OBSERVATIONS FROM SCOTLAND                            02_May 2008                                GM1SXX

The vertical revisited

You'll have seen adverts of this sort in the glossy radio magazines.... 'miracle vertical, needs no radials'.

Some people even believe this rubbish. Now I'm not against companies making an honest bob or two but I suspect that radio amateurs are in general a fairly gullible lot. They must be, to believe any of this rubbish. 

Your average vertical aerial is an electrical quarter wave.  It might in fact be a physical quarterwave ... more or less, or of course it can be 'loaded' by means of some inductance to appear as an electrical quarterwave.  So  on 21M, lets say 14.235Mhz, a quarterwave is a bit over 17feet , an entirely manageable height for a simple vertical.  On 40M, say 7.05Mhz, it's a less manageable 34.9 feet. In order to make it more mangeable, a shorter aerial loaded with some inductance can be used.  In this way, a radiator shorter than a physical quarter wavelength can be made to work.  The loading coil doesn't somehow 'make up' the 'missing' length... it merelay adds some inductance to null out the effect of the capacitive reactance that your radio would otherwise 'see'... because a radiator that's too short exhibits capacitive reactance.  On the other hand, if your radiator happens to be too long for the frequency of interest, it appears to have inductive reactance, so you need to add some capacitance to make the load seen by your transceiver appear like a 50R resistance.

A vertical quarterwave is effectively 'half of a dipole'.  Since a radio aerial is an electrical circuit, it follows that there needs to be a second 'half' of a dipole to complete the circuit.  In essence, the ground system of your vertical provides a mirror image of the vertical radiator.  You'll note I didn't say ground, I said 'ground system'.  What you and I call ground, the stuff we walk on, is not necessarily a good radio-frequency ground. Often it's nothing like a good RF ground, so we can help things along by building a proper ground system. This can consist of a number of plain or insulated wires of quarter wavelength fanned out around the base of the aerial and connected to the co-ax braid.  The wires need not be dead straight, they can be bent around obstacles should the need arise.  How many wires should you use?  Well, I'd say as many as you can manage. If you operate on multiple bands with say a commercial trapped vertical, try to have a set of radials for each band used.

How many wires to use is a subject of much debate. More is definitely better.  The Civil Aviation Authority operate a fair number of 'Non-Directional' aircraft navigation beacons in the 300-400Khz band and although they are low power beacons, they put out fairly strong signals.  They do this using a short lattice mast with a 'capacity hat' on top,  or by means of what looks like a large fishing pole (with a wire up the inside) mounted vertically.  A close look at the base of the aerials reveals that they use a massive web of radial wires connected to a metal grounding ring at the base.  Good RF 'ground systems' obviously have a big effect on the strength of the signals from these little 35 or 50W transmitters.  A quarterwave aerial at 300Khz should be Eight Hundred and Twenty Feet high, yet these aerials are rarely over 30 feet tall.  As a result of course they require a lot of inductance to cancel out their capacitive reactance, and this is done by an aerial matching unit (AMU) containing a huge air-spaced tapped coil at the base of the aerial. Since these aerials are really puny, the 'ground system' is obviously having a major effect in the performance of the overall system. Installations vary a bit from site to site, but it's not uncommon for the whip type aerials to have well over 100 radial wires. 

73 AL.
GM1SXX