Foundations of Amateur Radio

The single largest topic of conversation in Amateur Radio is about Antennas. The discussion often starts with one amateur telling another amateur about some or other amazing antenna, followed by a heated discussion about the merits or pitfalls of that same antenna and why they would never ever consider using it and why it's a waste of money, or some other rationale.

Let's take for example the discussion of Dipole versus Vertical. There are those who will tell you that they'd never ever use a Dipole and similarly those who'd say the same about a vertical. Assertions of suitability aside, let's have a look at what we're talking about, first of all.

In rough terms, a dipole is an antenna that is generally suspended between two sky-hooks, its fed from the centre and has pretty much an omni-directional radiation pattern. That is, signals arrive and depart from this antenna, pretty much evenly in all directions. Now, before you get all excited. It's not exactly the case, since it's not an isotropic point source, which you might recall is a theoretical antenna that we can prove not to be physically possible, but a dipole is the next best thing.

A vertical is an antenna, which is often supported from a pole of some description, has some form of radial ground-plane at the base and while it's also omni-directional, there are parts of the signal that don't arrive nearly as well and other parts of the signal that fare better.

Often a statement when comparing a dipole to a vertical will be something like this: "A vertical is better for DX and a dipole is better for local contacts."

Now, let's just investigate that for a moment. If you've ever seen the radiation pattern for a vertical, you might have seen that there is a particular angle at which there is gain when compared to other angles. What this means is that signal arriving and departing from the antenna in essence favour that angle. Similarly, a dipole doesn't display this phenomenon nearly as sharply. There is some asymmetry between the sides and the ends of a dipole, but it's not particularly strong. Not that it's non-existent, just not pronounced.

If you were to overlap the radiation pattern of a dipole and that of a vertical, you'd notice that apart from the single angle where the vertical favours radiation, the dipole pretty much has the same level of gain all round.

In essence, this means that to all intent and purpose, apart from a single little angle, the dipole is pretty much the same as a vertical.

I hear you say, "Yes, but..."

Indeed.

Think of a vertical as an antenna that favours a particular angle of incidence. It's more prone to hear signals from that angle than any other angle. Similarly, any transmitted signal is likely to favour that particular angle.

As you know, the ionosphere is a moving feast. Signals arriving at one angle one moment may not be arriving at the same angle the next moment. If your vertical hears a signal one moment and not another, does that make for an effective antenna?

Another aspect that separates a vertical from a dipole is the behaviour of vertical signals, so called NVIS, or Near Vertical Incident Signals. Things that are nearby. A vertical antenna all but ignores that aspect, where a dipole has no such behaviour.

So, we're getting to the heart of it, imagine for a moment that the differences between a vertical and a dipole is their difference in filtering of signals. That is, a vertical filters signals from above, where the dipole doesn't. Similarly, a vertical filters all but the bits from a particular angle of incidence, where a dipole doesn't.

If you've followed along, you might begin to realise that there is not a single "best" antenna. It's horses for courses. Your antenna choice is based on what you aim to achieve, not which antenna is better than any other antenna.

So, the question: "Which antenna is the best?" should really be: "Which antenna is the best for this particular activity?"

Something to try next time you have a chance. Get a two-way coax switch and hook up both a vertical and a dipole and listen to the same station with each antenna in turn. Take your time, listen throughout the day. You'll be amazed how they differ and how it changes over time.

I'm Onno VK6FLAB