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.
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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.

Step 2:

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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!
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 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  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 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?
RangerJ2 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!
gnomedriver5 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?