433 MHz tape measure antenna -suits UHF transmitter tracking!

Picture of 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.
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Step 1: UHF tape measure Yagi

Picture of UHF tape measure Yagi
Tape measure based Yagi antenna were popularized by Joe Leggio ( WB2HOL) =>  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!

Picture of 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

Picture of VHF versions
Here's a "classic" lower frequency 144MHz 2m  VHF version. Numerous tweaks to this are discussed at =>

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.

manuka (author) 7 months ago
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
manuka (author) 7 months ago
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...
Nice! Can i use copper wire?
manuka (author)  ondrikczech1 year ago
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!
Mr.What1 year ago
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.
manuka (author)  Mr.What1 year ago
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.
Mr.What manuka1 year ago
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.
manuka (author) 1 year ago
Agreed-I'm a PICAXE fan ( see => )& 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

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