Saturday, November 11, 2006

Antena Know How

Since the late 1970's when I got interested in HF mobile operation, I experimented with a variety of antennas to find the best practical design installations. You see all antennas are limited by their aperture size. The larger the aperture size, relative to wavelength of operation, the greater antennas gain. There are however constraints upon this; you can't simply make big antennas and expect them to function. You can and should control impedance matching and antenna efficiency. Antenna efficiency is improved by bigger size conductors. This is to say; larger, fatter, conductors which have less resistive losses due to their large surface area, and consequently higher antenna Q. Smaller antennas have less aperture gain, on the HF bands typically negative gain, and also less efficiency because they have greater resistive losses.
Since we can only have relatively small antennas on a vehicle, we have to concede that all our HF mobile antennas will have negative gain, which means less than unity gain. Even the body of the vehicle itself, which is the antenna counterpoise, is usually only a very small fraction of the HF wavelength in use. Thats not all that bad though; even HF Yagi antennas hardly have any positive gain! Whats important on mobile antennas that by their nature are small, and close to the ground, is that we need as much signal efficiency or radiation efficiency as can be managed. So, how is this achieved?
Antenna efficiency and Q are achieved by minimizing resistive losses. Resistance is accumulated within small wire conductors; this includes even the stainless steel wire or rod the antenna whip itself is made from. It is most notable in the coil that loads the tuned circuitry of the antenna. Big large diameter wire coils are much more efficient than smaller thin wire coils. Also of great importance is the resistance of the antennas counterpoise, where the antenna connects to the automobile chassis.
It is common for many Ham's these days to attempt to install all of their mobile antennas on some sort of clamp-on mechanism. The worst of these is some sort of luggage rack that has little metal surface area, and much higher accumulated resistance. These attempts at "bolt-on" installations are usually the most significant worst first step inexperienced Ham's make. Connecting the antenna directly to the metal body of the car in a nice low resistance fashion is best. Best of all is doing this in the middle of the cars metal roof. You just have to get over that concern of drilling holes in your car!
As a comparative analogy; several years ago Jerry Sevick W2FMI ran some tests with a roof top mounted tower antenna atop a building. He progressively doubled the number of counterpoise radials beneath this tower and took measurements each time. With each addition he lowered the feedpoint resistance, and improved the high current radiation of this antenna installation. We can do much the same thing on a vehicular installation by making the best low resistance mounting conditions possible, and by bonding the metal parts of the vehicles chassis and body together. Another trick is to bond the cars exhaust system in several places along its length to the vehicle chassis with short braided strap conductors.
Coil placement is also important. Base loaded antennas are less efficient than are center loaded antennas, and while its a bit of a misnomer, what are sometimes called top loaded antennas are yet even more efficient. At least this last statement is true if the coil is big and fabricated from large diameter wire! The Hustler "top loaded" antenna, and venerable "Texas Bug Catcher" are the best examples of this. Also, capacitive loading at the top of the antenna will improve its efficiency by raising the high current point further up the antenna structure. Capacitive loading above the coil itself will improve frequency bandwidth coverage.
So what are some practical antenna installations and methods of providing good mobile antennas between the frequencies of 3.5 to 54 MHz. Since I have been interested in not only HF mobile communication but, also working DX and local Ham's on the 6 meter wavelength band, I have favored the Hustler, so called, top loaded antenna. Its large surface area half inch diameter 54 inch long mast provides lower resistance than would a skinnier wire antenna, and by using the higher power Kilowatt rated coils; their larger wire size makes them higher Q, efficient tuned radiators. Also, this half inch diameter 54 inch long mast I referenced earlier forms a quarter wavelength whip for the 6 meter band. Any resonator coil above this mast is not 'seen' during 6 meter operation. This is because the coil functions as an RF Choke at 50 MHz. and higher frequencies. By this means the antenna will work on both 6 Meters, and also on whichever HF band I have a resonator coil installed for. You can even put 3 or 4 such coils atop the mast at the same time but, this extreme inductive reactance limits antenna efficiency and bandwidth.
The Texas Bug catcher is a good antenna. It is one of the best mobile performers at the lower end of the spectrum on 40 or 80 Meters. At these lower frequencies small wire coils of other antennas high resistance coils really demonstrate how bad things can get! On 10 Meters a simple stainless steel whip antenna that is 98.873 inches long (which is 1/4 wavelength, or 90 electrical degrees in length) is very efficient. This same antenna (or one like it that is 108 inches long) would work as a halfwavelength radiator {180 electrical degrees}on 6 Meters; if it was fed with a quarter wavelength transmission line transformer to provide proper impedance matching transformation.
So, which antennas should probably be avoided? The ATAS 100 is cute but, its not very good from an RF signal efficiency standard. Antennas that consist of mostly coil should be avoided. Antennas that are wire wrapped around a plastic or Fiberglas tube are a compromise antenna. They are inexpensive but, not very signal efficient radiators.
Build some antennas yourself, and see what works best. You can fabricate proto-type coils with #14 AWG solid copper wire on cardboard tubes. I use such discarded tubes from industrial plastic shrink-wrap, or smaller tubes from blank label rolls. These coils also make excellent wavelength traps or end loading coils for wire dipole antennas. These are great for Field Day and other radio field trip expeditions. Antenna tests, commonly called Antenna Shoot-Outs, are a popular competitive forum. If your club is in the local Orange County area of Southern California, I would love to help your club organize such events.
