Introduction: Medium Wave AM Broadcast Band Resonant Loop Antenna.

Picture of Medium Wave AM Broadcast Band Resonant Loop Antenna.

Medium Wave (MW) AM broadcast band loop antenna. Built using cheap 4 pair (8 wire) telephone 'ribbon' cable,& (optionally) housed in cheap garden 13mm (~half inch) irrigation plastic hose.

The more rigid self supporting version better suits serious use, as it can better null offending local noise or stations and even DF (direction find) when rotated towards remote signals.The weak signal enhancing performance (especially on classic 'deaf' AM radios) of either type has been found ABSOLUTELY OUTSTANDING - signals just leap off the bench!

As they can be built much cheaper (& faster) than traditional tediously wound & mounted loop antenna,this approach suits tight budgets,educational resonance demonstrations,remote weather forecast needs & travellers unable to erect a long wire outdoor antenna.

Step 1:

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The compact version allows easy storage -suitable portable & traveling needs. 3 metres (~10 feet) of cheap 8 wire cable will resonate nicely over most of the upper 500kHz -1.7MHz MW Broadcast Band with a common 6-160 pF variable capacitor. However use longer lengths for stations at lower MW frequencies, OR add a 2nd capacitor in parallel to the variable.

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The idea with such a loop relates to tuning the simple coil (L) capacitor (C) parallel combo so that the pair "resonate" at a frequency in the band of interest. The loop's variable capacitor is tuned so this station's frequency is also that of the loop, & then even loose coupling (by just placing the receiver nearby) will hugely boost the signal. The 8 wire version is the most convenient to use,as it lies flat,stores more compactly & offers a broader wire intercept to the signal.

The well know "1920s "Wheeler's Formula" relates L to the number of turns & coil diameter - fewer turns being needed at higher frequencies. EXPERIMENT!

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There's nothing new about loop antennas, as they dominated receivers for ~50 years until the 1960s transistor radio ferrite rod takeover-itself still a loop of course. Here's a WW2 era "Spam Can"(SCR-536) Walkie Talkie c/w broadside loop,which usefully allowed some directional finding (DF). These AM sets operated between 3.5 & 6 MHz,with a range of a few miles, so the loop no doubt allowed insights into just where your pinned down buddies were!

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Rather than tediously winding multiple strands of wire around a frame,the approach here is to simply connect the cables offset wire ends,thus making a 8 wire loop! Classic 4 wire computer grey ribbon cable could also be used, BUT the coloured wires of the phone type used here make for much easier assembly and less confusion.

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In fact,with the same 60-160pF varicap,6m of 4 wire flat phone cable gave LC resonance in the mid-upper MW band almost as well as 3m of 8 wire cable. (Check the 2 formula perhaps to justify this, but don't get too hung up on the maths, as significant inter-wire capacitance arises with such close spaced phone cable). With just 3m of flat 4 wire cable it'd only START at ~1.6MHz & then cover into lower Short Wave (SW) frequencies - maybe even as high as the 3.5-4.0 MHz 80m ham band.

Ferrite rod pickups within most radios however are only good for the MW band,& telescopic whips or external long wire antenna are usually needed for lower SW freqs. Simple inbuilt ferrite rod inductive coupling may possibly hence be thwarted above 1.6MHz. It certainly was for me on such diverse MW sets as the esteemed Sangean ATS-803A (a.k.a. Realistic DX-440) where AM reception via the inbuilt ferrite rod stopped dead at 1620 kHz.

Perhaps explore other freq. loop performance (maybe down into LW bands?) using "cut & trim" of cheap 4 wire cable & quick connect screw terminals. Phone grade 4 wire cable is usually now very abundant as scrap, but as twice as much will be needed compared with the (preferred) 8 wire version,it thus new may not be so cost effective. But rather than wasting quality 8 wire cable,just shorten or lengthen 4 wire cable back until suitable resonant performance results. Then approximately halve this length for 8 wire.

Although the soldering/joining is trickier,flat 8 wire cable generally makes a neater,more cost effective & compact final job, with the wider wave intercept "front" usually giving a stonger signal.

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If you can't locate the preferred flat 8 wire cable, then perhaps hot melt glue 2 x 4 wire "silver satin" grade phone cables together side by side! Wire colour match-ups will now be trickier, tuning will probably be somewhat altered, & the 2 cable approach (once glued) won't lend itself so easily to bundling up for portable use.

