433 MHz Tape Measure Antenna -suits UHF Transmitter Tracking!




Introduction: 433 MHz Tape Measure Antenna -suits UHF Transmitter Tracking!

About: Retired educator/writer

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.

Step 1: UHF Tape Measure Yagi

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.  

Step 2: Easy Storage & Deployment!

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!

Step 3: VHF Versions

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.

Step 4: Alternative D.I.Y. UHF Yagi

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...

Step 5: YagiCAD

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!

Step 6: Irrigation Hose Fittings

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 !

Step 7: Parts Ready to Assemble

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)

Step 8: Hole Making

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.

Step 9: Driven Element Details

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.

Step 10: Connection to a UHF Set

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!

Step 11: Commercial UHF Radios

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.

Step 12: Cheap 433 MHz Modules

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

Step 13: Dorji Receiver Hack

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.

Step 14: RSSI Tap to the Synoxo RF IC

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.

Step 15: Assembled Version

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.

Step 16: Wireless Doorchime Use

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.

Step 17: Detection of Super Regen. Receiver

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

Step 18:

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.

Step 19: Dorji Transmitter Hack

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.

Step 20: Extended Range Testing

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

Step 21: Initial Field Testing

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.

Step 22: Field Testing Data

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.

Step 23: Longer Range Performance

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.

Step 24:

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).

Step 25: Checks Against a UHF Level Meter

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.

Step 26: Long Range (2km) LOS Check

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.

Step 27: Reduced Range (~1km) LOS Check

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.

Step 28: Rotation Readings at 1km LOS

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

Step 29: Background Noise Insights Etc.

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).

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7 years ago on Introduction

Re: Measurement of the driven element - does the 310mm include the gap between them? Or is it the case that you have two 155mm elements an arbitrary distance from each other?

Many thanks,


Reply 7 years ago on Introduction

Alec: Good question - but it didn't seems too critical. Check Step 22 (where it's varied from 310-322mm) & you'll note only modest performance changes. Such other aspects as Director & Reflector spacing, number & element thickness (the metal tape is really just used for covenience!) can influence F/B ratio, bandwidth & impedance matching. Yagi's suit being cut & try tweaked for the best overall performance given the antenna size, weight, mounting & weathering issues etc

However suggest you start the driven element halves slightly TOO LONG initially & progressively trim them shorter while monitoring performance. You should see a broad "sweet spot" when they're ~half wavelength across.

A Yagi's radiating element has high voltage & low currents on it's ends (& thus Hi-Z), but low V & high I ( thus low Z) at the mid feed point.


Reply 7 years ago on Introduction

Thanks very much for the reply. The most important thing for my specific application is direction-finding ability - if it's "good enough" as-is then so much the better, otherwise I'll get trimming :)

Thanks for a really helpful Instructable :)


7 years ago on Introduction

Dorji's new DRA887RX receiver module may well suit this deisgn. Details to be published when modules to hand.


Question 4 years ago on Introduction

Hi, i want to change the frequency to DF to 350-400mhz. What length and spacing would you recommend for the antennas? Thank you.


Answer 4 years ago

You first need to more exactly specify just what freq. you'd in mind! Is it the US 315MHz ISM ? What modules - not all have such a handy RSSI tap. Are you just receiving (a lost model ?) & if so what is your setup?

As for the dimensions perhaps look at using the YagiCAD program I mentioned at Step 5, or simply scale using the classic lengths & spacings shown at Step 4.

NOTE: For lost model work the far superior LoRa approach may be best. See my "Introducing Lora " Instructable. Stan.


7 years ago on Introduction

After greater range ? Idle UHF antenna may be usefully persuaded! The mechanicals of the rugged 20 element Hills Yagi shown below (which seem cut for ~600-700 MHz UHF TV) are light & strong & with great mounting points. However such an antenna can't be used for lower freqs.unless it's first modified.

The driven element is far too short, so perhaps remove entirely & replace withcentre fed tape measure sections?

The rear angled reflector however is so generously large that it'd suit 433 MHz as is.

The existing. directors are ~160mm across & spaced ~100mm, making them hence ~twice the size needed for 433MHz work. Consider removal of all, then centre secure two at every other mount point so as to "double" their length? The overall 9-10 element Yagi should have gain ~12dB (& thus offer x4 the range)


7 years ago on Introduction

Alec: OK - I've been extremely gratified with the DF attributes of this simple tape measure approach when used with the Dorji RX & a DMM.

