Thursday, September 4, 2014

Simple Ham Radio Antennas. The Open-Wire Dipole (Doublet). Post #293

I'm still in the process of restoring my "antenna farm" after the passing of Tropical Storm Iselle on 07-08 August 2014.  I lost a few trees on my Orchidland Estates property, most of which were supporting several HF wire antennas.  I salvaged most of the wire and the 450 ohm feedline from the storm.  While I cleaned up the mess, I erected a simple multiband vertical antenna using a tall Norfolk Pine Tree as an antenna support.

Next on the list is the restoration of my approximately 134-ft/40.85 meters  center-fed horizontal dipole.  The antenna is fed with 450 ohm ladder line which goes into a W9INN 4:1 balun and then via a short length of RG-8X coax into a MFJ 941E Versa Tuner II.  This feed arrangement allows multiband coverage from 3.500 MHz through 29.000 MHz.

Although the antenna requires two tall supports (trees or masts), construction of the antenna is simple and the dipole doesn't require a ground radial system.

Using the general formula, 468/f(MHz)=L(feet) and a chosen frequency of 3.500 MHz, each leg of the dipole measures out to be 66.8-ft/20.38 meters.  You may want to cut the antenna a bit longer to allow for trimming and SWR adjustments.

Materials available:

Sixty-six-feet/20.12 meters of 450 ohm ladder line.

Three egg-shaped ceramic insulators.  One insulator would serve as the center connection point between the ladder line and the two dipole segments.  The remaining two insulators would be attached to the free end of each antenna segment.

Two dipole segments, each measuring 66.8-ft/20.38 meters. I used some #14 AWG wire I stored in the garage for the antenna elements.

Two, 100-ft/30.48 meters of nylon rope and two fishing sinkers (2 oz.)  The rope would be attached to the end insulator of each element and would be launched over tree limbs by a slingshot.

One, homemade slingshot.

One W9INN 4:1 balun.  The 450 ohm ladder line would be attached to the balun.

Two, 6-ft/1.82 meters lengths of RG-8X with UHF connectors.  One piece of coax would lead from the 4:1 balun to the window patch panel; the other would go from the patch panel to the "antenna tuner."

One MFJ 941 E Versa Tuner II.

One Ten-Tec Argosy II, J-38 CW key, and microphone.

One Heathkit Dummy Load.

One B&W low pass filter.

Assorted tools.

One "counterpoise bundle", consisting of one 1/4 wavelength wire (leftover #22 AWG) for each band of operation (80, 40, 30, 20, 15, and 10 meters).


The Open-Wire Dipole (Doublet) was built on the ground.  

The 450 ohm feedline was threaded through the center insulator and soldered to each leg of the dipole.  Each connection was wrapped with several layers of vinyl electrical tape.

Nylon rope (100-ft/30.48 meters) was attached to each end insulator and tied off to a fishing sinker.

I launched each antenna segment (with the rope and sinker attached) over a high tree limb with a slingshot.  The chosen branches were 40-ft/12.19 meters above ground level.  The trees were approximately 135-ft/41.5 meters apart, so the antenna fit was a bit tight.

Each segment was loosely tied off to a tree stump, while adjustments were made to give the dipole a uniform horizontal appearance.  Once I was satisfied with the nearly horizontal position of the dipole, I tightened the launching ropes around smaller trees, leaving some slack in the antenna to compensate for the wind.

I ran the 450 ohm ladder line to the W9INN 4:1 balun attached to the garage wall (approximately 8-ft/2.43 meters above ground level.  The ladder line passed within 8 inches/20.32 cm of the metal garage roof. This distance didn't affect the feed line too much, since I had no trouble adjusting the MFJ 941 E Versa Tuner II for a low SWR.

A 6-ft (1.82 meters) length of RG-8X with UHF connectors went from the 4:1 balun to the window patch panel.  Another 6-ft (1.82 meters) piece of RG-8X went from the patch panel to the MFJ 941 E Versa Tuner II.  Short pieces of RG-8X connected the tuner to the Ten-Tec Argosy II, the Heathkit Dummy Load, and the B&W low pass filter.  As a last step, I connected the "counterpoise bundle" to the ground lug of the antenna tuner.

Initial Results:

I was able to get a 1:1 SWR on all 80, 40, 30, 20 ,15, and 10 meter amateur radio frequencies with the aid of the MFJ 941 E Versa Tuner II.  I also got a 1:1 SWR using the old Drake MN-4 antenna tuner (excepting 30 meters--the old MN-4 didn't cover 30 meters).

Using a maximum power of 25 watts, I was able to maintain good CW and SSB contacts on the U.S. mainland and throughout the Pacific Ocean.  The best daytime contacts were on 20 and 15 meters, while 40 and 80 meters did well around sunset through the early morning hours.  Ten meters was quite noisy and no contacts were made.

This was a fun and inexpensive antenna to make.  Fortunately, I was able to salvage most of the wire from the antennas lost during the hurricane.  Try to get your dipole as high as you can.  My height of approximately 40-ft/12.19 meters was a little on the short side for 80 and 40 meter DX, but I did get a few DX contacts on those bands.  Performance on 20 and 15 meters was pretty good.

This simple antenna is easy to build and will deliver plenty of contacts, both local and DX.  By using a balanced feed line, you can use this antenna on several amateur radio bands....more bang for the buck!

Resources: (simple introduction to the theory of "open-wire dipoles" by Stan Gribilisco (W1CV).

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Until next time,

73 de Russ (KH6JRM).