4 wire phone grade flat cable is often extremely cheap & abundant,as it's traditional use in 15m (50')cord caddies is now pretty historic- thanks to the cordless,cell phone,ADSL broadband & WiFi takeover.

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If your soldering is not up to it, then these wire ends can even be joined by cheap screw terminal connectors. Naturally this will also give design versatility, perhaps should you want to quickly shorten the wire loop so it'll cover higher freqs.

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Trimmed with a scapel these terminals will also just fit (perhaps end to end) inside the 13mm plastic pipe.

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A serial D9 pair could also be used, but these are tricky to solder & more costly.

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Just basic household tools will do - the compact version can be mounted on a short piece of trellis offcut.

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Cut off 3 metres of cable & remove about 4 finger widths of the outer insulation.

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Avoid nicking (& thus weakening) the 8 inner wires- carefully bend back the outer insulation as you cut.

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A scapel will often do this most cleanly- side cutters are usually too savage.

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If soldering the pairs then "stagger" the joins by about 10mm to avoid shorting.

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Use both fine pliers & sidecutters to reveal the copper wire.

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An electronic "3rd hand" or "Helping Hand" will greatly assist in holding the wires steady during soldering.

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After soldering (or connector joining), use a DMM on resistance to check the wires are not shorted or broken. About 5 Ohms resistance is normal (subtract ~0.5 Ohms for the meter lead resistances).

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Rather than forcefully pushing the wires into the protective irrigation hose, it's probably easier to slit a short length with scissors. The hose saddles will hold it shut again afterwards,

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Hot melt glue can be used to keep any wire joins well apart- don't use too much insulating glue here or later resoldering may be difficult!

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Further hot melt glue can be used at the tube ends to secure the cable.

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Only low value (typically 60-160 pF)"polyvaricons" (plastic insulated variable tuning capacitors) are now usually available. Mounting for these can neatly be done with aluminium sliced from a drink can.

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Punch a hole through the thin aluminium, trim with scissors & fold the wings to suit the mount. Even use 2 such brackets if the first seems too flimsy.

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Voila-it looks quite professional. Discard the 2 side screws,as if screwed down too far these will usually hit the plates inside the varicap & stop them moving!

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IMPORTANT: Before fastening the capacitor to the mount, adjust the 2 small trimmers to a minimum (thus NOT overlapping)- this determines the upper frequency of course. However IF you want lower MW frequencies then adjust them to FULLY overlap (& thus more capacitance). These tuning capacitors have 2 sets of moving plates within, & they can be paralleled by joing the 2 side terminals. Fot most users however just the LH side & the centre terminal (as shown) will do- this accesses the larger variable.

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Finished. The portable design easily folds up for storage or travel.

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Clothes pegs fastened to a curtain make for a neat holding system. The loop doesn't need to be perfectly formed either, although it's directional pickup will naturally not be as good if irregular.

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Spot the antenna. Here the variable capacitor is up on the bookshelf, with the radio simply placed near the loop on the lower table. Simply move the radio around near or over the loop antenna for best pickup- this is usually when the radio's internal ferrite rod antenna is straddled at right angles.

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As most doors are about 2m high by 800mm wide, consider even simply fastening (Blu-Tack ? Velcro?)the antenna to the door itself! Even the lengthy 4 wire version could then conveniently allow simple DF & nulling just by suitably swinging the door.

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Simply tune the variable capacitor for maximum band signal- it can be quite sharp (thus a high "Q" factor). Signal enhancement on some stations is so strong that intermodulation may develop in the receiver,indicating nearby stations on frequencies where they don't actually transmit.

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Quite aside from now hearing NUMEROUS remote AM stations,some at night 1000s of km away,a sunset test with a cheap semi-digital radio found a weak NDB aeronautical beacon on 1630kHz. This was ~300km distant in the interior mountains from my location at the bottom of NZ's northern island, & can normally only be heard at sunset with a comms grade receiver and lengthy external antenna.

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YouTube demo of a weak 1630kHz NDB (Non Directional Beacon)signal being received with a (curtain pegged!) portable loop & a cheap semi-digital receiver.


