Effect of Antenna Height on Signal Strength

Electrical and electronic topics for small boats
jimh
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Location: Michigan, Lower Peninsula
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Effect of Antenna Height on Signal Strength

Postby jimh » Fri Jan 13, 2017 12:45 pm

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
Posts: 2531
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Re: Effect of Antenna Height on Signal Strength

Postby jimh » Sun Mar 12, 2017 10:53 am

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.


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