Friday, January 10, 2014

Simple Ham Radio Antennas: Antennas without "tuners", part 1. Post #254

Antenna transmatches or "tuners" are wonderful devices that make amateur radio operations easier.  They can help minimize swr on antenna feed lines and extend the band coverage of your present antenna.

However, there are times when such devices may not be needed, especially if you're designing an antenna for a single band or are contemplating an emergency or portable antenna for your "go" kit.  By carefully designing, cutting, and trimming your basic 1/2 wavelength dipole for the lowest swr, you can just connect a good quality 50 ohm coaxial cable feed line and run the band of your choice without much difficulty.

In this post, I will describe a few simple coax-fed dipoles and inverted vees that will serve your favored amateur radio band without the need for an antenna transmatch.  Of course, you could always insert a transmatch into the system to squeeze out the maximum power at the lowest possible swr if you so desire.  These antennas work either way.  For the purposes of this post, I will confine my choice of antennas to dipoles and inverted vees and save vertical antennas and their ground systems for another post.

In most cases, the dipoles and inverted vees cited in this article have been tested by me in both home and portable operations. Most of these antennas are single band affairs, unless stated otherwise.  However, by connecting a series of spaced dipoles to a single coax feed line, you can get multiband coverage.  Sometimes, this arrangement is called a "fan dipole."  As with the monoband dipole, an antenna transmatch can be used to remove any swr on the feed line.  So, if your design shows a bit of swr, insert the antenna transmatch and adjust accordingly.  I'll be covering the "Fan Dipole" in a later post.

The antennas I've built exhibit a swr of 2:1 or less across the band of choice.

MATERIALS NEEDED:

Depending on the amount of structures, trees, and masts available, you can build a standard 1/2 wavelength horizontal dipole or its close cousin the inverted vee.  The inverted vee requires only one tall structure, a wooden stake to support a telescoping fiberglass mast, and two end stakes to support the ends of the drooping antenna elements.  In my situation, I opted for the inverted vee designed for 40 meters.  By cutting the antenna for the lower end of 40 meters (below 7.080 MHz), I can use the third harmonic of this frequency to operate in the phone segment of 15 meters (21 MHz).

Sufficient wire to make two equal dipole elements.  Using the general formula 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 AWG #14 housewire for the antenna elements measuring 66.02 ft/20.13 meters.  Each antenna element would be one-half of the dipole length or 33.01 ft/10.06 meters.  There are some antenna experts that believe a better forumla to use for the inverted vee is 464/f (MHz)=L (ft).  In any case, cut your antenna elements a bit longer than these numbers to allow for trimming and low swr.  I was lucky to have my calculations result in an swr around 1.7 to 1.  A little trimming would be done later.  The antenna would use the third harmonic of 7.088 MHz for the SSB portion of 15 meters.

One 33-ft/10.06 meters MFJ telescoping fiberglass mast.  Try to get your antenna feed point as high as you can.

One 5-ft/1.82 meter wooden support stake for the mast.

One Budwig HQ-1 center coax connector.  You could also make a center connector out of pvc pipe or stiff plastic.

Two ceramic insulators for the element ends.

Two 10-ft/3.04 meters pvc pipes to support the drooping ends of the inverted vee.  Nearby structures such as fences, gates, or even tree limbs could be used.  The insulators would be tied off with pieces of dacron rope.

Fifty-feet/15.24 meters of RG-8X coaxial cable with UHF connectors at each end. This will be your feed line.

This completes the basic materials list for the 40 meter/15 meter inverted vee.

A similar materials list would be used to make separate inverted vee/dipole elements for 20 meters, 30 meters, and 10 meters.  Other than separate center coax connectors, the new elements would share the same mast, pvc support posts, and coax feed line used in the 40/15 meter inverted vee.

To change bands, I would only have to lower the mast, switch out the center coax connectors, attach the new dipole/inverted vee segments, raise the mast, and begin operating on the new band.

Once I had the original 40/15 meter inverted vee antenna built, I made dipole elements for 20 meters (14.200 MHz--16.47 ft/5.02 meters for each element), 30 meters (10.125 MHz--23.11 ft/7.04 meters for each element), and 10 meters (28.4 MHz--8.23 ft/2.51 meters for each element).

ASSEMBLY:

A separate dipole was made for the bands of my choice (40/15 meters, 30 meters, 20 meters, and 10 meters).

Depending on the band chosen for the day, I just attached the coax connector with its elements to the apex of the mast and secured it with nylon ties.  I hoisted the mast onto its wooden support stake, tied off the drooping elements so that the included angle between the elements ranged from 90 to 120 degrees, and ran the coax feed line to the window patch panel.  A short piece of RG-8X (3 feet/0.91 meters) ran to a SWR meter and then to my Swan 100 MX transceiver.  I bypassed the Drake MN-4 antenna transmatch for this direct feed.  I was lucky to find the swr for each separate antenna to be 2:1 or lower across the band of interest.  By inserting the antenna transmatch, I could get the swr down to 1.1 to 1.

So, there's one way to bypass an antenna "tuner" if you have to.  By using separate dipole elements cut for each band of choice, you can get an antenna that performs well without excessive swr.  Of course, a piece of equipment such as the MFJ antenna analyzer would make the task easier.

This is the system I use for my portable and emergency antennas.  To change bands, all I have to do is lower the mast, choose the appropriate antenna, raise the mast, and begin operating.

Next time, I'll describe another way of getting multiple band coverage out of one coaxial feedline without the use of a "tuner".  This antenna will be our friend the "Fan Dipole."

REFERENCES:

http://www.youtube.com/watch?v=8KHzsKYsZL0.
http://www.youtube.com/watch?v=IBncfqEORGM.
http://www.youtube.com/watch?=fyOWRTWdDKM.
http://www.hamuniverse.com/easydipole.html.
http://www.eham.net/articles/24060.
http://www.arrl.org/files/file/Technology/tis/info/pdf/9106923.pdf.

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Aloha, de Russ (KH6JRM).

BK29jx15--along the beautiful Hamakua Coast of Hawaii.
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