Original effort by me, Ed Loranger mailto:we6w@qsl.net (c) May 26, 1998. Permission to copy and distribute provided for non-profit use only. ---------------------------------------------------------- Here is a successful 160 Meter short TX loop antenna 4 feet high, 3 feet wide, and 6 inches deep. Ok. Thanks go to Alan/KB7MBI for alerting me to a 160 meter Transmitting loop -- "The Table Topper 160-Meter Loop", November 1997 issue of '73 Amateur Radio Today' (1). Briefly, the article describe a 6 turn spiral wound antenna 4'x3'x6". Unlike most "Box loop" sytle antennas, all of the wire is in the same plane, i.e. each turn of the loop is shorter than the previous. The article has the loop tuned with a TX style variable capacitor which has 300 pF/8 KV caps in series with each wire. The variable capacitor is a dual stator and Common Rotor. The Rotor may be grounded, but is shown as only common. Coaxial feed is via 'Common' and thru a 150 pF 8KV cap connected to junction of the loop outermost wire and one section of the variable capacitor. This is a VERY HIGH impedance point. PROTO I: (Saturday A.M.) I used one 5' seasoned wood, circular, about 2" diameter for the vertical support. Another 5' section was cut in half, giving me TWO 30" sections of wood. These 30" sections were then affixed horizontally at 8 inches below the top for one piece, and the second affixed lower on the pole by 36 inches. I took 6 section of 7 inch long, 5/8" diameter pvc pipe and cut deep 'V' notches every half inch from the outside edge. Six notches. I provide room at each end of the pipe for self drilling screws to hold the insulator thus created. Placement of the PVC sections were at each end of the two horizontal bars, the very top of the vertical rod, and just below the lower horizontal arm. Thus confiured, the resultant wire stringing results in a hexagon shape, slightly taller than wide. Wire is strung starting at the bottom of the pole, counter- clockwise, and using the outermost 'V' notches for anchoring the first turn. I used 14" long heavy duty wire-ties to slide into the pvc sections (6) such that the wire could be held in the notch. The wire tie is not tightened in the usual manner until all wire is strung. Continue winding until all of the .085 lambda (Wavelength) of wire is used up. Secure all wire ties and tie a NE-2 light bulb to the outermost ant. wire. Do not short the NE-2 leads. The NE-2 was my idea and lights up at 3 watts output, 1/3 aglow at 5 watts, and at 50 watts is about to explode! Connect each antenna lead to its respective Stator on the dual section capacitor with common Rotor. The Capacitor should be rated for high voltage -- You'll see. I recommend minimum plate spacing while closed to be at least 1/8 inch for QRP work. Identify the antenna/capacitor connection using the outermost antenna wire. This is the feed point. Connect the 150 pF/8 KV cap there and you are ready to test. Using an external portable receiver and remote grid dip oscillator at the design frequency, peak the GDO signal on the rcvr. Move the receiver close to the loop if you have trouble picking up the signal. Once you find a peak on the receiver, you now know the loop is tuned to the correct frequency and the variable capacitor should not be touched. Turn on the transmitter tuned to the design frequency. (Turn off portable receiver . Supply 1 to 3 watts of power, just enough to check the VSWR on you SWR Meter. WHAT?, You say? Yup! I also had trouble feeding the loop at the high voltage node. It don't load up. REGROUP AND MODIFYING THE FEED: I immediately went to a small input circular loop for the feed. Starting with about 4 inch dia. loop (very small!). I measured vswr. I continued increasing the size of the small loop until VSWR was below 1.5:1. Now I used my old Johnson Viking II rig. So success occured when I had a minimum Plate current (dip) and VSWR = 1.5 or less to 1. I was successful with a final feed loop circumference of 48 inches (approximately 1/100 wavelength) which is about 1/10th the length of the antenna wire used in the loop. Also, the circular feed resulted in 1.4:1 VSWR at best. Later I found that a TRIANGULAR feed (loop) provided about 1.1:1 match. Square and circular feedloops and even an HOURGLASS-shaped feed I made, all with VSWR's about 1.5:1. The HOURGLASS feed was an abstract Idea I had. I found it lowered the antenna 'Q' ever-so slightly but made tuning the rig a little easier. But in the end, a triangular feed, like a pyramid -- widest at the bottom, ended up best. (However: I worked W6ZH with the Hourglass feed) =============================================================== NOTE