Part 3 - Aerials (height, gain and angles)

Here we are again and something to ponder. Mike Banz, AA3RL did a study (I'm sure many others have too but I found his easy to understand) of a dipole's gain, its height above ground and its main angle of transmission. I've just converted his tables to graphs.

Please note that gain is not the same as efficiency. Efficiency is the aerials ability to transmit the power it is supplied. Gain is the aerials ability to direct its transmitted power in a particular direction in relation to a standard dipole. If that confuses you I've done a bad job but read on.....

We haven't talked about gain yet - it's a measure of how much better the aerial performs in relation to a standardised model aerial. The model is called an isotropic dipole and the gain is measured in deci-Bells (tenths of a Bell), the units are written as dBi. You can forget you read this paragraph now!

Let's do gain first. Can you understand the graph above? Height above ground as a fraction of the wavelength (we are still on 40 m) for the horizontal x-axis. Gain is shown down the left hand vertical (y-axis).

We all want more signal to be transmitted so more gain is good, in this case it's when the aerial is really low but to be honest it's not too bad across the whole range. But there is always a snag....

So we have height again but now we are measuring the main angle that the transmission is made (0 degress is horizontal and 90 degrees is vertical). Remember we want low angles for DX and not high angles. Now we want to be further to the left as possible to get those low angles (not quite true but it fits with the picture!). So the best gain in the previous graph was for transmissions that go straight up.

This may seem of no use to anyone but the waves are sent up and bounce off the ionosphere (a layer of gas in the sky - propogation comes later) and return to earth close by. Used by the Army and others in the frequency range 1.8 to 15 MHz.

My 7 MHz dipole is sometimes as low as 14 foot off the ground (which is pretty far over to the rght of the graphs) and I can make contacts about 500 miles away. Never let anyone tell you a low dipole is useless - it's just not good for DX - but then what's your definition of DX?

Summary

I hope you are you gettig a feel for our friend the dipole and how it works with the constraints we've given it. Is you visit the link here (the same as the top of the page) you'll see all those great plots and see that what I'm telling you here is a very simplified version of the story.

When things are made complicated they cost more money and we think they are more specialised and then we believe we can't survive without them. How did the SOE operaitves in WWII survive (see the Duxford site) with their B2 sets and a piece of wire hung from a tree or in an ampartment (no TVI then!). OK so they didn't want to send CW to the US or the far east but hopefully your getting my drift

£300 for an aerial is a bit expensive if two wires are all that's needed if you can mount them high enough. Aleternatives to spending £300 follow.