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.  

<p>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?</p><p>Many thanks,<br>alec</p>
<u>Alec:</u> Good question - but it didn't seems too critical. Check Step 22 (where it's varied from 310-322mm) &amp; you'll note only modest performance changes. Such other aspects as Director &amp; Reflector spacing, number &amp; element thickness (the metal tape is really just used for covenience!) can influence F/B ratio, bandwidth &amp; impedance matching. Yagi's suit being cut &amp; try tweaked for the best overall performance given the antenna size, weight, mounting &amp; weathering issues etc<br> <br> However suggest you start the driven element halves slightly TOO LONG initially &amp; progressively trim them shorter while monitoring performance. You should see a broad &quot;sweet spot&quot; when they're ~half wavelength across.<br> <br> A Yagi's radiating element has high voltage &amp; low currents on it's ends (&amp; thus Hi-Z), but low V &amp; high I ( thus low Z) at the mid feed point.
<p>Thanks very much for the reply. The most important thing for my specific application is direction-finding ability - if it's &quot;good enough&quot; as-is then so much the better, otherwise I'll get trimming :)</p><p>Thanks for a really helpful Instructable :)</p>
<p>Dorji's new DRA887RX receiver module may well suit this deisgn. Details to be published when modules to hand.</p>
<p>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 &amp; strong &amp; with great mounting points. <strong>However such an antenna can't be used for lower freqs.</strong> <strong>unless it's first modified</strong>.</p><p>The driven element is far too short, so perhaps remove entirely &amp; replace withcentre fed tape measure sections?</p><p>The rear angled reflector however is so generously large that it'd suit 433 MHz as is.</p><p>The existing. directors are ~160mm across &amp; 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 &quot;double&quot; their length? The overall 9-10 element Yagi should have gain ~12dB (&amp; thus offer x4 the range)</p>
<p>My version of the antenna is pictured below. It was used as part of a team-based hacking event called &quot;HackFu&quot; (Google it, or search #hackfu on Twitter), whereby teams tackle various cyber security challenges. This year's theme was Wild West, and one of the challenges I set revolved around the recovery of some rustled cattle. </p><p>Each rustled cow was fitted with a radio beacon consisting of an Adafruit Trinket, a 434MHz transmitter, and a small beeper. The Trinket was sending out a pattern of AM-modulated bleeps that you can hear once tuned to 434MHz; the pattern of bleeps was different for each cow to disambiguate them from one another. </p><p>The antenna (cable not yet fitted in the picture) was installed onto an appropriate handle (goes bang when you pull the trigger), and the cows could be tracked via a laptop with an ezcap DVB dongle on a USB port and sdrsharp or similar software. Once you get close enough to the cow such that the received signal is too strong to offer a useful bearing, you fall back to your ears and listen to the audible beeper on the transmitter for the final bit of cow hunting.</p><p>The antenna worked very well for its intended purpose - the signal was definitely stronger when pointing at the cows!</p>
Marvellous! Were full sized cattle or simply those models used however ? What sort of ranges &amp; terrain ? In the field the DorjiRX approach I'd offered may have been more compact than a laptop I'd say however. Stan.
<p>We had strict instructions to keep clear of the real cattle as they'd just calved :) Four rubber cows were deployed (this was the intent all along; the antenna is visible as the cow's &quot;tail&quot;). They were called LongLong, LongShort, ShortLong and ShortShort - these names matched up with the pattern of bleeps heard at 434MHz. The cows wore a QR code around their necks, and the challenge was to match the QR code to the name of the cow.</p><p>Range was constrained by the site; suffice to say you were never more than about 150m from a cow at any given time, although there was a large mansion house in between them all so you'd never get a strong signal from all four at once. They were concealed in woodpiles, nooks in walls, piles of brush, etc.</p><p>Much to my surprise, a very faint signal was present even in the basement of the mansion, which had a stone vaulted roof!</p>
<p>Alec: OK - I've been extremely gratified with the DF attributes of this simple tape measure approach when used with the Dorji RX &amp; a DMM. </p><p> 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 &quot;fox hunters&quot; ! 73s - Stan. (Ham ZL2APS since 1967!)</p>
<p>What are the dimensions of the antenna- element lengths and spacing between elements? </p>
<p><strong>Ah-have you viewed all the steps ?!</strong> Measurements are outlined in #5 &amp; #22, but (as mentioned) scope exits to adjust these, &amp; a 4th element also looked worthwhile. <strong>EXPERIMENT !!</strong> </p><p>Yagi element spacings, lengths &amp; thicknesses are a compromise for gain, bandwidth, impedance matching, F/B ratio &amp; even beam practicalties of course anyway-few aspects are absolutely sacred. See <a href="http://www.picaxe.orconhosting.net.nz/yagi433.jpg" rel="nofollow">www.picaxe.orconhosting.net.nz/yagi433.jpg</a> for &quot;cotanga&quot; type insights.</p>
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.
<strong>Some Sept. 2013 comments from a keen Kiwi (Andrew &quot;Brightspark&quot;) re PICAXEing the Dorji receiver: </strong><br> <br> <em>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!?<br> <br> 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.<br> <br> 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 &lt; 100 then 'threshold of squelch&quot;.<br> <br> 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 &lt; 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... </em>
Nice! Can i use copper wire?
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!
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.
What's your &quot;short range&quot; &amp; environment ? <strong>Close in UHF signals tend to bounce all over the place if metallic objects are nearby.</strong> 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.
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) <br> <br>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. <br> <br>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.
Agreed-I'm a PICAXE fan ( see =&gt;<a href="http://www.picaxe.orcon.net.nz" rel="nofollow">http://www.picaxe.orcon.net.nz</a> )&amp; such a micro extension indeed tempts!<br> <br> However the resolution of even a cheap DMM is so outstanding that it'd be perhaps a &quot;down grade&quot;. For skinflints &amp; occasional DF users, the convenience of just plugging in their already handy DMM, for both bearing &amp; power insights, has great appeal of course.<br> <br> That's if they even want DF- the quest&nbsp; arose as a cheapo&nbsp; Dorji module hack to persuade it into simple &quot;<strong><em>is my %$#@&amp;! 433 MHz&nbsp; transmitter actually working? </em></strong>&quot; 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

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Bio: Retired educator/writer
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