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published: Sunday 24 April 2016
modified: Sunday 24 April 2016
author: Hales
markup: textile

Selectable turns AM loop antenna

I’ve made a couple of AM loop antennas before, but I have never been sure of how many turns to wind and the results (when connected to my radio) have been lacklustre. Today’s job was to make another but with a variable number of turns.

Construction

I started by cutting and marking out a sheet of plywood to mount and wind the antenna on. The circle was made using a stick with two holes, a nail and a pencil. The other lines were for centre and square-line finding:

I drilled out 16 holes around the perimeter of the circle and hammered in wooden pegs. Despite the saying, square pegs do fit into round holes (nice and snugly):

Nothing fancy on the backside:

I then struck ten nails in at the bottom of the loop and used these as taps for my windings. I wound each loop of the antenna with a seperate piece of wire and terminated each end on the nail farm:

In the picture above you can also see a few other features:

Results

It works wonderfully! Different parts of the AM band work better with a different number of turns selected on the antenna. At certain ranges I found that 7 turns picked up something nasty that sounded (by ear) a good 10 or 20 more dB louder than everything else. At other ranges it provided the loudest/clearest.

Misc details


Jim Morrison - Wednesday 29 August 2018

I have been searching the net for just this kind of thing, but i have been searching "tapped loop" antennas. A guy on YouTube did a tapped loop around a milk crate. Then this finally showed up. It makes total sense to me but you just don't find it! Good for you! I am just about to wind a 4 foot square loop. The frame is done and the winding starts tomorrow. It will now be tapped. Thank You.

Hales - (site author) - Sunday 2 September 2018

Thanks Jim. As I briefly mentioned above: I have tried making AM loop antennas before, but they've always been garbage, even when I've followed people's advice on how many turns to use. Antenna impedance and matching are too complex of a problem to attack without more expensive gear -- I think tapping turns and listening 'by ear' are much better solutions for receivers.

Klas - Thursday 3 June 2021

Very cool! Think I may try this. Probably a silly question, but: Do you just tap into each loop and leave the rest of the turns connected, or do you have to cut connection to the remaining loops somehow?

Hales - (site author) - Thursday 3 June 2021

Hey Klas, good question. My switch just taps on to different points of the coil, the rest of the loops technically stay connected. I suspect this is not too much of a problem at AM frequencies (very long wavelengths, hundreds of meters) but it would probably have notable effects at higher ones.

Klas - Friday 4 June 2021

Thats what it looked like, but I figured I'd better ask. I guess it might not matter anyways, since even disconnected you could probably risk the connected loops receiving stuff from the disconnected ones like a pickup loop? This antenna business is pretty new to me I must admit.

Hales - (site author) - Friday 4 June 2021

Hmm. Perhaps a shift in how we think about antennas & radiowaves might help here. I'll presume you know the relationship between frequency and wavelength in free air, if not then I suggest you read up on it before continuing.

Electric fields and magnetic fields have intertwined behaviours, especially once you're a long distance from the transmitter, but it's still sometimes worth thinking about them separately.

For the majority of antennas: explanations for how they work rely on electric field interactions and skip the whole magnetic field side of things. These sorts of antennas tend to be somewhere around the size of the wavelengths being received (eg you don't use a 1cm long antenna to pick up 10M wavelength signals, but you can use a 15M long antenna to reliably pickup 10M signals).

AM radio transmissions are relatively low frequency, so the wavelengths are very large (hundreds of meters). Most of the time the antennas picking up AM broadcasts are not anywhere near hundreds of meters large, so how do our small AM radios receive anything?

This is where coil/loop antennas and magnetics fields come into play. Small coil antennas interact with the magnetics fields in free air more than they interact with the electric fields. Commercial AM radios for instance tend to use a wire coil with a ferrite bar inside ("ferrite rod antenna") but the design I made on this page only uses coils in free air.


Now let's split the antenna I made up into "connected windings" (both ends are wired to some load, ie the radio itself) and "non-connected windings" (only one end is connected to anything). Draw a diagram if it helps. For the windings to interact with the magnetic fields in the air: current must be allowed to flow through them (ampere's law). The connected windings are connected to a load, so current can flow and therefore magnetic fields in the loops are interacted with. The non-connected windings have no load attached (ie extremely high impedance), so basically no current flows and therefore basically no magnetic field interactions take place.