Terms used in this article
Impedance matching: All antennas whether small or large in their relative wavelength aperture must be properly matched to their feedline and exciter impedance. Even if the antenna is capable of producing tremendous antenna gain, it will all be lost without proper impedance matching.
Antenna efficiency: This would more fully be stated as "antenna radiation efficiency". Antennas vary in their ability to effectively radiate a signal. Large surface area, properly tuned antennas radiate well. By contrast, a common light bulb can be used as a "phantom antenna". It will radiate a signal, and you might even communicate with another station while using it. A good antenna would work much better!
Antenna Q: The Q or "Quality factor" of any antenna or tuned RF circuit governs how much of the signal is efficiently conveyed, or conversely, how much of it simply becomes waste heat!
Radiation efficiency: See antenna efficiency
Resistive losses: In an alternating current circuit, such as an RF circuit, larger surface area conductors minimize resistive losses. This is because AC currents flow on the outside surface of the conductor; this is called "skin effect". Hence, the more surface area the conductor has, the lower is its net AC resistance!
Loads: Coils are commonly called inductive loads. They "load" what would otherwise look like a short or capacitive circuit.
Counterpoise: All unbalanced antennas, such as mobile whip antennas, require a "ground plane" or "counterpoise" to work against. The main currents flow in the radiating whip but, without an efficient counterpoise, this does not happen!
Counterpoise radials: A home station vertical whip or tower radiator would use a wire counterpoise strung radially around its base. Kept sufficiently high above the dirt, these radials will provide a nicely symmetric and low loss counterpoise. The more of them that are used, the lower the feedpoint resistance will be, and consequently the greater the antenna efficiency.
Feedpoint resistance: In an automobile whip installation this is most practically achieved by making good low resistance connections to larger portions of the automobiles metal body. The roof is best, the trunk would be next, and a nice clean and solid connection to a fender will work well. See counterpoise radials (hint: we can't make a car into a symmetric counterpoise but, we can keep the resistivity of the connections down)! Bonding is critical if the antenna is trunk mounted. Jumper the hinges with braided straps! Actually the more straps the better! Two for each hinge, and one near each hinge side trunk corner.
High current radiation: A halfwavelength wire dipole has its high current point at its center. A "Ground Plane" antenna, such as on a car, has this high current point at its base. Actually, its at the base if the antenna is one quarter wavelength long and linearly loaded, such as a stainless steel quarter wave whip. If the antenna is inductively loaded, the highest current point will be the inductive coil itself.
Bonding: No this isn't something you do with your budd's. If you make nice low resistance bonds between welded portions of your cars body, you will lower radiation resistance, and improve radiation efficiency. Look at my article on, "The Waynis DC Connector"
Base loaded: This means physically placing the inductive loading coil near the base of the whip. This is mechanically convenient, but its bad for signal efficiency.
Center loaded: Nearly all mobile whip antennas place this inductive coil near the center of the antenna. Its about the best you can do to improve signal efficiency.
Top loaded: Some antennas are said to be top loaded, they are really (electrically and schematically speaking) center loaded.
High current point: Like I explained earlier, the high current point, or point of maximum radiation is at the bottom of a quarter wavelength whip. If though you place a capacitive "top loading hat" at the top of the antenna, you will raise this current point further up the antenna and, the antenna will be a better efficient radiator!
Bandwidth: Width of frequency coverage or bandwidth is limited by highly inductive circuits. The large inductance incorporated to make an automobile whip antenna work on 80 Meters is the best example of this. You often can move frequency by only 20 or 30 KiloHertz in an 80 Meter mobile. Adding capacitive reactance is a means to broaden this effect.
RF Choke: A large inductance provides an extremely high impedance for relatively high frequencies; this stops higher frequencies full stop, it chokes them off. Lower frequencies are accommodated with a lower resistive path in such coils, and current can flow more easily.
Resonator coil: This is Hustlers name for the means by which they provide a resonant tuned circuit on their whip antennas. It is descriptive of the function though!
90, 180, and 360 degrees of electrical length: An AC circuit of one wavelength provides 360 degrees of electrical phase rotation. A halfwavelength is therefore 180 degrees, and a quarter wavelength is 90 degrees in length. This becomes important when you start charting impedance (and current flow) at different electrical lengths along an antenna or transmission line.
Quarter wavelength transmission line transformer: This is sometimes called "an impedance inverter". In my example in this article, it would allow a very high impedance to be converted to a lower impedance suitable for coaxial feedline.
Wavelength traps: A coil in series with a conductor (such as a wire) merely provides circuit loading. When installed along with a suitable value capacitor, you can trap or stop wave travel. In other words, you can make the antenna appear electrically shorter or longer to different frequencies, even though you make no physical changes.
Dipole: Di means two, or two poles; like the poles in a motor or generator. The two poles referenced here are the quarter wavelength wires that are on either side of the feedline of a halfwavelength dipole antenna (Hint: such a dipole has a 73 Ohm feedpoint impedance; a quarter wavelength Ground Plane antenna [when the counterpoise is perpendicular to the radiating element] has a 36.5 Ohm feedpoint impedance.

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