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A 2-Metre Elevated Groundplane Antenna

Novel design includes a matching section to match the impedance of the feed cable. It's easy to build too.

By Phillip Watson, VK2ZPW

This VHF antenna was originally constructed as part of the author’s research into the impedance of an elevated groundplane antenna, as set out in the June 1999 issue of SILICON CHIP. In particular, the author wanted to establish that an impedance matching section (or "Q" section) could be constructed, to match the 52Ω impedance of the feed cable to the 18Ω antenna impedance.

In fact, the finished device has proved to be a completely practical antenna. It is simple to construct, easy to mount and because it provides the correct load, it allows the transmitter to generate maximum power.

Click for larger image
This view shows all the pieces of the antenna just before the final assembly. The copper tube forms part of the matching section. The materials used are all readily available and you should be able to scrounge most of the parts for little cost.

This is important because not every transmitting device is completely safe from mismatch damage. Typical commercial power amplifiers ("afterburners") frequently carry a warning that an SWR above a specified figure will void the warranty, for example.

A feature of the unit is that the "Q" section is of solid construction. It makes a substantial "handle" which can be lashed or clamped to a mast or, if the mast is in tubular form, the "Q" section can sit inside the mast, along with the coax cable.

Another feature of the unit is the use of screw-on radials which can be easily detached for transport. In fact, this antenna has proved extremely useful as a temporary base antenna during WICEN exercises. Alternatively, as a permanent base station antenna, it would suit any situation requiring an omnidirectional VHF antenna for 2-metres (144-148MHz).

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