Medium Wave AM Broadcast Band Resonant Loop Antenna.

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Introduction: Medium Wave AM Broadcast Band Resonant Loop Antenna.

About: Retired educator/writer

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:

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:

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:

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:

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:

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:

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:

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:

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

Step 9:

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

Step 10:

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

Step 11:

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

Step 12:

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

Step 13:

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

Step 14:

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

Step 15:

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

Step 16:

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

Step 17:

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:

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:

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:

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

Step 21:

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:

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:

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:

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:

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

Step 26:

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:

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:

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:

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:

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:

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

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    37 Discussions

    You make a single loop 2x2 meter to a Pioneer 44X tuner(in my garden), and it will work great.
    Some other tuners will only allow 1x1meter, but a replacement of the standard 15x15cm loop is a good idea! And PVC pipes of 3m only cost 80 cents €.
    I receive 153 khz Satelor with 9/9 signal on a Pioneer near the Belgain coast. 1026khz at midday about 6/9. Caroline will always be 9/9 with my antennas!
    You can buy measurement equipmant that will look for µH ... to check your loops ...

    Yes ours is round and twisted also aluminium will it still work ?

    Hi, I want to make a groundplane antenna same as the one described in this PDF document.
    https://www.arrl.org/files/file/Technology/tis/info/pdf/0791033.pdf
    I want my one for 433 Mhz. So It seems the design here for 440 Mhz will work. I want it for a mobile application using one of those cheap Chinese garage 4 channel garage remotes. I purchased the 1000 meter controller for it from the same manufacturer.
    My question is: To make the antenna more robust, can I zip tie it to a sheet of 6mm thick polycarbonate sheeting ?

    4 more answers

    Hi AussieF1: This Instructable was for low freq MW band (~1MHz), yet you are after presumably enhanced coverage for UHF. Although I've done lots over the decades with 433 MHz this confuses about your need- best you clarify VERY FULLY ! FWIW often a directional Yagi may best suit if you know the signal direction, otherwise the esteemed "Slim JIM" design can be ideal for low angle omnidirectional work. In any case at UHF a CLEAR PATH & ELEVATION can be everything. If you have "junk" in the way then it's unlikely that 1km 433 MHz ranges will arise with the legal 25 mW TX power UNLESS you can elevate RX/TX antenna significantly. Regards - Stan. ( Ham ZL2AJZ since 1967)

    Hi Stan,
    Thank you for taking the time to help me out.
    I am a licenced and fully accredited Macropod Harvester ('roo shooter) operating in Queensland. Harvesting skins for leather only.
    I have access to huge areas of freehold land in many regions of Queensland. Most Kangaroos are in what is known as the 'channel country'. The highest numbers I harvest are in a location about 100 Km west of Longreach. The stations I operate on there total about 250,000 acres. Others I also operate on near Charters Towers total about 600 square miles. I harvest around 1000 'roo skins each year, and have done for the past eight years. I go on harvest expeditions for about three weeks at a time.
    The land is clear and flat.
    There are absolutely no other transmitters in the area. Not a UHF mobile or cell tower or any wifi transmitters, nothing. The only comms I can use is a Thuraya satellite phone.
    The citizens living there have only recently (six months ago) been given access to the NBN satellite internet service.
    Before that it was underground landline only. Before that it was above ground party line. Telephone party line has been available for about the past 60 years.. Before that it was weekly overland snail mail delivery only. A visit to town (100 km) was no more than once per month, if that.
    The same family on the Longreach property have handed the station down through generations for about 160 years.

    Without the winch, hauling the 'roo carcasses (up to 90 Kg) back to the vehicle for skinning is an almost impossible gut busting task. I often need to do that up to 40 times each night.
    With the winch it is easy and much quicker.

    I have tried to attach two photos of the 433 Mhz receiver module and the 4 channel transmitter that I currently use. They are just the cheaper (about AUD$25 each) Chinese garage door type, but have proven to be reliable.
    I don't think it is anything more complex than simple analogue. Data is not involved. The receiver does use a code to recognise the transmitter.
    You can see that the antenna on the receiver module is just a coiled wire and the antenna on the transmitter is a telescopic pull out ( it is easy to break the pull out telescopic). I have replaced it with a tuned length of insulated copper wire.
    I have been using the set up for at least five years.
    The one problem is the effective range.
    Even a good antenna on the receiver should improve the range don't you think ?


    I think the omni groundplane idea on both the receiver and the transmitter antenna is more functional if it can be made to work.
    Have looked at the yagi style but can see it would be too large for convenience.
    It is a one way communication, no voice, no data. Hold down a button on the transmitter and a relay on the receiver closes one of its n/o contacts.
    The receiver is mounted on the winch attached to the vehicle and at a height of about 2.5 meters above ground. The transmitter goes with me. I often hold it at arms length aloft to get the connection.
    I am always well away from the vehicle (at least 50 meters) when I need to use it.

