Topband: Polarisation and Power Coupling

[Steve Ireland]

GM3POI replied:

>Steve VK6VZ Said
>"That being said, for 95% of the time my inverted vee dipole at 90'
>outperforms my Marconi-T with a 20m vertical section over a full density
>1/8 wave earth screen - to the amazement/disbelief of many people on this
>reflector.  IT WORKS FOR ME (but not necessarily for you)."
> Perhaps that should read that my Vertical with its Compromise ground system
>does not work as well as an Inv Vee.  My results here with a short Vertical
>51ft compared with a  Low (80ft) Inv Vee show the Inv Vee to be down 6-10db
>on all signals other than within 200 Miles. However that short Vertical has
>a near perfect  earth system under it with the shortest radials being 130ft
>and most being over 210ft. I now keep the Inv Vee as a fall back Antenna.

G'day

Clive GM3POI is right when he says my 1/8 wave ground screen is a
compromise, but is actually far less of one than it appears to be and the
reasons for my lack of success with vertical polarisation owe more to
losses in the far field of my Marconi-T (which a ground screen can't reach
into) and my relatively low latitude.

Losses that are due to far field earth conductivity and latitude are beyond
our control.

Work by people such as Eric N7CL has shown that if the radius of a
full-density ground screen is reduced from a quarter to an eighth of a
wavelength, the smaller ground screen will only result in a 0.3dB reduction
in signals produced by the vertical antenna mounted above it.  The
theoretical impedance of my Marconi-T antenna is about 22 ohms - and the
measured impedance of the antenna is about the same - given a perfect match
with a 22.5 ohm tap on a W2FMI unun.  The antenna has around an 80KHz 2:1
SWR bandwidth, so my near field losses are VERY low and the overall 'Q' of
the antenna system is very high.

However, I live 30km away from the sea, on rocky/gravel soil, and thus my
far field losses very significant, owing to poor ground conductivity.  In
contrast, Clive GM3POI lives on an island, with his antennas very close to
the sea - with what should be MUCH lower far field losses.  

The other important reason as to why it is hard to make a vertical antenna
work well for me - and other people at a similar latitude - is the
often-overlooked phenomena of power couping (see ON4UN's Low Band DXing
book).  For those living in Europe and North America, vertical polarisation
is usually the way to go because of their relatively high (geomagnetic)
latitude, where as for those continents closer to the equator (such as
large parts of Australia, Asia and Africa) horizontal polarisation is more
suitable.

What happens is that vertical polarisation used at high latitudes couples
best into the ionisphere, whereas at low latitudes horizontal polarisation
couples in better.  At the latitude I live at, vertical polarisation should
JUST have the edge over horizontal, but because of the very poor earth
conductivity where I live the coin falls the other way and horizontal
polarisation works better.

If I lived right by the sea, I have a feeling that vertical polarisation
would probably be better for me than horizontal. But I don't, so it
(mostly) ain't.

Vy 73,

Steve, VK6VZ