ANTENNAS 101 OR JUST THE BASICS (detail)

All Hams are going to have to buy, build, or install a variety of antennas during their Ham Radio career. It's interesting though that as simple an induc-tive/capacitive (LC) circuit as an antenna really is, many Hams never learn the fundamentals of how the antennas they use work. Consequently, much time, effort, and radio frequency (RF) power are wasted!

We will talk here about only two simple antenna configurations as the basis of this presentation, and as a fundamental model for nearly all antennas. These antennas are the "Dipole" and the 1/4 wavelength "Ground Plane". We may (through the verbal course of

this discussion) get to "voltage fed" antennas but, these two antennas we will use as models are most often utilized as "current fed" or low impedance coax cable fed antennas.

The Dipole or Half Wavelength Dipole Antenna

A "Dipole" antenna simply means an antenna with two poles, each usually a 1/4 wavelength long. Most Hams are familiar with wire dipoles strung between trees or posts, and fed with coaxial cable. A "J" antenna, sometimes though inaccurately called "J-Pole", is also a dipole. It differs only in the respect that its two 1/4 wavelength poles are a contiguous half wavelength long tube or pipe mounted above a linear impedance matching transformer. A dipole must have both of its "poles" to work! If the antenna was less than a half wavelength long, it wouldn't be a dipole, and would need some other sort of "counterpoise", to make up for the missing portion.

The "Ground Plane" or Quarter Wavelength Ground Plane Antenna

Most Hams have used a "ground plane" at sometime. This is true even if they simply used the metal roof of an automobile to temporarily enhance the communications signal efficiency of their two meter hand held transceiver, by placing it atop a car roof while they completed their communication. The most basic of "Ground Plane" antennas is simply a quarter wavelength long "vertical radiator", working against some sort of ground plane "counterpoise". This counterpoise might be 3 or 4 radialy placed wires or rods, it might be the roof or body of a car, or it might be the dirt in your back yard.

This last mentioned dirt counterpoise, is often used as the simplest way to provide a counterpoise for a "High Frequency" (HF) "quarter-wave vertical". It has some severe efficiency limitations however, and wastes most of the antennas value through "ground losses" and high "radiation resistance". The greater the number of radials, the greater the efficiency!

Wire or metal rod counterpoise "elements" are probably the best and most efficient method of providing a "counterpoise", and if installed with care and attention will provide excellent antenna efficiency. The way this is done best is to place the antenna, including its counterpoise, as high as is possible. Placing the antenna at ground level is quite a compromise situation, and using the earth or dirt as a counterpoise is much much worse!

Choices and Considerations, Questions and Answers

(Q) Which antenna is best?

(A) Where do you want to talk? Is your communication path Ground Wave, Direct Wave, or Sky Wave? Consider whether you want to illuminate the ionosphere directly above so that the reflected signal comes down to earth within a short distance, say a few hundred miles. Or, do you want a lower angle of radiation that will convey the signal greater distances upon each of its "Skip" hops or bounces.

(Q) How should I install it?

(A) As high and clear of conductive objects as possible. Higher is always better! Conductive objects include houses, buildings, and trees! Always use only high quality coax or transmission line! I prefer Belden coax such as Military Specification (Mil-Spec)\par RG-213/U C17D ultra-violate protected and non-contaminating jacketed cable. Why spend big bucks or big effort on an antenna, and then use some crap transmission line to feed it!

(Q) How much should I spend or invest in an antenna? (A) How much is your communications worth? Shop or build wisely! You can invest money or effort, each will get you what you want. My preference is design and construction effort because the results are more satisfying!

(Q) Should I install an antenna tower, or can I get away without one?

(A) Antenna height is fundamental. The higher the antenna the better it will work. Ground losses hamper antenna performance. HF beams should be at least a half wavelength removed above ground. That's not a bad formula for HF vertical omni-directional antennas either. VHF and UHF antennas should be very high for best efficiency, say 3 to 5 wavelengths minimum for VHF. For UHF, 50 feet \par ( 50 to 100 wavelengths ) is only moderate height.

(Q) Are used antennas a decent investment, or should I buy new, or build my own?

(A) Complex antennas suffer quite a bit of trauma while in the weather or being de-installed. Wave Traps get damaged, and matching systems suffer deterioration and damage. If you buy new, buy wisely and make your bucks count for the intended communication. My solution is to invest my money in test equipment and build antennas for only a few dollars per unit. You can often times build a much better antenna than you can buy!.