SubramoniamS1 (author)2018-01-12

I did one some years back with 10 colour ribbon cable you get in electronic stores. It was easy to connect them to form a coil. Stuck it on the former that came with a steering cover! It worked well to pull in stations from India sitting at Abu Dhabi UAE. i have to see if it is lying around somewhere. Sad if i threw it out.

JamesA41 (author)2017-10-19

Excellent improvised design. Maybe even a hoola hoop would work too, though the garden hose is more likely to be found around more common. Thank you for sharing!

Amir175 (author)2016-04-05

Hey, I just got the 4-ball. It was very good; be thanked.

manuka (author)2015-10-31

PeterT51: Great- any pictures please ? Yes - variations abound, but of course the LC circuit needs to be resonant at the band of choice. Larger loops intercept more signal too I've found. Stan.

PeterT51 (author)2015-10-31

Before I read this great article, I had already made two BCB tuning loops. I made one using a hula hoop for cable support and the other 12" in diameter. Both have a pickup loop for direct connection to a radio (not necessary but useful) and both use CAT5. Both loops tune most of the AM band. I use the 12" loop by the side of my bed. Both work great!

My loops probably have greater inter turn capacitance so I used fewer turns and a larger (about 400pf) tuning cap to get around this. There are many ways to get the job done :).

manuka (author)2014-03-03

Great- glad it worked well for you too! I'd not tried with a valve/tube set (assumed of pre ferrite rod vintage?) but the principle is still sound. Stan.

colin225 (author)2014-03-02

This really does work well! (acedemic theories about inter-winding capacitance etc seem to be not an issue) My coil was taped to a loop of thick coat-hanger wire as a coupling winding directly to the aerial and earth connections of a valve radio,I also tried it in place of the aerial tuning coil in the radio (yes, I am zap!)

manuka (author)2013-10-29

OK- but the variable cap. used here readily adjusts to compensate for this extra capacitance. If need longer/shorter cable lengths or extra/fewer turns could be trialled too.

halamka (author)2013-10-27

I have found that the capacitance between coil turns might possibly make tuning impossible.

manuka (author)halamka2013-10-27

Capacitance between turns? This may certainly be an issue at much higher frequencies, but we're not talking GHz here my friend, only low (or even sub) MHz. Build a unit yourself & you'll see how well it works!

halamka (author)manuka2013-10-28

Shortwave coils. I have a digital capacitance meter. I measured almost 10 pf with heavy gauge wire on a 1 inch coil.

manuka (author)2013-01-20

Your receiver shown at  looks rather too well shielded for inductive coupling! Why not try something simpler, perhaps a  plastic cased portable ( perhaps with an inbuilt RF stage)? If you are really isolated perhaps local radio spectrum noise may well come from the likes of a battery to mains inverter- I've just been in the remote Philipines & found this was often a MAJOR issue on the MW band 550-1600 kHz.  PLEASE OUTLINE JUST WHERE YOU ARE & WHAT YOUR ELECTRICAL SETUP INVOLVES.

  "After doing the math with MATLAB i found that antenna is resonating @1kHz-6kHz gap " is unclear - just what did you mean ?  Stan.

koko44 (author)2013-01-10

Hello, im still stuck here can you help me please?

koko44 (author)2012-12-31

Yeah i see that there is no need to connect but my reciever is this one and it stands in a cabin so i cant simply get it close to antenna, antenna should be free and connected with cables. Anyway, after searching a little bit i found that it can be connected to radio in parallel to capacitor, and i hooked it up yesterday night. Loop is attached to curtain. I found one or two stations but with high noise(distortion). I'll try to house it with garden hose for better signals, but before i wanted to calculate the freq that LC circuit resonates. After doing the math with MATLAB i found that antenna is resonating @1kHz-6kHz gap. And this is not the gap it should resonate i guess. I'm stuck here. Could there be any problems with designs or formulas? Or am i doing something wrong? I need a working formula because i want to develop this antenna for all AM bands tuned by varcaps. Could you calculate your antennas freq please?

manuka (author)2012-12-22

THERE'S NO NEED as the loop couples inductively via the receiver's inbuilt ferrite rod! However a wire can perhgaps betrialled running from the loop tuning capacitor. Also experiment with a small & simple pickup loop (wound around the large loop) that runs to the receiver AM antenna input terminal

koko44 (author)2012-12-22

Is there any way to connect this antenna directly to reciever? My reciever has an am antenna input?

