Topband: 'Uzi' 2-element delta loop array performance

[Steve Ireland]

G'day

I have received a number of inquiries about the 'Uzi' 2-element delta loop
array that I currently use (three-quarter-sized loops, linear loaded in the
bottom horizontal portion of each loop by folding the remaining quarter
wave, with folds one metre apart).  The tops of the two loops are supported
from a fibre-glass pole out of the top of my tower, at about 27 to 28
metres high and only about a metre apart, while the bases of the loops are
about 32 metres apart.

For those who are interested in the antenna, Jim K1PX has kindly prepared a
EZNEC file for me of the antenna in its most useful configuration - i.e.
with one loop configured as a parasitic reflector and the other as a driven
element - which I would be pleased to email anyone who is interested.

The antenna pattern, when in the main configuration (as per the EZNEC file
- driven element, parasitic reflector) looks far poorer than it is in
practice at VK6VZ, with a relative poor front-to-back ratio.  This is
because the antenna appears over-coupled, owing to the tops of the loops
being too close together.

In practice, at my QTH, the overcoupling seems far less than it looks on
EZNEC, possibly because all the stuff between the elements (like the VK6VZ
house, etc!) seem to lessen the coupling between the two elements, as you
would expect.  The front-to-back ratio on ground wave signals and
intercontinental signals that I get is 18dB or more.

If you can space the tops of the loops about 12 metres or so (see ON4UN's
book - page 12-7) this brings down the overcoupling.  I should add that any
overcoupling shouldn't have an effect on the forward gain of the array.

The antenna is predominantly vertically polarised and sits above an
eighth-wave radius ground screen.  At a QTH which doesn't usually suit
vertical antennas, in its major direction (north) after using it for six
months, I believe it is definitely the best DX antenna used at this QTH.

It should be noted that the antenna has quite a pronounced 'squint' - the
VK6VZ antenna is fed at the eastern corner and tends to fire about 45
degrees west of where you think it would - owing to the corner feed-point.  

The pattern/gain/front-to-back of the antenna when fed as a horizontally
array looks poor in EZNEC and this has been born out in practice - when I
can tune the back element as a reflector, it gives no noticeable
gain/front-to-back on DX signals, although it makes one to two 'S' points
difference on received signals from VK6HD 600km away!

In its vertical polarised configuration, the antenna shows consistently
shows an excellent front-to-back (on DX) of five 'S' points or more,
drastically reducing noise picked up by the antenna, particularly from the
rear.  Signals that are unreadable/lost in the noise on one loop (and are
in the direction of fire) are Q5 copy.

This phenomena makes actual gain difficult to measure, but I estimate one
to two 'S' points in its favoured direction.  

My only problem with the 'Uzi' is that its directivity/cardioid pattern is
more of a nuisance than I expected - seem to get out better to the USA than
before, but not so good into Europe, owing to the orientation of the
antenna.  It also makes having a good 80 and 40m antenna in my half acre
plot virtually impossible.

If you have a desired part of the world where you want a simple way to
maximise your signal the antenna, the Uzi is great.   However, as others
have found in past, it is very easy to have too much directivity on 160m
(if you only have one antenna!) and I may be forced to go back to my
inverted vee dipole at 27 metres in order to get around this problem (and
get back on 80 and 40m). 

Vy 73

Steve, VK6VZ