245 lines
12 KiB
Plaintext
245 lines
12 KiB
Plaintext
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HIGH FREQUENCY ANTENNAS
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JD DELANCY, K1ZAT/3
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1. Long Wire Antennas:
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a. The fundamental wire-type antenna is the horizontal half-
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wave center-fed antenna. The nominal impedance of this type of
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antenna is about 73 ohms, which will give a 1.5:1 standing wave
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ratio (SWR) for a 50 ohm transmitter/receiver. Various matching
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networks (delta and gamma types for example) can be used for 1:1
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ratio, but the power loss with a 1.5:1 is minimal, and the value
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of messing around with matching networks is questionable. Like
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all balanced antennas, the dipole should be fed through a 1:1
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balun (balanced to unbalanced transformer). The problem with
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half-wave dipoles for 75 meter frequencies (3.9-4.0 Mhz) is that
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the antenna is approximately 120 feet long. The antenna
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should be erected horizontally as high as possible. In most
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cases, the antenna will not be at least a 1/4 wavelength above
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ground (at 75 meters that equates to 55 to 60 feet). This means
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the directional effect of the dipole is virtually non-existant,
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so any geographical orientation can be used for omni-directional
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use. Dipoles are sometimes referred to as "long wire" antennas,
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an erroneous term since a true long-wire is at least one
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wavelength long. Variations of the half-wave dipole in use
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includes the commonly called "coaxial" dipole, which uses coaxial
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cable for part of the radiating element. The coaxial dipole
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seems to have a better bandwidth, probably due to the diameter of
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the coax used as a radiating element versus the number 14 or 12
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wire diameter used with a regular dipole. The half-wave dipole
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provides a good match only at the frequency for which it is "cut"
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and it has a high angle of radiation (60-80 degrees) which is
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correct for the 100-500 mile operating range.
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b. The trap dipole is a variation of the half-wave
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horizontal dipole which provides multi-band operation. Wave-trap
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networks are inserted along the length of the dipole and act to
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disconnect part of the antenna on higher bands to permit matching
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at different frequencies. The characteristic impedance of the
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trap dipole is also about 73 ohms, the same as the basic dipole.
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The length of the antenna is still the same on the lowest band,
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so a large amount of space is still required. The traps are
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frequency-dependent networks, and thus provide a relatively
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narrow bandwidth of matched operation. These antennas are also
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non-directional at the heights normally used.
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c. The inverted "V" dipole is a variation of the half-wave
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dipole that can be used in restricted space. The center-fed
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portion of the antenna is supported at a height of 40-50 feet,
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and the radiating elements are ruin diagonally down to the
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ground. The angle between the elements should be about 100
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degrees for a good match to 50 ohms. The element lengths are
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approximately the same as a regular dipole. The angle of
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radiation is about 50 to 70 degrees which is suitable for
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operation in the 100-500 mile range.
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d. The rotatable dipole is a center-loaded dipole about 40
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feet long with aluminum conduit elements. When mounted about 50
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feet high, where the directional effect becomes apparent, it can
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be rotated and permits desired signals to be peaked, or undesired
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signals to be suppressed. Initial testing of such an antenna has
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indicated no real advantage over a dipole or inverted "V" that
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would make it worth the time and effort.
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e. True long-wire antennas are at least one wave-length
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long. Such antennas are directional off the ends of the antenna.
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If one end of the long-wire is terminated in its characteristic
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impedance (500-600 ohms), the antenna can be made uni-
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directional, obviously, a good sized backyard is needed for a 75
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meter long-wire antenna (it's at least 220 feet long!!).
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2. Parasitic Arrays:
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a. The "beam" antenna commonly used is a three-element
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parasitic array - reflector, driven element, and director. The
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antenna is directional and will give 3-5 DB gain. The elements
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are half-wave in length, which means they are practical for 10,
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15, and 20 meters -- get a little unwieldy for 40 meters -- and
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just about impractical for 80 meters!! The characteristic
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impedance is about 73 ohms, with most antennas having a gamma or
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delta matching network for operation at 50 ohms.
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b. The YAGI antenna is the "pure" form of the beam antenna.
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YAGIs usually have 5 to 10 elements. The driven element in a
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YAGI is a folded dipole, making the characteristic impedance
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about 300 ohms. A 6:1 balun is normally used with YAGIs for a
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match to 50 ohms. A ten element YAGI will have a gain of about
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10 DB, with a narrow bandwidth. Again, the physical size of the
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antenna with half-wave elements is the limiting factor in low-
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frequency usage.
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c. The Quad or delta-loop antenna has become popular in
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recent years. the biggest advantage of Quad antennas is their
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ability to provide gain with broad bandwidths. The gain of a
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Quad is similar to that of a beam or YAGI (with an equal amount
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of elements). The quad is a directional antenna with an
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impedance of about 200 ohms (very approximate) and thus requires
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a matching network for 50 ohm operation. Again, the limitation
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in use for HF work is the physical size, since the Quad is a
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square-shaped antenna with each side 1/4 wave length long, while
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the delta-loop is triangular with each side 1/3 wave length long.
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Needless to say, there are not many 80 meter Quads around.
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3. Vertical antennas:
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a. The most common vertical for fixed-station use is the
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trap vertical, such as made by Hustler and Cushcraft. As with
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the trap dipole, this antenna uses frequency dependent networks
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to isolate portions of the antenna for proper resonating on
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different bands. Although the antenna can be mounted at ground
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level, greatly improved performance will result from installation
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on top of a 20-30 foot high mast. The efficiency of the antenna
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is dependent upon installation of adequate radials. At least
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four, cut to 1/4 wave length, should be installed for each band.
