Recently I've designed & built a seven element 2m yagi and figured that someone might be interested in it. I've tried to include enough information here for someone else to build their own and/or improve on the design. The design goals were:
-Optimised for max forward gain
-3m boom, as 3m lengths of aluminium are easy to buy (Bunnings et. al.) and transport
-50 ohm feedpoint
-Wide enough bandwidth to cover the whole band
The yagi was designed in the free antenna analysis software (http://hamsoft.ca/pages/mmana-gal.php). The programme has a built in optimisation routine, so the design was built by adding an element about 50cm off the end, optimising, adding another etc. This was repeated until the boom was 3m long.
The final simulation result predicts 12.2dBi forward gain (10.2dBd) in free space and an SWR of 1.01 at 146.5MHz
- MMANA-GAL model
The simulation model can be found here: http://www.fireflydesign.net.au/7el%202m%20yagi.maa
The boom chosen was a 19mm square aluminium extrusion, all elements were made from 10mm tube. Both of these were in stock at the local Bunnings. Elements are currently attached with a single M3 bolt with a small patch of pushbike tyre inner tube squashed between the element and boom. This keeps them in place well enough for testing, but in the long term I'm hoping to get a friend with a 3D printer to make up some plastic brackets which hold the elements in place and stop them rotating. As it stands the elements will rotate the second a small bird lands on them.
It is possible to mount the elements through the boom (in simulation it made no difference if they were insulated from the boom or not) but since the driven element must sit above the boom I figured mounting them on top would result in a better performing antenna (probably not measurably though). In simulation having the driven element 2cm above the plane of the other element makes little difference.
Coax connection was done via an N-type chassis socket (Jaycar PS-0674) mounted on a small piece of aluminum which is in turn riveted into a PVC insulator. The insulator is a ~25mm PVC pipe which has been cut in half lengthwise.
- Feedpoint
To remove the requirement of driven element boom isolation it is possible to re-optimise the antenna for forward gain in MMANA-GAL to reduc the feedpoint impedance and allow for a gamma match to be used. I found that this arrangement was simple enough to be built, but it's probably mostly up to personal preference and experience, along with what tools/materials are on hand.
Here's a snapshot showing the partially built antenna:
- Antenna Size
Click for full size
Lastly, this post wouldn't be complete without an on the air comparison. The remote station, VK2YCJ (Jamie), was about 50km away in his car, running about 70W into a magnetic mount antenna on the car roof. The comparison antenna is a homebrew vertical dipole mounted about 1m lower than the yagi on a different mast. The yagi is about 10m above ground, but close to a 2nd story steel roof. There is some QRM crackle over the Rx audio, not sure what it was but it went away a few minutes later.
The simulation model can be found here: http://www.fireflydesign.net.au/7el%202m%20yagi.maa
Hopefully someone can find this interesting and helpful.
Cheers,
Brenton
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