    Any system needs to be economical and relatively simple.
    I have attached images of the receiver and transmitter to give you a good idea of them.

    Please let me know if you need any more detail.
    If I had a mail address I could forward you some images of the receiver and the transmitter.
    My business address is: wish@newforceptyltd.com.au
    Cheers,
    Geoff

    Remote control receiver.jpgRemote control transmitter 4 channel.png

    Thank you Stan,
    The application for the 433Mhz system is as a handheld portable remote control for a small gasoline powered winch clutch.
    The controller (transmitter) is a 4 channel and the receiver is a 4 channel.
    The receiver is 12 volt DC with small relays mounted on the circuit board.
    I use one of those to switch a external 12 volt DC 40 amp relay (coil current about 30 milliamp) to engage the winch electric clutch (about 8 amp engaging and 4 amp holding).
    Two other relays on the circuit board start and stop the gasoline motor.
    The fourth controls a LED light. This winch is mostly used at night.

    The winch is very small and powerful. It will easily haul 300 kg load sliding along the ground at a quick walking speed. It is mounted on the tray of my 4WD one tonner tray back.
    The handheld controller has a 9 volt battery and lives in my pocket.
    I can manually pull the winch cable out for up to 300 meters. That means I am standing beside the load to be hauled. Then when the load is attached I simply push and hold the appropriate button on the controller to engage the winch to haul the load to the 4WD. Saves time because I don't have to walk back to the 4WD before being able to engage the clutch. Plus I can guide the load around obstacles as it is hauled.
    It certainly saves a lot of 'gut-busting' effort.

    Distance is an issue.
    The problem I want to solve is that the remote does not always engage the clutch. I sometimes need to walk part way back to the vehicle before it will engage.
    Low transmitter battery voltage is a common problem. But there is more to it.
    The antenna on the receiver is simply a coiled wire. I feel that a good receiver antenna would make a big difference.

    The antenna on the transmitter is also a length of flexible wire.

    One of these 'Groundplane' antennas as described by Zack Lau of ARRL look ideal for both.
    https://www.arrl.org/files/file/Technology/tis/info/pdf/0791033.pdf

    I need to make it a lot more rugged. It needs to withstand the vehicle movement and the odd brush with a tree branch.
    I wanted to know if I can zip tie it to a sheet of 6 mm thick polycarbonate sheet.
    Also the transmitter antenna should be a stronger type. It could also be attached to a stiff plastic frame of some kind.

    I have had some experience with this type of electronics, but am not a radio tech expert. I am unsure if close proximity to the polycarbonate will null the signal.

    I need your advice.
    With Zack Lau's 'groundplane' design am I looking at the best antenna design for both jobs ?

    Yours faithfully,
    Geoff Douglas

    Hi Geoff (assumed in Australia?) & thanks for the very full account, although I'm bemused by what you're hauling like this late night - kangaroos ? Fish ? Gold bullion ?! QUICK RESPONSES

    * Polycarb. is transparent to UHF signals.
    * Ground planes & whips give omindirectional radiation
    * As your need seems very directional a beam antenna may better suit. The classic for this is the esteemed YAGI
    * Are you working in an area with other 433 Mhz devices ?
    * If your battery is weak then get better ones - try a Lithium ?
    * What exactly is your TX/RX control device?
    * Budget ?
    * Is the comms one way or 2 way ( duplex) ?

    FWIW a major issue may be that even better 433 MHz devices struggle to give reliable comms at 300-400 metres. Almost anything obstructive (terrain, rocks, trees, buidlings etc) in the signal path will attentuate or even block transmissions!! This is especially an issue in damp environements such as rainforests.

    Hence I can easily get relaible 500 m line of sight (LoS) data links from a nearby hillside to my beach side home here in coastal NZ, BUT at street level am struggling with even a to timber homes, garage doors, power lines & significant trees.

    NZ/Aus.regulations limit to just 25 mW (about the power of a glowing LED!) & you'll only really get further with ELEVATED ANTENNA, different modulation techniques ( LoRa especially), superior receivers or (illegal) higher power transmitters.

    A small 433 MHz data repeater may have some mileage - these are quite legal in Aus/NZ.

    Otherwise a possible approach could be to use "PRS" UHF CB data Ch. 22/23 reserved for telemetry/control. I once put an outback type onto these for stock water monitoring - he was previously trying to cover several km with just 433 MHz . Several firms market gear just for these slots I recall.
    Stan. ( Ham ZL2APS since 1967)

    0
    user
    manuka

    5 months ago

    Extra: Australian firm Electrosense => http://www.electrosense.com.au/ offer UHF CB "PRS" based monitoring & control systems, as do Kiwi firm Gallagher. Prices are NOT cheap, but at least check out their approaches.  Stan.

    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.

    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!

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

    0
    user
    manuka

    2 years ago

    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.

    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 :).

    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.

    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!)

    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.

    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!

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

    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.