DIY Build Next-G Yagi
VK4ION
Before we begin...
You can download a PDF document for this project. Size 480kb
What's the Frequency?
This antenna is for Telstra Bigpond Next G Wireless Internet which uses two frequencies: uplink 839.8MHz and downlink 884.8MHz [Commonly referred to in Australia as "Telstra 3G 850MHz".]
This is NextG wireless we're chasing - keep these frequencies in mind as you troll for Telstra Transmission tower information in your area.
Materials and making a start:
First the Boom - it should be non-metal so I was looking for rigid but light.
I started at a hardware store - from the gardening section pick up a 'riser' - a thick walled (sturdy) black plastic pipe. They're sold for attaching taps ... hence the name Riser and why it's threaded both ends. Your final antenna will be around 700mm so the 900mm Riser will allow some leftover for mounting - cost is about $6.
Take the riser to the Plumbing section and buy a straight PVC joiner which will fit over the riser - costs about $2.
Get 2 stainless 3mm bolts, long enough to go through your 10mm alum tubing, (excess length won't matter). 2 washers to suit, and 2 small lugs to solder to the coax tails.
Elements are 10mm aluminium tube - you need just under 1200mm of aluminium tubing.
Cut all 8 elements at the lengths shown - mark the D.E. and put aside till later.
Drill all 8 holes in the black plastic boom at the correct distances, and fit SEVEN elements - not the D.E. Yet!
The last hole (Director #6) should be out near the end of your boom, leaving excess at the back, behind the Reflector.
This leaves plenty of boom to make a mount which you brace onto your mast (whatever suits your setup).
I stuffed the ends with styrene (lightweight filling) and a blob of silicone, to keep the bugs out.
The Specifications
Dimensions
Reminder: You can
download a PDF document for this project
To clarify some points:
Measurements are Centre to Centre - You can measure left side of one element to left side of the next and still retain the C to C relationship [be sure not to take left side of one piece to right side on another piece].
You begin at the Back element - the Reflector - and each measurement is taken from that, so do note the "Element Spacings from Reflector" measurements .
10mm Aluminium Tubing was used but 12mm can be used with the same measurements.
Construction Process
Step By Step Build
Next-G Yagi
Press the elements in, they should be fit tightly (if not, consider gluing them in place)
Later you will cut the boom & place the White PVC joiner into the boom.
The Driven Element was cut at 154mm long, then cut in half.
I put a short old pencil inside the aluminium 'halves' to keep them about 1mm apart.
Pre-drill 2 small (3mm) holes through the Driven Element quite close to the ends - coax will attach here. Note I have 2 bolts sitting in the holes.
1 hole thru centre, size of D.E.
1 hole off-centre size of Coax - off-centre will aid attaching coax as it's a tight space inside PVC joiner.
Pull coax thru so you can prepare the tails [ends].
Remove some inner plastic dielectric, expose the centre wires. Twist into single thin tail.
Keep tails short as possible - tin & solder a lug on each tail end.
Ensure they can screwed into the D.E. without touching. Cover with insulation tape.
1 tail must be longer due to the angle of coax.
Bolt lugs onto D.E. (this is why you pre-drilled those holes)
You'll need a long skinny screwdriver to fit 3mm stainless bolts & washers. Screw down tight to the D.E., while retracting coax back out of the hole so it doesn't kink.
Test the coax continuity to ensure no short circuit from all the pulling & twisting.
Cut the Boom.
Take a deep breath and cut through the centre of the hole that was drilled for the D.E. earlier.
Duble check the dimensions!
If you push the black riser into the PVC joiner and it doens't 'go all the way'... then shave some off the ends of boom.
You're working with the critical dimensions here!
sThe distance between Reflector & D.E., when it's assembled, must be 74mm.
The 'roll' in the image is an 'ugly balun' - 6 turns of coax taped together.
The BALUN avoids interference from short lengths of exposed/unshielded coax at the Driven Element.
I use flexible RG58 CellFoam low loss coax which suits my balun.
Buy what you can afford but just appreciate that you get what you pay for.
Be prepared to 'nibble' small amounts of the boom end inside the PVC. You need to make passage so the coax can exit from the boom.
You're also checking your specifications along the way to ensure your distances are accurate.
This is trial and error - cut a little - measure - cut more until correct spacing is achieved.
A little more info...
Finish the job
The final stage - how to mount - is up to you. Make attachments you need to get it in the air.
You can leave the rear end behind the reflector as long as you need to mount and brace.
Polarisation:
This may take a leap of faith as all the experts will tell you that Next G technology is a vertically polarised medium and hence they say you should make vertical antennas.
Research YOUR nearest transmission tower & you will discover that some are Vertical and many are SLANT polarisation.
Research on Whirlpool.net.au is helpful as people build and test antennas and compare results... proving that if your Telcos transmission tower is SLANT polarisation - you will get better download speed from a Horizontally polarised yagi.
Conversly if your nearest 850MHz transmission tower is Vertical - then you stick with Vertical polarisation too.
Performance:
What about the signal-strength lights and bars on your modem?
The number of bars is simply an indication of the noise in the air that the modem can see/hear. It is NOT a true indication of the directional gain you're achieving with your yagi. (I could prove this to you with lots of testing equipment but that would bore you).
A well-built antenna filters out all the noise and you'll only be seeing the signal you're looking for so 2 bars/lights might be great speed.
I'm not an expert by any means but I research propagation continually, and I can build an antenna to achieve the results I want - and at home handyman prices and that's good enough for me.
I hope this project has been of some help to you.
Important update 2020
Telstra Australia has published the following:
" Telstra will be switching off 3G in 2024. Before switch off, you can use handsets supporting 3G on 850MHz... After switch off you will still be able to access the Telstra Network on 700MHz. Find out more: tel.st/goodbye3G "
ยท I have decided to leave this page live on the internet as the 3G mode [and 850MHz frequency] is still used in other parts of the world.
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