Wednesday, June 5, 2013

Antenna Topics: A Field Day Delta loop. Post #199

Over the course of the next few weeks, I'll be describing some of the simple amateur radio antennas suitable for the ARRL 2013 Field Day Communications exercise.  These antennas will be transportable for remote operations, easy to build, easy to take down, and will offer good performance under "field" conditions.  Most of the materials for these antennas can be found in the nearest hardware or home building supply outlet.

When you are done using these antennas for field day, store them in your shack for future use.  If these antennas perform well for you, consider erecting them on your property for regular contacts with your ham friends.

In my last post, I described a sloping dipole fed by 450-ohm ladder line.  Along with its associated 4:1 balun and a hefty antenna transmatch ("tuner"), this antenna rendered service on 40, 20, 15, and 10 meters.  I still use my sloping dipole when I operate on the weekends at a public park or some deserted pasture above my home QTH.

My next Field Day antenna suggestion is the versatile and quiet delta loop which can be horizontally and vertically polarized  to fit your requirements.  Unlike the half-wavelength designs common to dipoles, inverted vees, and slopers, the delta loop is a full-wavelength antenna, which can be fed with 50- ohm coaxial cable for single band use or with 450-ohm ladder line for multiband use.  If ladder line is used, a 4:1 balun and an antenna transmatch are required.

Like the verticals, slopers, and inverted vees I've used in the past, the  delta loop needs a mast to support the apex of the triangle-shaped antenna and three support stakes--one to support the mast and two for tie-off points for the delta loop.

One of the first steps involved in the construction of this determine the lowest band of operation for the delta loop.  All bands above the lowest chosen frequency can be used if a 4:1 balun and an antenna transmatch are part of the antenna system.

My original delta loop was designed to be used from 40 meters through 10 meters, with the chosen resonant frequency of 7.088 MHz (the frequency of the Hawaii Afternoon Net).  Using the trusty Drake MN-4 and a W9INN 4:1 balun I was able to get a SWR of 1.5 to 1 across 40, 20, 15, and 10 meters.

So, let us begin:

MATERIALS:

A length of #14 AWG house wire suitable for a full-wavelength delta loop at the chosen frequency of 7.088 MHz).  Using the general formula, 1005/f (MHz)=L (ft), the delta loop would measure141.78 feet/43.22 meters.  I rounded off the length to 141 feet/42.98 meters.  Each side of the triangle would measure 47 feet/14.32 meters.

A telescoping  33-foot/10.06 meters fiberglass or pvc mast.  I had a spare MFJ mast in my garage that fit the specifications.

Three ceramic insulators.  One would support the apex of the delta loop at the tip of the mast.  Two would be used as tie-off points for the bottom element of the delta loop.

Short pieces of dacron rope would be used to tie-off the bottom portion of the delta loop to two, 5-foot/1.52 meters, wooden stakes.

One 5-foot/1.52 meters wooden stake to support the mast.

One 5-foot/1.52 meters wooden stake to support the feed line before it reached the operating position.

Three ceramic insulators.

Nylon ties, vinyl electrical tape, basic tools.

One 4:1 balun.  I happened to have a spare W9INN balun in the shack.  I've also used baluns from DX Engineering.  These baluns are well-made and will last a long time.

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

Twenty-five feet/7.62 meters of 50-ohm coaxial cable (RG-58, RG-8, or RG-8X).  The coax would run from the 4:1 balun into a suitable antenna transmatch.  I used the Drake MN-4 from my home QTH.

Short pieces of coaxial cable to connect the Drake MN-4 to my Yaesu FT-7 QRP rig.

A dummy load for tuning the Yaesu FT-7.

A deep-cycle marine battery.

Some small solar panels to keep the battery charged during daytime operations.

A small table for the transceiver.

A comfortable chair or portable seat for the operator.

Survival supplies, including food, water, insect repellant, and a first aid kit.

Logging materials.

ASSEMBLY:

I built the antenna on the ground.

I laid out a full-wavelength of #14 AWG wire (141 feet/42.98 meters) on the ground and passed the wire through each ceramic insulator.

I attached the top insulator to the tip of the fiberglass mast. At the top insulator I attached each side of the apex to the ladder line, with one line running down to the right and the other line running down to the left.  I secured the ladder line to the mast with nylon ties.  The ladder line was run down to the mid-mast position (approximately 16-feet/4.87 meters).  The ladder line would then run through the middle of the delta loop to a nearby wooden stake and be tied off at the 4:1 balun.  A twenty-five foot/7.62 meters piece of RG-8X with UHF connectors would go to the Drake MN-4 antenna transmatch.  Short pieces of coax would connect the Drake MN-4 to the dummy load and the Yaesu FT-7.

I then hoisted the fiberglass mast onto its support stake, "evened out" the delta loop, and tied off the loop to its wooden support stakes.

RESULTS:

When I tested this antenna last month at a spot above my home QTH, I got good reports on both CW and SSB despite the relatively low power of the Yaesu FT-7 (about 10 watts).  With a more modern rig, such as a K-3, Yaesu 817 ND, or even an ICOM-703, your results will be better.  With the Drake MN-4 in the line, I was able to keep SWR below 1.5 to 1 on 40, 20, 15, and 10 meters.  Generally, I was satisfied with the performance of this quickly-made antenna.

REFERENCES:

http://www.youtube.com/watch?v=kmdQMgBHeT0.
http://www.youtube.com/watch?v=WF1Tlfbl8Xo.
http://www.youtube.com/watch?v=ettbawdhqLc.
http://www.hamuniverse.com/kl7jrdueldelta1012.html.

Have fun!

You can follow our blog community with a free email subscription or by tapping into the blog RSS feed.

For the latest amateur radio news, please visit my news site...http://kh6jrm.com.  I've included a few items of general interest at the bottom of this post.

Thanks for joining me today!

Aloha es 73 de Russ, KH6JRM

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