Hi all
It has been interesting to read the comments about rhombic antennas.
We used a rhombic beam antenna at ZL6QH. It had 100 metre legs (to make a
total wire length of 400 metres) located at a height of approximately 20
meters AGL. The beam was made reversible by using a relay and open wire line
to switch the terminating resistor to either end of the antenna.
As antenna modelling predicts, this rhombic antenna was a cloud warmer for
160M, although it was sometimes useful as an alternative receiving antenna
on this band. Its performance was generally not as good as vertical antenna
on 80M but it did perform well in the designed direction on the higher
bands. However, the narrow nature of the forward lobe meant that it was only
useful for reaching a relatively small geographic area at the remote end of
the path. The loss of up to 3 dB in the termination resistor was also a
negative factor.
We found that unterminated long wire antennas in the form of vee beams (with
300 metre legs) were more effective than the rhombic due to them having
broader lobes in the azimuth plane. The lack of termination resistors also
meant that stations could also be worked simultaneously in both the front
and back directions. The combination of these factors meant that vee beams
more useful for working stations over a wider geographic area in contest
scenarios. Vee beams were also easier to construct as they only required 3
supporting masts. However, like the rhombic, they were cloud warmers on 160M
and were really only competitive on the 40M and higher bands.
Although we erected a number of vee beams and found them to be useful, they
were still not effective as yagi beam antennas erected at a similar height.
Our long boom mono-band yagi antennas for 20M, 15M and 10M all performed
better than the long wire antennas in contests. Antenna modelling shows that
these yagi antennas had broader patterns than vee beams in both the azimuth
and elevation planes, and without sacrificing much in the way of overall
gain. The broader range of take-off angles and directions meant that more
stations could work us on the yagi antennas.
The downside of the yagi antennas was the difficulty of keeping them intact
and up in the air during the gale force winds that regularly swept the ZL6QH
coastal site. In order to address these issues, the yagi antennas were
fixed (not rotatable) and they used a ruggedised design consisting of
aluminium elements mounted on a rope supporting structure.
See http://www.zl6qh.com/cvradiofest.pdf for more information about the
antennas and the ZL6QH story.
73
Brian VK3MI ZL1AZE
_________________
Topband Reflector
|