Mods to the NextG 850MHz Yagi for the Optus 900Mhz network.

12 April 2011

Optus 900MHz
Optus has installed 900MHz towers in many regional centres around Australia including a few ks away from where I live.

A number of people have asked me what mods are required to the NextG Yagi to make it work on 900MHz so I used MMANA to optimise it for that band. The only changes are to the element lengths and element spacing, the new Optus measurements are in a table at the bottom of this page.

The contruction details are the same as the NextG Yagi so please refer to [that page] for more information.
Here are the details for the Optus version:

Polar plot
The gain is 13.5dBi with a front-to-back ratio of about 15dB

SWR response
The SWR response is excellent:


Overview of elements:


Element Overall length in mm Spacing: distance from reflector in mm
reflector 161
driven element 148 71
director D1 134 119
director D2 131 194
director D3 131 302
director D4 125 411
director D5 123 527
director D6 121 644

The majority of phone towers in Australia use VERTICAL or SLANT POLARISATION (45 degrees, axial).

Mount your Yagi first so that it's elements are VERTICAL and not flat as if it was lying on a table. If slant is in use on your tower, the tower probably has two slant antennas at + and - 45 degrees, so the best starting point is still vertical.

The difference in loss you might possibly suffer between slant and vertical if you "get it wrong" is only 3dB.

The main reason is because mobile antennas are vertical. That's the opposite of most TV antennas which are usually horizontally polarised so it takes some people by surprise.

If you feel inclined to experiment, slowly rotate your Yagi along it's boom (lay it over half on it's side, to the left or right) to see which polarisation gives you the best signal.

If you want to be sure about your tower then you can look it up on the ACMA web site by typing the name of it's location in their site search form [here].

Bear in mind too that when you mount your Yagi vertically you cannot put a metal mast anywhere but behind your antenna, so you need to extend the boom long enough out past the reflector to allow mounting on a mast.