Ya' make my drool, Jim, thinking that I'd have a yard nearly big enough to stick up a 60 or 120 foot full-size vertical plus a bunch of 60 to 120 foot radials around the base for 80 or 160 meters. I even have to dream of having enough room to do that on 40 at some future date.
If an end fed wire is 1/2 wavelength long, the ground is unimportant for efficient operation. The impedance at the feed end will be in the thousands of ohms. Unfortunately, the whole rig is likely to be at the same voltage as the end of any dipole and that's not so good unless you relish the feel of RF burns and a tendency for logic circuits to go "freaky" with stray RF <G>. I look at an antenna system as three resistors in series - which it is once the reactance is tuned out. One resistor is the radiation resistance of the antenna. All the power consumed in this resistor becomes electromagnetic waves - the good stuff! The other two resistors are the ground resistance and the resistance in the conductors of the antenna, including the conductors in the antenna tuning network, any loading coils, feedlines, etc. In most cases, the conductor resistance is small - a total of a couple of ohms. Still that can be significant if hundreds of amps of RF are flowing through the wire, such as can happen on a feedline at very high SWR's. That's basically why open wire feedline is so much better than coaxial feedlines. It has a higher impedance than coax, so the currents flowing at the current loops is proportionately less at the current loops and so the resistance losses are less. The ground resistance is a different matter, and where most of the losses in an antenna working "against ground" come in. Of course, the easiest way to eliminate the ground resistance is to use a balanced antenna that does not require an RF ground, but sometimes that isn't practical. If a ground connection needed, how good it must be depends a great deal on the radiation resistance of the antenna. For example, if you drive a stake in the ground, or throw a hunk of wire on the ground, you can expect to see a ground resistance of at least a couple of hundred ohms. Say 300 ohms for a working number. If the antenna is 1/2 wavelength long, it might show a radiation resistance of say 3000 or 4000 ohms. Remember, those resistances are in series. So, for a given RF current, about 10% of the power is dissipated in the ground and 90% of the power is dissipated in the radiation resistance and becomes the electromagnetic wave headed off to that DX we're trying to raise. 90% is an very efficient antenna! On the other hand, if the antenna is only 1/8 wave long, or less, the radiation resistance might be as low as 2 or 3 ohms. Now, with a 300 ohm ground, about 99% of the RF power is dissipated in the ground connection and only 1% or 2% becomes the electromagnetic wave "tickling the ionosphere". "Tickling" is the right word unless you're running a huge amount of power. Of course, most antennas are somewhere in between, but all end-fed antennas live by the same rules. Divide the ground resistance into the radiation resistance, and the bigger the number the more efficient the antenna. Unfortunately for most of us, when we use an end-fed wire we're restricted to a length of 1/4 or less, so the ground resistance is crucial for decent efficiency. Most of us have a hard time measuring ground resistance, but one way to estimate it is just like Jim did. He knows that a 1/4 wave radiator should show a radiation resistance of about 35 ohms. When he sees a total resistance of about that using an antenna analyzer at the feed point, he is sure the ground resistance is a fairly low value. One might ask, why not use a half wave end fed wire on 20 meters and above if it's so efficient? That's a wonderful idea if your shack is up on the second floor or higher and the wire is out in the clear. Remember, the maximum radiation is taking place at the current loop on the antenna, and at 20 meters that's only 16 feet from the voltage loop. It doesn't make much sense to have an antenna that's 99% efficient but so close to the house, fences, trees and the ground that 95% of what it radiates is absorbed before it can get over the back yard fence! When an end fed wire like that is used on the higher bands, it's common to work out a way to put it up in the clear and feed it with a transmission line. One technique that used to appear in all the handbooks that hasn't been seen a lot lately is to put the half-wave antenna up in the clear, then couple one end to a parallel-connected capacitor and inductor resonant at the operating frequency. A low-impedance feedline is link-coupled to this "tank" circuit. It works FB, even though nothing is connected to the "ground" end of the tank circuit. Another very famous variation is to connect a 1/4 wave length of open-wire feedline to the end of the 1/2 wave long radiator. One side of the feedline connects to the end of the antenna wire and the other side of the open wire line connects to nothing at all. It is insulated. The 1/4 wavelength of feeder converts the very high impedance at the end of the antenna to a low impedance at the rig end, where it can be fed by any low impedance balanced source. This configuration is famous as the "Zepp" (short for Zeppelin) antenna used on the big airships of the 1930's where the wire trailed behind the dirigible and was fed at the end. In theory, if the antenna is exactly 1/2 wave long, the impedance at the end approaches infinity so no current is drawn; It's a strictly "voltage fed" wire. If there was no current, then there'd be no current flowing in either side of the open wire line at that feed point, so the feeders would be balanced and would not radiate. However, a real antenna doesn't approach infinite impedance and does draw some current. But neither are insulators perfect, so there's some leakage from the unconnected side. In practice the balance is good, provided the antenna is exactly 1/2 wavelength long. The problem with both of these configurations is that the antenna length or matching network up at the end needs to be adjusted when changing frequencies! That's not a popular idea today. Ron AC7AC -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim Wiley ...For field day, we often use vertical antennas for 80 and 40 meters. These are full size verticals, made from aluminum tower sections plus tubing "stingers" to get the right length. There is no reason that these same ideas cannot be used for the higher bands as well. Each vertical antenna is surrounded by a batch of 16 to 24 radials...We then put an antenna coupler box right at the base of the antenna, and use the MFJ antenna analyzer to adjust the system for resonance at the desired operating frequency. BTW - if the MFJ box is connected directly to the antenna, it usually shows about 35-40 Ohms impedance at resonance - exactly what a 1/4 wave vertical should read! ... If you are using an end fed wire (frequently mis-named a long wire), the principles are exactly the same. Any end fed antenna, if operated against earth ground, needs a fairly good ground system to be effective. Antennas with one radial (or two, or three or four) may tune up easily and show good SWR, but then so does a dummy load. An interesting aside - to be a true "long wire", an antenna must be at least one wavelength long, preferably several wavelengths. _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
