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Double Delta HF Loop Antenna

This Double Delta Loop is a 30 ft. free-standing ground-mounted no-radial HF vertical antenna of innovative design. It has superior performance compared to other verticals of the same height. Especially unique among verticals, it is horizontally polarized, affording up to 3dB less band noise. Being higher in noise than horizontal antennas is a well-known shortcoming of verticals. Tucked away in a back yard, it is far less noticeable than a beam on a tower. My “no antenna” HOA has said nothing in well over a year. What’s more, the loop retains the same low-angle radiation popular of conventional verticals. Also, without being deployed in a phased multi-antenna array all by itself it has 5-7 dBi of electrically steerable directional (azimuth) gain for 360-degree coverage. Although a small antenna, the double delta loop exhibits high efficiency on 20 through 6

meters and greater efficiency than other small verticals on 40 and 80 meters.

Horizontal Polarization Respected ham author L. B. Cebik, W4RNL (SK) stated, “Most human-made noise [QRM] [and atmospheric noise, QRN] is vertically polarized and of ground wave propagation. Hence, [conventional] ground-mounted verticals are more susceptible [to noise]. A [horizontally-polarized] antenna generally shows an immediate 3 dB reduction.” So while a classical vertical is vertically polarized; a tall narrow delta loop is very largely horizontally polarized.

Figure 1: Tucked away. in a small back yard. Less noticeable than a beam on a tower.

Figure 2: EZNEC current display (purple). Amplitude indicated by distance, phase by direction from wire.

Further, it radiates from higher up the antenna. Higher radiation reduces ground loss and improves gain. In Figure 2, note that the currents at the bottom of a tall narrow inverted delta loop are nearly equal and opposite. Therefore, radiation at the bottom is low. A conventional vertical radiate from the bottom. At the top of the loop, current is in one direction and unopposed, causing the loop to radiate mostly from the top. And even further advantage is directivity. Figures 2 shows the EZNEC azimuth radiation patterns of the loop over average soil. A conventional, ground-mounted, vertical of the same base height is omni-directional. It has no azimuth gain.

Feeding the Loop The total length of the wire in the double loop must NOT be resonant inside any ham band, otherwise the gain will be less. The natural resonant frequency of this loop is roughly 22 MHz. Notice Figure 3. It shows the antenna’s feed impedance, reactance and SWR by band, measured with a Rig Expert A-1400 antenna analyzer. Note that there is no ideal 50+/-j0 Ohm match to coax on any ham band. This means that the loop cannot be fed directly with coax.

MHz R jX SWR:1 Z 3.8 8000 +8700 349 11,800 7.2 1 -4900 480,000 4900

14.2 11 -800 1168 800 28.5 19 +105 14.5 106 52 108 -335 23.4 352

Matching, at the base of the antenna, is mandatory. For multi-band operation, most users choose a base-mounted remote auto-tuner, Figure 4. A home-brew relay-switched matching network would also be satisfactory, but IN NO CASE should you attempt to use a tuner in the shack followed by a run of coax connected directly to the antenna. If matching is located

Figure 3: R, jX, SWR and Z for 20 ft narrow inverted double delta loop.

Figure 3: EZNEC azimuth radiation patterns, 80/40/10m outward.

other than at the base of the antenna, severe SWR loss in the feedline will result, even for a short run of low-loss coax. Specifically, do not use an automatic tuner built into your transceiver followed by a run of coax; you may damage the tuner.

Next, it is necessary to provide a 1:1 current balun right BEFORE the input of the tuner to prevent common-mode current (RF on the coax shield), Any commercial or home-brew balun of this type, rated for the power you’ll be running, is suitable. Do not, however, place the balun AFTER the tuner and do not use a 4:1 balun. Also, insulate the tuner from ground. In this configuration it is operating in “balanced mode” despite its input and output connectors, or internal circuitry Some have asked if a 4:1 balun could be substituted for an auto-tuner. CATAGORICALLY NO. .

Steering the Loop’s Gain With the two loops in

series, maximum radiation occurs at 45 degrees to the plane of either loop. The remote-relay switch box, Figure 5, lets you remotely reverse the polarity of one of the loops to rotate the gain by 90-degrees of azimuth. With the antenna’s broad beam and bidirectional gain, remote switching affords 360 degrees of coverage. 12 VDC (black cable) remotely controls the relay. If you have a favorite direction, you may want to reverse the loop connections to allow the relay to not be energized for the most-often used case. The SO-239 connector shown is for a short low-loss coax

Figure 4: LDG RT-600 remote auto-tuner and remote directivity switch box.

Balun just out of view.

Figure 5: Interior of remote switch box for 90-degree rotation of directivity. Black cable, 12 VDC relay control. Schematic at end of article

jumper to the auto-tuner. Do not connect this jack directly to coax from shack.

