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.
Remove these adsRemove these ads by Signing Up

Step 1:

Picture of
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.

Step 2:

Picture of
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!

Step 3:

Picture of
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!

Step 4:

Picture of
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.

Step 5:

Picture of
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.

Step 6:

Picture of
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.

Step 7:

Picture of
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.

Step 8:

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

Step 9:

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

Step 10:

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

Step 11:

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

Step 12:

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

Step 13:

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

Step 14:

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

Step 15:

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

Step 16:

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

Step 17:

Picture of
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).

Step 18:

Picture of
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,

Step 19:

Picture of
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!

Step 20:

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

Step 21:

Picture of
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.

Step 22:

Picture of
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.

Step 23:

Picture of
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!

Step 24:

Picture of
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.

Step 25:

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

Step 26:

Picture of
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.

Step 27:

Picture of
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.

Step 28:

Picture of
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.

Step 29:

Picture of
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.

Step 30:

Picture of
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.

Step 31:

Picture of
YouTube demo of a weak 1630kHz NDB (Non Directional Beacon)signal being received with a (curtain pegged!) portable loop & a cheap semi-digital receiver.
manuka (author) 1 year ago

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.

colin2251 year ago

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 carefull...no zap!)

manuka (author) 1 year ago
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.
halamka1 year ago
I have found that the capacitance between coil turns might possibly make tuning impossible.
manuka (author)  halamka1 year ago
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 manuka1 year ago
Shortwave coils. I have a digital capacitance meter. I measured almost 10 pf with heavy gauge wire on a 1 inch coil.
manuka (author) 2 years ago
Your receiver shown at http://www.ingyen-aprohirdetes.com/wp-content/uploads/2010/11/534494.jpg  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.
koko442 years ago
Hello, im still stuck here can you help me please?
koko442 years ago
Yeah i see that there is no need to connect but my reciever is http://www.ingyen-aprohirdetes.com/wp-content/uploads/2010/11/534494.jpg 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) 2 years ago
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
koko442 years ago
Is there any way to connect this antenna directly to reciever? My reciever has an am antenna input?
RangerJ3 years ago
Ok - this is cool! Anything to do with a radio - especially an AM radio - is right up my alley. Excellent Instructable, very enjoyable.
FL_Bill3 years ago
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) 4 years ago
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 manuka4 years ago
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)  Johenix4 years ago
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.
shobley4 years ago
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.
shobley4 years ago
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 shobley4 years ago
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!
gnomedriver6 years ago
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.
rimar20006 years ago
Very good idea, very good work. The steps 2, 3 and 10 are BRILLIANT for its simplicity and effectiveness.
manuka (author)  rimar20006 years ago
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) 6 years ago
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!
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?