Low SWR Does Not Mean Good RF Propagation

In recent conversations with some of my ham radio friends, I can always count on some heavy discussion having to do with a topic that most radio operators obsess over: how to make and use an antenna effectively. This fuel was kindled and further fed by the recent publication of the ARRL's Annual Antenna Issue.

Antenna Talk

We had spoken recently about how to set up "stealth" antennas which are difficult to see by the casual onlooker or passers-by. Stealth antennas are especially crafty in housing developments and neighborhoods with covenants that prohibit the erection of unsightly decorations - such as ham radio antennas.

In one such conversation, someone I was talking with was especially proud after putting up an antenna in his front yard garden. After seeing the photo of his garden, it was true: the antenna was virtually invisible. However, provided the positioning of the antenna relative to the abundant greenery within the garden as well as the adjacent walls presented by his house, the question of good signal propagation and reception came up.

My friend defiantly proclaimed that he was doing okay in these departments, all the while solidly grounding his argument with the fact that he has been able to make some decent contacts as well as enjoy the obvious efficiency of his antenna system - "proven" by a low standing wave ratio (SWR) reading on his antenna tuner.

This is where I begged to differ.

Not Just an Antenna, but a System

When setting up a complete antenna system for one's transceiver set, one has to keep in mind that the antenna assembly truly is a "system". The system I speak of in this case not only includes the antenna itself, cabling, connectors, baluns, matchers, and any other hardware or software (such as tuning) that factor in - but it also includes the lesser-known environment in which the antenna system is expected to operate. This portion of the antenna system is a significant part of the entire system. All external factors have to be taken into account.

While there's plenty of documented discussion as to what external environmental factors may affect good signal propagation, I was able to make my point very clearly through a real-life example.

Back Seat Proving Ground

"Let's go for a quick walk," I said to my friend. It was a very quick walk -- right out his front door and straight to my car.I opened my car door and pointed to the back seat to have him take a look.

What I was showing him was my 10-meter radio's magnet-mount antenna (a Browning 54) in its stowed position in the back of my car. The entire vertical antenna assembly (less the cable) is about 5 feet tall, and poses a challenge when I try to shove it all into the back seat of my small passenger car. Thankfully the top portion is a forgivingly flexible steel whip which allows me to curl the whip downward from where it first strikes mid-ceiling in the rear passenger cabin, and curves down towards the bottom of the rear passenger seat.

In this stowed position, it goes without saying that if I attempted to key up and expect a response, I would see some pretty poor signal performance.

Good SWR. Bad Propagation.

To further make my point, I did indeed turn on my transceiver set and keyed up. My SWR meter showed a ratio of 1.6 to 1. Not perfect, but well within acceptable and useful limits.

What made the SWR so good despite the less than ideal positioning? Because the magnet mount portion of the antenna set is so large, it provides a pretty good ground plane to counter the driven portion of the vertical antenna element. However, despite having a good SWR reading, I explained to my friend that this was a good example of where having a seemingly good SWR reading could be deceptive.

Equating good SWR to good signal propagation is a misnomer. In the case of my antenna sitting in the back seat, here are a few reasons why the signal I was putting out was no good in spite of the good SWR reading:

• Poor polarization. Given that the antenna was resting at approximately 45° or more, any signal transmitted at such an angle would not show matching polarity to most of the intended receiver antennas outside (assuming the majority of them were vertically polarized).

• Shielded environment. My car is essentially a metal shell. Thinking of it this way, any signal transmitted from within would either be blocked, absorbed, or otherwise reflected around and rendered ineffective.

• Bad take-off angle (or no take off). Given that my antenna was all bundled up within a car comprised of convoluted metal components, any signal attempting some kind of long-distance trek would fail miserably.

SWR is not Signal

Given more time, I can cite more reasons for bad signal propagation -- but the point has been made. In spite of any good SWR reading, there is a host of environmental factors that can play significantly with how well your signal gets out -- or comes in for that matter!

In the case of my friend's garden-bound antenna, I would think that having his system set up so closely to his home (and the adjacent walls) would significantly hamper his transceiver's performance. Especially in the case of low wattage DX performance, having so much foliage (think: thick juicy leaves flapping in the wind, etc.) may come into play also.

Following all of this conversation, it didn't come as a surprise when I found that my friend had rigged up an identical antenna assembly and tested it in various places around town and in the countryside. At the conclusion of his exploration, he was clear that external environmental factors do greatly affect signal propagation - both on the sending and receiving sides.

Green-Tenna

In spite of all this, he left his garden antenna where it was. He said this: there is no argument the garden variety antenna enjoys a good view in the cool shade! And no doubt some good company, given how well his plants are growing in his front yard!

73!

- John

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ABOUT THE AUTHOR: John Melendez is a freelance writer reporting on technology, the environment, sustainability, alternative energies and "green" issues. John Melendez is a writer for hire. To email him, go to http://www.emailmeform.com/fid.php?formid=19595

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