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A Five Band 30" Diamond Loop Antenna

This note describes a loop antenna in a diamond configuration, 30 inches on a side, covering from 14 to 30 MHz. The antenna is made from ½" copper pipe and can be disassembled into sections for easy packing. On the air comparisons indicate that the loop performs nearly as well as a full size dipole if the dipole is placed at the modest elevations typically attainable during portable operation.

Overview

This note describes the portable antenna shown in Figure 1 which covers the 14-30 MHz frequency range. It consists of two main subsections: the diamond shaped loop radiator and the "H" shaped base. The diamond and the H may be separated by simply pulling a pin in the cross piece (c) of the H section. These two major components easily fit into the back of a mini-van. For the ultimate in portability, the antenna can be broken down as shown in Figure 2. It takes under 10 minutes to reassemble it from this state which is comparable to getting a dipole / vee strung in a tree... well... assuming there are trees! The antenna provides full coverage from 14 to 30 MHz and it can be tuned to 1:1 SWR eliminating the typical trans-match requirement. With these points in mind, most operators will find this antenna to have significant advantages over the typical dipole in portable use. h i f g

n k a b j d e c l

Construction

Aside from the butterfly capacitor and tuning motor i, the bulk of the components are easy to find and copper tubing is very easy to work with. It is not obvious from any diagrams, but the copper "T"s have been bored out so the pipe slides through them. The boring was done on a small lathe, but a drill press would probably do as well. The T at the base of element b has a slit cut in it so that it can be secured to element a with an automotive hose clamp. Careful study of figure 2 shows exactly which T and right angle fittings were soldered to the pipes. Figure 3 give the dimensions of the copper pipes as well as details on the pseudo gamma match used to couple RF to the loop. a b c e d f g

Figure 1 Assembled diamond loop with remote tuning unit (l) covers from 14 to 30 MHz . A bungee cord (k) provides tension for the antenna and works against the tension provided by the spring loaded tuning drive cord (n).

h

i m l k j n

Figure 2 The loop disassembled for maximum mobility. The lettered parts are shown assembled in Figure ­1. The pulley (i) has a brass spindle that slips over the shaft of the butterfly capacitor which is mounted on ¼" plexiglass (h). Two u-bolts hold the copper elements (f&g) to the plexiglass. The elements have banana jacks in their ends that serve 2 functions. First they are used to connect the butterfly capacitor and secondly they protrude through the copper pipe into holes in the plexiglass to stabilize the angle between the elements to 90 degrees. A 23 pF capacitor made from a 10" section of teflon RG8 cable (m) is shunted across the butterfly to hit 14 MHz. The feeder components (j) shows the BNC center conductor soldered to a 3/16" brass tube. A 1/8" brass tube is soldered to the loading capacitor and this slips inside the 3/16 section upon assembly. The two RF chokes for the DC motor feed are also part of this assembly. The DC motor is mounted to the base of section (b).

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02/18/03

Dick Benson - w1qg - Saratoga CA ([email protected])

Figure 4 diagrams the electro-mechanical construction. With the exception of the butterfly capacitor, the components were all intended for light duty. My portable rig emits 40W max and the components appear to be adequate for this power level. The remaining figures in this note provide more clues on construction for the interested reader.

Remote Tuning

Initially the loop was made without the benefit of remote tuning. There was no point in enhancing the loop if it did not perform well. After a few hours of operation two things became clear: the loop worked quite well for its size (several contacts to Europe on 20 meters), and tuning it was a real pain! With the two capacitors the feed point impedance can be set to 50+j0 ohms at any desired frequency. But given the high-Q nature of the loop, the bandwidth is just too narrow for most operators unless you're rock bound! The setting of the loading capacitor on the gamma match is relatively uncritical and needs to be only set once per amateur band segment (cw/phone) to obtain a reasonable feed line SWR. Therefore only one motor drive is required.

Figure 3 Dimensions of diamond radiator, feed-point section, and simple "H" support base.

Ball bearing butterfly cap, 65 pF per section. Pully made from 5 AOL CDs, 4.75 inch diameter.

Dual section variable cap, 210 pF per section (sections in ||).

14.5 inches (3/16 and 1/8 dia) brass tubing about 0.5" from element.

Gearhead DC motor, Maxon 12 RPM @ 4VDC, capstan 3/8" diameter

22 uH RF chokes BNC connectors and 2 SPDT pushbutton switches, 4 V gel cell, 0.01uF cap and 100 uH choke form the remote tuning control.

Figure 4 Diagram showing electro-mechanical construction (not to scale). The white plastic jar (l) in Figure 1 holds the remote tuning control. RG58 coax carries the RF as well as the DC for the tuning motor. Two spdt pushbuttons control the direction of the tuning motor. The motor drive is not a luxury, it is essential.