ON LOOP FEEDING: Polarity of the feed is important -- If in doubt, measure the VSWR, reverse the input leads and measure again. The feedloop HOT lead is the lead closest to the exit location of the innermost spiral antenna wire. (This is the Opposite of the Direct Feed center connection) =============================================================== PROTO I RESULTS: Heard my first-ever activity on 160 meters. N7DM calling CQ on 1807 KHz at 10 PM local time. SSB ragchews on 1854 and 1862.... I was stoked. But time for bed. I had a fun and productive Saturday. PROTO II: (Sunday A.M.) First I measured all of the loop demensions, logged the data, and verified tuning range etc. It was time to re-build. After walking around the house and playing with the kids, ( I told them the loop antenna was a "Kid Finder" :) :), I replaced the outer antenna wire with 44 Feet of #12 AWG solid copper wire with blue outer sheathing, 600 Volt rating. The inner loop was red #12 AWG wire, same stock. I was careful to use needlenose pliers to ensure wiring was solid. I then routed the High-Voltage wires thru PVC pipe affixed at the bottom of the loop. I routed the feedline safely away from the high voltage wires and into black flexible pvc tubing. All was secured at the base of the antenna. The heavy antenna wires were attached to the tuning capacitor and left to hang freely for now. FINAL TESTS: As advertized, this loop does IN-FACT, and totally unbelievable to me, but I measured it, have directivity AND Front/Back ratio. Yes, off axis nulls are excellent and measured signal levels on a FS meter show over TWICE the signal to the front than the rear. I measured 1.5 on the FS meter at 13 Inches from both sides and Rear, but the reading was measured a full 29 inches from the front of the antenna. I know this is near-field, maybe someone else wants to test at far-field. Tuning Range of Proto I: 1.35 to 2.6 MHz Tuning Range of Proto II: 1.6 MHz to 2.3 MHz. Tuning Range of Proto III: 1.75 - 2.1 MHz (TBA) Summary: I like this antenna. Hey, it is small, 2xQRP at 900 miles confirmed! It has some very HIGH VOLTAGE points when transmitting and ----------- YOU ARE RESPONSIBLE ------------------ for taking appropriate safety precautions. OK, :) It's not that dangerous, but it does spark if over driven, dusty, or you've got low voltage parts! I'm on 160 Meters finally. And I hope you give it a try. My next loop (Proto III ???, will probably use copper tubing for all the parts. Some 5/8" feedloop and 1/4" icemaker tubing for the spiral antenna. To scale the antenna I recommend: .085 wavelength of wire for the spiral, and .0085 wl for the triangular feed loop. Make a frame that can hold 6 turns for the spiral and you are done. I hope this practical report is useful. Best Hamming to all, Ed Loranger WE6W References: (1) 73 Amateur Radio Today: "The Table Topper 160-Meter Loop" by Richard Q. Marris G2BZQ, 35 Kingswood House, Farnham Road, Slough WL2 1DA, England UK. Posted to QRP-l email list on 5/26/1998. ---------------------------------------------------------- I think I have a few moments to do an ASCII Dwg. of the 160 Meter band short TX loop. (I know it is a ruff drawing. :) Summary: Inner loop feeds Co-planar spiral loop which is resonated at the design frequency with tuning capacitor. Inner loop is 1/100 Wavelength. Outer Loop is 1/10 Wavelength. Capacitor is 27-270 pF TX type, common Rotor, split stator. /\ / \ / \ / \ / \ 48 Inch Circumference / \ Coupling loop from Transmitter / \ / \ / \ / \ / \ / \ +----------+ +----------+ | | TRANSMITTER---O-----><-----------------+ | | | ------><-------------------+ O O O O +--------^--------+ ANTENNA ===========> /+--------^--------+\ //+--------^--------+\\ ///+--------^--------+\\\ ////+--------^--------+\\\\ NOT TO SCALE! /////+--------^--------+\\\\\ oooo//////oooooooooOooooooooo\\\\\\oooo Also: Shown 8-sided, but ////// O \\\\\\ is really only six sided! |||||| O |||||| |||||| O |||||| |||||| O |||||| |||||| O |||||| |||||| O |||||| |||||| .^. |||||| |||||| / O \ |||||| |||||| / O \ |||||| |||||| / O \ |||||| NOTE: Coupling loop inside>>>> |||||| / O \ |||||| And driven at * * terminals >> |||||| / O \ |||||| |||||| '-----* *-----` |||||| ooo\\\\\\ooooooooooOoooooooooo//////ooo \\\\\+=========----------+///// \\\\+--------(----------+//// \\\+--------(----------+/// \\+--------(----------+// -+--------(----------+/ + +--------/ | | | | | | / \ / \ / \ | | --- --- <27/270 pF> --- --- <1/4" plate spacing> | | //////////// Note that ALL wires are in the same plane. That is that they can all lay flat on the floor. Regards, Ed Loranger.