Topband: Switching/tuning on 2-element delta loop array

[Steve Ireland]

N6WG said: 
>You didn't mention how much spacing you have between your two loops.  If you
>have enough, I suggest you try phasing them.


VK6VZ replied: 

Hi Bob

The loops are spaced about 28 metres at the base and the tops are very
close together (about half a metre).

My switching/tuning unit is based on one by G3LNP/A45ZZ (from RadCom
magazine in December 1993).  I use a half wave of coax to each loop (corner
fed) and either feed one loop as an omni (pattern 1), feed the two loops
180 degrees out of phase as a figure-8 with bi-directional gain and pure
vertical polarisation (pattern 2), firing north as a cardioid (pattern 3),
or firing south as a cardioid (pattern 4).

You could look at the antenna as two pairs of quarter wave-ish verticals
(with each vertical in each pair spaced about a quarter wave apart), and
with each vertical pair spaced about 1/8 wave behind the other - in other
words, close-spaced phased verticals...    

My original LC network (switched into the designated reflector) consisted
of 42 turns of 14 gauge wire on a 7.5mm former, in series with about a
900pf variable capacitor.  This gave me a large frequency swing, making the
reflector tuning quite sharp - which is really useful when first setting
the antenna up.  After setting the antenna up, I usually change to a lower
Q tuned circuit, as this means the tuning capacitor is less likely to arc
over when I transmit.

The series LC network is switched into the designated reflector and simply
tuned for maximum coupling between the loops (I tune for maximum SWR and
then check this conforms to maximum F/B by placing a small (0.5W)
transmitter about one wavelength behind the array, coupled to a 4
metre-long antenna of the same polarisation as the loops).

Been playing about with loops like this for about five years now, on
various bands - great fun!!!

Vy 73

Steve, VK6VZ