433 MHz Tape Measure Antenna -suits UHF Transmitter Tracking!

This Instructable relates to the design & evaluation of a simple tape measure based 433 MHz 3 element Yagi antenna.  An effective receiver was made by  "persuading" a ~US$4 Dorji 433 MHz ASK (Amplitude Shift Keying) data module into analogue signal reception,perhaps from a companion PICAXE driven tone transmitter.

When used with the tape measure Yagi antenna, DF (Direction Finding) performance over line of sight ranges to 1km was quite remarkable,with a DMM (Digital Multi Meter) RSSI signal strength display allowing extremely fine bearing resolution.

Tape measure based Yagi antenna were popularized by Joe Leggio ( WB2HOL) =>http://pages.videotron.com/ve2jmk/tape_bm.htm  and are often used for field work and hidden transmitter "fox hunts". Most are VHF (~146 MHz on the 2m ham band) & are rather too large for bushy  terrain & indoor work- they also tend to flutter badly in winds.

In contrast higher freq. UHF antenna approaches are more compact, and can be both readily carried thru'  snagging vegetation & rapidly deployed. The receiving and transmitting electronics can also be very simple if  based on the license free (but low power) 433 MHz ISM band. This perhaps suits DF (Direction Finding) for  learners /scout groups working in a smaller area (such as a park), or local interference tracking.  



 

Discrete storage - not only does a compact UHF tape measure antenna stuff away in a small bag, but it can even be discretely used while deployed  within a folder or plastic bag!

Here's a "classic" lower frequency 144MHz 2m  VHF version. Numerous tweaks to this are discussed at => https://sites.google.com/site/tapemeasureantenna/

Note: Although lower frequency Yagi antenna will be proportionally larger (arising of course from wavelength considerations), VHF signals will have better penetration of vegetation and buildings than UHF, which is a significant reason why they're popular for outdoor "fox hunting"and animal tracking etc.

Simple Yagi designs follow long established "half wave dipole" techniques, often using imaginative hardware - this "Cotanga" ( coat hanger !) based 433 MHz design has previously shown itself a good performer, offering perhaps 6dB ( = range doubling) gain over an isotropic antenna. However the rigid wire elements do NOT suit easy storage or quick deployment, and they may be an eye hazard at close quarters...

YagiCad design ( refer => http://www.yagicad.com/ ) of  a 433 MHz ( ~70cm wavelength) tape measure Yagi indicated a good front to back ratio would result with tighter Driven Element - Reflector spacing. Additionally this compacts the design for discrete field use. 

Numerous antenna design programs exist, and although helpful their values should NOT be taken as sacred - the proof of the antenna is in it's actual performance!

Diverse construction techniques were explored, but garden irrigation fittings & black hose eventually proving the cheapest & most versatile. Shop around however, as some garden outlets often sell these fittings at quite high prices , & the "X" cross pieces are not as abundant as the "T"s.  These  "Pope" branded ones came from the NZ/Australian Bunnings Hardware chain & cost US$1 - $2 each. The black hose can be simply push fitted onto the connections- perhaps use hot water or a hot air gun to assist.

Securing with self tapping screws proved rugged and cost effective. The tape measure itself can usually be bought for a couple of dollars at coin stores etc.  Handy hint- more is less: Combo  imperial/metric tape measures -which are seemingly still made for USA users- are often a hassle to read, & as a result (here in all metric NZ anyway) they may be MUCH cheaper than just metric ones.  The quality combo  Bunnings Hardware sourced one used here cost only ~US$1 !

Here are  the antenna parts ready for assembly - no specialized tools were needed. Although the metal tape readily cuts with sheers it's edges will be sharp & should be promptly rounded/chamfered to prevent cuts !  Tape lengths of 355mm, 310mm, and 304mm were cut - although these lengths can perhaps be adjusted, so cut LONGER initially.

NOTE: It's crucial to recognize that the middle 310mm "Driven Element" tape portion MUST be cut in half,with the 2 parts mounted so that they are  insulated from each other.  (Refer Step 1 for an assembled view)

Use a nail to punch guide holes in both the hose fittings &  tape for the self tapping screws. This is simple & effective - drilling the thin steel tape tends to cause it to tear anyway.

Paint on the iron tape measure will probably need sanding off  to expose the metal at the 2 halves of the Driven Element connection, which can perhaps be solder covered to prevent weathering. Perhaps use lemon jouce flux on the tape's bared metal, and very hot iron ( or 2 !) to  achieve this. A toothed "star" washer will bite into this well for  reliable contacts.

Connect a short length of coaxial cable  to the 2 separated parts of the driven element, & it run to a suitable UHF radio antenna input. 

Note:  A ½ wave dipole has nominal centre impedance of 75 Ω, but this is influenced by adjacent antenna elements.  For reception, and if the run to the receiver is small, then almost any convenient shielded coax. (here flexible mike cable) can be used.