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02/18/03

Dick Benson - w1qg - Saratoga CA ([email protected])

Initially a high quality silver plated 2 section ball bearing capacitor was used for the tuner. The stator sections were wired in series using the rotor for the coupling between them. Therefore, no loss is associated with rotor's wiping contacts. The entire frequency range of 14 to 30 MHz was covered with 180 degree rotation. There were two problems with this capacitor: 1) the spacing was not sufficient for the 40 W 2) it was not "free wheeling" due to the rotor wiper contact. Arc over (#1) and corona are actually quite hard to see. You can get a clue about it in the dark, but you have to get uncomfortably close to the loop while applying power. I am in favor of minimizing collateral brain damage, I do enough already! The best way to detect this problem is to first tune the antenna for resonance and minimum SWR using 5 W or less. Then carefully watch the SWR as the power is increased. At the onset of corona the SWR will rise and with a little more power, go completely to hell when an arc is initiated. You will be tempted to change tuning adjustments but this will only lead to confusion. The capacitor appeared to be ok at the 15W level but there is still problem #2. I did not want to mount the DC tuning motor up by the capacitor due to the high RF potentials and Q. Loss is a real performance killer for small loop antennas. The motor is preferably mounted near the bottom vertex of the loop where there is low RF potential. The mechanical link to the tuning capacitor will then be physically long and any friction in the tuning capacitor rotation inevitably leads to poor tuning control. Yes the wire(s) to the motor could run inside the copper element, but this would complicate the assembly/disassembly. For lower power applications one might be able to remove the rotor wiper to reduce the friction, but I wanted the loop to handle the 40W level. The solution to both problems was to abandon this capacitor in favor of a small butterfly unit. It was extracted from a mil surplus VHF transmitter that had automatic tuning. Therefore the butterfly had ball bearings. However, its maximum capacitance of about 65 pF per section limited the low frequency coverage to about 18 MHz. However, the 0.065" plate spacing was sufficient to handle the 40W and with a little 3-in-1 oil, the rotor spun freely. Aside from having no wiper loss, the butterfly has continuous rotation without any mechanical stops. Not only does this eliminate any need for limit switches and/or mechanical "clutches", but tuning in one direction will always (eventually) find resonance. Given the free wheeling rotor and the large "AOL" CD pulley, a spring loaded nylon cord with 2 turns around the motor capstan couples the motor to the capacitor pulley. This simple

tuning system is remarkably precise, there is negligible backlash. To reach the 20 meter band, the butterfly is shunted with a fixed, low loss, high voltage capacitor made from a 10" section of teflon insulated RG8 coax. You must trim the braid back from the ends by 3/8" or more to keep the coax from arcing over even at 40W. A fuse holder soldered between the butterfly capacitor stator plates serves as a quick disconnect for this homebrew capacitor. It can be seen in Figure 5 with the coax capacitor installed for 20 meter operation.

Figure 5 The butterfly tuning capacitor with the coax capacitor in for 20 meter operation. The banana jacks serve as the connection points for the braided wire connecting each stator. The jack mounting screw protrudes beyond the copper into the ¼" plexiglass to pin the position of the end of the pipe.

Figure 6 The Maxon DC gearhead tuning motor is mounted with hose clamps to a short piece of brass soldered to the copper pipe. Minigator clips make the connection from the RF chokes to the motor. The gamma match brass tubing heading to the BNC connector is clearly shown.

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02/18/03

Dick Benson - w1qg - Saratoga CA ([email protected])

I dwelled (too long?) on the tradeoffs involved with the tuning capacitor since folks are more likely to have a regular dual section cap on hand. By the way, night dew is a killer! You must keep the condensation off the capacitor. This problem has not been solved yet, but I suspect a carefully chosen Rubber Maid container will do the trick.

Stay tuned and 73, Dick, w1qg

i

Tune Up

Tuning is always more challenging the first time. But once the loading capacitor settings for the various bands have been established, they can be marked on the frame of the capacitor and simply dialed up. Starting with 10 meters, set the loading capacitor to about ¼ mesh on the plates (100pF). Then adjust the tuning capacitor for a clear peak in receiver noise. Its best to turn the AGC off since there are clicks when the buttons are pushed and released. If you don't hear a clearly defined noise peak, you don't have an antenna yet! Once peak is established, apply a few watts of RF and retune the tuning cap for minimum SWR. If the SWR is below your threshold (1.5:1?) you have arrived. Otherwise, tweak the loading capacitor a bit and repeat the cycle. When you find the sweet spot for the loading capacitor note it or mark it on the capacitor frame. Those blessed with an RF network analyzer will make quick work of this process!

The Maxon DC gearhead motor was generously provided by Ted Wolf, WA6TJN

Substitutions

The Maxon motor won't be found at your local Radio Shack. However, the business end from an electric screwdriver could probably be adapted. Granger offers a line of 12DC gearmotors and stock number 2L006 would do the job ($35 plus shipping). As for the tuning capacitor, a free rotating ball bearing design is essential if the simple dial cord mechanical coupling is used. You might be able to make your own simple butterfly with just one rotor plate. A ball bearing could be mounted on the plexiglass plate along with a couple of stators. The capacitance range would probably be limited so band-switching could be accomplished with different length coax capacitors. And by the way, it is not essential that AOL CDs be used to construct the pulley. It's just satisfying to find a use for them other than shimming up the damn clothes dryer J.

Performance

Let's face it, the old axiom: "the bigger & higher the better" still rules. Comparisons between this antenna and my KT-34 at 50 feet (a modest antenna by most standards) were disappointing. The KT-34 was usually 10+ dB better. However when the loop is compared to a full size dipole that is 10-15 feet off the ground, the loop was on par. It will be interesting to compare this loop with a screwdriver style whip antenna.

Figure 7 The loop in action on its first road trip to Seacliff State Beach. The antenna was erected in well under one minute. A ssb contest happened to be in progress which made for considerable band activity. Contests are good for testing new gear, and that's about it ! Using 40 W pep on 15M SSB, the following stations were contacted: AD1T, K2BM, AA5NT, A Without great strain H7DX, KH6CQH, CL0C, JF2FIU, JH7BZR, JA2BNN, KH6GMP, JA7BJS, LU3FZW, VK2CA. The VK2 contact was made just after the sunset picture (inset) was taken. Proof enough for me: it radiates !

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02/18/03

Dick Benson - w1qg - Saratoga CA ([email protected])

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