TECHNICAL

Improvements to the YG5-4 antenna
Published on 25 May 2011 by Marcel Pertus F5DQK
5-ele yagi after modifications

Problems with the original design

The Moonraker YG5-5 is a 5-element yagi with gamma-match feed. It may work well when brand new, but after 3 months on the mast, problems rapidly occur with rain and moisture: several hams complained about the bad behaviour of this yagi after no more than half a year.

On investigation, it seems that there are problems with the feed section: the four screws fixing the SO-239 connector aren't secured with internal nuts, but are staying in place just because their heads jam on the SO-239!! That’s very bizarre !!

Secondly, the gamma-match has too much play on its retaining screw, which can be over-tightenend, thus shorting out the co-axial series capacitor inside the gamma-match tube.

Finally, with an equal director spacing, it is not possible to get an optimised front-to-back ratio anyway.

The YU7EF re-design with 4NEC2

After a discussion, Pop YU7EF did a re-design for me, using the 4NEC2 simulator, setting a target of keeping the same boom length (so the reflector and front director stay in their original places).
The gamma-match was replaced by a totally-insulated dipole, and the element spacings were optimised using the simulator. The old and new dimensions were also simulated using MMANA: the table below shows the results.

Simulated Moonraker designSimulated YU7EF design
SWR SWR
Polar pattern Polar pattern

The new design yielded a front-to-back ratio better than 18dB, with the same forward gain (7.9dBd), but no more problems with the rain.

Practical results

The first measurements were immediately a success: I just had to add the boomwidth correction (2cm more). The bandwidth is really marvellous now.
Thanks again to the the ham spirit of Pop YU7EF who really had a great idea!

The full details of this project are available here: this is a pdf version of PowerPoint slides showing how to “resurrect” this antenna and make it more durable.