-
Basically, the Inverted-L is just a shortened vertical with a "top-hat" capacitive load provided by the horizontal portion. For greatest efficiency, the vertical section should be as tall as possible. Since the radiation resistance (efficiency) is a function of the square of the vertical length, even a few feet can make a difference. Of course, as has been stressed, the overall effectiveness hinges upon the availability of a good radial ground system with as many radials as possible. The last word on DXing on the low bands cannot avoid the problem of local noise and, as stated previously, vertical antennas are susceptible. While some are able to erect dipoles (or inverted-Vs) that are high enough to be useful radiating - as well as receiving - antennas, it isn't easy to find room for a 260 foot-long antenna! That means verticals are the usual choice for the low bands. How does one get around the noise problem? A common practice is to use the vertical for Tx, taking advantage of its low radiation angle and "footprint", and use a second "low-noise" antenna for Rx. The subject of 160m Rx antennas is a book unto itself, so those who may be interested are directed to the references below. Having said this, let me add that on 160m the Inverted-L is a good all-around choice for anyone with limited space. The reason is that the horizontal portion (which usually turns out to be longer than the vertical part) seems to provide a good "lower-noise" receiving compromise than would a true vertical. As evidence of this last observation, consider my "late-life" adventure on 160m. Living on a relatively small (18m x 30m) urban lot, I felt lucky to have been able to put up a 20m-high crank-up tower for my 5-band quad. After almost 40 years as a licensed and active ham, I had never tried 160m because I had been told that it was " ... only for hams with plenty of green-space ...". Well, after some limited but unexpected success in working DX with an 80m Inverted-V hung from the tower, I decided that it was definitely possible for even a city-slicker to enjoy Top Balun! I put up an Inverted-L for 160m: 40m of 12gauge stranded/insulated copper wire running approximately 12m vertically up a small oak tree in the rear, and going out horizontally for 28m to a pine tree in the front yard. I then ran 10 radials varying in length (20m - 40m) in every nook and cranny of the yard and, with permission, sneaking a few out on the property behind and adjacent to mine. Did it work? Well, after some 5 years, I had over 100 DXCC entities confirmed, while having as much fun as I did when I first began to work DX in the middle 1950s! Of course, it was not easy and required a lot of late night/early morning listening through a LOT of noise, along with developing a new understanding of propagation, receiver sensitivity/selectivity, and plenty of P.E.P.S.I.!!. But I will tell you that it was, for me, a feat of immense satisfaction that I never imagined possible! Topband - try it, you'll like it! Directional antennas: these are the ultimate for DXing beacuse they offer not only gain, but also front-to-back rejection that is invaluable in DX pileups. One of the simplest directional antennas is a full-wave vertical loop antenna (figure). The actual loop geometry is not critical, and a diamond, triangle, or square configuration should all work well.
The vertical loop provides a bi-directional dumbbell-shaped radiation pattern, with major lobes extending outward from the plane of the loop. Full-wave loops also provide low-angle lobes even at low heights, giving them a distinct edge over both dipoles and verticals. These features are illustrated in the figure below.
To understand why a loop is directional, one can envision it as two verticals joined at the feedpoint and at the top that cancel the radiation in their common plane. Illustrated in the figure below is the way that the constructive interference and destructive interference of the emitted waves form the net radiation pattern of a full-wave vertical loop.
This is typical of the way that all directional antennas create the desired focal radiation patterns by using constructive/destructive interference of emitted waves from multi-elements resonant on the desired band. The most economical configuration is that of a "driven element" fed via a transmission line, along with additional resonant but usually unfed elements (called "parasitic" elements) that absorb and re-radiate the energy to create the interference that results in the final radiation pattern in a preferred direction. The usual arrangement of the elements consists of a "director" element placed behind the "driven" element, along with one-or-more "director" elements placed in front. By strategic spacing, the parasitic elements can produce the appropriate interference among the emitted waves in order to provide highly focused beam-like radiation patterns in the direction of the "director(s)". For this reason, multi-element directional antennas are referred to as "beam antennas" or just "beams"; and these may be fixed or rotatable. Because the size of the elements of directional antennas is approximately 1/4 wavelength each, and the element spacings are approximately 0.15 - 0.25 wavelengths apart, rotatable antennas are mostly found on the higher frequency bands (20m and above) where the size becomes more manageable. Of course, some compact beams are available for 30m & 40m, and even a few full-sized rotatable beams are being used on 80m! If space, neighborhood regulations, and budget allow, then a rotatable beam antenna mounted as high as possible is the unqualified best all-around antenna of choice for DXing, especially for the higher bands. On the lower frequency bands (40-160m), if an acre or more of space is available, directional arrays of phased-verticals are a good choice. One should consult the References below for more information. The the most popular rotatable beam design is the Yagi-Uda (or more commonly, "Yagi"), consisting of a half-wave dipole with one or more similar elements mounted in parallel on a "boom" and spaced approximately 0.15 - 0.25 wavelengths apart, as seen in the illustration below. The picture shows two popular yagi models in a "stacked"
configuration (i.e., one-above-the-other). The larger of the two is a 20m "monobander", with 5-elements - reflector on right, followed by driven element, and 3-directors - an extremely potent weapon for DXing! The smaller antenna above it is a dual-band (15m & 10m) yagi - looking closely, you can see the multi-band "traps". Almost as popular as the yagi is the cubical quad which consists of a full-wave loop and one or more adjacent closed loops spaced 0.15 - 0.25 wavelengths apart, and typically mounted in a "cube" configuration as illustrated schematically below.
A 5-band (20/15/17/12/10) version is shown in the picture, using a single feedline connected to all 5-director element "loops" through a 2:1 impedance matching transformer (note the choke balun, consisting of coiled coax, attached to the mast and just visible above the top of the tower).
Reference Websites
|
)
)
)
)
)
)
)
)
|
|