Effect of Antenna Height on Signal Strength

[jimh]

I have long been interested in finding some good research or experimental data on the effect of antenna height on received signal strength, particularly for frequencies in the VHF region (or even better, in the VHF Marine Band) for signals propagating over open water. Unfortunately, I have not found any original research on precisely those conditions, but I did come across a rather interesting article on the topic of antennas in general for use with FM Broadcast band transmission and reception. FM Broadcast is in the 88 to 108-MHz band, a bit lower than the VHF Marine Band (156 to 162-Mz), but the propagation characteristics and general theories of signal paths and effects should be reasonably close.

The article to which I refer is from an older edition of the NAB Engineering Handbook (National Association of Broadcasters), in a chapter about "Antennas for FM Broadcasting." The article mostly discusses transmitting antennas, but it also remarks on receiving antennas, particularly their elevation. Here is an excerpt from a section "Calculating Service Contours", which discusses the strength of received signals.

The signal received in an automobile whip antenna at 5 ft. (1.5 m) above ground is unfortunately much lower than one that may be received at 30 ft. (9.14 m). Of course, it is impossible to have to have receiving mobile antennas at the higher height. But in very smooth terrain, there is nearly a direct relationship [of signal strength] with height, and in the example, the signal at the standard dipole measurement height [i.e. 30-feet] will be nearly 6 times stronger (7.78 dB). However, in areas where there is terrain roughness or in mountainous areas, there is even greater increases with the same height increase. [16]

16-Measurement of Service Area for Television Broadcasting, Robert S. Kirby, National Bureau of Standards, Boulder Colorado, IRE Transaction on Broadcast Transmission Systems, February 1957, pages 23 to 30.

It seems reasonable to conclude that calm seawater is a good candidate for the description of "smooth terrain."

The salient information here is that received signal strength is very proportional to receiving antenna height, even if the station transmitting has its antenna on a very tall tower or at some very high elevation as would be expected in the case of receiving a signal from a commercial FM Broadcast station (or the Coast Guard). Although it may be impossible to get a receiving antenna on your automobile to be 30-feet in the air, it is not impossible on a boat. And if you can't get to 30-feet high, then you can get to more than 5-feet high. Since received signal strength is just about directly proportional to height, doubling the height (say to 10-feet from 5-feet) should tend to double the strength of received signals from transmitters that are not closeby.

The information in this older article tends to affirm what has already been deduced: increasing antenna height, particularly when starting from very low antenna height, produces immediate improvements in radio reception and thus increases range of communication.

[jimh]

I have reviewed and republished a new version of an article I wrote a few years ago about two interesting models for predicting propagation over water and the influence of antenna height on the received signal strength. The revised article is now available at

VHF Propagation Over Water
http://continuouswave.com/whaler/reference/VHF_RadioPropagationOverWater.html

I'll use this thread to answer any follow-up questions on this topic.

[Dr T]

Nice article, and a trip down memory lane.

Back in the 80's and 90's when I was working on systems, we needed to worry about how close we would have to get in order to see something (context intentionally left blank).

Since I was trained as a pure mathematician, I relied on our physicists and antenna engineers to get the models right. Then I would have to build systems models to determine where we needed to be.

I thought it was pretty neat the first time I saw the Longley Rice model (see, e.g., http://lrcov.crc.ca/main/) integrated with a digital topo map. On the other hand, we did not have the surface driven problems and short antennas that you are dealing with on small boats.

Thanks for the article.

BTW, if you don't already have one, Skolnik's "Radar Handbook" is fascinating light reading on RF propagation problems. However, parts of it can be a cure for insomnia...

[jimh]

Recently I came across a published dissertation that studied the propagation of VHF signals over water in a very scientific and systematic way, looking at signals coming across the English Channel. I tucked it away for further study on some cold winter evening, with comprehension augmented with a generous pour of Irish whiskey. Any new insights will be revealed in a summary article, but not until next winter and after the boat is laid up.

And, yes, the soporific effects of too much analytical mathematics in the study of radio wave propagation is a hazard one must be wary for--particularly for me, whose mathematics education barely survived linear algebra.

I used that on-line calculator you mention for the Longley-Rice propagation model a few years ago in an article about radio range in the vicinity of Lake Superior's Isle Royale. See

http://continuouswave.com/ubb/Forum6/HTML/003322.html

[Dr T]

After doing the stuff for several years in the days of my youth, I decided that once you considered every filter and amplifier as a noise-contributing operation in an inner product space, then simple linear algebra is about all you need. Even the power spectral density of a signal is just the norm of a basis projection All the equations become much simpler when you dig yourself out of the details of the calculations and just think about what they mean. Perhaps I should join you in a glass of pinot noir--at least in spirit. [Note: Feynman's "Lectures on Physics" are great in the focus on intuition, not equations. Great books.]

My basic feeling on antenna height is that the higher you are, the farther you can see. With the right receiver (think real low noise figure), you can see a 1 watt signal in VHF from 400 nm when your antenna is at 35000 feet. I have often wondered if the CG has ever considered using a tethered lighter than air craft as a relay point over some of the more remote sites in the Great Lakes region. The three main arguments against would tend to be weather, budget, and other air traffic.

But it would be interesting to see the study.

[jimh]

I amended my article to make clearer the influence of antenna height on signal strength as described in the FCC's chart model. Using data from the chart as an example, I show how signal strength increases by 6-dB when antenna height is doubled in the FCC's chart data.

[jimh]

I just came across another excellent reference for the path loss of VHF signals over open water, the ITU-R P.1546-5, "Method for point-to-area predictions for terrestrial services in the frequency range 30 MHz to 3 000 MHz." The recommendation describes it purpose:

This Recommendation describes a method for point-to-area radio propagation predictions for terrestrial services in the frequency range 30 MHz to 3 000 MHz. It is intended for use on tropospheric radio circuits over land paths, sea paths and/or mixed land-sea paths up to 1 000 km length for effective transmitting antenna heights less than 3 000 m. The method is based on interpolation and extrapolation from empirically derived field-strength curves as functions of distance, antenna height, frequency and percentage time. The calculation procedure also includes corrections to the results obtained from this interpolation/extrapolation to account for terrain clearance and terminal clutter obstructions.

I have further amended my original article to introduce a chart taken from the recommendation. I extract several conclusions from the data in the chart. In general, the chart shows that antenna height has a very strong effect on received signal strength and that signal loss with distance is always much greater than predicted for free-space paths. The recommendation can be downloaded at no cost from the ITU website. See

https://www.itu.int/rec/R-REC-P.1546/recommendation.asp?lang=en&parent=R-REC-P.1546-5-201309-I

And, once again, the data show that doubling antenna height increases the signal strength by a factor of four ( 6 dB).

[jimh]

In addition to all three models of radio propagation showing a significant benefit from increasing antenna height, they also show that signal strength decreases with distance at a much faster rate that is often suggested when the free space model is used. In free space, signal strength decreases with the inverse square law, that is, a doubling of the distance reduces the signal by a factor of 1/4. In these models based on real world observations, signal strength decreases with distance at a much faster rate. When the distance is doubled, the signal decrease is by a ratio of 1/16 or higher, perhaps as much as 1/32 with each doubling of distance.

[jimh]

I have revised the original article to look at a fourth model, used by the NTIA, the National Telecommunications and Information Agency. Their model has plots for path loss at 157-MHz for two stations with antennas of 3-meters and 10-meters above ground, in three combinations, modeled over seawater.