Friday, May 9, 2014

Simple Ham Radio Antennas. A 5/8 wavelength vertical antenna for 20 meters. Post #274

How would you like to have a simple, effective antenna for 20 meters thast will give you some gain over a ground plane antenna and exhibit a radiation angle of approximately 15 degrees?

You can grab more DX (distance) at a modest cost by building a 5/8 wavelength vertical antenna working in conjunction with 1/4 wavelength radials beneath the main radiating element.

I've built 5/8 wavelength antennas for 10 and 15 meters and they work very well.  If you desire multiband performance out of this antenna, you will need a balanced feed line (such as 450-ohm ladder line), a 4:1 current balun, and a sturdy ATU (antenna tuning unit).  If you prefer to use this antenna on one band only, you can establish resonance with a base loading coil that tunes the antenna to 3/4 wavelength  resonance.  Standard 1/4 wavelength wire radials are used with both versions of this antenna.

According to William I. Orr (W6SAI) and Stuart D. Cowan (W2LX), "a 5/8 wavelength antenna provides improved performance DX-wise over the ground plane and is recommended for those amateurs with restricted yard space who have nowhere to go but up."

So, let's get started.  I built and tested this 5/8 wavelength vertical antenna for 20 meters on Friday, 09 May 2014 after I completed my morning yard work.  The process was done at a leisurely pace and took approximately 4 hours to erect and test.

PRELIMINARY WORK:

Using the general formula 585/f (MHz)=L (ft) and a chosen frequency of 14.200 MHz, I cut some #12 AWG house wire to a length of 41.19 ft/12.55 meters.  This would be the vertical element.  Each 1/4 wavelength radial wire (there would be four elevated radials in this system) was cut using the general formula of 234/f (MHz)=L (ft).  Each radial came out to a length of 16.47 ft/5.02 meters.

I had one remaining MFJ telescoping mast in the garage.  The mast was 33-ft/10.05 meters fully extended. Since the length of the 5/8 wavelength vertical exceeded the length of the mast, I would run the required extra length to a nearby tree limb approximately 35-ft/10.67 meters above ground.

I also had a good supply of 6-ft/1.82 meters metal tomato support stakes in the garden storeroom.  Four of these stakes would support the slightly sloping radial wires.

Six ceramic insulators were found in my junk box along with assorted pieces of nylon rope.  Four  insulators and rope would tie off the radial wires to the support stakes.  An additional insulator would serve as the junction/connection point for the 450 ohm feed line and its attachment to the vertical element and the radial system.  A final insulator would be attached to the end of the vertical element.

One 5-ft/1.52 meters wooden support stake for the fiberglass mast.

Five, 5-ft/1.52 meters wooden support stakes to keep the ladder line off the ground until it reached the shack.

Fifty-feet/15.24 meters of 450 ohm ladder line.  This would serve as the antenna feed lline.

One W9INN 4:1 current balun attached to the outside shack wall (i.e. the garage).  The balun was approximately 8-ft/2.43 meters above ground.

Twenty five feet/7.62 meters of RG-8X coaxial cable.  The cable would lead from the 4:1 balun, through the window patch panel, and onto the Drake MN-4 transmatch or ATU.

One "counterpoise bundle" consisting of 1/4 wavelength pieces of #12 AWG household wire for 20, 15, and 10 meters.  This "counterpoise bundle" would be clipped to the Drake MN-4 ground lug.  The station grounding system was also attached at this ground lug.  The ground system consisted of copper braid from an old piece of RG-8 coax attached to a ground rod and 3 buried radials (each being 33-ft/10.06 meters long).

Short pieces of RG-8X coaxial cable would connect the station transceiver (Ten-Tec Argosy II), Heathkit Dummy Load, and Low-Pass filter to the Drake MN-4 ATU.

ASSEMBLY:

The antenna was built on the ground.

I extended the mast to its full length of 33-ft/10.06 meters.  Since I wanted an elevated radial system (ground plane), I attached the bottom of the 5/8 wavelength vertical to a point 16-ft/4.87 meters from the base of the mast.  The antenna element was threaded through the eyelet at the top of the mast and drawn out to its full length.  I then attached a ceramic insulator to the top end of the vertical element. The remaining 25.19 ft/7.67 meters of the vertical element would be attached to 100-ft/30.48 meters of nylon cord and shot over a nearby tree limb approximately 35-ft/10.67 meters above ground.  The antenna would be partially vertical and partially horizontal.

I joined the bottom of the vertical element and the four-wire radial system to the 450 ohm feedline with the help of  a ceramic insulator, with one leg of the feed line soldered to the vertical element and the other leg soldered to the four, slightly sloping radial segments.

I carefully hoisted the mast onto its support stake.  The dangling remainder of the 5/8 wavelength vertical (25.19 ft/7.67 meters) element was attached to a ceramic insulator, a nylon cord and fishing sinker, and shot over a nearby tree limb with a slingshot.  I pulled the nylon rope until the top portion of the 5/8 wavelength vertical element was nearly horizontal.  The rope was tied off to a bolder.

The 1/4 wavelength radial wires were led off at a slight angle from the the 16-ft/4.87 meters spot on the mast to four, prepositioned metal stakes, approximately 5-ft/1.52 metes above ground.

The 450 ohm feedline was attached to five, 5-ft/1.52 meters wooden stakes with thumbtacks and led to the W9INN 4:1 current balun on the garage wall.  The feed line was kept at least 4-ft/1.21 meters above ground until it reached the wall-mounted balun.  A 25-ft/7.7.62 meters length of RG-8X coaxial cable ran from the balun, through the shack window panel, and onto the Drake MN-4 transmatch.  Small lengths (3-ft.0.91 meters) of RG-8X coax with UHF fittings connected the Argosy II, dummy load, and low-pass filter to the Drake MN-4.  I also connected the previously mentioned "counterpoise bundle" to the ground lug of the Drake MN-4 transmatch.

PRELIMINARY RESULTS:

With the Drake MN-4 transmatch in the antenna system, I was able to get a 1:1 SWR across the entire 20 meter band.  With careful adjustment, the antenna can also be used on the 15 and 10 meter bands.  After construction, I was able to work both mainland U.S. and Hawaii stations with ease.  SSB reports varied between 57 and 59, while CW contacts ranged from 579 to 599+.  The old Argosy II was running 50 watts. Not bad for few hours work.

References:

The ARRL Antenna Book.  ARRL.  Newington, CT, 06111.  Fourteenth Edition, copyright 1982. pp. 8-8 to 8-30 and 13-12 to 13-13.

Hood, William.  "Home Brew Hf/VHF Antenna Handbook."  Tab Books, Inc.  Blue Ridge Summit, PA, 17214.  First Printing, October 1977. pp. 152-153.

Turner, Rufus P.  "The Antenna Construction Handbook for Ham, CB & SWL."  Tab Books, Inc.  Blue Ridge Summit, PA, 17214.  Second Printing, January 1981.  pp. 141-143.

Orr, William I. (W6SAI) and Cowan, Stuart D. (W2LX).  "Simple, Low-Cost Wire Antennas For Radio Amateurs."  Radio Publications, Inc.  Box 149, Wilton, CT, 06897.  Third Printing.  pp. 115-116.

Orr, William I. (W6SAI) and Cowan, Stuart D. (W2LX).  "The Radio Amateur Antenna Handbook."  Radio Publications, Inc.  Lake Bluff, IL. Seventh Printing, 1988.  pp.108-109.

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Aloha,

Russ (KH6JRM)

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


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