Antenna Height: Some Considerations.

Fate delivered me to a spectacularly good home site for operating HF radio.  As a result, I've always been fascinated, from the outset, why this site was so good.

Apart from the most mineralised ground imaginable, the site is modestly elevated at 300feet amsl, with sloping ground to a nearby coastline.

What became clear very early on to me was that extremely low 'take-off' angles are available here.  Measurements in the far field show strong radiation slightly below the horizontal.  In other words, the radiation pattern isn't restricted by the pseudo-Brewster angle as in the kind of normal plots you see, but can be considered 'tilted', such that the peak pattern occurs at or below the horizontal, matching the slope of the ground. 

Now, we all strive to place our antennas as high as we can possibly manage.  But is this necessarily an informed way to proceed?

I modelled a dipole with the ARRL HF Terrain Assessment software, and here is the output with a dipole operating at 14MHz, set at 10m:

First of all, it's evident that this location gives very good ground gain from very low angles (peaking at about 6.5dBi), right across to higher elevations.  It's important to note from the profile that the software clearly cannot cope with elevations that are zero, or are less than zero, so the peak gain should almost certainly be to the left of its drawn position if it were to reflect the reality of down-sloping ground.

What happens if we take the dipole down to a superficially-pathetic 2m above ground?

Very surprisingly, the peak gain shoots up to 10 dBi!  Not only that, but the peak lies on (and, with sloping ground, somewhat below) the horizontal.  At elevations above about 3 degrees, the gain is now in negative territory.

Now, ordinarily, to cover all the potential arrival angles of signals, one might say that the 10m-high antenna is clearly the one to opt for.

But, when there is an additional 3.5dBi - a doubling of gain - available at angles considered to be crucial to best DX performance (think verticals at the seaside), then a very low dipole on a hillside clearly has something spectacular to offer.

Situations where the low dipole might be of considerable interest are (a) long path working of the antipodes and (b) long skip DX to, for example, the west coast of the US from the UK, when other stations are unable to access them.

These are all things that the late Les Moxon alluded to in his 'HF Antennas for All Locations' book, now many years ago. He had a very open, enquiring mind, which make his texts, now rather dated in style, nevertheless still very relevant to those who think radio is all about height and power.