Demi-Quad Antenna

The demiquad is a single-element 1 quad antenna. The length of the antenna is,like the cubical quad beam antenna (see Chap. 12), one wavelength. Figure shows a type of demi-quad based on the tee-cross type of mast.

The impedance-matching section is a quarter-wavelength piece of 75-Ω coaxial cable (RG-58/U or RF-11/U). The length of the matching section is determined from:

where
L is the overall length, in feet 

FMHz is the frequency, in megahertz

V is the velocity factor of the coaxial cable (typically 0.66, 0.70, or 0.80)

Improved G5RV Antenna

 

The original G5RV antenna was developed by Louis Varney G5RV for 20 meters. Although his design was a good one, he used the 450 ohm ladder line as a feed-line-to-antenna impedance match, and without the use of a BALUN. We discovered that feeding the 450 ohm ladder-line directly with an antenna tuner, left us with a shack full of RF…HOT mics, hum, and in some cases, we had "squeals" from rectified RF getting into the microphone audio path, within the transceiver, a sure sign of RF-Feedback (base rectification).

To make the G5RV more "user-friendly" and with less RF exposure within the HAM-shack, we added an MM11 BALUN at the lower end of the 450 ohm ladder-line, and from the asymmetrical input of the MM11 BALUN (outside the HAM shack), we used 50 ohm (low impedance) coax to reach the antenna tuner inside the HAM shack. We’ve found that this improvement to the G5RV has put more of our transmitted RF into the elements of the antenna, and made the antenna virtually noise free and reduced re-radiation as much as 85 percent.

Without using an external antenna tuner, we’ve found that our transceiver will work into the 50 ohm coax and the MM11 BALUN with VSWR below 2:1 on the bands the G5RV is cut for.

By making the additional BALUN and coax improvement to the original 20 thru 10 meter G5RV, it is now possible to build the G5RV for more bands, and thus cover lower bands and frequency’s. We now have a means by which we can have an antenna that fits almost any real-estate configuration, from as little as 27 feet (8.2 m), (20 thru 10 meter bands) to 207 feet (64 m) (160 thru 10 meter bands).

80, 40, 20 and 10 M Multiband HF antenna

 

Multiband antenna using paralleled dipoles, all con-nected to a common 50 or 75-Ω coax line. The half-wave dimensions may be either for the centers of the various bands or selected for favorite frequencies in each band. The length of a half wave in feet is 468/frequency in MHz, but because of interaction mamong the various elements, some pruning for resonance may beneeded on each band.

The Multee Antenna

Two-band operation in limited space may be obtained with the multee antenna. The portion identified as H should remain as vertical as possible, as it does the radiating on the lower frequency band.

Two-band operation may be obtained on 1.8/3.5 MHz or on 3.5/7 MHz within the confines of
the average city lot by using the multee antenna shown in Fig. Dimensions are given for either pair of bands in the drawing.  If built for the lower frequencies, the top portion will do little radiating on 1.8 MHz; it acts merely as top loading for the 52-foot vertical section. On 3.5 MHz, the horizontal portion radiates and the vertical section acts as a matching stub to transform the high feed-point impedance to the coaxial cable impedance.

Since the antenna must work against ground on its lower frequency band, it is necessary to install a good ground system. Minimum requirements in this regard would include 20 radials, each 55 to 60 feet long for the 1.8/3.5-MHz version, or half that for the 3.5/7-MHz version. If not much area is available for the radial system, wires as short as 25 feet long (12 feet for 3.5/7-MHz) may be used if many are installed, but some reduction in efficiency will result.With suitable corrections in length to account for the velocity factor, 300-Ω TV twin-lead may be substituted for the open wire.  .

bobtail curtain thorne antenna

 

The bobtail curtain antenna is a fixed array consisting of three individual quarter- wavelength elements spaced a half-wavelength apart, and fed from the top by ashorting element or wire. The inverted bobtail curtain, or Thorne array, consists of an upside down bobtail curtain as shown in Fig. 12-11. The radiator elements are each a quarter-wavelength long. Their lengths are found from

The lengths of spacing between the elements are exactly twice above the value or

Feed method  worked out by the late J. H. Thorne (K4NFU/5), feeds the end elements from the shield of the coaxial cable, and the center element of the array is fed from the center conductor of the coaxial cable. A coaxial impedance-matching section is used between the cable transmitter and the antenna feedpoint.

Bisquare loop antenna

The bisquare antenna, shown in Fig, is similar to the other large loops, except that it is wavelength/2 on each side, making a total wire length of two wavelengths. This antenna is built like the diamond loop shown earlier (i.e., it is a large square loop fed at an apex that is set at the bottom of the assembly). In this case, the loop is fed either with an antenna tuning unit (to match a 1000-Ω impedance) or a quarter-wave length matching section made of 300-Ω or 450-Ω twin-lead transmission line. A 1:1 balun transformer connects the 75-Ω coaxial cable to the matching section.

The bisquare antenna offers as much as 4-dB gain broadside to the plane of the antenna (i.e., in and out of the book page), in a figure-8 pattern, on the design frequency. It is horizontally polarized. When the frequency drops to one-half of the design frequency, the gain drops to about 2 dB, and the antenna works like the diamond loop covered previously.

Delta loop antenna

The delta loop antenna, like the Greek uppercase letter “delta” (∆) from which it draws its name, is triangle-shaped (Fig. 14-8). The delta loop is a full wavelength, with elements approximately 2 percent longer than the natural wavelength (like the quad). The actual length will be a function of the proximity and nature of the underlying ground, so some experimentation is necessary. The approximate preadjustment lengths of the sides are found from:

 

The delta loop antenna is fed from 52-Ω coaxial cable through a 4:1 balun trans former. The delta loop can be built in a fixed location, and will offer a bidirectional pattern.

QRP Magnetic Loop Antenna for 40,30,20 Metres