OBSERVATIONS FROM SCOTLAND                            28 May 2008                                GM1SXX

Small lots.

With my recent emphasis on aerials...bugs have antennae!, I'd like to say a little more about ones for small lots.  Many of us who live in small lots seem to favour commercially made vertical aerials.  I personally find this a very strange choice.  Here's why.

Many verticals operate as quarter-waves at their resonant frequency.  This of course is achieved by loading them, adding inductance or capacitance as appropriate to bring the radiating element to resonance.  Now, such an aerial is effectively half of a a dipole disposed vertically.  It's a quarter-wave monopole over an RF ground 'mirror'.   In short, the ground , forms the 'missing' half of the dipole.  What should we take from that?

Well, it's rather obvious, or should be. The ground is EVERY bit as important as the aerial.  STOP!  Have a serious think about that... roll it around in your brain for a bit.  It has important implications for users of such aerials.  The current flow into the RF ground matches (or should match), that going to the resonator.  If it doesn't, where do you think it might be headed for?

Let me draw a practical parallel.  I live close to an airfield. Airfields generally make use of NDB's ... Non-Directional radio Beacons.  These beacons operate on Long-wave and are used for 'non-precision' navigation.  (They work just like 'Fox-Hunting' ... AKA radio direction-finding).  They are very simple things... just a CW transmitter beacon connected to a short vertical aerial (sometimes just a whip, at other times a short insulated tower with a capacity hat).  Not much to look at really.  Because the aerial is electrically *very* short, a large inductor is mounted at the base to make it resonant on the chosen frequency. The same tapped inductor is also used to match the PA to the aerial.  What's interesting about NDB's is that despite being puny, these beacons almost always produce a 'rock-crushing' signal from that short whip or tower.  How can such a poor aerial produce such big signals? 

The cleanest airfield in Britain?

Traigh Mhor beach on Barra.

Barra (Callsign BRR 316.0 Khz) in the Outer Hebrides.The short NDB tower is clearly visible alongside another tower  carrying the airfield anemometer and wind-vane.

Well, it should now be obvious that the ground is VERY important for quarter-wave verticals. In fact it's just as important as the radiating part, yet how many adverts do you see trying to sell short verticals that 'don't need radials'?  Don't believe it. It's just marketing hype.  The real 'secret' of why these airport NDB aerials are so effective is to be found below ground where most of the money is spent.  Typically they use at least 180 radials connected to a metal ring at the base of the mast. That's why they are so effective.  A single 8 foot stake hammered into the ground in your back garden just won't cut the mustard.  Yes, you will get a signal out, but the overall efficiency will be lousy.

Now that we understand the need for an effective ground system for verticals, the next question is sort of obvious.  Why use verticals at all?  If you have the space for radials, you may as well use a wire dipole, mounted as high as possible,  a delta-loop, or inverted vee aerial instead.

A favoured option for restricted spaces is loop aerials. Loops don't need to be square or triangular (delta), just bend the wire into whatever shape it needs to be to fit the space available, try to keep it away from children and animals, and support it with decent insulators.  It helps if you can place it as high as possible, but if you can't, don't worry.   It'll still work, but not quite as well.  The biggest advantages of such loops is obvious.... they don't need a ground (although their radiation pattern is affected by it).  They can also be fitted into awkward spaces.  Most of all, for me at least... they cost next to nothing :-)

Some ideas from elsewhere  http://www.eham.net/forums/AntennaRestrictions/2908

How dipoles work http://en.wikipedia.org/wiki/Dipole_antenna

Ground Image Vertical antenna Ground Image Vertical

Loops  Loop Calculator  20M Delta Loop 

73 Al.
GM1SXX