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Saturday, July 13, 2013

My favorite stealth antenna. Post #212

Amateur radio operators who live in deed restricted homes and apartments face a variety of antenna problems.  The amateur radio press is full of stories describing the "no outdoor" antenna rules of OHAs and CC&Rs.

Despite these reports, many amateurs have been able to build effective indoor and outdoor antennas to pursue their radio interests. From flagpole antennas to attic beams, hams have used creativity and "stealth" to get and stay on the air.

In my case, the adoption of "stealth" antennas was forced on me by natural circumstances and not necessarily by highly critical neighbors.  In fact, my neighbors are good people who tolerate my amateur radio pursuits as long as I don't ruin their television reception or interfere with their entertainment systems.  Although many people in my neighborhood get excellent television via cable television providers, my immediate neighbors get their tv programming over the air using the familiar deep fringe antennas marketed by Radio Shack and others.  The adoption of digital over the air television transmissions has lessened potential tvi problems, but, just to be safe, I always use a low pass filter in my antenna systems and operate my station at qrp power levels (10 watts or less).  I also lower my antenna masts (vertical and inverted vee) after I close out my radio day.  So far, this arrangement has worked.

Now to the stealth part.  When hurricane "Iniki" struck Kauai head on in 1992, I learned first hand how fragile most antennas were. Fortunately, the storm just grazed Hawaii Island.  Kauai's communications infrastructure was a mess, with amateur radio operators providing most of the initial communications links with the outside world.  In the past, high winds sweeping down from Mauna Kea have damaged my pvc and fiberglass masts, despite my best efforts at guying the structures.

Since Hawaii gets occasional visits from tropical storms and hurricanes, I thought it a good idea to have a protected antenna at the home qth.  The antenna would be protected from the weather, capable of providing emergency communications throughout the state of Hawaii, and be nearly invisible from peering eyes.

The antenna would be a full- wavelength 40 meter loop under my house.  My home is built on a post and pier system tied to concrete pads, an arrangement that provides some protection against earthquakes and floods.  The house is approximately 3 feet/0.91 meters above the ground.

The antenna would be fed by a length of 450-ohm ladder line, coupled to a 4:1 balun.  A short piece of coaxial cable would connect the balun to an antenna transmatch and then onto the rig and associated station equipment.

MATERIALS and ASSEMBLY:

I designed the loop to be resonant on the daily Hawaii Afternoon Net frequency of 7.088 MHz.  Using the general formula 1005/f (MHz), I calculated an antenna length of 141.78 feet/43.22 meters.
I used #14 AWG household wire for the loop antenna.

The antenna fit fairly well under my house, with a short turn into the garage to make the total length come to what was calculated.  I attached 20 feet/6.09 meters) of 450 ohm ladder line at an intersecting corner of the loop.  That connection was soldered and wrapped with several layers of vinyl  electrical tape.

The ladder line was run under the door of the shack and was connected to a W9INN 4:1 balun.  A 3-foot/0.91 meters piece of RG-8X coaxial cable with UHF connectors ran from the balun to my trusty Drake MN-4 transmatch.  Several pieces RG-8X coaxial cable connected the transmatch to the Yaesu FT-7, the low pass filter, and the dummy load.  I also attached a 33-foot/10.96 meters piece of #14 AWG housewire to the ground lug of the Drake MN-4. 

With the help of the Drake MN-4, the under the house loop works on all bands between 40 and 10 meters.  Since the loop antenna is close to the ground, most of the signal goes straight up.  This low mounted NVIS (near vertical incident skywave) antenna is perfect for local statewide coverage out to about 200-300 miles/320-480 kilometers. 

The performance of the full wavelength loop is acceptable on 20, 15, and 10 meters.  This certainly is no DX antenna.  But it does work well for local and statewide coverage.

So, if one of my inverted vees or verticals succumbs to the forces of nature, I still have a dependable antenna for local emergency HF work.  The antenna can't be seen from the street or from any of my neighbor's homes.

A low mounted loop may be the solution to your antenna problem.  You could also run a loop around the ceiling of your apartment and feed it with 450 ohm ladder line into a balanced antenna tuner.  If you operate at qrp levels, rfi will be kept to a minimum. 

ADDITIONAL RESOURCES:

http://voices.yahoo.com/stealth-ham-radio-antennas-guide.
http://www.radiosurvivalist.com/antennas/build-stealth.asp.
http://www.eham.net/articles/29676.
http:/www./ac6v.com/antprojects.htm.

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For the latest Amateur Radio News, visit my news site--http://kh6jrm.com.  I've included a few headline stories at the bottom of this post.

Thanks for joining us today.

Aloha es 73 de Russ (KH6JRM).

BKJ29jx15--along the beautiful Hamakua Coast of Hawaii Island.