Now that you've got some power on 1296 MHz, you'll need a good antenna to make the most of it. Along comes Antennes Tonna of France with a new wrinkle for 23 cm: a 55-element long-boom yagi* The boom is 15-feet, three-inches fong and the manufacturer claims 21.25 dB gain over an isotropic dipole. Now, that's a lot of antenna!
The antenna comes completely disassembled with the various elements and holders in separate packages, unlike the 432 Tonna yagis that come largely assembled. But don't let the apparent complexity of the antenna scare you: Tonna has earelully color-coded the ends of the various elements so you can tefl them apart. The elements themselves are made from £10 enameled wire that is filed flat but still quite sharp, so be careful. Select the elements by their color and push them through the one-piece molded element holders. YouhU need a ruler, preferably one with a metric scale, to center the etements.
\ have assembled two of these antennas and they both took about 2 hours of careful work from start to finish. Tonna includes an extra element in case you bend or break one. This extra element is as long as the reflector. As wath all Tonna antennas, the drive element is a seaied dipole with a supplied RG-213 pigtail. On every Tonna antenna I ve ever used (and that includes five 432 MHz 21-element yagis and four 1296 MHz 23-element types) the dipole-driven element exhibits better than a 1,2:1 match in the desired bandwidth. Not only is this an excellent matching systeiu but it's also very durable There are no mechanical joints to corrode o: work loose, and I wonder why more manufacturers don t use it, You'll notice that the antenna comes with two boom braces. One problem with 23 cm yagis (and higher frequency yagis) is that the antenna mast support usually is a significant portion of the element length, and a 2-inch mast in front of one of these directors detunes the antenna quite nicely. Tonna recommends either mounting the antenna on top of a mast section, or using an outboard sidearm with the boom attached to the masl at its end. Since the elements stand off from the boom, mounting the boom at mast end ensures that the elements are sufficiently clear of the boom and mast The top clamp makes this attachment, and the second, lower brace and clamp serves to level the entire antenna. As you might expect, the boom exhibits a bit of sag,
Once the antenna is assembled. you will probably have to break the boom at least in half to get it out of the door! I forgot the boom length and assembled the antenna inside forgetting that my door wouldn't allow the clearance to get it out!
The next order of busfness was to verify the claimed gain spec. Steve Katz WB2WIK of CO magazine also received one of these monsters for evaluation, and he suggested we pool our efforts to set up a test range in which the gain figures could be verified. Sieve did some research and came up with a formula from the publication "Technique of Microwave Measurements/ from the M l.T. Radiation Laboratory
G = Gain expressed arithmetically (not in dB) \ = Free space wavelength in units R = Range of separation of antennas in same units \
Fig. 1. Formula for calcuiating gain of either of two identical yag-s
73 for Radio Amateurs * April. 1986 83
series. The formula calculates the gain using two identical antennas that are a known distance apart. The formula is given in Fig. 1 + Basically, all you need are two of the same yagi antennas and the ability to measure the distance between the two, as well as the power delivered to one antenna and the received signal from the other.
We set up the two 55-element yagis so that the front-most directors on each yagi faced each other at a distance of 75 feet (1OQ wavelengths at 1296 MHz), Using a Kenwood TR-9000 and SSB Electronics t T23S Transverter as a signal source, a Bird 43 wattmeter was put in the line right at the RG-213 pigtail, meaning it was about 1.5 feet from the dipole-dnven element. At the receiving antenna, we mounted a Boonton 92 rf millivoltmeter with 50-0hm probe and tots of Bunji cords. The power output was set at 5 Watts on the Bird 43 and allowed to stabilize for a few minutes. The rf millivoltmeter was also warmed up and set to the + 20dBm range, Steve then swung the receive antenna back and forth slightly to peak the indicated output, At this point, the measurement taken from the millivoltmeter was +13,5 dBm (22.4 mW).
Plugging this into the formula resulted in a figure of 265.98 for both antennas. One antenna would exhibit 3 dB less, or half the gain: 132.99. Expressing this number as gain in dB would be 10 logio 132.99, or 21.24 db. The manufacturer claims 21.25 db. Close enough for you? I would as* sume from this that Tonna's gain figures probably aren't overrated.
The pattern is very sharp. Fig, 2 gives the nomograph for the H-plan of tfte antenna, The 3 dB beamwidth is claimed to be about 11 degrees. The nomograph and our experiments would bear this out. The 55 element F9FT is a sharp antenna, and while it exhibits a considerable amount of forward gain, the sharpness of the pattern might make it unwieldy for the average 23 cm operator. Still, if you understand the antenna and its advantages and disadvan-tagesJ it can make the difference for your 1296 MHz station. For example. the K3YTL contest group used eight of these during the recent September VHF QSO Party and worked 65 stations on 23 cm. It definitely works!
The Tonna 55-element antenna costs $70. US Importer: VHF Shop, 16 S. Mountain Blvd., Mountaintop PA 18707.
Peter H, Putman KT2B Morris Plains NJ
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