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Copyright: 1996 - 2024 BSEE - KB3UON RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while typing up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived... All trademarks, copyrights, patents, and other rights of ownership to images
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The Novice 90 Antenna |
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See all articles from Popular Electronics. The Novice 90 Antenna Simple tuner makes long-wire antenna effective on 40 and 80 metersBy Jay Stanley  Simple tuner makes long-wire antenna effective on 40 and 80 meters  Locate
the antenna tuner for the shortest possible ground wire while
maintaining accessibility for tuning. A remote waterproofed
tuner installation is possible but this would make tuning more
difficult. PARTS LIST C1-350·µµf. (or larger) midget TRF single-gang variable capacitor J1-RCA type phono jack (or suitable coaxial cable connector) L1-Six turns of B&W 2"-diameter Type 3900 coil stock 3-Stand-off insulators 1-Baseboard 1-Fahnestock clip Transferring power from transmitter to antenna can be tricky business, and this is particularly true when you try to feed most ordinary long-wire antennas with the typical pi network stage of a transmitter. In many cases, the antenna represents a high-impedance load - and feeding it from the low-impedance output of the transmitter makes for a big waste of precious power. This problem is neatly sidestepped with the "Novice 90" antenna, which is designed to be end-fed with a low-impedance line. Thus, the antenna is very simple to erect and, even more important, you can adjust resonant length right in the operating room. The adjustment is easily made using a 10-cent flashlight bulb which shows you the point of maximum antenna current. This current is the power being soaked up by the antenna. Construction The "heart" of the antenna is a simple tuner, consisting of a TRF-type single-gang tuning capacitor (C1) and coil L1. These parts are mounted on a convenient board as shown in the diagram. Stand-off insulators support the coil and the antenna terminal. Variable capacitor C1 and the Fahnestock clip for the ground connection are mounted directly on the board. The RCA phono input jack J1 (a regular coax connector could be used instead) is mounted on a small metal angle bracket screwed to the board. Jack J1 connects to the coaxial cable jumper which runs to your transmitter output terminal. The coil for the antenna tuner consists of six turns of No. 14 wire approximately 2" in diameter. Turns are spaced approximately 1/8" apart. Wind the coil yourself or cut some turns from a piece of B & W 2"-diameter No. 14 wire coil stock (Type 3900). The antenna itself is a 90' "L"-shaped length of wire. How much is horizontal and how much is vertical is not particularly important: for example, 30' up to 60' out is ideal. However, other combinations, such as 40' up and 50' out, will work as well. For best results, keep the ground lead under 10' in length, the shorter the better. If it must be longer, subtract the length of the ground lead over 10' from the antenna length in feet. For example, if the ground lead is 14', make the antenna 86' instead of 90'. You can build an efficient ground as shown in the diagram. Use a standard TV ground rod (the longer the better) and then improve the conducting characteristics by chemically treating the ground nearby. Copper sulphate crystals are excellent, as is rock salt. Ordinary table salt will work well also, but of course is more expensive. Dump in the chemicals, soak down the area with a garden hose, and you will have a ground far more efficient than the usual water pipe ground. The latter should be used only as a last resort and cannot be expected to work as well. In a typical station layout, the antenna tuner should be mounted alongside a window sill. The ground should be just outside the window, close to the house. The antenna tuner can be connected to the transmitter by any convenient length of 50- or 52-ohm (approximately) coaxial cable. Antenna Tune-Up First, attach a two-turn loop of wire to a No. 44 dial light bulb or a bulb from a two-cell flashlight. The loop and the bulb provide an indicator for antenna current. Then, connect the coaxial cable from the transmitter to the antenna tuner. Set the capacitor on the antenna tuner approximately one-half "open." Next, set your transmitter for 80 meters, and tune it following the manufacturer's instructions. Load up the transmitter final by adjusting the antenna loading and plate tuning capacitors in the transmitter. Now, couple the pickup loop and bulb (as shown in the photograph) to the antenna coil, and adjust antenna tuning capacitor C1 for maximum brightness. When you find this point, you have the antenna at resonance and it is taking maximum power. On 40 meters the adjustment is quite similar, although the resonance point is usually very broad, as shown by the tuning loop indicator. On this band, set the antenna tuner capacitor to as high a capacity as possible (plates almost closed) where the tuning indicator is at maximum brilliance. The last step is to adjust the transmitter tuning for proper load as indicated by the milliammeter in the final of the transmitter. Don't make any big changes in the antenna resonance adjustment already achieved, but check for maximum brilliance of the bulb. Posted 3/18/2013 |
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