HFTA In The Real World

Recently Larry, KA1VGM posted a request to the Club reflector, seeking recommendations for an economical Triband antenna. Now the availability of small, rotatable antennas is like the Salad Dressing aisle in your local grocery store…there are a lot of choices. And the ultimate decision will be his.

I suggested that considering his local terrain and its effect on antennas is important also. That’s where HFTA comes in. While this is not intended to be a tutorial, I will briefly describe the steps involved in determining where the antenna should be located…particularly at what height.

HFTA (High Frequency Terrain Analysis) takes several variables into consideration: Local terrain, gain and takeoff angle of an antenna and the arrival angles of signals coming from various parts of the world. The result is the ability to vary the antenna height, in software, and chart the gain and takeoff angles at that height.

The steps taken to get HFTA to provide useful data are as follows:

1) Determine tower Lat/Lon
2) Obtain Elevation Data for that location - Available from a number of government websites
3) Generate a terrain profile every 5 degrees around the tower using MicroDem software
4) Using the terrain profile, an Elevation Statistics File from HFTA and antenna height, generate HFTA charts

So let’s take a look and see what HFTA has to tell us. First we’ll look at the terrain surrounding Larry’s tower.

DEM.JPG

This shows the Digital Elevation Model after loading it into MicroDem and producing the terrain profiles. The circle center indicates the tower location.

 

Let's look at something that is a little more meaningful.

Terrain_EU_AF_SA.jpg
From left to right - 50 degrees (Europe), 110 degrees (Africa) and 180 degrees (South America)

This is a Contester's and DXer's dream. The terrain sloping down away from the tower. Couple this with the elevation, approximately 1270 feet, and the antennas don't have to be that high.

Here's a look to the West...

Terrain_OC_US_JA.jpgLeft to right - 270 degrees (US and Pacific) and 335 degrees (Japan)

Not so much of a "dream location" anymore. We don't want to see the terrain slope up away from the tower. Unless Larry has a big bulldozer, there is not too much he can do about it. There are locations that are a lot worse off. The 270 degree shot is certainly workable.

So what happens when we start putting aluminum in the air? I ran plots on 3 bands...10, 15 and 20, to the major locations in the world. From top to bottom the plots represent Europe, Africa, South America, Oceania (Pacific), the US and Japan.

EU.jpg

Europe - No problems here. The dips at 9 degrees on 20, 6 degrees on 15 and 5 degrees on 10
are probably caused by the "saddleback" about 2500-3000 feet out.

AF.jpg

Africa - Again, not too bad in this direction. Note the peak arrival angles
are lower as indicated by the vertical bars.

SA.jpg

South America - This is a great direction with pretty much all of
the arrival angles covered at any of the 3 heights.

OC.jpg

Oceania (Pacific) - Here we see the effects of the rising terrain towards the West.
We are losing gain at the lower takeoff angles. A possible cure is to go higher.
Unfortunately with a single tribander you risk going too high for certain bands.

US.jpg

United States - Although we are pointing in the same direction as we do for the Pacific,
the signals are coming in at a higher angle. The rising terrain is not as much of an issue.

JA.jpg

Japan - This is a tough shot from this location. The arrival angles are very low
from Japan and Asia. In this case, higher will usually win out.

I have created a pdf of the plots in a larger, easier to read size. HFTA Plots

Doug - K1ZO