Does this explanation help? Or is it more confusing?

Klas - Friday 4 June 2021

Aaah makes sense! Thanks for the explanation!

Tyler Akins - Saturday 23 October 2021

I am looking to replicate your success, but am wondering if it's important to use a 12 position switch. Do you find all of the different numbers of windings to be useful, or do you tend to skip settings 1 through 5 and focus more on the longer runs? What number of loops works well for stations that you can pick up?

Hales - (site author) - Saturday 23 October 2021

Unfortunately now disconnected due to a room re-arrangement :|

5 or less were never useful for any station. IIRC some stations were good at 7 turns, others good at 11 turns. It probably depends on lots of factors, including the antenna matching circuit already in your radio receiver.

Klas - Sunday 30 January 2022

Hey, and thanks for the help so far! Ive now started my prototype. Got my antenna set up with a varable capacitor and it's tunable near the ferrite of my reciever. So far so good. Now I'm trying to set it up with a pickup loop so that i can place it away from the receiver.

I soldered a solid copper 6 inch loop with one end to each pole of a coaxial cable and plugged the core into the am contact of the receiver, however it seems this loop is acting as its own antenna, and is not at all picking up my tuned loop antenna. Any idea what I'm doing wrong?

Best,
Klas

Hales - (site author) - Sunday 30 January 2022

Hello Klas,

I'm very confused. You're using both a transmitter and a receiver? Or just a receiver? But you have two loop antennas, sometimes near each other and sometimes not? And a ferrite loop antenna as a third?

It might be easier to draw a quick picture of your whole setup (everything). Upload it to somewhere like imgur and past a link here.


> I soldered a solid copper 6 inch loop with one end to each pole of a coaxial cable

Coax may cause you trouble here.

Coaxial cables are "unbalanced" cables that are best used with "unbalanced" antennas (eg single-wire whips). Twin-wire or twisted pair cables are "balanced" cables, best used with "balanced" antennas (loop antennas, dipoles, etc). Balance is an important thing to match, otherwise your cable becomes part of your antenna too (which can often heavily de-tune your antenna, although that might not be as much of a problem with loop antennas? Not sure).

For mine I used some twisted pair wire out of an ethernet cable. It's probably wrong in many ways, but it was good enough to work.

Regards, Hales


Klas - Sunday 30 January 2022

I'm just using a receiver. I just used the built in ferrite to pick up the antenna in order to test the capacitor. Put it next to the radio and switched to internal antenna basically.

The issues started when I connected a pickup loop to the radio via the external am contact. The pickup loop acted like its own antenna, but never showed any interacton with the actual antenna when put near it. Maybe I've plugged it in wrong? With only the core connected, it acted as an antenna. With the coax shield plugged into the ground terminal, no reception at all.

The reason I used a coax was because they did so here http://www.mtmscientific.com/pickup.html

Figured it was the way to do it with tuned loop antennas.


I'll draw you a diagram tomorrow!

Best, Klas

Klas - Sunday 30 January 2022

Alright, here is the current setup.

When ground is connected to the receiver via the LMK/AM contact i get nothing, when only the core is connected to the antenna side of the LMK/AM contact, the pickup loop acts as an antenna.

https://imgur.com/a/EzQONM9

Best, Klas

Klas - Monday 31 January 2022

Slight update! I've managed to pick up the antenna with my pickup loop, but I had to make the pickup loop the size of a full loop, and add it to the antenna. (I also went over to twisted pair for the pickup, which doesn't seem to matter much). The antenna seems to only fare slightly better than the built in ferrite in the receiver at the moment, but i think I'll add some more taps to loops closer to the medium wave specter to see if i can get any channels there.

Any idea why a smaller pickup won't work?

Slightly bewildered greeting,
Klas

Hales - (site author) - Monday 31 January 2022

Oh wow, you're doing something MUCH more complicated than me.

