Sunday, June 16, 2013

Antenna Topics: A Field Day Dipole. Post #202

One of the simplest and most efficient Field Day antennas is the common half-wavelength flat top dipole or doublet stretched between two trees or portable masts and fed with 50-ohm coaxial cable (RG-58, RG-8, or RG-8X) for single band use or fed with 450--ohm ladder line, a 4:1 balun, and an antenna transmatch ("tuner") for multiband use.  I 've used this type of antenna in various configurations, included the inverted vee and the half-wavelength sloper.  For single band use, I've also employed 75-ohm RG-6 coaxial cable with "F" to "UHF" connectors.  Used with an antenna matchbox, the RG-6 serves as a suitable  feed line if 50-ohm cable is unavailable.

Placed a height of 30 to 50 feet above ground (9.14 to 15.24 meters), the  doublet will give you plenty of local and DX contacts.  Just cut your 2 antenna elements to the lowest frequency of use, feed with ladder line  into an appropriate balun and antenna matchbox, and you have an antenna capable of operating on several bands.

A few years ago, I was part of an antenna raising team that placed such an antenna in use during a Field Day operation at Hilo's Wailoa Visitor Center.  Using a Kenwood 430 (remember those?), our operators ran a hot operation on 40, 20, 15, and 10 meters.

Here's a list of materials our team used for the half-wavelength flat top dipole:

Two 33-foot (10.06 meters) telescoping fiberglass masts.

Three 7-foot (2.13 meters) wooden stakes.  Two stakes would support the fiberglass masts.  The remaining stake would be used to provide some strain relief to the center of the dipole and the feedline.

Fifty-feet (15.24 meters) of 450-ohm ladder line.  This would be our feedline and our ticket to running several bands off one antenna.

Three ceramic insulators.  One  insulator would be at the end of each dipole element.  The third insulator would serve as the center junction of the ladder line and the two dipole segments.

One 4:1 balun.  I had an extra W9INN balun for this purpose.

One Drake MN-4 antenna trasmatch from my shack.  I still have this "tuner".  It works very well.

Twenty-five feet (7.62 meters) of RG-8 coaxial cable.

Sufficient wire to make the dipole antenna.  We chose to design the antenna for 7.088 MHz, the frequency of the Hawaii Afternoon Net.  With the ladder line in play, the antenna could be used from the lower portion of 40 meters through the limits of the 10 meter band.  Using the general formula 468/f(MHz)=L(ft), we cut each dipole element to a length of 33.01 feet (10.06 meters).  The wire used for the antenna was #14 AWG house wire.

150-feet (45.73 meters) of dacron rope to support the fiberglass masts.  The rope would serve as "guy wires" for the masts.

Six plastic tent pegs to support the guy ropes.

A basic operating position, consisting of a covered lanai (porch), table, two chairs, logs, and various other record keeping items.

A 10 kw gasoline powered generator and extra fuel.

Assembly of the doublet antenna:

The antenna was built on the ground and later hoisted into position, with one team raising one mast and the other team raising the second mast.

Insulators were attached to the end of each dipole element and were tied off to the tip of each mast.  The center insulator was used to support the 450-ohm ladder line, one line of each was soldered to each antenna element.

Three guy ropes were attached to the mid-point of each mast (16-feet/4.87 meters above ground).

The masts were hoisted onto their support stakes and the guy ropes were attached to each tent peg (3 tie-offs for each antenna mast).

The ladder line dropped from the center insulator to a 7-foot (2,13 meters) wooden stake below the center connection.  The ladder line was tightened slightly, allowing a bit of "sag" in the doublet.  The 450-ohm ladder line was secured to the wooden stake with nylon ties. Then, the ladder line was led to another 7-foot (2.13 meters) stake, allowing the feed line to remain above ground level.

The ladder line was secured to the 4:1 balun on the stake.  Twenty-five feet (7.62 meters) of RG-8 coaxial cable went from the 4:1 balun to the Drake MN-4 antenna matchbox.  Several short pieces of RG-8 coax connected the Drake MN-4 to the Kenwood-430 and the dummy load.


For a quickly-made 40 through 10 meter antenna, the doublet did very well.  The antenna was used throughout the day and night to contact stations on the mainland U.S., Canada, and Asia.  Most of the time, the old Kenwood 430 ran approximately 75 watts output.

After the Field Day exercise ended, the antenna was broken down and stored away in less than 15 minutes.

If you have only one tall support, try the doublet as a half-wavelength sloper or an inverted vee.  If you wish to include 80 meters in the mix, cut each dipole element to 66.85-feet (20.38 meters).  That should put your antenna resonance at 3.500 Mhz--perfect for some cw DX.


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Thanks for joining us today!

Aloha from the Big Island of Hawaii,

Russ Roberts (KH6JRM)

Bk29jx15--along the beautiful Hamakua Coast.