Thursday, August 22, 2013

The Bent Dipole or Inverted "U" antenna. Post #223

Have you ever wanted to erect a full half wave dipole antenna for your favorite amateur radio band, but just didn't have enough horizontal space to put the antenna?  This could be a problem for 80 and 40 meter dipoles, which can stretch out to 135 feet/41.15 meters (80 meters) and to 67 feet/20.42 meters (40 meters).

According to an article by Claude Jollet (VE2DPE), most of the rf radiated and received by a dipole is concentrated "in the middle 60% or so of the antenna...the ends can be dropped down from the horizontal without much adverse effect."

That was great news to me, since my back yard has a maximum length of 50 feet/15.24 meters bordering the rear of my house.  So, if I bent the horizontal dipole at the 60% part of the flat top and let the remaining length hang down well above ground, I should still retain most of the dipoles good qualities.

I decided to erect a quickly built bent or inverted "U" dipole to see what results I could get.

MATERIALS:

Using the general dipole formula of 468/f (MHz)=L (ft) and a chosen frequency of 7.088 MHz (the frequency of the Hawaii Afternoon Net), I cut a piece of #14 AWG house wire measuring 66.02 feet/20.13 meters.  I rounded off the length to 66 feet/20.12 meters.  I cut the wire into two equal segments for the dipole...each segment measured 33 feet/10.06 meters.

Two ceramic insulators to isolate the segments from the fiberglass masts.

Two MFJ telescoping fiberglass masts...each mast was 33 feet/10.06 meters long.

Two 5-foot/1.52 meters wooden stakes to support each mast.

A homebrewed fiberglass center insulator to support the feed line.  The insulator had a hole drilled in each end to attach the feed line to each dipole segment.

Fifty feet/15.24 metes of 450-ohm ladder line.  This would be the antenna feed line.  The ladder line, used in conjunction with a 4:1 balun, a few feet of RG-8X coaxial cable, and an antenna transmatch would enable me to get multiband coverage from the dipole antenna (40 through 10 meters).

A W9INN 4:1 balun.

25 feet/7.62 meters of RG-8X coaxial cable with UHF connectors on each end.

My old but trusty Drake MN-4 transmatch.

Station equipment, including a Swan 100-MX transceiver, the Drake MN-4, a low pass filter, and a dummy load.

ASSEMBLY:

The antenna was built on the ground and later hoisted into position.

I first attached and soldered each dipole segment to the 450-ohm feedline.

I marked each dipole segment at its 60% distance from the center connector.  In this case for the 40 meter dipole, that length was 19.8 feet/6.03 meters.  The total horizontal span would then be 39.6 feet/12.07 meters.  The remaining segment lengths would be 13.2 feet/4.02 meters for each side.  The space between the ground and the bottom of each dipole segment would be 19.8 feet/6.03 meters.  A ceramic insulator was attached to each bent portion of the dipole.

The bent portions of the dipole were secured to the fiberglass masts with nylon ties.

Each mast was hoisted onto its support stake.  The dipole now conformed to an inverted "U" shape.

The 450-ohm feed line was run to the W9INN 4:1 balun attached to the garage wall.  Twenty-five feet/7.62 meters of RG-8X coax with UHF connector ran from the balun, through the shack window via a homebrew patch panel, and then to the Drake MN-4 transmatch.  Small coax patch cords connected the Swan 100-MX, the dummy load, and the low pass filter to the transmatch.

INITIAL RESULTS:

Thanks to the Drake MN-4 transmatch, I was able to get a SWR of 1.3 to 1 or lower on 40, 20, 15, and 10 meters.  The best bands were 40 and 20 meters, where I received reports on cw ranging from 569 to 599 and on SSB varying between 54 and 59.  I was running 25 watts from the old Swan 100-MX.  Ten meters was very noisy, while 15 meters had a few good contacts in the mid-afternoon, Hawaii time.

The bent dipole performs a bit better than my inverted vee, especially on 20 meters, where my reports were generally about 1 "S" unit more than the signals sent from the inverted vee.

This antenna is uncomplicated and performs better than expected.

REFERENCES:

http://www.hamradiosecrets.com/ham-radio-hf-antenna.html.
http://www.w6ier.org/images/97050561.pdg.
w4rnl.net46.net/uyagi.html.

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Aloha es 73 de Russ (KH6JRM)

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