Saturday, April 5, 2014

Simple Ham Radio Antennas: The Ghost of Antennas Past--the simple vertical. Post #269

Over the past few posts, I've been describing some of the antennas I designed, built, and used during my days as a novice amateur radio operator (1977-1978).  Most of the working designs were copied into school theme books and saved for future reference.

One of my favorite homebrewed antennas was a simple vertical  antenna supported by a high tree limb terminating with a slightly angled ground plane consisting of 10 radials.  The antenna was designed for 40 meters and worked very well for contacts throughout the Pacific Rim and the mainland United States.

Last weekend, I decided to duplicate that antenna with some spare wire, ceramic insulators, coaxial cable, and basic tools.

As mentioned earlier, the process of moving to a new home often uncovers items you once thought were lost.  Such was the case here when I found several 50-ft /15.24 meters rolls of #14 AWG house wire in the garage.  That wire would serve as the vertical element and the rudimentary ground radial system.

To function properly, vertical antennas need a  radial system to supply "the missing half"of the antenna were they to be used as ordinary dipoles.

Fortunately, I had several tall Norfolk Pine trees in my back yard that could support a vertical wire.

ASSEMBLY:

My first vertical was cut so that I could operate cw on the lower portion of 40 meters (below 7.100 MHz) and the upper portion of 15 meters phone (approximately 21.240 MHz).  I wanted to save this antenna for when I became a general class licensee, so, by cutting the antenna for this length as a novice, I expected some SWR on the antenna.  When I added my Drake MN-4 transmatch to the system, the SWR was easily accommodated.

The 40 meter vertical antenna was built on the ground and later raised into position by a length of nylon rope.

My chosen design frequency was 7.088 MHz, the "watering hole" for many Hawaii amateur radio nets.  Using the general formula 234/f (MHz)=L (feet), I cut a length of #14 AWG wire measuring 33.01 ft/10.06 meters.  This would be the vertical element.

I also cut 10 pieces of #14 AWG wire for the radial system.  Although some antenna experts recommend that radials be cut approximately 5% longer than the vertical element, I decided  to follow my original plans and cut each radial wire to match the length of the vertical element (33.01 ft/10.06 meters).

I had 50-ft/15.24 meters of RG-8X coaxial cable with UHF connectors in a plastic storage box.  The coax would serve as my antenna feed line.  I also had a spare Budwig HQ-1 center coax connector to attach the feed line to the vertical element and the radial screen.

To support the vertical element and to terminate each radial wire, I found a dozen ceramic insulators in the junk box.  I would use one of the  11 insulators to secure the top of the vertical to a tree limb and the remaining insulators to tie off the radial elements to nearby tree limbs.

I had 100-ft/30.48 meters of nylon rope with a fishing sinker attached.  The rope would be attached to the insulator at the top end of the vertical element and shot over a convenient tree limb, taking the vertical element high into a notch approximately, 40-ft/12.19 meters above ground.

Once I had all the wire laid out on the ground, I soldered the vertical element to the + terminal of the center connector and soldered the radial wires to the - terminal of the center connector.  All joints were covered with several layers of vinyl electrical tape.

I attached a ceramic insulator to the top end of the vertical segment and to the free end of each of the 10 radial wires.

Before I attached the RG-8X coax to the center insulator, I made a 6-turn "choke balun" from the coax measuring approximately 8-in/20.32 cm in diameter. The coil was held together by nylon ties.  The coax was then attached to the center coax connector.

The nylon rope with sinker attached was tied to the insulator at the top end of the vertical element.  I then shot the rope over a limb about 40-ft/12.19 meters above ground level.  I slowly pulled up the vertical element into the tree notch.  The remaining part of the rope was secured to a nearby tree, so that the vertical segment stood straight up and down from the tree branch.  I made sure that the radial wires didn't get tangled as I lifted the vertical element into place.

The bottom of the vertical element was approximately 7-ft/2.13 meters above ground.  I then led each radial wire off from the center connector at a slight angle and tied off the end insulators to nearby trees or rocks.  The antenna resembled a low angle ground plane.

I fed the coax feed line through the nearby trees to the patch panel in the shack window.  A 6-ft/1.82 meters piece of RG-8X coax led to the Drake MN-4 antenna transmatch.  Short patch cords made from RG-8X interconnected the transmatch to the transceiver (Ten Tec Argosy II), dummy load, and low-pass filter.

I also connected a 33-ft/10.06 meter counterpoise wire to the ground lug of the Drake MN-4 transmatch.

RESULTS:

Without the Drake MN-4 in the line, the SWR across the 40 meter band measured 1.5 to 1.  The SWR on 15 meters was a bit higher at 1.9 to 1.  With the transmatch in the line, I was able to get the SWR near 1.1 to 1 on both the 40 and 15 meter bands.  Although I could load the antenna on 20 and 10 meters with the Drake MN-4, I decided not to strain the old ATU anymore than necessary.  For 40 and 15 meters, the antenna worked very well, just like its predecessor back in 1977.  The antenna can be converted into a multiband antenna with the use of 450 ohm ladder line and a 4:1 current balun attached to the Drake MN-4.  I received good signal reports from both Hawaii and mainland U.S. stations.

This simple, inexpensive antenna was easy to build.  When I was a "wet behind the ears" beginner in amateur radio, this antenna delivered many enjoyable hours of excitement and discovery.

REFERENCES:

http://www.arrl.org/hf-vertical.

http://www.arrl.org/files/file/Technology/tis/info/pdf/7806016.pdg.

http://www. dxzone.com/catalog/Antennas/Vertical.

http://www.youtube.com/watch?v=642N0oWgY51.

http://www.youtube.com/watch?v=jiHKFAdu18U.

Thanks for joining us today!

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

Aloha es 73 de Russ (KH6JRM).

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

Enhanced by Zemanta