Some Things Just Can't Be Explained!

There is nothing magical about antennas. There is nothing magical about ground planes. There is nothing magical about adjusting an antenna tuner. In fact, there is nothing magical about any part of amateur radio! If there is anything magical, it is in the eyes of the beholder! However, you'd think some things were magical reading the threads herein. My word, where's all of this misinformation come from? Here's a few examples.

What's with all of these references to OFC (off-center fed) dipoles being the epitome of antennas? Every single one of them, no matter how they are fed, result in a mismatch, their use of baluns (no matter their configuration) notwithstanding. This mismatch is so pervasive, no matter the balun type in use, that common mode currents on the fed line are a feature! In fact, in most cases the feed line is the major radiating element! Yet, on-line references about OCF dipoles would have you believe they are the ultimate, magical radiator! They're not!

A G5RV is a decent HF antenna if you except the fact that it was designed to be a three band antenna (20, 40, and 80). What it is not, is an all-band antenna. However, you'd think so by reading some of the threads about them. Just because you can use an antenna tuner to load up some object, thing, chunk of wire, or an G5RV, does not mean, in itself, that it is an efficient all-band, magical antenna!

Grounds! Grounds! Grounds! How many times have you read or heard this? 'I added a ground strap to the antenna (mount?), but the SWR is still to high!' Geez, you think a ground strap was the answer for every single antenna problem ever encountered! I'll agree it is important to DC ground antennas and coaxial lines, for lightning and static protection. However, a ground strap is not a replacement for a ground plane! Unless of course, it's long enough to be a radial, and it isn't the only one. Please note I said ground plane. Perhaps you'd prefer the term image plane, but that's even harder to explain to a neophyte. What it isn't, is a counterpoise! That's a different animal altogether, regardless of the mystifying misuse of the term.

Quarter wave verticals are wonderful antennas (and inexpensive) when properly implemented. Every single one of them requires a decent ground plane. A ground rod, a piece of pipe, a chain link fence, a metal roof, or rain gutter, isn't an adequate ground plane. It is important to remember, a vertical is one half of a dipole, hence the (uncommon) name monopole. The missing half must be a ground plane typically made up of radial wires. The ground plane must be adequate enough to prevent, or limit, common mode currents flowing on the feed line. Not following this rule, will often result in RFI problems, regardless of the SWR reading. Speaking of which...

Viswaritis! I don't know who coined the term, but you'd think about 75% of the amateur population suffered from it! While standing waves are an important consideration in a lot of cases, SWR alone has nothing to do with efficiency, angle of radiation, RFI levels, common mode currents, suitability of purpose, or bandwidth. This isn't magic, it's fact!

Speaking of bandwidth. There is a formula floating around the Internet that goes like this. An antenna's Q (assumedly an antenna system, but that's not given) is equal to 360 times the operating frequency in MHz, divided by the 2:1 VSWR bandwidth in kHz. Again, an antenna's bandwidth is not in any way directly related to the SWR (nor its -3 dB points). It might be an indication perhaps, but one is in no way dependent on the other. That's not magical, that's fact!

Nowadays, almost every manual antenna tuner is a "T" type network. That is, there are two series capacitors. Between them is a shunt inductor. If your antenna tuner falls into this category, then the tuning procedure is always the same. That is to say, the one where the capacitors are at their greatest reactance consistent with a minimum SWR. What's more, depending on the antenna system in use, the lowest SWR indication isn't necessarily the best tune. With that in mind, there is an upcoming article to appear in the February, 2009 issue of QST, written by Eric Nichols. He suggests that an RF ammeter should be used (instead of an SWR indication) to adjust a "T" type network tuner, and he just might be right. I for one am looking forward to it. Let's hope it's not magical!

Here's one magical thought brought home just recently. Resonant antennas are nice to have, because they can be fed with coax (if for no other reason). However, if you want to operate the whole 75/80 meter band, chances are you'll still need a tuner. Some will argue that what should be done is to feed the antenna with ladder line through a good tuner. Good idea most of the time, although in some installations (like mine) that isn't a practical solution. The distracters will argue about the loss differences between coax fed, resonant antennas, with no tuner, versus ladder line fed, non-resonant antennas and requisite tuner. Well, here's a fact that's sure to rankle feathers. The overall loss in an average amateur's installation which uses coax to feed a resonant dipole antenna, versus the exact antenna fed with ladder line and a properly adjusted tuner, is in favor of the latter. While small, it is still a fact. Move off resonance, or increase the distance from the shack to the antenna by two or three times, and the difference really gets astounding. And, I didn't pull this out of a magic hat!

There are a few points to reiterate.

Antennas, feed lines, and networks all follow well-defined parameters. They possess no magical properties, regardless of the faux-pundit palaver to the contrary. When all else fails, pundits always argue that experimentation is good. Well, it isn't if you don't learn anything in the process! For example, you can't make (or assume) one measurement, and apply it to something you didn't measure. The aforementioned Q formula is a typical example.

No matter what antenna you use, or how you feed it, or how you tune it, it is what it is. But when you justify your on-air successes, signal reports, or low SWR readings, to some magic property your antenna possesses, you're fooling only yourself.

Alan, KØBG

www.k0bg.com

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