Topband: More on 2-element 160m yagi

[Steve Ireland]

Thanks to everyone on the reflector for their ideas about building a low
2-element horizontally polarised yagi for 160m, with a special thanks to
Joe K3RR and Mike AB5XP for some excellent data.

I thought some might be interested in an update as to where this 'project'
is heading. Everyone ain't going to agree with the stuff below, but this is
what I believe/am going to try.

The problems in a nutshell with this kind of antenna seems to be that with
the elements so low to the ground that there are problems in getting them
to couple properly.  If you use a conventional parasitic array, adjusting a
reflector or director to get maximum front-to-back or forward gain is
almost impossible, as trying to 'predict' what length you need for the
former/latter is almost impossible.

Similarly, with a conventional phased array, with both elements driven
through differing lengths of coaxial of cable, it is similarly hard to
adjust/optimise owing to the unpredictable effects of the ground and
surrounding objects on the antenna. 

What is required is an 2-element antenna where you can adjust the currents
and phase in each of them from the shack via a matching/phasing network,
effectively 'driving' both elements.  Such a technique was invented some
years ago - called critical coupling - by the genius Les Moxon G6XN and
described in his book 'HF Antennas for All Locations'.  

My antenna guru friend Phil VK6APH and I have buit a number of antennas
based on Moxon's idea of critical coupling over the last few years (for 20
and 40m) and it works amazingly well.  It is one of the best kept secrets
in the antenna world, perhaps unfortunately owing to Moxon's book being a
bit harder to understand than some.

We are going to feed the 2-element yagi reflector with a quarter wave of
coaxial cable, followed by a quarter wave network and then a series tuned
circuit to tune the reflector. The reflector will be U-shaped in the
horizontal plane, with its ends pulled in towards the ends of the inverted
vee and finishing in same plane as the inverted vee, but around 1' below
it.  The antenna design has been optimised in ELNEC (and the phasing
network designed) by VK6APH.

One of the nice things about critical coupling is that the characteristic
feed impedance of a driven element is not decreased by the presence of a
reflector (when the array is correctly adjusted). 

According to ELNEC modelling carried out by VK6APH, we should be able to
get around 4dB forward gain, dropping by about 0.5dB if we tune the antenna
for absolute maximum front-to-back (i.e. approaching infinity) .  

Perhaps almost as important is the gain/front-to-back attainable is that
the centre of the major lobe of the 2-element array (at the height erected
at the VK6VZ QTH) is at about 45 degrees above the ground - as compared to
the 90 degrees of the inverted vee dipole currently used here.  There
should be a excellent reduction in atmospheric noise on this antenna, as
compared to the inverted vee. 

To tune the array, we will place a dipole several wavelengths away behind
the array in nearby state forest, feed it with a small battery-powered
transmitter and tune the array reflector for an F/B of at least 3 'S'
points (on the small Tx signal) in the shack. The shack will be equipped
with metered current probes on both the driven element and reflector
feedlines, plus a double beam oscilloscope to compare relative phase. 

 
Vy 73,

Steve, VK6VZ





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