Airband Radio Aerials: Maths in Action

I've been interested in aircraft and airshows for over 40 years - anything military or civil, and I've blogged in the past about how to use a spreadsheet to track down where to watch the Red Arrows fly past on their transit flights.  You didn't think that post was about geometry without some real-life applications?  What is this - "Another day I haven't used algebra"?

Anyway - I've been particularly interested in the Red Arrows and their air-to-air chatter, and the communications between pilots and air traffic control.  Yes, I take my airband radio along to airshows and to airports, and listen to the pilots request and receive clearance to take off or land.  Getting to airports is more of a challenge than it used to be - my children aren't as interested as I am in the whole thing, and standing at the end of a runway in poor weather isn't as much fun as it sounds!

So, I've started developing my home-based receiver.  In other words, I spent my birthday money on an airband antenna and an extension cable to connect it from outside (cold and sometimes rainy) to my desk (warm and inside) so that I can listen to pilots flying nearby.

Now: nearby is a relative term.

From Stoke on Trent, I've been able to pick up pilot transmissions from about 35 miles away, on the southern edge of Manchester Airport.  That's with a very basic antenna, set on my garden gatepost and about two metres off the ground - not bad for a first attempt.

My dad, on the other hand, has been tracking radio transmissions for decades.  His main areas of interest are long wave (around 200 kHz), medium wave (500-1600 kHz), and TV (UHF, 300 Mhz to 3GHz).  

Airband falls into the Very High Frequency range, around 100-200 MHz.

Here comes the maths:
All radio transmissions travel at the speed of light, c = 2.998 * 10^8 ms-1.
c = f w

Where f (sometimes the Greek letter nu, ν) is the frequency, and w (usually the Greek letter lambda, ƛ) is the wavelength.

So, if we know the frequency range that we want to listen to, we can calculate the wavelength of that transmission.  And this is important, because the length of the antenna (or aerial) that we need will depend on the wavelength.  Ideally, the aerial should be the length of one full wavelength, for maximum reception effectiveness.  Alternatively, a half-wavelength or a quarter-wavelength can be used.

So:  we know the speed of light, c = 2.998 * 10^8 ms-1
And we know the frequency of the transmissions we want to receive, which is around 118 MHz.

c/ν = ƛ

ƛ =   2.5 metres

Which is feasible for an external, wall-mounted aerial.  Can you see where this is going?

Exactly.  And here it is:  

It's just over two metres from end to end, with a feed at the midpoint.  This is the Mark One; the Mark Two will be the same aerial but even higher up, and closer to vertical (with a bracket that will enable it to dodge the eaves of the roof!