Antenna Basics for Fun
(Part Four – ¼ Wavelength Verticals – Tricking the Universe with Mirrors)
For a proper discussion of the classic ¼ wavelength (λ) vertical, let us start with our previously explained antenna, The Dipole. Please refer to previous articles in this series. I will be Assuming that you have read and understood the previous articles. They are readily available at the www.KC2RC.com or www.KingsCountyRadioClub.com website.
Now, what would happen to our ideal dipole in the vacuum of space if we inserted an infinite sheet of conductive metal between the two ¼ λ monopoles that the dipole consists of (keep in mind the sheet of metal DOES NOT touch either monopole)?
Sometimes nature can be easily fooled. If you take one of the monopoles away and connect that wire’s feedline to the sheet of metal, the Universe acts as though the sheet of metal is a mirror and it creates a “virtual mirror monopole” to replace the actual monopole that you removed. As far as the Universe is concerned your dipole antenna is “sorta there”! Now rotate it in your mind by 90 degrees and make the monopole vertical instead of horizontal and shrink the infinitely long sheet of metal into a bunch of wires arranged radially near the base (usually something near ¼ wavelength themselves) and you have a ¼ wave dipole in all its glory!
What’s different from the dipole it was derived from? Well the impedance, for one. Is it going to be that elusive 50 ohm nominal impedance that we Hams are always lusting after? Nope, sorry the nominal impedance for a ¼ wave vertical at resonance is half that of a real dipole – 36 ohms of pure (radiation) resistance, all the inductive and capacitive reactance is cancelled out! Can we “trick” the Universe into making the resonant vertical a 50 ohm impedance device? Yes we can! As you bend the spokes of the radial wires at the base further and further “away” from the upper vertical the impedance begins to inch up higher and higher. Perhaps you have noticed that more than a few ground plane verticals have radials that dip 45 degrees below their base. The reason why is at that angle its impedance IS 50 ohms!
How is its radiation pattern effected? Well, it really isn’t affected much at all. You have the same “doughnut” (or “bagel”) shaped radiation pattern, but since you rotated it in space 90 degrees it is radiating and receiving in a ring ALL AROUND its base. It is considered omnidirectional (but NOT isotropic). There are pluses and minuses for this characteristic – you don’t need a rotator, BUT verticals tend to be noisier – you can’t null out noise coming from a direction different from the station you are trying to receive, and you can’t expect any antenna gain from steering a tighter “beam” of radio waves in the specific direction of your intended receiving station.
But why did we set this model up in the vacuum of space? Are we just “outer space nuts”? No, it’s because things begin to change in the real world as you drop the vertical closer and closer to “Mother Earth”! Elevated verticals can get by with only three or four radials (you can use less, but you will distort their omnidirectional characteristics if you do that). As verticals get closer than ½ wave above ground the story changes. When your vertical lands just above a ground installation, there are new things you have to deal with. Close to the ground, your three or four radials are not good enough to act as that infinitely long mirror. Your radials become very poor reflectors and your antenna becomes more and more coupled to the Earth’s ground, all that RF you would like to propagate into the ionosphere will, rather, go into heating your ground locally – it might help with snow reduction on your property but I don’t think that many of us really had that in mind!
Is there anything you can do about it? Sure, the more radials you add, the better the situation gets. Many of our obsessive compulsive brethren have done extensive experimentation on how many radials are enough. The point of “diminishing returns” appears to be past the 36-42 radial numbers. Of course, if you have a lot of salt water just below the surface of your ground, or you’re living on an ocean, you have nature’s best radial system already there for you to avail yourself of!
Does a ¼ wave resonant vertical have to measure ¼ wavelength long (remember that RF travels SLOWER in wire than in free space, so that a quarter of a wavelength on a vertical piece of metal is shorter than it is in the vacuum of space)? No, we can play more tricks with the Universe, but in this department its much more wary of the games we play, and it demands “a price” for any tricks we may try to use.
We can take a shortened vertical and turn it into “an electrically resonant vertical” by the use of a “tuning network”, i.e. a bunch of capacitors and inductors of a specific value in a number of configurations. A simplification of the problem is that at resonance a vertical antenna’s inductive and capacitive inductance is cancelled out. As you shrink a vertical below its resonant length you lose more and more of the inductive reactance and the capacitive reactance has a greater and greater effect, making your impedance mismatch greater and greater with all the awful high SWR that we loathe. You can use “Pi” networks, or “T” networks or “L” networks (or a lot of other kinds of networks). A little simpler approach is to put a coil, an inductor somewhere on the vertical – either at its base, or somewhere in the middle (top loaded coils are usually discouraged for a number of reasons) and sometimes a “top hat capacitor” is crowned at its top. Does it work? Sorta You have to pay Nature’s Price – the antenna’s Q factor increases, decreasing the antennas acceptable bandwidth and the components will have their own energy losses and will couple with each other causing further decreases in their overall energy efficiency. That’s why “Hamsticks” sorta work. They are better than nothing and can “get you on the air”, but then so can a light bulb mounted six feet up a fence can – if the Gods of Propagation are kind to you. Some people swear on Hamstick, many people I know swear AT them!
What about those radials that are some other multiple of their working wavelength? Something like 5/8 λ or ½ λ verticals? Well, you’ll have to deal with the impedance mismatch such verticals provide – either with a matching network, or a transformer balun. The radiation pattern is a bit different – 5/8th λ has a lower elevation angle than the one for ¼ wave verticals.
Do verticals HAVE TO BE electrically resonant? Well, maybe not directly, you can use the equivalent of a random wire with a very good antenna tuner with as little feedline loss as you can manage. That’s what those commercial 43 foot verticals are all about! You can also use “traps” – resonant LC circuits in-line with the vertical, effectively blocking shorter wavelength transmissions from traversing the whole length of your vertical. Some verticals have numerous traps built in. They do work (if they haven’t developed a fault), but they also insert a loss to the antenna system and mean a less efficient radiator of your transmitting power.
What about those ads for “NO RADIALS REQUIRED” verticals? Well, they are fibbing to one degree or another. Some just renames radials as “counterpoise” – six of one, half a dozen of the other! Only one counterpoise/radial will get you a vertical that sort of works, but beware of ads for verticals that require no radials or counterpoise at all – you’re getting very close to the antenna quality of that light bulb on a fence post that I mentioned earlier. Half wavelength verticals are an interesting variety that have not been very well researched. They should not require a radial system, but still require some kind of a ground connection. This is a fresh area for experimenters to add to our knowledge of antenna theory.
That’s all for today… Any questions?
Until the next time…
Roy S. Laufer, AC2GS (I can be contacted at TheEditor@KC2RC.com)