Oversized Antennas for Undersized Wavelengths
From the Olde Antenna Lab
Dave Clingerman W6OAL
As of late a lot of experimentation has been done in the Rocky Mountain Region on the 13 cm band. Antennas are always a subject that comes up with the use/operation on any band and so the 13 cm band experimentation was not an orphan. For directionality the gamut has been run; yagis, loopers, dishes and the like. But, how about for mobile or just plain ole’ omni-directional operations? Yeah, the vertical monopole comes to mind, the semi-radiating dummy loads that they are. They have their place, but not in connection with any sort of microwave DX’ing. Then there is the dipole in both vertical and horizontal configurations but still not a real omni. There are the hoops and loops and square things and heavily capacitive loaded plates and halos and squalos and turnstiles and you name it. All of them with all of their patterns containing lumps and bumps and cardioids and nulls; depending on installation anything else imaginable for a pattern. Still, most of which, are based on some configuration of a dipole (bent here, curved there) and not producing what might be called a good omni pattern. Furthermore, the current in a dipole of any configuration is not constant along its length. Along one arm of a dipole the impedance rises from 73 ohms at the center to ~2500 ohms at the end and thus the current falls off drastically beyond the first eighth wavelength of that quarter wavelength arm. This situation is rectified by using a folded dipole, to some extent. Current being constant throughout a series circuit, of which a folded dipole is allows radiation from the entire folded dipole structure and not just that 1/8 portion of the one side of the conventional dipole. It is still not an omni as its pattern is as bidirectional as the single wire dipole. A pair of folded dipoles in a turnstile configuration will produce a pattern that starts to approach an omni but still has four significant nulls around the periphery of its pattern. The stacking of folded dipole turnstiles can fairly much obviate this discrepancy.
Through many years of experimentation with units in operation both amateur and government, many published articles, received letters of acknowledgement and papers of praise – it has been determined that the Wheel antenna (three one wavelength cloverleaf looking loops centered on a hub) has the most omni-directional pattern of any array in practice, in print or on the market today. The gain is good, claims of 5 dBi have been read of and experienced. The current is constant throughout the loop(s) as they are closed loops. The pattern is consistent save for three 0.3 dB nulls, one every 120° of the periphery. The current is cancelled in the paralleled quarter wavelength sections of the loops that lead back to the hub and are attached to the half wavelength periphery sections ends. The Wheel can be modeled as three paralleled broadband monopoles. A monopole has a 36 ohm terminal impedance, the paralleled three exhibits a 12 ohm impedance which is matched to a 50 ohm transmission line with a stub. The stub is less than a quarter wavelength long and therefore appears capacitive and cancels the array inductive reactance.
Now many profess the Wheel to be a true omni (or the closest thing to one) the subject matter of the paper can be broached. An omni for the 13 cm band would consist of three loops (skeleton clover leafs) measuring an average diameter (if visualized as circles) of 4.138 cm or 1.629”. Possible to build, yes, difficult to tune, yes, and not one whole heck of a lot of radiation from one due to the very small area of the peripherys’ single dipoles. So now, how about the old adage of ‘the more metal one has in the sky, the better one receives a signal’? And visa versa due to the laws of reciprocity. The statement (by Frank Jones – W6AJF – SK) that if your antenna array lasted through the winter it probably isn’t big enough, propagates a lot of truth. Well, we don’t want to get that big on the 13 cm band; however, we can get bigger than a three and a quarter inch periphery of wire that would make up the radiating portion of a normal size 13 cm Wheel antenna. Cute yes, but not terrible efficient especially in the far field.
Many of us have used our 2 M yagi to operate on 432 MHz and our 70 cm yagi to operate on 1296. It works, and in many cases it works well. The gain will not be the same as it’s a compromise. Back in the ‘70’s the Los Padre’s Microwave Group of Ventura County in CA consistently used their 2 M antennas on 432 MHz to work from Mt Pinos near Bakersfield, CA to Las Vegas, NV, and also found the 2 M beams to work fairly well on 23 cm to work the folks up in the San Joaquin Valley to the north of Bakersfield.
The case in point, and in an effort to get more metal in the sky (so to speak), a 13 cm Wheel antenna was created that is three times the size it would normally be. Each element was cut to 39 cm or 15.35 inches. This produced a radiating area of 7.675”, ~19.5 cm which is three times the size of a 13 cm dipole and end impedance was preserved (~2500 ohms). The otherwise one quarter wavelength sections from the ends of the three times normal size dipole back to the hub were 9.75 cm long, again preserving impedance – high impedance at one end of a quarter wavelength section reflects a low impedance at the other. A three quarter wavelength section is impedance-wise is no different than a single quarter wavelength section. Twice the 9.75 cm (19.5 cm) sections, hub to dipole end lengths added to the radiating portion of 19.5 cm, three dipole lengths returns to the total length of the element, 39 cm. In other words we built a Wheel antenna for 768 MHz and operated it on its third harmonic. So, did it work?
Art – WØBA mounted this over sized antenna on his vehicle and running ~3 watts SSB went mobile from the Olde Antenna Lab (DM79QL), 77 miles north to his residence in Fort Collins, CO and never lost contact during the entire trip to DN70KM. The set up at DM79QL was 2.5 watts to a pair of 45 element Loop Yagis at 60 feet. Through all the ups and downs and twists and turns the signals on both ends were always Q-5 but with widely varying signal strength from S-0 to S-9 on occasion. The 13 cm “Three Times Normal Size Wheel” now resides on the roof at the WØBA location and our Saturday evening 13 cm get-togethers allows Art to work fairly consistently over the 77 mile path down to W6OAL and Q-5 to Don – NØYE & Bill – KØRZ in Boulder, CO, a distance of 38 miles. Also, 7 miles up the road from his QTH he works Phil – W6HCC in Wellington, CO. The band does exhibit some very interesting characteristics from daylight to dark and from season to season.
In summary, let me suggest that antennas will work on the odd harmonics (3, 5, 7, 9 etc.) of their design frequency, directly driven omni‘s especially. Why I make this distinction is that gamma matched Omni’s or other antennas employing any sort of capacitance in the matching system don’t do a very good job. To wit; Art – WØBA has a 2M H.O. Loop (clone) that he operates on 70cm and 23cm with a very good signal up and down the Front Range of Colorado.
We would like to see more interest shown in the populating of the 13cm band as it’s good for, at least, 100 mile QSO’s along the Front Range. In the very near future there should be a considerable number of beacons on the band (2304.3MHz) due to the auspices and good fortune of a group of hams from the Canon City Area acquiring some old Farinon telecom equipment that operated in the 2 GHz portion of the spectrum. A lot of these have been distributed throughout Colorado and we hope to have a 13 cm beacon network up and running very soon.
Remember we are limited in our endeavors only by our imagination!
Olde Antenna Laboratory 41541 Dublin Drive Parker, CO 80138