Roof Mounted Antennas

by Ramon Gandia AL7X

Numerous studies made; the best known one from Okumura, who made extensive studies, both theoretical and by measurements, in Tokyo and the countryside, provide the basis for this article.

In general, there is such a thing as "antenna height gain." You need to understand what he is talking about.

In a situation where the antennas are in visual line of sight of each other, you would assume that the signal goes straight and that any extra height would not improve the signal.

This is not the case.

You have to understand the distinction between line of sight, which is basically "range", and "signal strength."

Line of sight on a spherical earth is 1.4x the square root of the height in feet. Thus an antenna at 25 feet will have a line of sight of 7 miles. This is 1.4 times 5, the 5 being the square root of 25 feet.

An antenna at 100 feet will have a line of sight of 14 miles.



However, within the line of sight, there is the issue of signal strength. Two walkie talkies at 5 foot height are within line of sight of each other, talking over a flat field. Say, half a mile. What happens if one or both of them go to a greater height?

I think Colby would say "not at all."



That is not true. Okumura predicts, and confirmed with measurements, that the signal strength increases at about 8 db per octave. An octave is 'doubling the antenna height.'

In the case of the walkie talkies, going from 5 to 10 feet on ONE of the radios gives you 8 db, and raising the other one similarly gives another 8 db. A total of 16 db. This is a dramatic improvement, even though both situations are line of sight.

What is 16 db? It is a 40x increase in power. Thus, the radios at 5 feet would require 200 watts whereas the radios at 10 feet only 5 watts, for the same signal strength.

It is also true that line of sight improves as you go higher, but that is not what I am talking about here. I am talking signal when in both situations the radios are in line of sight.

When an antenna is situated on a tall building, a tower, or a hilltop, what is the "height?"

Look at this image. It shows the bulbous antenna pattern in free space, with no obstructions nearby.

The antenna radiates in a bulbous, donut shaped pattern. If you look at a typical VHF vertical, the radiation patter resembles a number 8 laying on its side. Towards the horizon is the peak power, decreasing as the angle lowers towards the ground. Straight down there is no signal transmitted.

This is why a tower is a very good way to raise an antenna: the tower does not impinge the signal being radiated. Buildings and hilltops, on the other hand, intercept the signal in the downwards direction.

This is the antenna mounted on a tower, notice that none of the transmitted energy is intercepted by the tower.

Understand that VHF signal is not one straight line, but that due to athmospheric diffraction, scattering and rolling terrain, a lot of the signal that arrives at the receiver is not the energy being sent directly to it. It is not a flashlight beam.

This is an antenna mounted low on a roof. The shaded area is the energy being intercepted --and effectively lost-- by the roof.

If the antenna is situated on a flat top roof building, and the antenna is at a low height above that roof, then a lot of the signal is intercepted by the roof. Ditto for a hilltop, unless it is a very steep one indeed.

In this picture, it looks like 40% or more of the signal is intercepted and lost.

In the case of a small roof, or a peaked one, the intercepted signal is small. In a large roof, say the New Hospital,   the higher elevation above ground would give a farther range but not neccesarily a stronger signal.

Let's parse that statement a bit. Let us say that we have two identical antennas mounted on twenty foot pipes. One antenna is mounted on the roof, and the other is staked out on the ground. What signals do we have?

Within the line of sight range of the Lower antenna, about 12 miles or so (to a typical, 5 ft high HT or mobile), the signal strength would be about the same. Once the distance goes past the line of sight of the lower antenna, then indeed the roof mounted antenna is king.

What if we compare a 50 ft tower mounted antenna on the ground, vs a 20 ft mast mounted antenna on a 70 ft roof?

In that case the range of the ground mounted antenna is about 13 miles, and the roof antenna (total height 90 ft) would have a line of sight range of 17 miles. But within the 13 mile radius, the tower mounted antenna would have a stronger signal, by a significant margin, than the roof mounted one.

Keeping things in perspective

In trying to figure out how a repeater is going to perform, we need to look at all the parameters and the expected performance.

In most cases, we are talking about servicing HT talkies within the range of the repeater.

For Nome, Alaska, a town that is only about 2 miles end to end, just about any repeater antenna will be "line of sight" to the HT. Thus, we do not worry about distant rural coverage, but the signal and service provided to the HT in town.

