Saturday, May 25, 2013

Antenna Topics: Reworking a basic 10 meter ground plane antenna. Post #196

Reworking a basic 10 meter ground plane antenna.

After two fun-filled days of experiments with my horizontal 10 meter dipole, I decided to lower both masts, disassemble the dipole elements, and store them in the garage for possible emergency or portable use.  For some reason, the second mast made my "antenna farm" noticeable from the street and I opted to use only one mast for my next 10 meter antenna project.  I selected the fiberglass mast with the light green paint for antenna projects because it blended in well with the garden and the surrounding vegetation.  The spare MFJ mast would be used as a backup and emergency mast.

Still, I wanted to work a bit more with 10 meters, which has shown some excellent propagation during the past month.  I had already used a 10 meter vertical dipole with great success during the past year.  This time, I wanted to experiment with a 10 meter ground plane antenna that could be easily built, erected, and taken down without undue problems.  With four elevated radials, I hoped to avoid an extensive radial field and to get a good match for some RG-6 coax I had stored in the garage.  Although the RG-6 is a nominal 75 ohms (versus a nominal 50 ohms for RG-8X, RG-8, and RG-58), I felt any small mismatch could be handled by my trusty Drake MN-4 matchbox ("tuner").

As I mentioned a few days ago, I had a supply of RG-6 left over from a radio station studio rebuild several years ago (I was working as a news person then).  I was able to get a few Radio Shack "F" to "UHF" connectors to make using the RG-6 easier.

MATERIALS:

Since this was going to be a busy weekend (I had to announce the "Memorial Day Drag Races" at the Hilo Drag Strip), I lined up all supplies on Thursday and built the 10 meter ground plane on Friday (24 May 2013).

One 33-foot (10.06 meters) MFJ telescoping fiberglass mast.

One 5-foot (1.52 meters) support stake for the mast.

One Budwig HQ-1 center coax connector (available from Fair Radio Sales in Lima, Ohio).

Five ceramic or plastic insulators.  One insulator would be attached to the top of the mast and would support the vertical element of the 10 meter antenna.  The other four insulators would be attached to each elevated radial.

Four, 5-foot (1.52 meters) wooden or metal fence posts to tie-off the radial elements.

Some dacron rope to attach the end insulators of the elevated radials to the posts.

Fifty-feet (15.24 meters) of RG-6 coaxial cable.  This would be the feed line.  The feed line would be connected to the anti-static unit beneath the shack window.

10-feet (3.04 meters) of RG-6 coaxial cable to run from the anti-static unit, through the shack window, and onto the Drake MN-4 matchbox.  Small pieces of RG-6 coaxial cable would connect the Drake MN-4 to the Swan Transceiver, a low pass filter, and a dummy load.

Approximately 50-feet (15.24 meters) of #14 AWG housewire.  Although the length of the vertical element and its four elevated radials would amount to only 41.15 feet (12.55 meters), I wanted some extra length, just in case I needed to trim the antenna.  Some antenna experts advise making the radials about 5% longer that the radiating element, but I preferred to keep all antenna lengths the same.

Nylon ties, vinyl electrical tape, dacron rope.

ASSEMBLY:

The mast was lowered to the ground for ease of antenna installation.

I wanted the antenna to be resonant at 28.400 MHz, right in the center of the novice/technician phone band (28.300 MHz to 28.500 MHz).  I find there is a lot of action in this portion of the band when 10 meters is open.

Using the general formula, 234/f (MHz)=l (ft), I cut each wire (the vertical element, plus four elevated radials) to a length of 8.23 feet (2.51 meters).

I attached an end insulator to the top of the mast and connected the vertical element to it.  I led the vertical element down from the top 8.23 feet (2.51 meters) and secured the wire to the fiberglass mast with nylon ties.

I attached the vertical element to the + side of the Budwig HQ-1 center connector.  I attached each elevated radial to the - side of the Budwig HQ-1 center connector.  All connections were soldered and covered with several layers of vinyl electrical tape.

End insulators were attached to the end of each elevated radial.

Before I connected the RG-6 feed line, I wound a "choke balun" near the "F" to "UHF" connector.  The balun would help keep stray rf from entering the shack by the shield of the coaxial cable.

Once erected the mast, the feed point of the antenna would be approximately 25-feet (7.62 meters) above ground level.  I ran the feed line down the mast to a level of approximately 16-feet (4.87 meters) above ground level.  The feed line was secured with nylon ties.

I hoisted the mast onto its wooden support stake and spread out the elevated radials, separating each wire by 90-degrees.  The elevated radials were led off the mast to pre-positioned wooden fence stakes.  Although the angle was not at an optimum of 45-degrees, the elevated radials appeared evenly spread around the mast at a uniform height at the post tie-off point.  The elevated radials offered some support for the mast.

I then led the feed line to the anti-static unit beneath the shack window.  A short length of RG-6 went from the anti-static device, through the shack window, and onto the Drake MN-4 matchbox ("tuner").  Small patch cords connected the Swan 100-MX transceiver to the Drake MN-4, the low pass filter, and the dummy load.

INITIAL PERFORMANCE:

With the Drake MN-4 in the antenna system, I was able to get a 1.3 to 1 SWR in the 28.300 MHz to 28.500 MHz segment of 10 meters.  A little trimming will probably improve that figure.  I was able to get a SWR of 1.7 to 1 across the entire 10 meter band with the help of the Drake MN-4.  Tuning was a bit "touchy" at the bottom 50 KHz of the band.  For now, I'm satisfied with the performance of the ground plane.

I've made only a few contacts on 10 meters since I erected the antenna on Friday.  So far, CW contacts have reported 559 to 579 on my signals, while SSB contacts have ranged from 54 to 57.  Nothing spectacular, but the antenna works.  The rig runs cool and the old Drake MN-4 seems able to take care of any mismatch on the band.  I've been running the old Swan 100-MX at 20 to 30 watts.

The antenna was assembled at little cost, since I had most of the materials in the shack.  I'm sure you can find  mast sections, wire, and rope at the nearest hardware store or home improvement center.  I leave the choice of coaxial cable up to you.  I used RG-6 because I had a supply of the cable from my old days at KKBG-FM/KHLO-AM.  "F" to "UHF" connectors for the RG-6 can be found at Radio Shack stores.

If you want a simple 10 meter antenna that requires only one support and doesn't need an extensive ground radial system, try the ground plane antenna with elevated radials.  The antenna is fun to build and it works.

REFERENCES:

http://www.dxzone.com/catalog/Antennas/10M.
http://www.youtube.com/watch?v=aLgFeTpIX4A.
http://radiosurvivalist.com/antennas/for-10-meters.asp.
http://www.comport.com/~w5alt/antennas/?pg=3.

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Aloha es 73 de Russ (KH6JRM0.

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