The end-to-end length in feet of dipole antennas should be equal to 468/f ,where f is the frequency in megahertz. It's possible to connect several antennas cut for different bands to a single transmission line. They should fan out as shown by the dotted lines.
MOTHER EARTH NEWS STAFF
During a recent New Directions Roundtable session, we talked with good guy Nicholas Johnson about citizen radio and control television in the U.S. Nick—a former FCC Commissioner who currently heads the National Citizens' Committee for Broadcasting—is a long-time proponent of getting the media out of the hands of big money and into the hands of the people. As he pointed out, the real "product" marketed by the communications industry is the listening audience ... which is sold to advertisers as so much per thousand head. Any action which tends to upset this state of affairs will therefore be fought by the networks and by the FCC, which takes the position that commercial stations are operating in the public interest.
Public Broadcasting, alternative radio such as the Pacifica stations, audience participation via telephone, and the use of ham rigs for serious communication are, Nick thinks, steps in the right direction. His book, How to Talk Back to Your Television Set (published in hard cover by Little, Brown and Company in 1970 at $5.75 and available in paperback from Bantam Press for 95¢), contains other ideas for citizen action.
Here's one area where grassroots projects might be effective: Among the members of our society who have least access to electronic communications media are those groups which have been shortchanged in other areas too: native peoples, the poor, Blacks, Chicanos, etc. Because such cultural minorities are commonly much less into the written word than the average middle class American, possession of audio-visual devices such as radio and TV is that much more important to them.
Billy Mitchell, who works on the Mohawk Nation's newspaper, Akwesasne Notes, has asked whether we can help our Red brothers—who are scattered all over the country—get it together via radio. I pass the question along to you and would like to hear from those who have ideas and/or the desire to get involved.
Canada is carrying out a lot of interesting activities along these lines, including the establishment of a radio and videotape network—RAVEN—to link Indian groups in British Columbia ... and the installation of hundreds of small TV and radio transmitters in isolated areas. I asked Nick Johnson about the possibility of the FCC's authorizing similar short-range, low-cost, "community" TV stations. He said that the Commission had already been approached with this idea and had voted it down. Unfortunately, Nick sees little hope of any such development in the near future. (After all, we have to keep watching those commercial channels so we can be sold!)
Now to a couple of good words I'd like to pass along. First, here's a suggestion from Curt Barnes (WB6 EUN): If you try for a license, take the Advanced Class test at the same time you tackle your General exam. You have nothing to lose, since failing the higher level has no effect on your passing the General, and at least you'll have met the exam face to face. If you do pass, another bonus is that you'll get the higher-class license at no additional cost (whereas coming back later to take the Advanced test would cost another $9.00).
Another helpful thought comes from Jim Bandy, Jr. (WA3HKR), who reminds us of the existence of the National Traffic System (NTS), a network of hams who will transmit and deliver short written messages—(15 words or so)—all over the country, free of charge, for radio freaks or anyone else. (If you're "anyone else" you can gain access to the service by tracking down your nearest ham club.) Jim suggests that New Directions use NTS to help set up schedules, to announce the topics of upcoming Roundtable sessions to interested persons and perhaps even to modify our net schedules on the basis of short-range propagation forecasts.
Jim also mentioned that he'll be operating the University of Maryland's amateur radio club station (W3EAX) this fall and winter. State residents who would like to participate in the Sunday New Directions Roundtable sessions are invited to contact him at 828 Denton Street, College Park, Maryland 20742. He'd be happy to have non-hams join him at the school station, and a Maryland-only WATS line also offers some possibilities.
Incidentally—you other college students and teachers out there—why not track down a sympathetic radio freak and get involved this fall through your own school's amateur station? Very often, such groups' facilities are located in the engineering building. Carry over an issue or two of MOTHER EARTH NEWS and see if you can't stir up some interest.
On another front, folks interested in computer/ham radio tie-ins are hard at it. Bob Wallace, of the OPEN Information Network in Seattle, sent several of us a long letter on the subject along with a big packet of technical info. Marshall Allen (WB6VQC) reports that Dan Taylor (WA6LRA) has gotten together a San Jose-based group that includes Paul Bailey (WB60VC) and Jack Unger (WA6WTX). This bunch hopes to start a Bay Area information depot, and Dan is looking at the possibility of constructing a 2-meter rig for reliable alternatives-oriented communication in and around San Francisco.
