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.

[jimh]

About two-and-a-half years ago I wrote:

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...

A bit later than I expected, I revisit the article I mentioned above, a research paper presented by Chow Yen Desmond Sim, a Ph.D. candidate at the University of Leicester in 2002 and presently a professor in the Department of Electrical Engineering at Feng China University in Taiwan. (You can find the entire 202-page article on-line as a PDF for your own reading pleasure.)

The paper presents research conducted on the propagation of VHF and UHF signals over seawater. (The then) Mr. Sim collected a great deal of careful measurement of signal levels that propagated over two paths of open and unobstructed ocean between three locations in the Channel Islands of the United Kingdom. As might be expected, his paper contains a great deal of mathematical descriptions and details of his experiments, but otherwise it is quite readable, particularly the introductory portions and the narratives describing the behaviors observed.

The method of testing employed two transmitting locations which directed signals at a third receiving location, with the distances involved as 33-km and 48-km (20.5 and 29.8-miles). The antenna heights were fixed at approximately 15-meters above the mean height of the sea. The tide variation in the area typically averaged about 5-meters, but the lowest tide was only 0.5-meters and the highest tide was 10-meters. As a result, the data collected includes a variation in the antenna heights at both the receive and transmit sites, from about 10-meters (33-feet) above the sea at highest tide to about 20-meters (66-feet) above the sea at lowest tide. This data provides a method to investigate the influence of antenna height on the path loss. Dr. Sim found there was a linear correlation between signal strength and tide height (and thus antenna height). He noted:

5.2.2 Calm sea (Jersey to Guernsey)
The first major effect observed from the sea parameter...[is]...the signal amplitude (in dB) displays approximately linear relationship with the tide height (in metres).... Lower signal amplitude was observed at high tide with a reduction of approximately 1.1 dB per metre increase in tide height...

To reword slightly, when the effective antenna height above the sea was decreased (because the tide height increased) the signal level decreased in a linear manner, with the relationship of a reduction in signal of 1.1 dB for each meter decrease in antenna height above the sea.

The Summary section of the paper repeats these observations:

During times of calm sea state for both, Jersey-Guernsey and Jersey-Alderney showed a linear relationship between the signal amplitude and the tide height although the Jersey-Alderney data showed some fluctuation in the signal across the linear region. The signal amplitude decreases as the tide height increases and this accounts for approximately 1.1dB in signal amplitude reduction for every metre of tide height increase.

In the Conclusion section of the paper these observations are again mentioned:

Chapter 7: Conclusions / recommendations and suggestions for further work
The objective of this thesis was to investigate the current understanding of over-sea VHF/UHF propagation at different path lengths, antenna heights, season and weather conditions...

Measurements from both receiving sites showed a linear relationship between the signal amplitude and the tide height (approximately 1.1 dB in signal amplitude reduction for every metre of tide height increase). This is consistent with the smooth earth diffraction theory obtained from ITU-R P.526-7, which suggested that smooth earth diffraction (especially over sea) is the dominant propagating effect for distance below 40 km [25-miles] between the transmitter and the receiver.

My interpretation of these observations and conclusions: increasing antenna height above the sea for VHF Marine Band communication is an effective method to improve the strength of your transmit signal as received by other ships when the distances are in the range of 20 to 30-miles. Even with antennas that were already mounted substantially above the sea at 66-feet, decreasing antenna height by 3-feet (at both ends of the path) caused a noticeable and consistent 1.1 dB loss in signal.

Note that for the VHF signal Sim used a frequency of 248.375-Mhz. I suspect this frequency was chosen because it would be free from other signals and was available or authorized for this experiment. With a bit of thought one can see that conducting the experiment using a frequency in the VHF Marine Band would not have been particularly wise, as transmissions from other ships could have interfered with the tests, and authorization to transmit would have been likely very difficult to acquire. I believe the ITU allocation in Region 1 for 248.375-MHz is to Fixed-Mobile services. Perhaps a special temporary authority was obtained to use that frequency from the U.K. for the experiment's duration. The methodology was quite extensive, including testing of both vertical and horizontal polarization, with paths involving co-polarization and cross-polarization, and with signals with both amplitude and phase modulation.

[jimh]

Regarding the effect of antenna height on range of signal reception, one can make an inference from the practices of broadcast stations, and in particular broadcast stations in the VHF FM band. In allocation of an effective radiated power (ERP) limitation by the FCC onto stations in the broadcast services, there is a trade-off between antenna height, ERP, and coverage which works in a manner that as the antenna height is raised the effective radiated power allowed will be reduced so that the intended coverage remains constant.

Effective radiated power or ERP is the power delivered to the antenna (allowing for loss in the transmission line) multiplied by the gain of the antenna. Antenna height is measured as being height above average terrain, which is calculated by using the actual terrain heights in the coverage area of the station. If the station were located in extremely flat terrain, then the HAAT would be the same as the antenna height. Also the antenna height is computed as from the center of radiation of the antenna.

In broadcasting there are various categories of station license, based on the anticipated area that will be covered by the station's signal. For each category ("class") of broadcast stations there are limits on effective radiated power (ERP) and antenna height, intended to produced a particular range of signal. The signal level is specified with a basis for the percentage of receivers that will receive the signal and the percentage of time they will be able to get that signal level. In the example below the signal level target is a field strength of 60 dBu (50,50) which means that 50-percent of the time, 50-percent of the receivers in the coverage area will get a signal of 60-dBu.

For an example, we can use the FCC regulations for a Class A 6 kW station. The FCC limits on ERP and antenna height are designed to produce a signal of a particular level at a particular distance, in this case a signal of 60 dBu (50,50) over a radius of 28.3-kilometers. According to FCC calculations, to create such a coverage area will require an ERP of 6 kW and an antenna height of 100-meters. The regulations impose for broadcasters a hard limit on ERP, so there is no increase available in ERP if the antenna is lower than the target HAAT, in this case 100-meters. But if the HAAT exceeds 100-meters the ERP must be reduced so that the same coverage distance is maintained. This is a real-world practical example on how height increases coverage distance. In the case of a Class A station, the coverage area is designated as a 28.3-kilometers (17.6-miles) radius.

The FCC has provided an on-line calculator that computes the reduction in power that must occur if the antenna height is raised above the regulated model HAAT in order to maintain the same signal coverage. See

https://www.fcc.gov/media/radio/fmpower

Using a Class A 6 kW station, we can see the power reduction necessary to maintain coverage as the antenna height is increased. For example, the effect of a doubling of the antenna height to 200-meters results in a reduction in ERP to 1.55-kW from 6-kW, a power reduction of 0.2583 or -5.88dB. So we see a 200-meter antenna height with 1.55-kW ERP produces the same signal strength at a distance of 28.2-kilometers as would occur if the antenna were half as high and the power were 6-kW ERP. In other words, doubling the antenna height improved the signal by 5.88-dB or a factor of almost four (3.88). This relationship is quite close to the simpler rule that doubling antenna height increases received signal four times.