In the interests of receiving simplicity no impedance matching has been used here, but this should be considered if the antenna is used for transmitting, otherwise an impedance mismatch & poor SWR (Standing Wave Ratio) may result !!   Such matching approaches  as "T", Gamma, Beta ( "hairpin") & Delta are traditional, BUT the background theory to these can be involved.

Here an old Kenwood TH-28A ham band transceiver (& known for it's sensitive 433 MHz  FM receiver) conveniently connects to the tape antenna lead via a BNC input. Such a setup allows significant reception enhancement when directed towards the transmitter, and the receivers signal strength bar indicator can even allow simple direction finding.

Modern UHF sets often have only "5 bars" signal strength meters & are usually too coarse for fine bearings, but this may aid in interference tracking from rogue UHF devices etc. Tests with the sensitive Kenwood TH-28A  & the tape measure antenna over  ½ km LOS showed signals peaking over a broad sweep ~45 degrees either side of the transmitter's known position.  This hence only allowed rough bearing indications, & triangulation (or closer fixing) may be additionally needed. Naturally the local terrain may not always suit this!

Although offering sophisticated features & sensitive receivers, for simple reception work suitable UHF radios can be complicated, costly or even requiring a ham radio license! Their use may hence be an overkill for basic work, and additionally many offer only NBFM (Narrow Band FM) reception.  AM (Amplitude Modulation) is more commonly used in DF work.

The recent availability of  cheap ( ~US$4) Chinese sourced Dorji  ASK  433 MHz wireless data modules however tempted! Although the Dorji receiver is only a modest performer beside a professional UHF set, it's capable of remarkable work with a decent antenna!

The Dorji receiver bandwidth (quoted as 200kHz) is wide enough to accept signals not exactly on the nominal 433.920 MHz, but was found selective enough to ignore nearby high power UHF CB transmissions around ~470 MHz.  Classic 433 MHz ASK receiver  modules are often very influenced by out of band signals.

The matching Dorji transmitter is more powerful than classic ~2 mW level 433 MHz offerings, but at 25 mW is still legal (in most countries- the USA may be an exception?).

Refer a wireless data slanted evaluation of these Dorji ASK modules  => www.picaxe.orcon.net.nz/dorjiask.htm

A LED & small piezo transducer speaker allows the received audio to be both seen & heard. Although the LED may be hard to see outdoors & the piezo hard to hear, this is particularly valuable when identifying a specific signal and it's transmission "schedule"!  In urban areas the local license free 433 MHz ISM band may be full of weird transmissions arising from garage door openers,car remotes, back yard weather stations & wireless energy monitors etc.

Exploring the Dorji modules SYNOXO SYN470R  RF IC  revealed that a pin 13 CAGC tap offered a valuable RSSI ( received signal strength ) voltage output.  This undocumented AGC (Automatic Gain Control)  output  was of only very low current, but a high impedance DMM on DC volts readily showed a ~1.2 V signal strength variation, ranging from ~1.3 V  when the receiver was beside the transmitter, to ~2.5 V with no signal. ( This later value seemed related to the freshness of the 3 x AA batteries )

Note: Moving coil meters & even low quality DMMs may swamp this RSSI output, in which case a buffering current boosting common collector  "emitter follower" enhancement may be needed.   ( This is under exploration for a final PCB based design )

NOTE: Soldering a wire to this pin can be tricky, as the fine IC pin spacings are only half the normal 2.54mm (1/10th inch). A useful approach involved first crimping a small protoboard flying lead pin over the pin & then soldering while it was thus secured.

Here's the completed 433 MHz antenna, complete with the Dorji receiver & monitoring  DMM. A small snack box suits holding the electronics & 3 x AA batteries, and also makes a convenient operator grip. For directional work a household "Lazy Susan" may suit,although it's metallic bearing my confuse unless the antenna is lifted well clear,perhaps with a block of wood etc.

For initial 433 MHz monitoring and antenna evaluation  a cheap wireless door chime RECEIVER can be used- the super regenerative circuitry within radiates a weak signal some metres away ! This one was somewhat enhanced with a short external  ¼  wave (~160mm) whip antenna.

LOS (line of sight)  detection of the door chimes super regen. receiving  radiating circuitry  from across a lawn  ~10 metres away !

The benefits of adding a 4th element ( Director #2 - spaced & cut the same as Director #1) were  explored some weeks later, and were found to be marginally worthwhile! However the overall antenna then becomes somewhat inconveniently long. For demanding situations however a 4th (or even more) director(s)  may be beneficial. Further trials with the design however proceeded with just 3 elements, largely because this was most convenient for portable use.

For extended range work the  door chimes's transmitter could of course be used, but these typically have ranges of only 10s of metres, and continual button pushing would cause their small battery to soon run flat !

