Helpful Colin

Radio Wave Power And How It Gets Reduced By Distance

Introduction

Some people are very concerned that radio wave power coming from radio transmitters’ antennas will harm people. In particular people are troubled by mobile phone masts causing long term health problems when they are near to their homes.

I just want to make it clear how much a transmitter’s power diminishes as it spreads out from a transmitter. I’m sure some feel it must follow a simple linear law such as:

double the distance and get half the power,
triple the distance and get a third of the power,
quadruple the distance and get a quarter of the power.

When in fact it follows an inverse square law which is nonlinear and gives this type of result:

double the distance and get a quarter of the power,
triple the distance and get a ninth of the power,
quadruple the distance and get a sixteenth of the power.

This means the power (and damaging energy) is much lower than might be expected at any particular distance. Even within a short distance the power can drop considerably.

Keeping Birds Off TV Aerials

Introduction

In my previous post “Freeview Aerial Direction” you may have noticed something unusual about my Yagi TV antenna/aerial. Yes, amongst the array of directors in front of the dipole and on the reflector at the back there are some Zip Ties (Cable Ties – often used for anchoring a cable to something else). In this case their purpose is to prevent birds from landing on my aerial. The problem I have is that when they take off they tend to deposit their droppings on my solar panels and so reduce their efficiency. So this post describes a simple method of keeping birds off TV aerials.

My Method For Keeping Birds Off TV Aerials

I asked my aerial erector if he could do something about it. He told me I could pay for some specific spikes to be fitted to my aerial or he would happily attach some zip ties for no extra cost if I had them to hand. I gave him a bunch and the result is shown in the featured image repeated below:

In between the directors they are fixed to square tubing so they don’t rotate under gravity to point downwards. On the reflector where some round bar is used they are left long and threaded through to stop them moving out of place. Being plastic (an electrical insulator) they have no effect on the signal.

Zip ties are commonly available in DIY and electrical stores. I used black ones because they are usually more resistant to UV (ultra-violet) light.

Addendum

April 2014

I have to advise that a wood pigeon has been seen perched in the centre of my aerial. It neatly fitted itself between two zip ties. If I could do it again I would increase the number if zip ties and have them closer together.

June 2019

There are no signs of any zip ties on my aerial. They have not stood the test of time.

Terrestrial Freeview TV Aerial Direction

Introduction

In November 2012 I had my terrestrial TV aerial relocated to another corner of my chimney stack so that it doesn’t overhang my solar panels. Birds were perching on it and making a mess on them. Consequently I needed to check my TV aerial direction to be sure it points to the transmitter.

The aerial engineer (Martin Downing) worked until it was dusk when he re-attached my aerial to the chimney stack. That left him little time to adjust my TV aerial direction. I could see from the ground that the initial direction didn’t align with the aerials of my neighbours. I checked the TV picture and on some channels I got interference on my digital Freeview signal causing blocking (corrupt squares of varying sizes) on the picture. So I asked him to rotate it until it matched the other aerials nearby. The picture has been good since.

In this article I show how terrestrial TV aerial direction is accurately set using a compass instead of measuring the signal strength.

I accept that measuring signal strength at the aerial is the best method especially when an aerial is pointing towards an obstruction such as a building or electricity pylon.