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These radials can also be used as the top guy wires for the mast.
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These antennas can be adjusted for a very low SWR at 50 ohms, but
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have a narrow bandwidth. The radiation angle is 30-40 degrees,
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making them perform well for long-distance operation (1500 mile
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range) while still usable for short-distance (500 mile range).
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These are omni-directional antennas.
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b. The single band vertical for 75 meter operation is
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usually a loaded type, either with a coil at the bottom for base-
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loading or a coil in the center of the mast for center loading.
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While top-loading would be preferred, the size and weight of the
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coil required would make for an unwieldy mechanical structure.
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Center-loading is preferred to base-loading since a large portion
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of the radiation is performed by the coil itself, and the
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elevated coil will provide better performance and efficiency.
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The antenna can be resonated for a 50 ohm impedance. Being
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frequency-dependent, due to the loading coil, the bandwidth is
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relatively narrow. Again, the efficiency of the antenna is
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highly dependent upon the ground radial system. There should be
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a minimum of four (1/4 wave) radials, with the more, the better.
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The antenna can be elevated on a mast for better performance.
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The radiation angle is similar to the trap vertical.
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c. Towers utilized to elevate VHF antennas or beams can be
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loaded and resonated as vertical antennas. The base of the
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antenna can be grounded or insulated. Adequate ground radials
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are required for proper operation. The radio amateur's handbook
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shows several methods of feeding and matching towers for use as
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vertical antennas, even for 160 meters. A common use for towers
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is as a counterpoise for "half sloper" antennas. This is a form
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of inverted-antenna with the tower acting as one of the radiating
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elements. The antenna is fed at the top of the tower with the
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coax shield connected to the tower, and the center conductor
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connected to the sloping element. A "slope" of approximately 45
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degrees will permit matching to 50 ohms.
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d. Due to mechanical problems involved, the only practical
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antenna for mobile HF operation is a vertical whip antenna.
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These can be base-loaded, center-loaded, or trap verticals with
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the body of the car acting as the counterpoise. The efficiency
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of such an antenna, though, is only about 5 to 10 percent
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(working DX from a car usually means driving to the top of Pike's
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Peak!!). For local (100 mile) operation, the mobile vertical
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whip does a good job. Two important considerations apply to
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safety in mobile antenna operation -- one, the antenna must be
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mechanically suitable to withstand the whipping and shaking
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involved in traveling -- two, the antenna height must be
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considered in clearing overhead obstructions, PARTICULARLY power
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lines.
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4. Examples of simple design antennas:
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\ctr\G5RV MULTIBANDER ANTENNA
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I I 51 FEET
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I I
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I I
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I I
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I I -------- 300 OHM LINE
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I I L = 36 FT FOR
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I I HORIZONTAL DIPOLES
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I I
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I I L = 29 FEET FOR
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I I INVERTED VEE DIPOLES
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I I
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I I
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I I
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--------------
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| | ----------1:1 50 OHM
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| | BALUN
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--------------
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I
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I
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I -------------------50 OHM COAX
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I (RG8) ANY
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I LENGTH
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XMTR/RCVR
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The impedance at the lower end of the 300 ohm line is 50 to 60
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ohms. A 1:1 balun is recommended for RFI or TVI suppression.
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The antenna is slightly "short" on 80 meters and the 300 ohm line
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section acts as a sort of matching stub on that band and acts as
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an impedance transformer on the other bands. Trim the 300 ohm
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line for best SWR on 15 or 20 meters (most critical bands). SWR
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should be 2:1 or better across any band. SWR of 1.3:1 can be
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obtained for the most resonant frequency in each band. Antenna
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is good for about 400 watts. For higher power, use regular
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amateur transmitting open-wire line (300 ohm). Shielded twin-
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lead can be used, but length will be different due to cable
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propagation factor. You will have to cut and try.
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\ctr\DJ4BQ DOUBLE-DIPOLE
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(55 feet) (55 feet)
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------------------------------O O------------------------------
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-----------------------O O-----------------------
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(40.5 feet) I I (40.5 feet)
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I I
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I I --------200 OHM OPEN-WIRE
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I I LINE, ANY LENGTH
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I I
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I I
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------------
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I I -------- 4:1 BALUN
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I I (50:200 OHMS)
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------------
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I
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I
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I ------------- 50 OHM COAX
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I (RG8) ANY LENGTH
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I
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I
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XMTR/RCVR
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This antenna operates from 80 to 10 meters. The longer dipole
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operates on 80, 20, and 15 meter while the shorter dipole
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operates on 40 and 10 meters. SWR of 1.2:1 to 1.5:1 are typical
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on each band. This is a 200 ohm antenna and a 4:1 balun must be
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used to match it to 50 ohm coax. Dipole wires should be spaced
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at least six inches apart to eliminate interaction of dipoles.
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Heavy-duty twin lead can be utilized for the dipoles (forming
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both dipoles at the same time) but the length of the dipoles will
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be different from the standard dimensions, and you will have to
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"cut and try" for the lowest SWR. Amateur transmitting 200 ohm
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open-wire line with plastic spacers should be used, with about a
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six inch separation from tower legs or mast. Inverted Vee
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operation may require shortening of the dipole lengths due to
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extra capacity to ground.
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\ff
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