Question About Radio Waves and Interaction With Water

Hello sawmilleng

The groundwater should not downgrade your transmission/reception at those frequencies, unless it rises high enough to cover the transceivers and/or the antennae = A Flood

It was found here some 40 years ago, that at certain frequencies, the rising sap during the Spring season did affect transmission/reception from/to radio repeaters which we had based on hilltop sites.

When I did a series of practical tests at that time, after it was realized that the radiated power at distances had dropped off so severely, the Monterey Pine (called Pinus Radiata in New Zealand) tree sap rise during the Springtime was found to cause major problems in the 90MHz through 450MHz frequency range.

Other trees were also found to cause losses at different frequency ranges.

Our solution was to raise the antenna masts some 20 metres, which gained some years breathing space, until the trees arrived at milling stage, normally in New Zealand around 30 - 40 years, and then it was ensured that replacement trees were not planted in the immediate line-of-sight from/to the repeater stations.

So, you should have no bother with your repeaters, but remember that reception is mostly line-of-sight at your frequencies too.

Ensure that you have good filters, so the repeater transmitter does not interfere with the repeater receiver - we used tunable silver-plated coaxial filters.

Field-Strength Tests - A Good Plan:

It is a good idea to have a field-strength test of your system, at various locations in the district, before handing the system over.

We used to do this by setting the transmitter of the repeater station, so that the transmitter remained transmitting for 5 seconds (a "tail") after the initial receiver lost the incoming signal from the mobile transmitter.

The transceiver in the testing vehicle (We used a Land-Rover because some terrain here was difficult) thus triggered the receiver of the repeater, which passed that signal through to the repeater transmitter (On a different frequency) and that repeater transmitter signal was then measured using a tuned field-strength meter in the mobile vehicle.

Signal positions were marked with a number on a Topographical map, and note made of the actual strength was made.

This was all done some 40+ years ago, before New Zealand got its first Computer, and later the field strengths were joined on the Master Map, same strengths were joined by lines, (similar to Weather Isobars), then that Master Map was copied off and a copy placed into each vehicle and installation.

So you have an explanation, along with some local (NZ) history.

Trust you have been assisted.

Kind Regards....

Hello again sawmilleng

<"....1. You implied that the propagation is unaffected by the conductivity of the ground--so that not much effects the radio signal as long as a clear line of sight is maintained between transmitter and receiver? The "ground plane" is kind of immaterial? (ignoring fresnel zones for the moment...)....">

The conductivity of the ground does have an effect, but more in the lower frequency bands up to around 30MHz.

At the lower frequencies, the ground conductivity has a major effect on the "vertical angle of radiation", and getting that angle correct is still important to "Short Wave Broadcasters", so that their transmission uses the correct angle at full radiated power to optimise reception in the intended area of the world.

In extremely poor ground conductivity locations, it is often necessary to bury an Earth Mat, which may be heavy copper conductors, welded into 2 foot squares, and extending for 2 wavelengths of the transmitted frequency away from the antenna structure.

At the frequency ranges you intend using, the local ground conductivity should have a minimal effect, because your transmissions are line-of-sight, local UHF/SHF, not requiring multiple bounces between the ground and the Heaviside Layer to have proper signal strength at the receiving point.

Note: The Earth is an oblate spheroid, and the Heaviside Layer reflects back radio signals - This enables long distance Short Wave Radio reception which uses multiple bounces of the signal path between the Heaviside Layer and the ground or seawater eventually arriving at the intended destination.

Kind Regards....