A final thought before I move on to more technical matters: Very much needed is an alternatives library or reference service where specific info can be made available to those who want it. This could include not only access to written materials but also a skills bank through which experienced volunteers could discuss problems with others. We radio folks would like to help with the communications end of any such projects, going or planned, and to hear from anyone who's into this field.
The radio antenna is used as a coupling device between a radio receiver or transmitter and space. During "receive" periods it intercepts incoming radio waves and converts" them into feeble electrical signals which are piped through a transmission line to the receiver input. During "transmit," the radio frequency energy generated by the transmitter travels up the transmission line to be launched into space by the antenna.
The mathematics behind all this is very fancy, and we'll leave it to the physicists and antenna engineers. One concept we do need to grasp, though, is that of resonance, which can be illustrated in terms of a pipe organ. Each pipe in such an instrument emits a tone at its particular resonant frequency: that is, the sound wave produced is exactly the length of the pipe (or, more typically, some integral multiple of that length). The pipe's frequency is inversely proportional to its length: that is, long pipes have lower resonant frequencies than short pipes, and thus produce lower-pitched sounds.
An antenna, too, has a resonant frequency which depends upon its length. The simplest and most common ham antenna is called a "half-wave center-fed dipole". Radio waves travel at the speed of light and have a vibration frequency of millions of cycles per second. The "half-wave" designation means only that the length of the antenna is about equal to half the distance between the crests of the radio "wave".
The antenna's actual end-to-end length in feet should be made equal to 468/f ,where f is the frequency in megahertz. For example, an antenna cut to exactly 7233 kHz (7.233 MHz) would be 468/7.233 =64.7 feet long. The transmission line should be attached at the precise center.
The material used for the antenna itself can be any copper wire—insulated or bare—that will support its own weight and that of the transmission line. Eighteen- or 20-gauge "bell wire" is satisfactory if a wooden pole is used to uphold the device at the center. If the antenna is supported by the ends only and the transmission line hangs down, 14-gauge is a better choice.
Normally, three insulators are installed: one at each end of the antenna and one in the center. These can be made of plastic, glass or porcelain. Shape, size and exact material are unimportant as long as they provide good mechanical support and electrical insulation.
If the transmitter power is 200 watts or less, the best transmission line is RG 58/U (a "50 ohm" coaxial cable commonly used for citizens' band radio installations and available through outlets such as Radio Shack and Lafayette). It's lighter weight, less expensive and easier to find than the RG 8/U type which is needed at higher power levels. RG 59/U and RG 11/U are "75 ohm" versions of the same transmission lines and are also satisfactory. Since the larger diameters (8/U and 11/U) have lower loss than the others, they should be considered if the required transmission line length is much over 100 feet.
Rosin core solder should be used to attach the center conductor of the "coax" to one half of the dipole, and the outer copper braid to the other half. The exposed end of the cable must then be weatherproofed with plastic electrical tape, and the cable clamped in some way to the center insulator so that the soldered connections don't support the weight of the transmission line. (A hardwood block, boiled in paraffin for 20 minutes or so, is OK for the center insulator.)
The dipole antenna is somewhat directional; that is, transmitting and receiving is best at right angles to the wire, and worst off the ends. Consider this fact when you plan where to string the device. Regarding height: the higher the better. Sixty or 70 feet above the ground is ideal, and you should try hard to achieve at least 30 or 35.
Nylon cord is a good support for the ends of the antenna (and can also serve as the end insulators at 200 watts or so). If the device is strung between trees which blow in the wind, it will break unless you leave sufficient slack to allow for the movement. Several screen door springs connected end to end, or a pulley and counterweight, can be used to take up the sag if desired.
A dipole cut for a particular resonant frequency will also cover a band of frequencies extending from at least 1% above to 1% below the one intended. In addition, it's possible to connect several antennas cut for different bands to a single transmission line. They should fan out. (The halves of a dipole don't need to lie in a perfectly straight line as long as the angle they make with one another is 90° or greater.)
Good sources of additional information on antennas are The Radio Amateur's Handbook ($4.50) and The ARRL Antenna Book ($3.00), both published by ARRL.