Topband: Small Tx/Rx antennas with front-to-back and front-to-size

[Steve Ireland]

G'day all

There has been quite a lot written on the reflector recently about the
advantages of having good front-to-back and front-to-side rejection on
Beverages or a phased vertical system - mainly by stations such as W8JI and
K9DX, who have spent long hours building very large antenna systems that
deliver amazing performance in these areas.

Whilst simpler systems such two phased or parasitic delta loops or inverted
Ls cannot offer the front-to-back and front-to-side noise rejection of the
four-element+ phased vertical arrays and staggered Beverages used by these
stations, they do offer do offer substantially improved noise rejection
ability to a single element antenna, as well as a small amount of gain.

Earlier this year I moved from a Marconi-T antenna (with a 20 metre/66'
vertical section) over an eighth wave ground screen (35 x 66' radials) to
two linear loaded (at the voltage ends) three-quarter sized delta loops.
The footprint required by the two vertically polarised antenna systems was
very similar  - the delta loop array requiring a bit more land of
approximately 46 metres (~150') by 32 metres (~100') in size.

The (predominantly) vertically polarised delta loop array lies over the
same ground screen as the Marconi-T did and is almost as noisy when either
of its loops are used as single elements.  However, phase them together as
a figure-8 array (no f/b but some 15 - 20dB f/s) or as a conventional
parasitic 2-element director/reflector array (even better), then the noise
REALLY drops away.

In my favoured DX direction, the noise levels is typically S7/8 on a single
loop and S1/S2 on the parasitic configuration.

Until I had this simple array, I only a vague idea of where my 'noise'
(atmospheric, domestic electrical) was coming from - and no way of
rejecting any of it, as my semi-rural half acre block is too small to run a
decent Beverage on (except during contests when a neighbour allows me to
run one onto their land).

As an antenna, I found a single vertically polarised delta loop at VK6VZ
can be pretty useless for Rx, as (like its Marconi predecessor) it is just
too noisy.  However, couple up another loop to it and I have not only got a
small amount of gain in the main DX direction from here, but lots of noise
rejection in the others (in particular with the cardioid type pattern
offered by the conventional radiator/reflector parasitic configuration).

If you can put up a 2-element loop or inverted-L array, do give it a try -
a half acre block is usually just about enough room.  The extra space
needed in comparison to a single element antenna is small, but the benefits
are potentially really large.

Vy 73

Steve, VK6VZ