TechTip #3 Radials and counterpoises

TechTips on Ham Radio Outside Outside the Box are occasional short posts on specific technical topics. But beware, along with regular posts here on this blog, the content is not a definitive technical treatise and should not be read as such. I write about my own opinions and experiences; your opinions and experiences may be quite different. I welcome constructive feedback either in the comments or, if you prefer, by direct email.

“All lies and jest …

Still a man hears what he wants to hear and disregards the rest”. Paul Simon and Art Garfunkel sang those words in their song “The Boxer”. The words reflect how hams often react when our long-held, cherished beliefs are challenged. “My antenna just works, I don’t care about all that technical stuff” is a view I have heard – especially when the antenna cost hundreds of dollars. And nothing creates more exothermic debate than the subject of radials. Or is that counterpoises?

Definitive definitions

On Ham Radio Outside the Box, when the word “counterpoise” is used, it refers to the “other half of an antenna”. The “other half” could be 120 buried radials, or it could be a “magic carpet” (Faraday cloth). It could be a ground rod driven vertically into the ground. It could even be the ubiquitous (and apparently blessed by angels) 17 feet of wire laid on the ground. That magic length of 17 feet has been proved by thousands of QSOs so why care about all that technical stuff? Eh?

When you read the word “radial” here on Ham Radio Outside the Box it refers to a length of wire. A counterpoise may comprise one or one hundred and twenty radials. Radials could be buried in a shallow grave inches below the surface or they might be raised above ground. Buried radials – or even radial wires laid on the ground – are detuned by the ground and can be any length. Longer radials work better on the lower bands.

“Crickets are singin’ and lightning bugs are floatin on the breeze”

It’s been a while since I went “Fishing in the Dark” (for contacts on the radio) but I do spend a lot more of my on-air time out in the Big Blue Sky Shack, usually on a sunny afternoon – even if it’s only out in the backyard. If we venture out to a park to spend a couple of hours playing radio, we really aren’t going to waste that time laying out 120 radials. If an antenna needs ground radials I lay out between 4 and 8 of them. I know my antenna’s efficiency will be compromised, and I acknowledge the physics, but field expediency is the prime directive. Heck, I usually operate QRP in the field too; maybe I should try “QLF” (sending CW with my left foot) too. Some of my code buddies claim I’ve already mastered QLF!

Floatin’ on the breeze

Compromising efficiency is not mandatory. If we raise the antenna (a vertical whip) above ground. Now just a couple of tuned radials is as efficient as a rat’s nest of wires on the ground. A couple of radials? Why not just one radial? I was asked that question recently by a Ham Radio Outside the Box reader. I had to put on my physics graduate gown and mortarboard while I thought about that.

I started with the idea of a vertical dipole – that’s a quarter wave of wire on top of another quarter wave of wire with the feedpoint in the middle. Now let’s bend that dipole a little. Now it looks like a vertical radiator with a quarter wave high flying counterpoise. If we keep that configuration well above ground it should still be as efficient as a dipole.

I got wires in low places

How low can we go? My theory (which is open to debate) is based on interaction between that raised radial and the ground. The current in the radial wire induces an out of phase image in the ground. In the near field the image wave and the wave in the radial wire tend to partially or fully cancel each other.

The amplitude of the image wave in the ground varies based on two things. (1) the amplitude of the current in the radial, and (2) how close the radial is positioned with respect to ground. A high radial has almost no interaction with the ground, whereas a low radial will have a much greater interaction.

Now, let’s consider this: we can reduce the amplitude of the current wave in the counterpoise if the current is shared between more than one radial. Two radials each carry half the current; four radials each carry a quarter of the current. The lower the current, the lower the canceling effect of the image in the ground.

So we can conclude that it is fine and dandy to use low radials if there are a lot of them. On the other hand, a single radial should be kept well clear of the ground. Does that make sense, or didn’t I wake enough brain cells with my morning espresso today?

But wait, there’s more …

If the objective is to reduce the interaction with the ground, and the amplitude of the current wave in the radial wire(s) is a factor, there is another way – reduce transmitter power. See, QRP ain’t so bad after all!

Incidentally, the image wave in the ground travels more slowly than the current wave in the radial. If the length of the radial wire is one full wavelength, or more, the two waves eventually are in phase and the current is reinforced instead of being canceled. That is how Grasswire antennas work.

Murphy’s opinion

Electrically, a single raised radial will be efficient, subject to the conditions discussed. According to Murphy’s Law, if you use a single raised radial, you better anchor that antenna down good or the tension in that radial wire is gonna pull your antenna over. Murphy says two radials, 180 degrees apart, will keep it up.