Monday, April 14, 2014

Simple Ham Antennas: The Novice 40 meter delta loop. Post #271

During that brief period between getting my novice amateur radio license and moving on to the technician and general class license (1977-1983), I built several dipoles, inverted vees, and verticals.  All of these antennas worked very well, considering the lack of space I had at the time.  When my xyl and I moved from Honokaa to Laupahoehoe, we occupied a larger home and had more backyard space to place antennas.

Just before I passed the Advanced Amateur License exam, I built what I considered my best and "most forgiving" antenna--a full-wavelength 40-meter loop.  The loop displayed some gain over a dipole at the same height, needed only one tall support, and didn't require a ground radial system to operate efficiently.  Using balanced feed line, a 4:1 current balun, and a Drake MN-4 transmatch, I could cover 40, 20, 15, and 10 meters with one antenna.  Later on, when 30 meters became available to amateur radio operators, I used a MFJ-941-E Versa Tuner II and my spare rig (Ten-Tec Argosy II) to cover 30 meters.

Last weekend, I decided to recreate that first "novice" loop just to see if it was a good as I once thought it was.

MATERIALS:

Fortunately, all of the materials for this loop project were available on site.  Based on what I found in my antenna notebooks for 1979, I began to revisit this old friend who sat beside me on those damp tropical nights while I "pounded the brass" in the QRM-infested frequencies of the 40-meter band.

Design frequency:  7.127 MHz--one of the novice hangouts "back in the day."

Antenna:  Full-wavelength 40-meter delta loop.  Using the general formula 468/f (MHz)=l (ft), I came up with a length of 141 ft/42.98 meters).

Antenna wire:  I had three rolls of Radio Shack #18 AWG speaker wire in the shack.  That wire would serve as my antenna.

Three ceramic insulators--one for the top of the delta loop and the other two for securing  the corners of the loop.

One 33-ft/10.06 meters MFJ telescoping fiberglass mast.  The mast would support the apex of the loop.  Back in the late 1970s, I made a homebrewed 33-ft/10.06 meters mast out of pvc pipe.  This time, however, I decided to use the fiberglass mast to support the loop.

Five, 5-ft/1.52 meters wooden stakes--one to support the mast, two to support the bottom section of the delta loop, and two to keep the balanced feed line off the ground until it reached the wall of the shack.

One W9INN 4:1 balun.  The balun would be attached to the outside shack wall and would serve to interface the balanced feed line with the ATU.

Fifty-feet/15.24 meters of 450 ohm ladder line. This would serve as the antenna feed line.

Four alligator clips.  Two would be attached to the delta loop at the left hand, bottom corner, and two would be connected to the leads of the balanced line. I could connect and disconnect the feed line easily in the event of storms or other emergencies.

Twenty-five ft/7.62 meters of RG-8X coaxial cable with UHF connectors, which would run from the 4:1 current balun through a small hole in the window patch panel and onto the Drake MN-4 ATU or MFJ-941-E Versa Tuner II.

Small lengths (3-ft/0.91 meters) of RG-8X coaxial cable to interconnect the rigs (Swan 100- MX/Ten-Tec Argosy II), the Heathkit Dummy Load, and the low-pass filter to the ATU being used.

Various lengths of nylon tie off rope, nylon ties, vinyl electrical tape, and basic tools.

ASSEMBLY:

The full-wavelength loop was built on the ground and later raised into position.

The length of the delta loop was calculated for a frequency of 7.127 MHz.  The 141-ft/42.98 meters antenna wire was shaped into a equilateral triangle, measuring 47-ft/14.329 meters per leg.

Three ceramic insulators were used for the delta loop--one for the apex of the mast, one for the bottom right hand section of the loop, and one for the left hand bottom section of the loop.  Once I threaded the two free ends of the loop through the bottom left hand insulator, I soldered a small alligator clip to each end.

I soldered two small alligator clips to the end of the 450 ohm ladder line that would be attached to the loop clips as they passed through the bottom left ceramic insulator.  The feed line then could be connected/disconnected as conditions warranted.

I pounded in three, 5-ft/1.52 meters wooden support stakes.  One would support the fiberglass mast, while the other two would be spaced 47-ft/14.329 meters apart to accommodate the bottom leg of the delta loop.
With each support stake sticking 4-feet/1.21 meters above ground, there would be some space separating the bottom element of the loop and its connection to the 450 ohm feed line from the surface of the lawn.

I carefully hoisted the fiberglass mast onto its support stake, tied off the corners of the loop with nylon rope, and adjusted the loop for a uniform appearance.

I next drove in two more 5-ft/1.82 meters wooden support stakes to support the ladder line as it was taken to the 4:1 current balun on the wall outside the shack.

I connected the clip leads from the feed line to the clip leads at the bottom left hand corner of the delta loop, led the ladder line away from the left hand corner of the loop, connected the ladder line to the pre-positioned wooden support stakes (keeping the ladder line 4-ft/1.21 meters off the ground), and finally connected the end of the ladder line to the 4:1 balun on the outside wall of the shack.  The balun was approximately 6-ft/1.82 meters above ground level.

Twenty- five feet/7.62 meters of RG-8X with UHF connectors was attached to the balun and run through the patch panel and onto the Drake MN-4 transmatch.  To be on the safe side, I attached a "counterpoise bundle" to the ground lug of the Drake ATU.  The bundle consisted of a quarter-wave length piece of #22 AWG hookup wire for 40, 30, 20, 15, and 10 meters.

RESULTS:

I was very pleased with the initial results of my old "novice" 40 meter loop.  With the Drake MN-4 and the MFJ-941E Versa II tuner in line, SWR across all bands was held below 1.1 to 1.  If you prefer to use this antenna for 40 meters only, a feed line made from 50-ohm coaxial cable and a 75-ohm matching section should serve you well.  Since I preferred to use the loop as a multiband antenna, I opted for the 450-ohm feed line used in conjunction with a 4:1 current balun and an ATU.

Local and DX contacts over the past few days have been excellent, with daytime 40 meter reports ranging from 56 to 59+ on SSB and 579 to 599+ on CW.  Twenty meters has been quite good, with signal reports ranging from 57 to 59 for SSB and 579 to 599+ for CW.  I was using the Ten-Tec Argosy II running around 30 watts.  Of course, results may vary depending on time of day and proximity to structures.

I plan to keep this antenna up for awhile, because it's so fun to use.  I may even add a director or reflector to increase its gain.  And best of all, the antenna was cheap, required no ground radial system, and needed only one tall support.

RESOURCES:

http://youtube.com/watch?v=WFITlifbl8x.

http://youtube.com/watch?v=0pdxbVc-gmc.

http://www.hamuniverse.com/kl7jrloopnotes.html.

The recent ARRL Antenna Handbook also contains a good discussion of loop antennas.

You can also search for more information on the internet by referring to "Delta Loop Antennas".

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Until next time,

73 de Russ (KH6JRM)

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






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