Saturday, November 23, 2013

Simple Ham Radio Antennas--the 40 meter NVIS loop. Post #244

In this age of natural and man made disasters, it's important to have back up equipment, antennas, and power available during periods of emergencies.  A set of homebrewed wire antennas can keep you on the air when your beam or tower have been damaged by forces beyond our control.

One of the best standby antennas is the NVIS antenna, which, because of its portability and ease of operation, can be tucked away in a convenient spot for future use.  NVIS (near vertical incident skywave) antennas are perfect for local or regional use out to about 300 miles/480 kilometers.  NVIS antennas are close to the ground (between 1/10 to 1/5 wavelength above ground) and shoot most of their signals straight up.  Sometimes called "scatter beams" or "cloudwarmers", these antennas can cover a wide area (especially mountainous terrain) with little power.

A NVIS antenna can be configured in several ways, including low-level 1/2 wavelength dipoles or low-level full wave loops.  If your present HF dipole is lower than a quarterwave length, you probably have the beginnings of a simple NVIS antenna without knowing it.  All you would have to add is a halfwave length "reflector" wire on the ground below the main dipole.  Make the reflector element about 5% longer than the main dipole antenna.  This crude arrangement works rather well in putting strong signals into a local or area net.  This is not a DX antenna.  But, for local emergency HF use, this dipole/reflector combination will do the job.

Although I've used such an antenna arrangement in the past, I felt a low-mounted loop would do a bit better in the gain and noise department.  I have an under-the-house 40 meter loop at my present location.  I feed the antenna with 450 ohm ladder line connected to a W9INN 4:1 balun.  The balun is connected to the station rig through a Drake MN-4 antenna transmatch  with a 10 foot/3.04 meters length of RG-8X coaxial cable with UHF connectors.  This antenna does an excellent job on the daily Hawaii Afternoon Net on 7.088 MHz.  The loop is mounted horizontally, approximately 5 ft/1.82 meters above ground.

At my new Puna District home, I've put up several "permanent" antennas to compliment the 80-10 meter flat top dipole (classic doublet) mounted about 50 feet/15.24 meters above ground in two Norfolk Pine Trees.  This past Saturday, while there was a lull in the scattered showers over Hawaii Island, I built a more or less permanent low-mounted 40 meter NVIS loop near the backyard garden. Now that I have more space than before, I'm trying a variety of full-sized antennas, including a "cloudwarmer" to use in emergencies or for general portable use.  The backyard NVIS loop is mounted close to the ground, just like the loop used at my Laupahoehoe qth.  The only difference is that the new loop was built in an open area far removed from power lines and neighbors.

MATERIALS:

Using the general formula 1005/f (MHz)=L(ft) and the resonant frequency of 7.088 MHz (the frequency of the daily Hawaii Afternoon Net), I cut a full wavelength loop measuring 141.79 feet /43.22 meters.  I would configure the loop as a square mounted close to ground level.  Each side would then measure 35.44 ft/10.80 meters.  I used #14 AWG housewire for the antenna.

Four 6 ft/3.04 meters wooden stakes to support the loop.

Four 6 ft/3.04 meters wooden stakes to support the 450 ohm feed running to the 4:1 balun mounted on the garage wall.

A W9INN 4:1 balun.

A Drake MN-4 antenna transmatch.

Ten feet/3.04 meters of RG-8X coaxial cable with UHF connectors to run from the balun to the Drake MN-4 antenna transmatch.  A 3 ft/0.91 meters piece of RG-8X coax connects the transmatch to the station rig (Swan 100-MX or the Ten Tec Argosy II).

Short lengths of RG8X coax to connect the dummy load and low pass filter to the antenna system.

Basic tools, including a soldering station.

Vinyl electrical tape, five ceramic insulators, and nylon ties.

Four short pieces of dacron rope to secure ceramic insulators to the wooden support stakes.

Fifty feet/15.24 meters of 450 ohm ladder line.  This will serve as the antenna feed line.  Used with a 4:1 balun and a sturdy transmatch, the loop will be capable of operating on all amateur radio bands between 40 and 10 meters.

ASSEMBLY:

Four, 6 ft/3.04 meters wooden antenna support stakes were placed at each corner of a square measuring approximately 35.44 ft/10.82 meters on a side.  The loop would be horizontal to the ground, approximately 5 ft/1.82 meter above the lawn.

The tip of each wooden stake had a ceramic insulator attached by a short piece of dacron rope.  The attaching tie off rope measured approximately 2 in/5.08 cm.

The full wavelength loop of antenna wire (141.78 ft/43.22 meters) was threaded through each ceramic insulator.  A fifth ceramic insulator would support the 450 ohm ladder line and its union with each end of the loop.  All connections were soldered and wrapped with several layers of vinyl electrical tape.

Once the feed line was attached to the square loop, I adjusted the tension of the horizontal square loop so that the loop had a uniform height above ground of approximately 5 ft/1.82 meters.

I ran the feed line at a 90 degree angle away from the attachment point to a series of four, 6 ft/1.82 meters wooden stakes, which led to the W9INN 4:1 balun mounted on the garage wall.  The balun was approximately 6 ft/1.82 meters above ground.

A 10 ft/3.04 meters piece of RG-8X coaxial cable with UHF connectors ran through a window patch panel and onto the Drake MN-4 antenna transmatch.  Small patch cords connected my rigs, low pass filter, and dummy load to the transmatch.

INITIAL RESULTS:

As expected, the low-mounted 40 meter loop did an excellent job for local and statewide nets.  Most of my SSB reports ranged from 57 to 59+10 and from 579 to 599+20 for CW contacts.  There was little or no DX worked at the time of testing (early afternoon Hawaii time).  During the evening hours, I was pleasantly surprised by some mainland U.S. contacts on CW, with signal reports ranging from 559 to 579.  My contacts were made with the Swan 100 MX running 20 watts on both SSB and CW.  When I used the Ten Tec Argosy II, I kept power down to 05 watts qrp.  Even at this power, local contacts ranged from 54 to 56 on SSB and from 559 to 599 on CW.

The antenna can be quickly assembled and broken down for portable use.  Best of all, this simple NVIS antenna is inexpensive, easy to build, and requires no ground radial system.  If you want a basic antenna that puts in a strong local or regional signal, try a NVIS antenna in one of its many configurations.  I found the full wavelength loop useful for my purposes.  A low-slung dipole may work better in your location.  Either way, add a NVIS antenna to your collection of amateur radio tools.

REFERENCES:

home.century.net/w9wis/NVIS.html.

http://www.emcomm.org/projects/nvis.htm.

http://dxengineering/com/techarticles/miscinfo/learn-how-to-build-a-nvis-antenna.

http://www.youtube.com/watch?v=aAshymGZ8Ww.

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Thanks for joining us today!

Aloha es 73 de Russ (KH6JRM).

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