Wednesday, December 25, 2013

Simple Ham Radio Antennas--going stealth mode. Post #250.

For most of my 36 years as an amateur radio operator, I've had to operate under the limitations imposed by HOAs, CC&Rs, and postage stamp sized backyards.  Like many of you, my creativity was sorely tested  as I tried to get reasonably efficient antennas erected for my home station.  In many cases, one antenna had to suffice for multiband operation.  And that antenna had to be inconspicuous, easy to erect and take down, and not present an "eyesore" to the neighbors.

Over the course of those years, I managed to enjoy ham radio despite the highly compromised antennas and low power employed at the shack.  There were a few multiband designs which proved successful for local and occasional DX.  Among them were inverted vees and 1/2 wavelength horizontal dipoles fed by 450 ohm ladder line connected to a 4:1 balun and a Drake MN-4 antenna transmatch.  When I did have a bit more space, I used full wavelength loops fed by ladder line for 20, 15, and 10 meters.  Height above ground was always a problem owing to the lack of nearby trees or other structures.  So, early on I bought several telescoping fiberglass masts which extended to 33 feet/10.06 meters.  These masts were easy to erect, take down, and store.  With a pulley and lanyard system, I could hoist pre-made antennas for single or multiband use in a matter of seconds.  The antennas were nested to ground level after use, lessening the impact on the local "environment."  My antennas were largely "out of sight, out of mind" since I operated mostly at night.

I generally avoided vertical antennas because I lacked sufficient space to lay in a decent ground radial system.  A vertical antenna depends on a ground radial system to function efficiently.

While I did enjoy working on these temporary antennas, I felt the need for more space so I could erect more permanent, full-sized  antennas for the bands I favored.  In May, my xyl and I bought a nice home on an acre of land in the sprawling Puna District of Hawaii Island.  At last, there was sufficient space to erect antennas for each amateur radio band.  The antenna "farm" is slowly taking shape--a project that will take a few months to complete.

Meanwhile, both of us are still living in a nice rental home in Laupahoehoe along the Hamakua Coastline. The house is quite nice, but it's confined by high voltage lines, numerous neighbors, and very little backyard space.  I needed a temporary, largely invisible antenna to continue my amateur radio activities while we gradually moved into the new place.  Most of the antennas I had used here were already packed away and stored at the new qth.

After a few days of research in my antenna books, the ARRL archieves (members only), and some inquiries into a few antenna websites, I found an antenna which is both "stealthy" and useful for my mostly local and regional contacts--a 40 meter full wavelength loop strung under the house and fed by 450 ohm ladder line connected to a 4:1 balun and my trusty Drake MN-4 transmatch.  Last year, I used this low-lying loop to good effect on the local Hawaiian Afternoon Net.  The loop is basically a NVIS (near vertical incident skywave) antenna that puts a strong signal out to about 300 miles/approximately 480 km--more than enough to cover the entire Hawaii Island chain.  So, I decided to reinstall the loop.  I'm glad I did.

My rental house is built on a pillar and pier system to withstand minor earthquakes and flooding.  The house is approximately 5 feet/1.82 meter above ground.  The loop can be squeezed around the perimeter of the house and part of the garage.  

MATERIALS:

Using the general formula 1005/f (MHz)=L (feet), I cut the 40 meter loop to a length of 142.55 ft/43.46 meters.  I used some #22 AWG hookup wire stored in the garage for the antenna.  The resonant frequency of the loop would be 7.088 MHz, the "watering hole" for the Hawaii Afternoon Net.

A box of push pins to secure the wire to the underside of the house.

One ceramic insulator to support the junction of the loop and the 450 ohm feed line.  The insulator would be attached to the underside of the living room floor about 5 feet/1.82 meters above ground level.  The feed line connections were soldered and covered with several layers of vinyl electrical tape.

Twelve feet/3.65 meters of 450 ohm ladder line.

One W9INN 4:1 balun.

Three feet/0.91 metes of RG-8X coaxial cable with UHF connectors.

Transceiver (Swan 100 MX).

Dummy load.

Drake MN-4 transmatch.

Low pass filter. Some of my neighbors get their television programs over the air.

Various coaxial patch cords to interconnect station equipment.

ASSEMBLY:

The antenna assembly is simple and takes only a few minutes.

I attached the 40 meter loop around the perimeter of the house and part of the garage.  Push pins secured the loop to the underside of the house.

The 450 ohm ladder line was attached to each end of the loop.  Connections were soldered and covered with vinyl electrical tape.

The ladder line was inserted under the back door and led into the living room where it was connected to the W9INN balun. The balun was approximately 3-feet/0,91 meters from the Swan 100 MX transceiver.

A 3-foot/0.91 meters length of RG-8X coaxial cable was attached to the balun and then connected to the Drake MN-4 antenna transmatch.  The Swan 100 MX was connected to the Drake MN-4 along with the dummy load and the low pass filter.

INITIAL RESULTS:

As expected, the under-the-house loop used  in conjunction with the 4:1 balun and the Drake MN-4 transmatch covered all amateur radio frequencies between 40 and 10 meters with a low swr (below 1.3 to 1).  Since the antenna was a NVIS design, most of the signal went straight up and covered the state of Hawaii from Kauai to Hawaii Island with SSB reports ranging from 57 to 59 using approximately 20 watts from the old Swan 100 MX.  In most cases, I will not exceed this power level because of rf and safety concerns.  When I use CW, the power level is held to 10 watts or less.  I've had a few mainland U.S. contacts on 40 and 20 meters with reports running between 55 and 57.

Given the self-imposed height and power restrictions, this low-lying loop is providing the performance I need until my antenna "farm" is done at my new qth.

The under-the-house 40 meter loop supplements the 80-foot/24.39 meters loop I have tacked around the interior of my rental home.  In any case, these antennas, when operated at low power levels, provide many hours of enjoyment without undue interference to computers, televisions, or entertainment systems.  

The new loop is invisible to neighbors, requires no ground system, and easily erected and taken down.

Perhaps a loop antenna is in your future.

REFERENCES:

http://www.g4ilo.com/stealth.html.
http://www.dxzone.com/catalog/Antennas/Stealth.
http://www.arrl.org/shop/Stealth-Antennas.
http://www.sgcworld.com/Publications/Books/stealthbook.pdf.
http://bvarc.org/pdf/HF_Antennas_by_KD5FX.pdf.
http://www.hamlearningnet.org/dl/Stealth_Antennas.pdf.

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Happy Holidays!

Russ (KH6JRM)
BK29jx15
Along the beautiful Hamakua Coast of Hawaii Island.