A better approach used a matching Dorji ASK transmitter module, persuaded to send PICAXE micro. generated tones.( Refer => http://www.picaxe.orcon.net.nz  ) The beauty of using a micro-controller is that a distinctive transmission scheme, perhaps Morse Coded, can be organized. Not only does this add to the tracking action, but it allows several stepped transmitters to be sequenced. Quiet times can be pre-programmed too, for "hunting" suspense, transmitter battery saving  and to also monitor any local interference.

Although the Dorji transmitter is only 25 mW, unobstructed detection ranges were greatly extended and easily covered a large park.

Signal strength readings over several hundred metres were made, with the DMM's high resolution further allowing remarkable "beaming" to the transmitter's location.  It built up areas of course assorted signal reflections may confound things, but cross reference triangulation can assist.

Results, of two tape measure antenna over 200m range, showed scope for further antenna dimension experimentation. This could involve variation of element lengths & their spacings, or even addition of a further director element for more gain.

Signals were still very strong, & the antenna still remarkably directional, at ~300m LOS.  The rate of  RSSI fall off steadily decreased, but from 1.98 V at ~200m, to 2.05 V at ~250m, 2.11 V at ~300m  and 2.18 V at ~350m.  As the system "no signal" level is ~2.5 V it implies signals could still just be detected at several km over similar open terrain? We ran out of suitable unobstructed room here to verify!

Note:  Objects crossing the propagation path were noted to suitably attenuate signals as well - indicating a possible security monitoring application for the setup.

The setup suits  spot signal checks when elevated above obstacles. The DMM  remains conveniently at ground level, & only DC voltage readings are made (for which simple twin wires are suitable).

The  Dorji DRA886RX  modules are stated as good to handle data at 1200 bps &  -107dBm  ( =1µV) receiver sensitivity.  This  hence implies a ~2.5V DMM noise floor reading equates to a ~1µV.   Use of a (mid 1990s & thus  now near obsolete)  PROMAX  TV/FM Level Meter (Model MC-160B) tended to verify this, with a 2.10 V (DMM) signal registering as 5µV (PROMAX).

 As with a 50 Ohms impedance 1µV =  -107dBm, then  5 µV = -93 dBm.   As every 6dB gain = range doubling, then 107-93 = 14 dB is  equiv. to ~2x2, or greater than 4 times the range at  300m.   This implies an open field range of perhaps 1½ km is feasible under similar park conditions with a head high Dorji 25mW transmitter & waist high tape measure antenna fed Dorji receiver, and was consistent with our initial findings.

To verify the predicted maximum range a near LOS link  of ~2km was investigated. Although signals were received by a UHF scanner (even with just it's rubber ducky antenna), detection was at the very limit of the Dorji DMM/tape measure Yagi system. Even with a 4th antenna element the ~2km distant transmitter was  (questionably) discernible on the meter above the (significant) background noise readings.

Note: The broadband nature of the simple Dorji receiver of course somewhat fails it here, as when overlooking such urban setting all manner of 433 MHz radiation is likely to be received from an "eagles nest" monitoring site!  In more isolated regions the Dorji would probably still perform well.

A ground level transmitter and elevated hillside receiving site was then used & a good link with sharp DF was possible at ~1km LOS. Weaker signals could still be detected at ranges of several 100 metres when within light vegetation as well. However solid hillsides totally blocked them.

Just on 1km LOS - the 25mW  transmitter (which used a simple ¼ wavelength  horizontal whip of ~160mm ) is outside a beach side house far below. A remarkably fine bearing could be made to it's known position, with the exact transmitter location able to be "fixed".

Bearing readings at this position- note they're somewhat unsymmetrical (perhaps due to terrain & background noise) -
   Angle         DMM 
(degrees)    (Volts)
0                  1.91 V         ( Front = pointed directly at the transmitter)
45                2.05 V
90                2.27 V
135              2.30 V
180              2.37 V        (Back =  pointed directly away,& enhanced by body shielding)
225              2.26 V
270              2.34 V
315              2.10 V

While at this elevated site a significant increase in background noise was noted at certain bearings. This seemed to arise from Wellington City (across the harbour some 10km away) & perhaps even Wellington Airport's hilltop  radar (~15km away) or the powerful Mt. Kaukau UHF TV transmitters (~12km away). Changes in antenna polarisation were beneficial in some cases - a camera tripod suits mounting.

NOTE: Although based around cheap 433 MHz  electronic hardware, this antenna could readily suit  70cm ( ~430MHz) ham band or ~470 MHz UHF CB ( "PRS") 2 way radio use when  dimensions are suitably altered.  For transmitting (if allowed by regulations) impedance matching will probably be crucial to ensure a low SWR (Standing Wave Ratio).