Signal Path affects and the natural environment

TECHNICAl BULLETIN

Signal Paths are almost always different.   Ever noticed how you can obtain one result in one location and then immediately move the digital television antenna and get a totally different result.   Why – obviously the path the signal has travelled is very different and you are able to see the consequence of that in the result obtained.

What are the key things to know about signal paths and why is this important to know.  As a professional television antenna technician it is valuable to know how signal is effected and what you can do about it.   This is what I know from years of observation and experience.

The higher the frequency the more predictable the signal.   The higher frequencies are impacted less by environmental manmade structures.  UHF signals penetrate and pass through with less impact on the integrity of the signal.  BER’s remain useable.    As you drop down the spectrum say into VHF Band 3 frequencies you do start to see consequences in your BER readings.   Power is often not impacted in a city environment as the signals are bounced from one object to another often with an amplification type of impact which often fools technicians because they expect good results and it doesn’t happen.   In another situation the BER’s will remain useable – signal path again is the answer as to why this happens.    It is important to start to understand the signal path as it can save many hours in testing throughout your career.  You are more component and the customer notices.

Some time back I visited a site where our technicians were having difficulty.  They were relative new to problem reception situations.    I scrambled onto the roof and the only thing I did was look at the signal path.   They had placed the antenna in a structurally good location but the path was poor which was immediately obvious to me.   I had them move the antenna some 4M along the ridge of the roof and at about 3M in height and bingo all good and problem solved.  It took me less than 5 minutes to identify the location.  What did I see when I scrambled onto the roof.   The antenna pointing into pine trees on the horizon which was only about 2 kilometres away due to the fact we were in a small valley.   Pine trees are a real barrier.   So we learn that pine trees are something to avoid.  They resonate and capture the signal rather than let it pass.  In energy terms they form a ridge so you have to get away from them, under them or over them.  My observation is they are even a bigger barrier in wet weather.

My father was in the Royal Australian Air force during the WW2.  He told me a story when I was a teenager playing with crystal sets.   His survival kit should he have to bail out was a 4” (100mm) nail and a length of single core copper wire.    The idea was to hammer the nail into a tree (with something) and send out a SOS.    All sounded pretty strange to me until when surveying during the upgrading of country services in outback NSW in the 80’s I observed a new phenomenon relating to UHF signals and believe it applies to VHF too but with less impact.    Signals travelling across granite ground as opposed to sandy ground will always do better.   The iron in the ground acts like amplification and keeps the signal going while the sand slows the signal down thus increasing the losses across the ground.  This was very clear from the many 1000’s of installations completed by my first company Skybeam Antennas when working in the sandy belt of Melbourne.   Signal levels were always lower and the lower the frequency the worse or more loss of signal resulted.   Channel 2 in those days was often quite difficult.   I have no real proof beyond observation but I believe the speed slowed to such a degree the noise in the signal became the dominant component.   This did not necessarily read as a lower DB.   Most meters cannot distinguish between a raw or wanted signal as opposed to a synthesised signal generated by noise in the signal path or circuit.

Going back to trees and their impact on signal.  Apart from pine trees other trees will impact on signal conditions but the ground they are growing in is more of a concern than the tree itself.   Clay has iron, magnesium, alkali metals, alkaline earths which all assist signal by contributing to the electromagnetic energy.   This in turn will affect the sap in the tree.     Further evidence of this was gained on the Murray River with the big red gums sitting in granite type soils.    I had a customer who had been receiving good television signals for years as a result of a DIY kit we put together for him and then suddenly he lost his pictures.    Lots of questioning resulted before I discovered the why.   The removal of rows of red gums on the side of the river.  The side the signal came from.   The rows of red gums where a natural amplifier and once gone so went the signal.

Getting back to the Royal Australian Air force they knew this and hoped the airman would ditch beside big trees in clay or granite type soils thus allowing the tree to act as the transmitter.

If involved in problem reception it is important to know something of the natural environment and how television signal travels.   Trees dependent upon the soil can be used to advantage.   treesTrees in sand type conditions become a deterrent.  The inertia of sands generally means lack of response and due to their hardness can reflect signal rather than resonate and amplifier.

If working in rural areas with some distance to transmitters a knowledge of the signal path is critical to getting results.   Decisions have to be made whether extensive testing is worth it and by understanding the environment you can make better decisions.   Next we will tackle the outcrops, hills and surfaces and environment of our cities.

Sally Garden

Founder