L. B. Cebik (cebik@utkux.utcc.utk.edu) Mon, 9 Dec 1996 08:14:53 -0500 (EST) The reactangular 40-meter loop proposed by W6RCA is an interesting antenna worth some more extensive modeling. So I made a model using the original dimensions of 20' vertical sides and 51' top and bottom. I put the bottom at 20' so the top came out at 40', a feasible ham installation. Remember, this is a vertically oriented rectangular loop. Frequency of interest was 7.15 MHz, and the wires was #12 copper. Feedpoint Feed Z Gain in dBi Mid-side 46 - j28 2.61 dBi at 20 degrees (max) Lower corner 61 - j39 2.37 dBi at 22 degrees (max) Mid-bottom 237 + j94 1.77 dBi at 22 degrees 5.19 dBi at 45 degrees 5.64 dBi at 64 degrees (max) In use as a low angle dx antenna, there is little to choose between the mid-side feed and the bottom corner feed. (Top or bottom corner feed makes no operational difference.) The mid-side feed is less convenient, but does exhibit a lower high angle gain: at 45 degrees, the mid-side version gain is -1.90 dBi, while the corner feed version is -0.30 dBi, about a 1/4 S-unit difference. Notice that changing the feedpoint either to enhance horizontal radiation or to enhance vertical radiation does have an affect on the resonant frequency of the antenna. The model proposed appears to be a compromise: long for a horizontal antenna, short for a vertical antenna (polarization-wise). If one wishes to use a dual feed, perhaps enlarging the antenna to resonance is the better bet. One would then have a feasible coax match at the corner (or mid-side), with a parallel feedline junction at the center of the bottom for use with an ATU (where precision resonance of the basic antenna is less important). Relays or hand changes would do the job, but one must remember to replace the feedline disconnected with a jumper to complete the loop. For comparison, here are some numbers for a full size 66+' dipole at 40' at the same frequency. Gain at 51 degrees 5.88 dBi (max) 45 degrees 5.81 dBi 22 degrees 2.95 dBi The performance exceeds that of the loop with a bottom center feed because the power is not divided between a higher and a lower wire. The gain at the 22 degree level exceeds that of the loop with side or corner feed, but there is also higher angle radiation going out and coming in, which means QRM and QRN for the dx paths. In addition, although the story is too long for this posting, one might be better satisfied with the patterns on bands above the fundamental with the dipole fed with parallel feedline. Some of the loop patterns are very narrow in beamwidth and some are quite high in take-off angle at some frequencies. If I had my druthers and some trees/poles/towers, I'd take a dipole for fundamental high angle paths and multiband use and a vertically-polarized antenna for dx paths. Perhaps the easiest low angle antenna to erect (because it is independent of the need for ground planes and counterpoises) is the half square. Since the half square is a half wavelength long horizontally, and needs about a 40' height to let the verticals dangle down to about 5' above ground (for nearly 3.5 dBi gain at about 21 degrees), ingenious builders might make a relay system to transfer from the corner to the center (with the verticals disconnected). But if you can have one and only one antenna, the loop is not too bad a choice for somewhat easier (manual) changes in polarization of radiation. Hope these details are useful to someone. -73- LB, W4RNL Cecil A Moore (Cecil_A_Moore@ccm.ch.intel.com) Tue, 10 Dec 96 07:54:00 PST >From: "L. B. Cebik" >In use as a low angle dx antenna, there is little to choose between the >mid-side feed and the bottom corner feed. Hi L.B., just a few comments based on EZNEC. EZNEC sez this rectangle fed on a side is resonant at 7.205 MHz with a feedpoint impedance of 47 ohms and a gain of 2.79dBi at TOA=21deg. Horizontal beamwidth is 2x87deg. For bottom corner feed, it is resonant at 7.24 MHz with a feedpoint impedance of 78 ohms and a gain of 2.6dBi at TOA=30deg. Horizontal beamwidth is 2x95deg. This configuration has a very large footprint. The zero dBi point occurs at a TOA of about 11deg. The horizontal radiation pattern covers almost 190deg and the vertical radiation pattern covers almost 160deg, the large footprint that I prefer. This antenna is comparable in performance to a dipole and has a few advantages. 1. Radiation is not straight up at low heights. There is a combination of useful medium and low take-off-angle radiation with a large footprint. It beats a vertical for medium take-off-angles and beats a low dipole for low take-off-angles. 2. Coax is minimized. Bottom corner feedpoint can be close to the shack in a location where a dipole cannot be centered. In this respect, it has the physical advantages of an end-fed and/or base-fed and the radiation advantages of a full-wave loop. Give credit to Kurt Sterba for mentioning this antenna in his Worldradio column, "Aerials". Here's the two configurations that I have tried. X is the feedpoint. 51ft 59ft +-------------------+ +------------------------+ | | | / | | | / | | 20ft on a side | / | | | / | | | / X-------------------+ X These antennas also load with a tuner and perform well on 20m,15m,10m. 73, Cecil, W6RCA, OOTC