In this case, the antenna that is mounted higher than its supporting structure will the winner. Clearly a 50 foot tower, shooting above sorrounding buildings will have a stronger signal than a New Hospital roof mounted antenna on top of a 20 ft pipe support.

I concede, of course, that the best scenario all around would be to have that 50 ft tower on top of the New Hospital roof. But I sincerely doubt that the administration would approve such a monster. For one, if you look at the building, you will see numerous air traffic red obstruction lights. That should be an indication that the building is, in fact, an air navigation obstruction already. [I disagree, I think the building is low enough not to require the lights, but they have put them in for liability reasons, safety concerns beyond the rules, or they may have helicopters flying into a pad.]

A 50 ft tower on top of that roof, plus the antenna itself, would be a 60 ft obstruction. FAA/FCC regulations state that on any building, without further ado, you can have a 20 ft antenna structure. That 20 ft is measured from the basic structure, not from "appurtenances" such as chimneys and vents, or other legal antennas. Thus, if the antenna proposed is a Diamond X300A, a 10.5 ft tall whip, then the supporting pipe would be only 9.5 ft above the roof before the FAA/FCC rules, or the perception of those rules, would create an objection.

An antenna mounted at only 9.5 ft above the roof, with perhaps some vents and other stuff around it, is not going to work very well at all. This being the reason that I am pessimistic about the New Hospital.

Consider the SPARC antenna on the Church Cross. The steeple is tall and skinny, like a tower. The Cross itself is part of the structure. The antenna is a mobile whip, only 3 ft long. This is clearly well within FAA/FCC regs ( 3 ft vs 20 ft above the structure). Yet, at its height of 80 feet, it sure puts out a signal. It can be heard a long ways out the Teller road because it has an 80 ft effective height. The SPARC receiving antenna, on the other hand, is on a 20 ft ground mounted pipe. It is questionable what the effective height is because there are several houses and buildings that can intercept the signal.

Indeed, the SPARC transmitter can be heard a long way from Nome, but it is impossible to hit the repeater from a mobile until the range has shortened. This is an unbalanced setup, and one of the pitfalls of a split site. It would be better, from a design standpoint, to put the receive antenna on the Cross, and use higher power at the low mounted antenna. But there are other considerations, such as bleedovers etc from transmitters near the Cross, so it is difficult to say what would give better performance.

For RAM, I am urging the club NOT to invest in "Low Ball" antennas. A nice, lightweight antenna that can be mounted high up will more than make up the presumed gain of a larger, but lower mounted antenna.

Last, but not least, consider a hilltop antenna. Unless the hill or mountain is really steep, a tower is needed on top of the hill as well or the performance will suffer. Why do you think TV stations, and commercial repeaters mounted on hilltops and mountains always have tall towers to boot? Wouldnt just mounting the TV broadcasting antenna right on the roof work as well? Of course not, and that is why they put up tall towers on top of tall mountains. Not to get the last mile, but to increase the signal strength. Work out the line of sight difference between an antenna at 4000 feet, vs one at 4080 feet. The difference in line-of-sight range is minimal, but the signal strength could well be dramatically better with that 80 ft tower.

How steep? Well, imagine yourself at the antenna. Look down at 30 - 45 degree angle to the ground. You should be looking beyond the mountain at this angle. If not, the mountain is not steep enough to qualify as a "steep" mountain and you need more tower height. There is a tradeoff here between hilltops that are easy to work on, land a helicopter or perch a building, but need a good tower, versus hilltops that require mountain climbing, long hikes, or slinging loads from a helicopter, but only need a modest supporting tower.

Towers are important on mountaintops.

Mechanically, it is very hard to make an antenna as strong as a tower. A diamond X510 whip is 17 ft tall. The wind whips it around. It is not vertical in any significant wind (go see Carls, KL0FP's he has an X510 Diamond). An antenna that leans like that in the wind not only beams a lot of its signal into the ground or up into the sky, but is also at great risk of breaking.

Unless there is a height restriction on a mountaintop, I would invest my money and effort in a taller mast rather than a long antenna. Fortunately, there seems to be a convergence of factors here; ie, both heights are hard to attain: the inconvenience of tower height and the fragility of tall antennas, so in the end, both a big and small antenna usually end up at the same height.