HOWEVER do keep in mind that this is only a 3or 4 element Yagi ! For really sharp DF over longer distances or with weaker signals you'll need both a superior comms grade receiver and probably a sharper antenna. Such gear is the domain of RF "fox hunters" ! 73s - Stan. (Ham ZL2APS since 1967!)


8 years ago on Introduction

What are the dimensions of the antenna- element lengths and spacing between elements?


Reply 8 years ago on Introduction

Ah-have you viewed all the steps ?! Measurements are outlined in #5 & #22, but (as mentioned) scope exits to adjust these, & a 4th element also looked worthwhile. EXPERIMENT !!

Yagi element spacings, lengths & thicknesses are a compromise for gain, bandwidth, impedance matching, F/B ratio & even beam practicalties of course anyway-few aspects are absolutely sacred. See www.picaxe.orconhosting.net.nz/yagi433.jpg for "cotanga" type insights.


8 years ago on Introduction

The Dorji RX module ANT/GND pin pair are slightly misaligned. Rather than stress them, best to work on the SIL socket as shown. Note the clean module back that suits an RSSI tap link.

Dorji Pin Alignment.jpg

8 years ago on Introduction

Some Sept. 2013 comments from a keen Kiwi (Andrew "Brightspark") re PICAXEing the Dorji receiver:

I do not like tacking on flying lead wires but an extra pin could be added to the 4 pins when setting up the Dorji as it usually comes with the SIL pins NOT soldered on (in my case) so you could ad a 5 pin SIL at the business end of the Dorji with one protruding under!?

The whole object has been to READADC simply and directly with least fuss the RSSI value direct from the Cagc line. = One Wire project The 10~50 MegOhm input impedance of the PICAXE pin is not likely to upset the radio This has least impact or issues or risk of loading the RF Rx superhet radio or causing any effects NO PARTS = One wire and a readadc 1, b1 command.

If you wanted to drive a moving coil meter why not use the PWMOut to scale and buffer the readadc value in software ? Squelch is also a great idea... It would be a s simple as saying 'if b1 < 100 then 'threshold of squelch".

Gating the radio for super battery life with the Dorji is all there and waiting since you have the enable pin to * Turn on the radio (low 4) etc * Sniff Cagc level * If b1 < 100 then (there is activity on the channel so time to wake up) *Keep squelch open or go back to 'sleep for x' If you want to 'fox hunt' then just convert the Cagc voltage to a pitch change and or pulse of the sound command to make it beep faster and at a high pitch as you get closer to the target etc...


Reply 9 years ago on Introduction

Of course-but solid core copper wire it is not as flexible as tape measure steel and will soon break if folded up often. Copper is costly too!


9 years ago on Step 28

How was the peaking at short range? My idea is to have a robot track a TX beacon. I worry that a yagi scanner (on a servo) might not show a good peak to track at close range.


Reply 9 years ago on Step 28

What's your "short range" & environment ? Close in UHF signals tend to bounce all over the place if metallic objects are nearby. A human may be able to factor this in (by considering a passing vehicle or garage door),but a robot wouldn't! However- as shown with my many test results- in open space with clear LOS (line of sight) the peaking can be very sharp indeed.


Reply 9 years ago on Step 28

Yea, indoors. I was afraid of IR because of the bounce problem, and not going through people and objects. Would 2.5 or 5GHz work better for DF indoors? If so... is it legal? I do have a ham tech license... so there might be some other bands available. (I might have to change the signal to be broadcasting my call sign, instead of a simple whistle, but I can live with that)

I do not really think it is critical to have a very sharp peak. I would plan to scan to about -3db points on each side of the signal, then assume the source is in the middle.

Thanks for the great write up! I have no experience with radio. I went to YagiCAD, and designed an antenna for max peak (both forward AND reverse), and plan to build it by putting 10ga copper wire in the channels of a chloroplast sheet (corrugated plastic). We'll see if/how it works. It is possible that the reverse direction peak will be sharper, but with less gain. At close range, I don't need gain anyway.


9 years ago on Introduction

Agreed-I'm a PICAXE fan ( see =>http://www.picaxe.orcon.net.nz )& such a micro extension indeed tempts!

However the resolution of even a cheap DMM is so outstanding that it'd be perhaps a "down grade". For skinflints & occasional DF users, the convenience of just plugging in their already handy DMM, for both bearing & power insights, has great appeal of course.

That's if they even want DF- the quest  arose as a cheapo  Dorji module hack to persuade it into simple "is my %$#@&! 433 MHz  transmitter actually working? " band monitoring duties.

Awsomesauce! Now just add an Arduino with an LCD as a sub for that mulit-meter! Could even get a graphical LCD so you have an arrow pointing the way. XD