Pikes Peak

Pikes Peak

Monday, January 28, 2013

20 Meter Moxon Antenna



   I was officially introduced to the Moxon Antenna by a local Ham (KH6IB) at our club's meeting this month, where he demonstrated a vertical Moxon built for 2 meters and claimed it had > 6 dB gain... I was Sold. 

Parts for my build:
  • 3 each, 10ft x 1" schedule 40 PVC (used for irrigation) 
  • 1 each, 1" four-way interconnect (went with 4-way in case I want to someday used a center mast)
  • 3 each, 1" PVC end caps
  • 4 each, small eyelet screws (for upper arms rope support)
  • Enough #12 or #14 stranded copper wire
  • Feed line
  • 1 can spray paint, flat, dark-green
  • A handful of plastic cable ties
  • Rope and/or 550 (parachute) cord
  • 1 each, small pulley
  • 1 each, 5-gallon bucket
My costs: about $40.


   Upon questioning KH6IB about building one for 20 meters, he referred me to the Moxon Antenna Project website (KD6WD's Moxon Antenna Project). It has pretty much everything you will need with many useful testimonials and photos of other HAMs' interpretations of the antenna. The key is the free-ware program to design and build the antenna.  As the Moxon is essentially a two-element Yagi, there is a need to follow the instructions to get your driven and reflector element cut and placed where you need them. 
   
   You also have to figure out how to support this antenna. I went for the vertical Moxon supported at the top-center with rope. I reasoned it would require the least supports (guys) for my situation, allow me to utilize the tall trees nearby, and provide a relatively quick setup and tear-down. I toyed with the idea of using really long fishing poles, plus extensions but realized this would not be as tactical as I would like. Don't get me wrong, this #20MeterAntenna is still a beast, with the legs at 25 ft long, and a spread of 9 ft. The big plus is that you only need to have it a few feet off the ground. This antenna models a 6 dBi gain and nearly 40 dB F/B ratio.

Construction:

   This part was pretty straight forward. I cut one of the 10 ft PVC pipes in half and fitted the two pieces into the four-way interconnect. Then I cut a 2 ft piece to act as my vertical lift point and provide supports to the ends of the upper PVC arms. I capped the three ends, then pre-drilled the points were I wanted my eyelets and attached my arm support ropes. Lastly I added a loop of rope at the top to lift from. No glue needed in my build as the PVC seated snug enough.
  
   I was warned that the software (#Moxgen) might produce an off-frequency antenna on the low side, and to expect cutting the antenna for 400-600 KHz higher. I found this to be true in my case. I set the software for a center frequency of 14.2 MHz. I do not own a antenna analyzer so I relied solely on my radio's SWR meter to get me as close to 1.1:1 as possible. I ended up closer to 14.7 MHz by design measurements.
  
   With my original measurements and the PVC laying on the ground I affixed the driven and reflector wires to the upper section first using the plastic cable ties; taking care for the front/back spacing. I measured out the length to the bottom PVC and attached the wires with cable ties also. On the driven element I used a spare insulator and terminal piece to connect my feed line at the mid-point just using crimp-on spade lugs. Once elevated it is very important to support the feed line out and away from the elements. I found this made a huge difference in the noise level. 

Setup:

  To keep the whole thing from swinging around in the breeze I just used a five-gallon bucket of water and tied one of the bottom ends to the handle with fishing line. To tune the antenna, I made small increases in the software's frequency and calculated the amount to be cut/re-positioned in inches; made the changes and retested. These testing cuts are made easier if you use two ropes: one tied at the top with a pulley, the other to raise and lower the antenna through the pulley. Once finished, I painted it flat-green to blend into the yard and trees. It is hardly visible to someone driving by on the street only 25 feet away. Bottom line, if you are looking for a weekend antenna experiment or something to haul out for Field Day, consider the Moxon Antenna.
73,
de KH7AL


Moxon Antenna


Top-center, lift point.

Close up of the PVC ends in storage mode.

Up in the tree, pre-painting.


Feed point connection.

Top center piece.
For another great tactical antenna, check out Honolulu Emergency Amateur Radio Club's (EARC) EFHW matchbox. Its a great deal; kit or finished product, shipped for under $50.

1st HAM Radio Post


  The biggest allure to me about HAM radio is that with a very simple setup, anyone can communicate wirelessly with another person thousands of miles away. I understand there are a lot of physics and a myriad of ionospheric variables going on; it just amazes me that a wire cut to a specific length resonates at a given electromagnetic frequency. Upon that frequency we can modulate information to be demodulated at some other place on the planet. In this day and age, it is something completely taken for granted.
  Rant aside, I really enjoy building antennas. The never-ending quest is to find the most bang for my limited buck while staying tactical. I live in a housing area which is not friendly or understanding to the needs of a HAM to communicate. My first antenna was an inverted-L for 20 and 40 Meters. It looked like a Frankenstein creation gone wrong. Hastily made ground radials were flung about and the vertical wires were tied with string up the nearest tree trunk outside my shack. I loved it because it was my first antenna and it actually worked. I managed to talk with someone in New Zealand from Alaska on it.
  Since then I have experimented with others. Luckily I have trees nearby to allow quick access to throw a line up and make things happen. My second vertical with upgraded ground plane connection plate was good and stealthy, until the community yard maintenance ate up my ground radials with their industrial lawn mower. My homebrew inverted-V with homemade ladder line worked well until someone questioned if I owned a monkey (ladder line into the trees) and the HOA came calling shortly thereafter. I had good success with an end-fed wire antenna, but it lacked one very desirable quality: directivity. And that brings me to the point of this all...Please see my next post on the Moxon antenna.