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Tuesday, September 3, 2013

An Enhanced 40 meter full wavelength loop. Post #225

When I was first licensed as a novice in 1977, I tried all kinds of antennas.  In the space of a year, I must have built and used 10 or more antennas, including verticals, loops, inverted vees, slopers, and half wavelength horizontal dipoles.  I've always been fascinated with full wavelength loops, but, because of limited space, I rarely built loops except for 15 and 10 meters.  I relied heavily on the standard 1/4 wavelength verticals with ground radials and the inverted vee.

Now that my xyl and I are remodeling our future home in the Puna District, I had a chance to build a full wavelength 40 meter loop without worrying about nosey neighbors or lack of space.  Our lot is one acre, giving me plenty of space to experiment with a variety of antennas.

Over the past few weeks, I've  built and used a half wavelength horizontal dipole and a half square antenna for 20 meters with outstanding results.  Now it was time to rethink the loop I built 36 years ago as a novice operator.

This antenna would be considerably larger than my previous loops and inverted vees.  I had plenty of horizontal space and two sturdy fiberglass masts to support the loop.

With the loop, I wouldn't need a ground radial system.  If I used 450 ohm ladder line, a 4:1 balun, and a transmatch, I would avoid using coaxial cable as a feed line.  Coax would be used from the balun to the transmatch, but otherwise would not be connected directly to the loop.  I didn't fear making the quarter wave matching section, but I didn't have any 75 ohm coaxial cable in the shack to help the match 50 ohm coaxial cable to the loop's estimated impedance of 105 ohms.

Another benefit of using 450 ohm ladder line was the ability to use the loop on all frequencies between 40 and 10 meters.

I designed the loop for a resonant frequency of 7.088 KHz, the frequency of the Hawaii Afternoon.  Using the general formula 1005/f (MHz)=L (feet), I calculated a wire length of 141.78 feet/43.22 meters for the full wavelength loop.  I rounded off the measurement to 142 feet /43.29 meters.  I chose #14 AWG housewire for the loop.

MATERIALS:

One hundred forty-two feet/43.29 meters of #14 AWG housewire.

Two 33-foot/10.06 meters MFJ telescoping fiberglass masts.

Two 5-foot/1.52 meters wooden posts.  The posts would support the fiberglass masts.

Five, 5-foot/1.52 meters wooden posts to support the 450 ohm feed line from the loop to the balun mounted on the garage wall.  The posts would keep the ladder line off the ground.

One W9INN 4:1 balun.

Fifty feet/15.24 meters of 450 ohm ladder line.

Twenty-five feet/7.62 meters of RG-8X coaxial cable with UHF connectors.  The coax would run from the balun to the Drake MN-4 transmatch.

Station equipment, including a dummy load, low pass filter, and a transceiver...in this case an old Swan 100 MX.

Basic tools, including soldering gun, vinyl electrical tape, nylon ties, ceramic insulator, and clear fingernail polish to cover the soldered joints.

ASSEMBLY:

I built the 40 meter loop on the ground.

The vertical sections of the loop were cut to 30 feet/9.14 meters.  The vertical sections were run from the top of each mast to a point 3 feet/0.91 meters above ground.  The horizontal segments of the loop measured 42 feet/12.80 meters.  The loop was attached and secured to each mast by several nylon ties.  In its final form, the loop measured 30 feet/9.14 meters high and 42 feet/12.80 horizontally for a total length of 142 feet/43.29 meters.

The 450 ohm ladder line was connected in the middle of the bottom horizontal element.

Each mast was hoisted onto its wooden support stake.  The loop was adjusted so it had a uniform shape.

The 450 ohm ladder line was attached to a 5-foot/1.52 meter wooden stake which was driven 2-feet/0.60 meters into the ground.  Three feet/0.91 meters of the stake was exposed above ground.  Four other wooden stakes were similarly placed in the ground supported the ladder line until it reached the garage wall.  The ladder line was attached to the W9INN 4:1 balun.  Twenty-five feet/7.62 meters of RG-8X coaxial cable with UHF connectors ran to the Drake MN-4 transmatch in the shack.  Short patch cords connected the Swan 100 MX transceiver to the transmatch, dummy load, and low-pass filter.

PRELIMINARY RESULTS:

I was generally pleased by my old novice loop, now extended to its full size.  With the help of the Drake MN-4, I was able to keep SWR below 1.3 to 1 on 40, 20, 15, and 10 meters.  Running approximately 25 watts from the old Swan 100 MX I received good reports on 40, 20, and 15 meters.  Ten meters was marginal because of solar conditions, but the transmitter worked into the 10 meter band without SWR problems.  My best bands were 40 meters and 20 meters with reception reports varying between 569-599 for cw and 55-59 for ssb.

The loop is very quiet and shows a slight gain over my horizontal flat top dipole at 35 feet/10.67 meters.  The loop works very well for local contacts.  I suspect I'm getting some high angle radiation from the loop because of its proximity to ground.  That's alright with me, since I built the loop for statewide contacts.  When propagation is good, I get decent contacts throughout the Pacific Rim and the mainland U.S.

For DX work, I rely on my 20 meter half sqaure.  That antenna far outdoes what I can get on the vertical and the inverted vee.

RESOURCES:

n1su.com/loop.html.

mysite.verizon.net/ka1fsb/loopcalc.html.

home.comcast.net/~n8itf/floop.htm.

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Aloha de Russ (KH6JRM)

BK29jx15--along the beautiful Hamakua Coast of Hawaii Island.