RangerJ (author)2012-04-26

Ok - this is cool! Anything to do with a radio - especially an AM radio - is right up my alley. Excellent Instructable, very enjoyable.

FL_Bill (author)2012-03-13

Thank you for this build. This is something I could 100% use! I will most likely build this one with my 8 year old son. Thank You!

FL Bill

manuka (author)2011-01-02

Sorry for answering delay ! MW radios have an internal ferrite rod, so just "link" by placing the radio within the loop. Such linking is magnetic so no extra connections are normally required! If you want however a permanent connection perhaps just run a wire to your MW radio from a tap point at the tuning capacitor. Experiment!

2011 UPDATE: Home wireless technologies have made classic 50' (15m) 4 wire phone cord caddies almost redundant. The 4 wire loop version shown at step 5 may hence have increasing appeal as the cabling will be very cheap/free. It'll be of course easier to connect too, & as the larger loop intercepts more wave front it may even have better performance. Stan ( ZL2APS)

Johenix (author)manuka2011-03-27

It seems to me that you could use ONE piece of 4 wire with only five solder joints (two at the capacitor and three to make the 4 wires into one) if you used twice as long a cable and double wound the cable.
A little shrink tubing added before soldering might hold the loop for constant size.
A trimmer capacitor might hold the loop on frequency for a single station.

manuka (author)Johenix2011-04-03

Johenix: Well said sir! 4 wires may indeed be easier, but the entire loop will be larger. Although this will intercept a larger wave front (& may usefully fit around a door- as shown) it will not suit rigid tube compacting.

shobley (author)2010-11-01

One final question - if I wanted to use this as the antenna on a home-made radio how would I hook it up?

Ground and signal on either side of the cap?

Or should I omit ground as I want the signal to resonate in the LC and ground would only leak the signal away.

I've only ever done this before with long wire antennas.

shobley (author)2010-10-31

Actually I have to question the comment about twisted wire not working. Traditional loop antennas have been made with Litz wire - multipe individual strands insulated from each other - every spool of Litz wire I've ever looked at has twisted strands in it - so surely CAT-5 has a good chance of working?

I might test this out later today and see what happens...

shobley (author)shobley2010-11-01

Actually I was wrong about this, once I read the article I realised that the 8 parallel strands themselved were forming the loop - I was thinking about using CAT-5 and making 8 turns with that.

I built this last night out of 2 lengths of the standard US 4-strand cable.I laid out a 10' piece of masking tape (sticky side up) and carefully placed the cables side by side. I then folded the tape over and sealed it up.

It's quite flexible and will spool up when not in use.

Thanks for finally explaining what all the trimmers do on a radio cap!

gnomedriver (author)2009-08-05

Great instructions. Ive got a small SW radio when I travel and normally use a long length of wire as an antenna. Ive experimented with making my own loops and wrapped the wire around a cardboard carton. It is all a little messy. But using computer cable is a great idea. It is cheap and easy to find. Feed it through a hula hoop or garden hose for some rigidity and away you go. I like the idea of direction tuning using the door too.

rimar2000 (author)2008-10-30

Very good idea, very good work. The steps 2, 3 and 10 are BRILLIANT for its simplicity and effectiveness.

manuka (author)rimar20002008-12-11

Thanks- following the addition of extra material to support an article in the Jan. 2009 "Silicon Chip" ,these steps are now renumbered 4,7 & 14.

manuka (author)2008-10-29

The cable really has to be flat-surely such 8 wire (4 pair) cable is available in the US? It's just ~(US$) ~50 cents a yard here in NZ via Jaycar (WB-1625)! Perhaps 2 x 4 wire cables could be used if held neatly side by side - maybe hot melt glued? Sorry,but twisted pair style cat5 isn't suitable, as the twist deliberately reduces pickup!

BigCommieNat (author)2008-10-29

this is a great idea! A question though... the 8 connector wire you have, it looks flat. here in the US the only 8 wire cable I can think of is twisted - cat5. would unshielded cat5 work for this? or would the twisting ruin it?

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Bio: Retired educator/writer
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