The Effect of Horizontal Yagi Height Above Ground on 144MHz.
Have you ever wondered why some stations come booming in every day on meteor scatter but others, who your neighbours on 2m can work with ease, are very difficult or impossible to work? Or why some stations seem to have super QTHs and can work extraordinary DX on 2m?
The graphics below were created using MMANA modelling a 9 element Tonna over "real" ground. I used Photoshop to create the animation. The pictures should change every 2 seconds.
Note how the antenna lobe is broken up because of the Fresnel Zone effect. The reflection from the ground in front of the antenna adds and cancels with the radiation from the antenna to produce a comb shaped pattern. Stations using fixed horizontal antennas (without any elevation) for EME know this effect well. As The Moon rises, signals appear, then dissapear and then come back again as its elevation angle moves up through the antenna pattern.
VHF Yagi over Flat Ground Raised by 1m Steps Between 8m and 14m AGL
The green dot indicates a takeoff elevation of 2 degrees = on meteor scatter a distance of about 1800km
The red dot indicates a takeoff angle of 4 degrees = on meteor scatter a distance of about 1500km
The red dot indicates a takeoff angle of 6 degrees = on meteor scatter a distance of about 1200km
The same effect applies of course to reflections from Sporadic E (Es) patches on 2m.
For super long meteor scatter, Es or tropo DX a station needs low angle radiation. So for a site with flat ground extending out to the target, a higher antenna is always better.
For meteor scatter or Es QSOs, at any given antenna height, some stations will be nicely in a main lobe and so easy to work, while others will be in a null and so will be significantly weaker or perhaps not detectable at all.
So for some QSOs a lower antenna may work much better than a high one. Don't forget this applies to the antennas at both ends of the QSO!
N.B.this is a computer model. The actual results from any QTH will vary depending on the local terrain for several km around the site. But the principle still applies.
The Effect of Sloping Ground
The late Les Moxon G6XN showed in his book HF Antennas for All Locations that horizontal antennas over ground sloping towards the target can be used to produce very low angle radiation which of course is exactly what we want for super DX. It works on VHF too.
This graphic below illustrates a very lucky station who has a 3 degree slope extending out towards the target direction. The red arrow represents horizontal radiation towards the horizon. The green line is the ground slope. N.B. the Fresnel Zone extends out up to several kilometres. A dream VHF QTH then might not be right on the coast (flat ground towards the target) but rather a few kilometres inland but with a gentle unobstructed slope extending out towards the sea.
Same VHF Yagi over Sloping Ground.
Note that this station will be using the second lobe for all but the most distant, 2000km+ meteor scatter QSOs.
For day to day troposcatter QSOs a low takeoff angle is important. Bob Atkins, KA1GT says, "Take-off angle has a strong influence on path loss. For a given path, a 1-degree increase in take-off angle will result in a decrease of around 10 dB (depending on the scattering model) in the received signal strength."
And if the antenna is raised too high the main lobe has a negative takeoff angle into the ground! Unlike the flat ground example above, on sloping ground the antenna can be too high!
The optimum height for this station's antenna for super DX is about 9m above local ground. But the mast may need to be higher to get the second lobe low enough for some shorter meteor scatter contacts.