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Gain by height !!!


#1

The height of the summit produces some extra gain (db) in the HF bands? Someone knows some study that shows it.

73 de Dani EA5FV.


#2

Hola Dani,

I have been told such things, but I think not really. A 1km height is less than 1% of the F layer height. I doubt that height would make a measurable difference. And the take-off angle (or angle of radiation) for practical antennas on HF bands is well above 20 degrees, often above 60. “reaching to the horizon” doesn’t happen when the majority of the radiation is up at 20+ degrees, nothing much remains at an angle of zero.

In fact people talk about being in “an RF valley” due to a 300m mountain that is 10 km away from them. What angle is arctan(300/10000), it is much much less than 20 degrees. 2?

But what I do believe is that by positioning my station antenna on the side of a mountain, I get a little bit of gain in the desired direction, at right angles to the hillside. When trying to make LP contacts into EU I often position my station on the hill side to favour the long path to EU on 20m. South east in my case. That does seem to improve my LP success.

edit: you asked about studies on this subject, I can’t give you any references.

Andrew vk1da/vk2uh


#3

HF Antennas for all Locations, by the late Les Moxon, G6XN has a discussion about antennas on hillsides, and gain in the direction of the drop-off. As Andrew said this is not related to height though!

David, VK2NU


#4

Hi Dani,
I have to agree with what Andrew says, the difference in height related to the height of the Ionosphere will be minimal - I also know of no study that gives a height vs db gain comparison. The question of gain by using the mountain as a reflector for your antenna is an interesting one and either the Book from Les Moxon that David mentions or the US radio corps book may give some estimations of such gains (but this would be gain by reflection rather than by height).

What I would say though is that what is more important than pure gain is the signal to noise ratio. By going portable and getting away from man-made noise you can lower your noise floor by 2 S-points or more - so looking at received signals that is effectively a 12dB gain (taking one S-point to be 6 dB). So a station that may not be able to be received at your home location can be clearly heard from a summit.

73 Ed.


#5

Hi Andrew and Dani
Your last part of your analysis I agree on. I remember last summer when I selected a spot for my activation slightly on the north side of a mountain. Not so many QSO in the log, I think I got one station from OH. So I started to tear down my antenna and repositioned to the south side of the mountain. Boom, the activity came and I was busy for long time with callers from Europe. So even if you are on the top of a mountain the side could play a role.
Jaan, SM0OEK


#6

Hi,

Are there any studies about radiation angle when the antenna is mounted on a summit peak with steep slopes around, so not flat ground as usually assumed in simulations?

Peter
OE5AUL


#7

Yes there are. There was an ARRL publication and program for working out the optimum height to benefit from ground gain. The program lets you enter the contour information for ground around the antenna and suggests a best height for the antenna. We use it for contesting but the program stops at 50MHz and we have interpolated the results for 2m and above. We use it whenever we contest in the unrestricted section and there is no height limit.

Of course, I cannot find a link to it now I need one!


#8

Generally, you need very expensive NEC software to simulate non-even ground. Probably you can get an idea with basic free EZNEC by slightly tilting the whole antenna and looking at how the radiation lobes change, but it won’t be accurate.

My guess, there will be more lower angle radiation but slightly lower maximum gain.


#9

I know of some studies investigating the optimum height for vhf antennas on a hilltop, using Fresnel zone concepts to work out how high the antennas should be to avoid having the ground create too big an attenuation. These studies were done by Glen VK1XX who is a professional engineer. They were presented to the Gippstech conference in 2016 and possibly 2017. Essentially unless your 2m beam is at least 6m above ground, it is compromised. And this means the 6m beam should be correspondingly higher, 18m.

However these are studies of vhf performance rather than investigations into whether elevation above sea level affects HF performance.


#10

Andy,

It was HFTA by Dean N6BV, I think the last windows version is included in the recent ARRL Antenna Handbook. It models horizontal antennas, single and stacks for your terrain in a direction. Then you optimise your antenna heights to match the averaged VOACAP predicted take off angles for that path. I looked at my home station and concluded higher the better and a single yagi, as I live in a valley.

Description here

I did tinker with it looking at a low dipole and sloping ground as you might encounter on SOTA. My conclusion was to get good low angle ground reflection gain you needed to be right on the edge of a reasonably steep slope. I concluded there was little difference in performance between a low dipole and a vertical with ground losses on a typical summit, so I now use Inverted L for convenience.

73 Gavin
GM0GAV