Introduction: 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.
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.
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!
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!
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.
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.
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.
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.
Trimmed with a scapel these terminals will also just fit (perhaps end to end) inside the 13mm plastic pipe.
A serial D9 pair could also be used, but these are tricky to solder & more costly.
Just basic household tools will do - the compact version can be mounted on a short piece of trellis offcut.
Cut off 3 metres of cable & remove about 4 finger widths of the outer insulation.
Avoid nicking (& thus weakening) the 8 inner wires- carefully bend back the outer insulation as you cut.
A scapel will often do this most cleanly- side cutters are usually too savage.
If soldering the pairs then "stagger" the joins by about 10mm to avoid shorting.
Use both fine pliers & sidecutters to reveal the copper wire.
An electronic "3rd hand" or "Helping Hand" will greatly assist in holding the wires steady during soldering.
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).
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,
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!
Further hot melt glue can be used at the tube ends to secure the cable.
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.
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.
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!
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.
Finished. The portable design easily folds up for storage or travel.
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.
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.
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.
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.
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.