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This second edition contains all-new BASIC computer programs for antenna design and impedance matching, expanded coverage of long-wire directional antennas and radio wave propagation theory, and new material on small loop direction-finding antennas. This book also covers circuits and methods for matching ante nna load impedance to an RF source or transmission line, plus information on antenna measurement and adjustment.
The VHF/UHF spectrum is commonly accepted to range from 30 MHz to 900 MHz, although the upper breakpoint is open to some differences of opinion. The VHF spectrum is 30 MHz, and the UHF spectrum is 300 MHz to 900 MHz. Above 900 MHz is the microwave spectrum. These bands are used principally for local "line of sight" communications, according to the standard wisdom. However, with the advent of OSCAR satellite, the possibility of long-distance direct communications is a reality for VHF/UHF operators. In addition, packet radio is becoming common; this means indirect long-distance possibilities through networking. For the low end of the VHF spectrum (e.g., 6-m amateur band), long-distance communications are a relatively common occurrence.
In many respects, the low-VHF region is much like the 10-m amateur band and 11-m Citizens Band: skip is not an infrequent occurrence. Many years ago, I recall an event where such skip caused many a local police officer to skip a heart beat. In those days, our police department operated on 38.17 MHz, which is between the 6-m and 10-m amateur bands. They received an emergency broadcast concerning a bank robbery at a certain Wilson Boulevard address. After a race to the county line, they discovered that the reported address would be outside of the county, and in fact did not exist even in the neighboring county (a number was skipped). The problem was traced to a police department in a southwest city that also had a Wilson Boulevard, and for them the alarm was real.
The principal difference between the lower frequencies and the VHF/UHF spectrum is that the wavelengths areshorter in the VHF/UHF region. Consider the fact that the wavelengths for these hands range from 10-m to 1m for the VHF region, and from 1m to 33 cm for the UHF region. Most antenna designs are based on wavelength, so that fact has some implications for VHF/UHF antenna design. For example, because bandwidth is a function of length/diameter ratio for many classes of antenna, broadbanding an antenna in the VHF/UHF region is relatively easy. If, say, 25-mm (i.e., 1-in) aluminum tubing is used to make a quarter-wavelength vertical, then the approximate L/D ratio is 790 in the 8-m band and 20 in the 2-m band. This feature is fortunate, because the VHF/UHF bands tend to be wider than the HF bands.
Another point to make is that many of the mechanical chores of antenna design and construction become easier for VHF/UHF antennas. One good example is the delta impedance-matching scheme. At 80 m, the delta-match dimensions are approximately 36 X 43 ft, and at 2 m they are 9.5 X 12 inches. Clearly, delta matching is a bit more practical for most users at VHF than at HF.
Types of antennas usable for VHF/UHF
The concept "VHF/UHF antenna" is only partially valid because virtually all forms of antenna can be used at HF, MW, and VHF/UHF. The main limitations that distinguish supposedly VHF/UHF designs from others are mechanical: there are some things that are simply much easier to accomplish with small antennas. Besides the delta match mentioned previously, there is the ease of construction for multielement antennas. A 14-element 20-m beam would be a wonderful thing to have in a QRM-laden DX pile-up, but is simply too impractical for all but a few users because of its size. If you look on embassy rooftops around the world you will see many-element Yagi and log periodic HP antennas supported on massive towers ... and some of them use a standard Size 25 tower (common for amateur use) as the antenna boom! A 14-element 80/75-m Yagi approaches impossibility. But at 2 m, a 14-element Yagi beam antenna can be carried by one person, in one hand, unless the wind is acting up.
Safety note Large array beams, even at VHF/UHF, have a relatively high "windsail area," and even relatively modest winds can apply a lot of force to them. I once witnessed a large, strong technician blown off a ladder by wind acting on a modest, "suburban" sized, TV antenna. It can happen to you, too. So always install antennas with a helper, and use hoists and other tools to actually handle the array.
Lower band antennas on VHF/UHF
Between 1958 and 1962, a friend and I had access to a radio club amateur radio station in a Red Cross chapter house in Virginia. The "antenna farm" consisted of a 14-element 2-m beam, a three-element triband HP beam (10, 15, and 20 m), and a five-band (80 to 10 m) trap dipole. All of the coaxial cables came into the station through a wall; they were kept disconnected and shorted out when not in use because of the senior Red Cross official's concern over lightning.
One night, attempting to connect the 2-m beam to the Gonset "gooney box" 2-m AM transceiver, my friend accidentally used the cable from the five-band trap dipole instead. We worked a lot of stations that contest weekend, and scored lots of points. Later, we discovered the error, and asked a more technically competent adult (we were teenagers), "Why the good reports?" He then gave us a lesson in longwire antenna theory. A good longwire is many wavelengths long. Consider that a half-wave antenna on 2 m is 80 m/2 m or 40 wavelengths shorter than an 80-m half-wave antenna. Thus, the 80-m antenna, counting foreshortening of physical lengths because the traps, was on the order of 35 to 38 wavelengths long on 2 m. We had a highly directional, but multilobed, pattern.
Sirnilarly, 40- to 10-m and 80- to 10-m trap verticals are often usable on VHF/UHF frequencies without any adjustments. Similarly, Citizens Band 11-m antennas, many of which are 5/8-wavelength (18 ft high), will sometimes work on VHF frequencies. Check the VSWR of an HF antenna on 2 in with a reliable VHF/UHF VSWR meter (or RF wattmeter) to discover the truth about any particular antenna. Always use the low-power setting on the transmitter to limit damage in cases where the specific antenna is not usable on a specific frequency.
The lesson to be learned is that antennas are often usable on frequencies much higher than the design frequency, even though useless on nearby bands. Care must be exercised when initially checking out the antenna, but that is not an inordinate difficulty...
|Introduction to the third edition|
|1||Introduction to Radio Broadcasting and Communications||1|
|2||Radio Wave Propagation||5|
|4||The Smith Chart||95|
|5||Fundamentals of Radio Antennas||123|
|6||High-Frequency Dipole and other Doublet Antennas||141|
|7||Vertically Polarized HF Antennas||173|
|8||Multiband and Tunable-Wire Antennas||203|
|9||Longwire Directional Antennas||217|
|10||Hidden and Limited Space Antennas||235|
|11||Directional Phased Vertical Antennas||249|
|12||Directional Beam Antennas||259|
|13||Antennas for Shortwave Reception||275|
|14||Large Wire Loop Antennas||291|
|15||Small Transmitting and Receiving Loops||303|
|16||Wire Antenna Construction||313|
|17||Antenna Modeling Software||329|
|18||VHF/UHF Transmitting and Receiving Antennas||337|
|19||Microwave Waveguides and Antennas||367|
|20||Antenna Noise Temperature||415|
|21||Antennas for Radio Astronomy||419|
|22||Adjusting, Installing, and Troubleshooting Antennas and Transmission lines||431|
|23||Antennas for Radio Direction Finding RDF||437|
|24||Impedence Matching in Antenna Systems||455|
|25||Mobile, Emergency, Portable, and Marine Antennas||477|
|26||Antennas for Low-Frequency Operation||499|
|27||Measurement and Adjustment Techniques||513|
|28||General Antenna Mechanical Construction Techniques||541|
|29||Grounding the Antenna: What Is A Good Ground?||557|