Mast and Ground Mount The mast is [ five ] nesting [ 6 ft. ] sections of aluminum tubing each with a 1 ft. overlap. The mast tapers from an outside diameter of [ 2 in. ]. to [ 15/8 in. ]. EZNEC simulations show that a metal mast has little effect on the radiation pattern or gain of the antenna. A tapering telescopic mast is easy to collapse for transport or when the antenna is not in use, Figure 5. One person can easily carry and erect the antenna without assistance. An application of silicon grease or Noalox or similar anti-seize compound is advisable if the mast is to be put up and taken down frequently. Free-standing, the antenna isf supported by 4 ft. of [ 2½ ] in. PVC electrical conduit buried in the soil. No concrete is needed at this length. A ground auger or a 2 in. water drill are easy ways to make the ground hole. It is also a good idea to bury a grounding rod or to use a metal ground-support pipe to provide a ground connection for the tuner and any added lightning protection. For portable use, a single set of light-weight non-metallic guy lines, attached at roughly 10 feet, is sufficient short-term support. An under-wheel car mount is also a good temporary portable mount.

Assembly Instructions

1. Use a permanent marker to draw a circle around each pole section, one foot from the bottom end. These marks are essential when telescoping the mast to full height.

Figure 5: Mast and spreaders collapsed. Notice PVC conduit

ground support pipe, top just visible. Auto-tuner not shown.

2. Insert all the pole sections into each other and fix a stainless hose clamp roughly 3 in. from the end of each. When the mast is fully collapsed, the clamps tightened prevent the tube sections from sliding down all the way. You will move and re-tightened them to hold the mast sections at full height.

3. Insert the collapsed mast into the ground support pipe and install the pole-top fixture.

4. Cut two individual 50 ft. lengths of 12 AWG stranded insulated wire, one for each loop.

5. Place the end caps on the four spreaders and thread the two loops through a spreader and through the top fixture. Then pass each loop wire through the opposing spreader. Do not insert the spreaders into the top fixture at this time.

6. Pull each loop through the top fixture and spreaders until both wire halves are equal.

7. Insert the spreaders into the top fixture and tighten the securing bolts. Adjust each loop until there is only a small arch crossing the top of the fixture.

8. Lay out all four wires on the ground in a straight line, not crossing. Avoid dropping the wires in a random pile. Wires have an innate tendency to snarl and kink. Keep your eye on them as you elevate the mast to keep them straight.

9. Then begin by pushing up the upper-most section (smallest-diameter) a short distance. Hold the section with your hand to prevent it from slipping down. Then loosen the stainless clamp and slide it down as you push the section farther up. Stop immediately when you reach the mark you made near the bottom of the section. Then re-tighter the hose clamp.

10. Repeat step 10 until all pole sections are at full height.

11. At the bottom, gather and insert the antenna wires into the securing collar, directly below the respective spreaders. Gently snug up all wires,

securing each with a single wrap around the collar and a further wrap around the down wire.

12. Cut each wire to roughly 6 in. beyond the securing collar and crimp on a male ¼ in. disconnect terminal. These terminals will connect to female ¼ in disconnect terminals on the wires coming from the directivity switch box.

13. Attach the auto-tuner and directivity switch box at the bottom of the mast. Connect the feed coax, the short coax jumper, the 12 VDC relay control cable and the antenna wires as shown in Figure 5.

14. Install a 1:1 current balun in the coax just before (not after) the tuner with an additional short coax jumper. Add lightning protection if desired.

15. Ground the ground terminal of the tuner to the ground rod or aluminum pipe section buried in the ground.

16. AGAIN: Do not place the balun AFTER the tuner.

Possible Modifications

EZNEC shows that moderately improved performance can be achieved by extending the antenna to a height of 40 or 50 ft. It will be easier for the tuner to tune the lower frequency bands. Add larger-diameter nesting pole sections at the bottom, more wire and longer-heavier top arms. Guying is also advisable at greater height. For optimum efficiency and gain, the width of the delta should be roughly 1/3 its height. A 30 ft. delta would, therefore, have 5 ft. arms, a 40 ft. delta, 7 ft arms. A steel or heavy-duty fiberglass masts is also very suitable. Two popular user choices are the MFJ 1906HD 38 ft. heavy-duty push up telescopic fiberglass mast and the Channel Master CM1850 40 ft. telescopic push-up steel mast. A very light-weight and highly portable version can be realized with a telescopic fiberglass windsock pole and lighter wire. However, do not make the wire too fine or efficiency will suffer. For this lighter version, use ¼ in. fiberglass bicycle flag whips for the top spreaders.

IMPORTANT: The total series length of the wire in the two loops combined should not be half-wave, quarter-wave or multiple resonant length on any ham band. It needs to be at least 1 MHz, high or low, otherwise the gain of the antenna will decrease. On-air experience, and from testimonials from around the world, have shown that a tall narrow horizontally-polarized inverted double delta loop vertical is an excellent performer. It has the same low-angle radiation of a conventional vertical and up to 3dB less band noise and less soil loss due to higher/top radiation. Plus, it has nearly the gain of a small beam at a modest height. As a portable, it is especially well suited for field day and other field situations. The collapsed mast, wire, cables, tuner and hardware travel well and are quickly set up. In a permanent installation, it is far less noticeable than a beam or a mast or tower. John Portune W6NBC, 519 W. Taylor St unit 111, Santa Maria CA, 93458, jportune@aol.com, w6nbc.com.