Date: Sat, 03 Oct 1998 02:46:11 -0700 Reply-To: utahfolk@konnections.com From: utahfolk@konnections.com To: "Low Power Amateur Radio Discussion" Subject: Loops A lot has been written about loop antennas. Some of it not so. A large loop is one whose circumference is at least one wavelength at the frequency of operation. This discussion relates to large loops. Reasonably resonant is a term used to mean that a practical network can be built to match the antenna feedpoint impedance to a common coaxial line. A half wave dipole at frequency F is resonably resonant at its odd harmonics: 3F, 5F, 7F, etc. The only practical case in the HF ham bands is: a 40M (7mhz) dipole works on 15M (21mhz). Some try 30M dipoles on 10M too. But in general not a lot of multibands from a dipole. Consider now the half wave folded dipole, this is a dipole in which the impedance of the feed point goes up by a factor of about 4. The antenna consists of two parallel half wave wires spaced quite closely and connected at each end. A feedline connects to the midpoint of one of the half wave wires. The total circumference or perimeter of this folded dipole is one wavelength of wire. The enclosed area is equal to one half wave times the distance between the two half wave wires. For our discussion, hardly any area at all ... call it ZIP! This folded dipole behaves just like any halfwave dipole when operated harmonically ... it is reasonably resonant on its odd harmonics only. Now take this folded dipole and "open" up the area enclosed by the two half wave wires. The maximum area you can enclose using a fixed length of wire is when the geometry is a circle. If you analyze a circular loop you discover it is reasonably resonant on ALL harmonics, you can match it quite easily on all the ham bands to coax. But a circular loop takes an infinite number of skyhooks (kinda expensive if perchance somewhat impractical) so settle for a square (1/4 wavelength of wire on each side -- at the lowest frequency of operation), takes four skyhooks, more likely to be found or accomplished. The impedance/matching problem has moved back towards that of the folded dipole, but a square is still quite large so the characteristics are much closer to the perfert loop than the ZIP area folded dipole. A square is 80%-ish of the area enclosed by the wire in a circular configuration. Make a loop triangular, rectangular (off square) and you move closer to the folded dipole and it's more restricted impedances ... so, what's the rule: When you put up a loop, enclose the maximum area possible ... use as many corners as you can, but don't settle for anything less than a square if you don't have to .... stay as far away from the folded dipole ZIP area as you practically can. The plane of a loop is that plane containing the loop wire. These loops are thus viewed as two dimensional objects. A one wavelength loop (this frequency is called the design or fundamental frequency) in free space has a dipole pattern normal to the plane of the loop ... that is, it radiates max perpendicular to the loop plane. At the second harmonic, the pattern splits into four lobes and these lobes move towards the plane of the loop. A similiar effect occurs at each higher harmonic, the number of lobes and nulls increases. The lobes (max values) fold towards the plane of the loop. Result -- here is antenna that operates all bands (tuning wise) and whose patterns fold into the plane of the loop as the frequency is increased. What happens if you mount the loop vertically? It radiates perpendicular to the plane of the loop (thus at a somewhat low angle above the earth horizon) on it's fundamental, but the lobes fold into the plane of the loop (begin to radiate towards the sky) as the frequency is increased. What happens if you mount a loop horizontally? The pattern on the fundamental frequency has lots of high angle signal, but not worse than a corresponding dipole!, for all practical purposes they are the same. As you incrase the frequency the lobes fold down into the plane of the loop (down towards the DX horizon!) and gain increases. Conclusion: If you hang a loop vertically you limit it's practical use to one band (the fundamental design frequecny band) and most loop users have been taught this use for a loop. They perform super well ... but are single band loops for DX purposes. If you hang a loop horizontally, you have dipole illumination of the sky but as you move to the harmonics (which are easily matched) the lobes move into DX elevations and shazzam you have a multiband DX monster! Thus, if you work one band, hang the loop vertically. If you intend to work many/multi bands, hang the loop horizontally. The gain is actually a bit more in the horizontal mode! For years folks have said the horizontal loop is a "cloud warmer" and doesn't give much DX results .... they have never USED a horizontal loop harmonically! In fact, until about 1980 or so, most loops users were sold the mount them vertical story. In 1985 a fella wrote an article called the LOOP SKYWIRE, a very practical presentation of the horizontal loop and an article that has been copied, "borrowed and rehashed as original text" by more than one folk out there! First published in November 1985 QST and since about 1990 included in the ARRL Radio Amateurs Handbook and the ARRL Antenna Handbook. Hundreds and possbily thousands of (horizontal) loop users have discovered this antenna to be the best multiband antenna they've ever used and few seldom take it down or remove it! I know of folks who took their beams down before their loops! The great advantage of the loop is that it can be operated as a grounded antenna (feed with coax) and thus does not suffer the major wipeout of precip static. Perhaps the most significant parameter is the loop has a considerable improvement in S/N ratio, in that it's response to noise is often 3-4 S-units lower than verts and dips. You hear things you haven't heard before. Feeding the horizontal loop is essentially your choice, the feedpoint impedance at the mid of one side of a square loop is a bit higher than at a corner, but feed a foot or so off the corner so you don't have to combine feedline mechanics with those of corner support mechanics ... There is a fundamental rule about horizontal loops: If you can hear them you can work them! 100 watts will do quite nicely! Go read the sidebar in the original LOOP SKYWIRE article where W8BO comments about his loop! 5BDXCC! He uses RG58 coax to feed it! This writers experience with loops goes back to 1957 and there is no antenna that beats it in my experience! I have many first place plaques, certs and listings in contests, many QRP! If you are interested in seeing the 160M version, check UPFRONT QST either Feb 96 or 97, the picture of the flat loop that took 4th place worldwide, made 50 WAS in 39 hours, and set the record for number of ARRL Sections worked (70) in the 1994 ARRL 160M DX contest .... QRP! A calibrated 4.8 watts on 160M ... warmin' them clouds!!!! The more of you who believe the popular "loops are cloud warmers" propagada the stronger my signal will remain on the band. Don't put up a loop! Use a dipole, a vertical or a random wire ... loops don't work, they attract lightning, bugs and birds. Neighbors don't like them either, and they radiate into your phone lines, cause TVI and BCI .... and one case of GDI (Garage Door Interference) ... I think I heard some Doctor say they cause brain twomores and mebbe pizza cravings! So, loops are hazardous! Don't put up a loop! Keep your antennas close to the ground. Load your extension ladder .... and getta 8KW furnace AMP !! .... de Dave NC7W Huntsville, UT ..... (way back when wuz w0mhs)