made a spiral loop with no formula whatsoever. I saw some pictures of spiral loops, and decided to make one that looked similar.

I made a 3.5 foot one where I wound a ton of old, enamelled wire on a crosspiece made of two 1x2's. I had a small varicap (the cheap plastic kind you could get for a couple bucks at Radio Shack), and soldered the ends of the loop to the varicap. Then I cut it to fit.

At first, it was a Longwave loop -- I had a bit too much wire on it for MW DXing, and it was covering 800 kHz or so down to 400 kHz or lower. Cutting down a few loops brought it into MW territory, covering 520-1600+ or so.

I logged stations in South Korea, Russia, a station in Colombia, a couple stations fairly deep in Mexico, and stations all over the Western US with the loop, DXing with that loop and a boombox.

The point being that when you're making MW DX loops, formulas really aren't as important as winding the loop, cutting to fit.
 

dlwtrunked

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Yes they certainly can work above 2.3 Mhz but you start to loose directional nulling of a signal. ...
Properly built loops work perfectly well for finding nulls up trough VHF and UHF and ones are made for doing that. (Personally my experience shows their null is better than the peaks of a Yagi for doing that.) I keep a pair in my car just for that.
The problem with loops on HF is not the loop but what is inaccurately called "polarization error" due the the angle above the horizon of the arriving signal. Of course, loops have significantly lower gain that can often be corrected with ampification.
 

ka3jjz

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The technical reprints from the International Radio Club of America - one of 2 well known AM DXing clubs in the US- are likely to have a lot of loop antenna information. They can be found on the EXer.ca website


Mike
 
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Properly built loops work perfectly well for finding nulls up trough VHF and UHF and ones are made for doing that. (Personally my experience shows their null is better than the peaks of a Yagi for doing that.) I keep a pair in my car just for that.
The problem with loops on HF is not the loop but what is inaccurately called "polarization error" due the the angle above the horizon of the arriving signal. Of course, loops have significantly lower gain that can often be corrected with ampification.
I need it for AM reception only.
 
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