Here is my wiring: https://halestrom.net/darksleep/blog/013_antenna_spiralam/comments_loopturns.png

My design only has one control: the number of turns hooked up. Changing it changes the inductance of the antenna, this affects how it interacts with with my receiver's inputs (making some stations sound better on some settings rather than others).

Your design has more controls: tuning capacitor (allowing it to resonate at a stations' freq?) and ratio of turns between your pickup loop and main antenna (transforming impedance of received signals to better match your receiver?).

Thoughts:

(1) Can your loops and variable capacitor actually resonate at the frequencies you want to listen to; or are their values off? Might be worth testing with a signal generator and your pickup loop (and possibly a scope, depending on how you think it out).

(2) Is the transformer (one loop coupling with 20) beneficial or problematic? On one hand it avoids your radio's frontend "loading" your LC coil and cap. On the other hand it might deliver such a small or impedence-mismatched signal to your receiver's frontend that it does more harm than good. This design requires finding a tradeoff, I have no clue how far away or close to it you might be, or how you would even begin to find out (other than through lots of trial and error).

Klas - Monday 31 January 2022

Yeah, I do realize that I am doing it a bit complex with the tuning cap and all. The idea was to create a tuneable antenna that was very "modular".
Unsure what i'll be able to pull off, but hey thats what prototypes are for I guess.

Well, it seems like I'm able to tune it to the wanted frequencies, and the signal seems to get better with less or more turns on the LC coil, depending on where I am on the AM band.
But all I have to go on is by ear. No fancy equipment except a basic multimeter sadly.

I can at least do a test by connecting a number of loops directly to the receiver like you, tune it to a station, and then do the same with the capacitor and coupling loop aiming at the same frequency to see if I'm doing more harm than good with this setup!

rs - Wednesday 16 February 2022

Hello,
Reading this confuses me you state its for AM band, but there is no specific band for AM. Since AM is from a few khz to above 30mhz, I dont see this any use for short wave listening. I have made up a 2ft square antenna with 10 turns this tunes from around 1.3 mhz to around 550khz with variable cap, I am now adding a 12 position switch to switch 1- to 10 turns. 1 turn seems to cover the 40metre band (around 7mhz)
I have a two turn receiver coupling winding.

metalinchains - Thursday 3 March 2022

Hey hales!!
I have a 30.000 mha power bank and i want to use it to power an atom netbook. It has two outputs 1 amp and 2.1 amps regular usb connection. I think it has enough juice to last a good couple of hours. Do you think its possible to make a circuit board to connect the output of the power bank to the netbook?

Hales - (site author) - Thursday 3 March 2022

Hey Metalinchains.

Yes, but it will be a bit complicated.

The netbook will need a specific voltage as the input, typically 19V or 12V (depending on its vintage & design). If you supply it with just 5V (what comes out of most powerbanks) then it won't do anything (it will think the charger is unplugged or faulty). You will need some form of boost power supply.

Some laptops also have a third wire in their charger's plug, to "sense" the wattage of the charger or its authenticity. See my latest article about my laptop for more info, I have a short bit about the charger right near the bottom. If you don't have this: you're in luck :)

Finally: many powerbanks only provide their higher current & power output levels after they have been negotiated via the USB protocol. If this is the case then you will need to buy a board that does all this negotiation for you. I don't know what these are called (USB power sink? USB power negotiaters?) but there should be lots available online.


Overall: it's probably easier to make your own whole battery bank that can power your laptop, rather than trying to use a USB power bank. But YMMV.

metalinchains - Friday 15 April 2022

Yep but someone told me that is a bit dangerous to solder my own battery pack. Perhaps using some kind of battery holder? And soldering that to the battery logic board? I have a faulty battery laying around to use its board to make some kind of adapter to interface between the netbook and the cells. So i can make some kind of base full of batteries.

Hales - (site author) - Monday 18 April 2022

Yes soldering directly to cells can be a bad idea. Sometimes you can get away with it, but if you can get battery holders then it's probably a better long term option anyway.

> some kind of adapter to interface between the netbook and the cells

A buck (or boost) converter is the minimum you will need. Depending on how you plan to charge your cells: you may need a "balanced" battery charger or some form of battery balancer.


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