The Quad Loop
quad loop yagi for 23cms and above was developed by Mike Walters
G3JVL and later sold as a commercial product by him. The design
is very well documented in the amateur press, including in such
publications as the RSGB VHF/UHF manual, several foreign publications
such as the UHF Compendium also carry the dimensions of designs
that can be assembled quite easily at home.
The Severnside group continues to market a very successful 18
element Yagi type aerial specifically designed for the television
portion of the 1.3GHz band. You can now also extend this aerial
using an extension kit; this will give you an additional 5-6 dB
and will still only have an overall boom length of 2 metres.
Helical Beam Aerial
type of aerial available for 23 cm use is the helical. This comprises
a continuous spirally wound element with a back plate reflector.
This type of aerial is really suited for satellite use, since it
produces circularly polarised signals.
The sense of the polarisation - clockwise or anti-clockwise - depends
on which way the spiral is wound. When used to receive a plane ploarised
signal such as our ATV signals, there is an inherent loss of 3 dB.
This figure must therefore be deducted from any published gain figures,
which for a helical aerial usually refer to circularly polarised
signals. However, this loss is not always as bad as it may seem,
since it often happens that a signal which starts off horizontally
polarised at the transmitter can become twisted during it's signal
path. Hence a plane polarised aerial would suffer some loss, but
the loss exhibited by the helical would remain constant at 3 dB.
A 20 turn helical beam used to be marketed by Sandpiper communications,
and has a claimed gain of 15 dB. For plane polarised signals, this
amounts to 12 dB, which is not a great deal for an aerial 1.5m in
length. Never the less, the bandwidth of these aerials is wide,
and it will cover the whole band with ease. The current cost of
the aerial is about the same as the 23 element Tonna Yagi.
Plate aerials are quite popular with home constructors due to the ease