Monday, May 5, 2014

Simple Ham Radio Antennas: A 5/8 wavelength vertical for 15 meters. Post #273

Over the past few weeks, I've been experimenting with a 5/8 wavelength vertical antenna for 10 meters.  The antenna is a marked improvement over my 1/4 wavelength ground plane vertical for 28.400 MHz.  Some antenna experts believe the 5/8 wavelength vertical can deliver a 3 dB gain over the conventional vertical presented in most antenna books.  According to William I. Orr (W6SAI) and Stuart D. Cowan (W2LX), the "5/8-wave vertical provides improved performance DX-wise over the ground plane antenna and is recommended for those amateurs with restricted yard space who have 'nowwhere to go but up.'"

With that in mind, I spent part of Saturday, 03 May 2014, building a 5/8 wavelength vertical for the 15 meter amateur radio band, with a preferred frequency of 21.260 MHz in the phone (SSB) portion of the band.  The antenna was simple, requiring some wire, a few wooden support stakes, some ceramic insulators, a "ladder lock" support for the feed line, a length of 450-ohm ladder line, a 4:1 current balun, a length of 40-ohm coaxial cable with UHF fittings, a sturdy ATU (antenna tuning unit or transmatch), a transceiver, and related station equipment.  With the use of a balanced feed line, this antenna can be used on 20, 15, and 10 meters.

If you prefer single band operation and plan to use 50-ohm coaxial cable as your feed line, you can bring the 5/8 wavelength antenna into  resonance by "the addition of a small tapped coil at the antenna base.  The number of turns in the coil is varied to provide a low value SWR on the transmission line...." (citing Cowan and Orr in this case).

Since I try to make my antennas serve several bands, I opted to use 450-ohm feed line, an ATU, and a balun to connect the antenna to the shack's transceiver.

MATERIALS:

For ease of construction, I built the antenna on the ground, attached it to a fiberglass mast, and hoisted the assemblage into position.

1.  One 33-ft/10.06 meters MFJ telescoping fiberglass mast.

2.  Five, 5-ft/1.57 meters wooden support stakes.  One stake would support the mast, with the remaining four stakes used to tie off the slightly sloping radial elements.

3.  Approximately, 75-ft/22.86 meters of #14 AWG house wire.  One wire would serve as the vertical element, with the remaining wires to constitute the radial system.

4.  One "ladder lock" balanced line support block.

5.  50-ft/15.24 meters of 450-ohm ladder line.  This would serve as the antenna feed line.

6.  Four ceramic insulators with small lengths of nylon rope to tie off the sloping radials to pre-positioned wooden support stakes.

7.  One W9INN 4:1 current balun.

8.  One Drake MN-4 ATU.

9.  Twenty-five ft/7.62 meters of RG-8X coaxial cable with UHF fittings.  This cable would connect the balun to the Drake MN-4.

10.  Several 3-ft/0.91 meter patch cords made from RG-8X coaxial cable to interconnect station equipment to the Drake MN-4 ATU.

11.  Ten-Tech Argosy II transceiver.  I usually run the rig at the low power setting (05 watts).  But, for this test, I ran the Argosy II at the higher power setting (50 watts).

12.  One Heathkit Dummy Load.  Transceivers should be adjusted off the air.

13.  One low pass filter.  Some of my new neighbors receive over-the-air television signals.  Cable tv penetration at my new Puna District home is sparse.  Many folks still get their programs from tv repeaters in Hilo.  A few neighbors subscribe to Dish TV and other satellite services.

14.  Various tools, including soldering equipment, vinyl electrical tape, nylon ties, pliers, wire cutters, etc.

ASSEMBLY:

1.  Using the general formula for a 5/8 wavelength antenna (585/f (MHz)=L (ft), I cut the vertical element to a length of 27.51-ft (27 ft, 6.1")/8.40 meters.  This would resonate the vertical element of a frequency of 21.260 MHz.

2.  I extended the MFJ fiberglass mast to its full length on the ground (33-ft/10.06 meters).

3.  I attached the tip of the vertical element to the apex of the mast with nylon ties, ran the wire down the mast, and brought the wire to a point approximately 6 ft/1.82 meters from the bottom of the mast.  The vertical element was secured to the mast by nylon ties.

4.  Four 1/4 wavelength radial wires were cut to frequency (21.260 MHz).  Each radial segment measured 11.00-ft/3.35 meters.  A ceramic insulator and nylon tie off rope were attached to one end of each radial wire.

5.  The 450-ohm ladder line was attached to the "ladder lock" support device and a wire from one leg was soldered to the vertical element, while the four radial wires were soldered to the other leg of the ladder line.  The "ladder lock" connector was affixed to the mast by nylon ties.  The junction was approximately 6-ft/1.82 meters from the bottom of the mast.

6.  I carefully hoisted the fiberglass mast onto its wooden support stake.  Each radial was carefully led to a prepositioned wooden stake, with the end of each radial wire being tied off at a height of 4-ft/1.21 meters above ground.

7.  The ladder line was led to the W9INN 4:1 balun attached to the garage wall.  The balun was approximately 8-ft/2.43 meters above ground.  The ladder line had a very slight sag leaving the lower portion of the mast, but it didn't touch ground.

8.  A 25-ft/7.62 meter length of RG-8X coax with UHF fitting was led through a window patch panel and then connected to the Drake MB-4 ATU.  I attached a "counterpoise bundle" consisting of wires 33-ft/10.06 meters, 16.5-ft/5.03 meters, 11-ft/3.35 meters, and 8.23-ft/2.51 meters long to the ground lug of the Drake MN-4.  I also had a station ground consisting of a ground rod driving into the soil with four 33-ft/10.06 meters radial wires attached.

9.  Several small RG-8X coax patch cords were used to connect the Ten-Tec Argosy II transceiver, the Heathkit Dummy Load, and the Low-Pass filter to the Drake MN-4.

INITIAL RESULTS:

With the Drake MN-4 in the line, I was able to get a 1.2 to 1 SWR across the entire 15 meter band.  Also, the antenna loaded fairly well on 10 meters and 20 meters.  For now, I'll just use the 5/8 wavelength antenna for 15 meters.  No sense putting additional strain on the aging Drake MN-4.

So far, results have been good.  Depending on propagation and time of day, my cw reports range from 579 to 599+, while SSB varies between 55 to 59.  I'm running the little Argosy II at 50 watts.  Now that I know the antenna works properly, I'll cut back my power to between 5 and 10 watts.

This was an enjoyable and fun project, since I had all of the necessary components in my "junk box."  And best of all, the antenna works and gives my hours of pure fun--as long as 15 meters stays open.

Next on the agenda, a 5/8 wavelength vertical for 40 meters.  For this project, I think the tall Norfolk Pine Tree in the back yard and my trusty WalMart slingshot will come into play.

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Aloha,

Russ (KH6JRM)
BK29jx15--along the beautiful Hamakua Coast.

REFERENCES:

Orr, William I. (W6SAI) and Cowan, Stuart D. (W2LX).  The Radio Amateur Antenna Book.  Radio Publications, Inc.  Box 149, Wilton, CT, First Printing, 1978.  pp.108-109.

Noll, Edward M. (W3FQJ).  73 Vertical, Beam, and Triangle Antennas.  Editors and Engineers.  Indianapolis, IN, 46268.  pp. 29-36.