Monday, January 13, 2014

Simple Ham Radio Antennas. Antennas without "tuners". Part 2. Post #255

One of the more popular amateur radio antennas is the multiband "Fan Dipole" and its close relative the multiband inverted vee "Fan Dipole."  According to Howard (W6HDG), the Fan Dipole "consists of  two or more distinct half wave dipoles which are mounted to a common parallel feed point so that a single feed line can be utilized."  With a little ingenuity and careful trimming, it is possible to make a single band dipole antenna work on several bands just by adding half wavelength dipoles for your favorite band to the center coax connector and by "fanning them out" below the dipole for the lowest frequency.  Add a 1:1 balun or a "choke" balun below the center connector, connect some 50 ohm coaxial cable, run the cable to your rig, and you're ready to go on your favorite bands without a "tuner" and a ground radial system.

According to Simone (IW5EDI), "an antenna like this works on multiple bands because the antenna presents a high impedance on the elements that aren't resonate, allowing the resonant elements to radiate ...."

A quick search of the data available, shows there are many approaches to building a "Fan Dipole"--some of them backed by research from the Standford Research Institute and years of individual ham operator experience.

My goal in this antenna project was not to duplicate the plans of these authors or to buy all the necessary auxiliary supplies needed to build these antennas.  All I wanted was to build, erect, and use a simple "Fan Dipole" with the materials I had at the qth or from the local hardware store.    A  secondary purpose was to build an antenna that was portable and inexpensive to make.

MATERIALS:

My property in the Puna District of Hawaii Island is blessed with an abundance of trees approaching 50 feet/15.24 meters in height.  Some of these trees are stately Norfolk Pines which have strong lateral branches every 5 feet/1.82 meters or so.  These trees would make perfect supports for the end of each inverted vee element.

One 33-foot/10.06 meters MFJ telescoping fiberglass mast.  This would be the main support for the multielement inverted vee and the coaxial feed line.

Seventy-five feet/22.86 meters of RG-8X coaxial cable with UHF connectors.  This will be the antenna feed line.

Six ceramic insulators to tie off antenna elements to tree branches.

One 5-foot/1.82 meters wooden stake to support the fiberglass mast.

One Budwig HQ-1 center coax connector.

One 'homebrew choke" balun to keep rf off the feed line and station equipment.

Approximately 150- feet/45.73 meters of #14 AWG housewire for the antenna elements.  You won't need the total amount, but it doesn't hurt to have some extra wire around.

Approximately, 200- feet/60.97 meters of 0.25 inch diameter/0.635 cm diameter dacron rope.

A MFJ antenna analyzer would be helpful for trimming purposes.  Since I didn't have one in the shack, I used the SWR meter in my Drake MN-4 antenna transmatch for any trimming needed.  Clumsy and slow, but it did the job.

ASSEMBY:

The antenna elements were made and connected on the ground.

The inverted vee "Fan Dipole" would cover the 40/15 (shared antenna), 20, and 10 meter bands.

Using the general forumla 468/f (MHz)=L (feet), my antenna elements looked like this:

40 meters (7.088 MHz/21.264 MHz)--33.01-feet/10.06 meters for each element (2).  By cutting the antenna for the cw portion of 40 meters, I could use the antenna on its third harmonic for ssb in the 15 meter band.

20 meters (14.200 MHz)--16.48-feet/5.02 meters for each element (2).

10 meters (28.400 MHz)--8.23 feet/2.51 meters for each element (2).

Each dipole was soldered to the Budwig HQ-1 center connector.

30-feet/9.14 meters of dacron rope was tied to each end insulator of the antenna elements.

A halyard and pulley system would be used to hoist the center connector and antenna elements into position.  I used 50-feet/15.24 meters of dacron rope and a marine pulley from "Ace Hardware" to complete the hoisting mechanism.

A "choke balun" consisting of 8 turns of RG-8X coaxial cable measuring approximately 8-inches/20.32 cm in diameter was positioned just below the connecting point of the center insulator and the coaxial feed line.  You could also use a commercially bought 1:1 balun.  The "choke balun" was taped to the fiberglass mast.  The coaxial feed line was led down to the 16-foot/4.87 meters point on the extended mast.  That part of the coax was taped to the mast and secured with several nylon ties.  The remainder of the coax would be lead off the ground to the patch panel in the shack window.

I slowly hoisted the mast onto its support stake.

I used a slingshot to fire each dipole over suitable branches in the two Norfolk Pine Trees.  The trees were approximately 40-feet/12.19 meters apart.  The top dipole elements belonged to the 40/15 meter antenna, and they were shot through branches approximately 30-feet/9.14 meters above ground.  The top dipole was nearly horizontal. The dipole elements were tied off at lower branches with the leftover rope.  I found my antenna length was pretty much close to what I had calculated, leaving the dipole with a swr of 1.6 to 1.

The 20 meter dipole was led off the center connector at an angle and secured to a branch 25-feet/7.62 meters above ground.  Extra rope was tied off at a lower branch.  The swr of this dipole wasn't too bad--1.7 to 1.  Five-feet/1.82 meters separated the ends of the 40 and 20 meter dipoles.

The 10 meter dipole was led off the center connector at an angle below the 20 meter inverted vee.   The 10 meter elements were secured to branches approximately 20 feet/6.09 meters above the ground.  Extra rope was tied off at lower branches.  The swr for the 10 meter antenna elements was 1.9 to 1.

The antenna  was adjusted so  the appearance of the inverted vees was uniform and balanced.  Before I started on-air contacts, I tested the antenna with the Drake MN-4 transmatch in the system.  I was able to get a swr of 1:1 on all bands. Without the transmatch in the line, the swr measured less than 2:1 across each band preference.

Your results will vary because of local ground conditions, proximity to structures, and space restrictions.  Even if you don't get a great swr, you can use an antenna transmatch to smooth things out. Try to keep the antenna elements separated as much as possible to cut down on coupling.

RESULTS:

Considering the materials available, I am satisfied with the overall results of this inverted vee "Fan Dipole." Running 50 watts from the old Swan 100MX, I had no trouble maintaining solid cw and ssb connections with stations in Hawaii and on the U.S. mainland.  Despite the relative crudeness of this antenna, it has performed very well at my new home site.  The antenna is fed at a current maximum for all bands and does not require a radial ground system.  Most of your material can be found at the nearest home improvement outlet or hardware store.  If you don't have tall trees on your property, you can use several telescoping fiberglass masts to support the antenna.  Be creative, save money, and build it yourself.

REFERENCES:

http://www.hamuniverse.com/multidipole.html.
http://www.hamuniverse.com/kb3pkbfandipole.html.
http://www.hamuniverse.com/ae5jumultibanddipole.html.
http://www.hamuniverse.com/w6hdgfandipole.html.
http://www.iw5edi.com/ham-radio/?20-40-dual-band-dipole0146.

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Aloha de Russ (KH6JRM

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