Having settled on a 30m vertical dipole as the "experimental" antenna, I decided to build a wire version of the Force12 Sigma 40 but with a vertical section long enough to not require the center loading coil. I started the construction process by looking for a convenient tree (highest and closest to the radio). Of course "The Tree" turned out to be close to others and made a straight wire copy of the Force12 impractical – there wasn't enough space for the end loading wires.
After a modeling session with EZNEC, I settled on a slightly different design using X shaped end wires – the modeled antenna looks like this:
The dimensions are as follows:
- Each leg of the X is 4 feet long
- The vertical wire is 26 feet long – two 13 feet lengths from the feed point to the X end load
The antenna is suspended with the bottom X 10 feet above a "normal" ground (EZNEC – Average, Pastoral, Heavy Clay).
Here are the field and SWR plots for the modeled antenna.
In a word – simple!
The X end loads were made with 4 foot bamboo plant stakes pushed into a 4" section of 2x4 lumber with eye hooks top and bottom. The wires were measured, soldered together and then spread via the bamboo stakes – secured with PVC tape and tie wraps.
The eye hooks provide strain relief for the vertical section wires – each 13 feet long before pruning and tie off the support rope (at the top) and the stabilizing rope anchored by a large rock (at the bottom) to minimize movement in the wind.
The antenna is fed by 50 ohm coax and a 1:1 balun. The balun is housed in a PVC "T" piece with end caps on the straight through portion of the T. Bolts are drilled through the end caps and secure the vertical wires to the balanced side wires of the balun.
The side connection of the "T" houses the balun and supports a three foot length of PVC pipe that helps take the coax away at right angles to the dipole wires.
For wire, grab what's at hand! I used PVC insulated 18 AWG wire – cheap by the 500' from your friendly hardware store…
Some pruning was required to bring the antenna to resonance – about a foot off each vertical section of the dipole. I used an antenna analyzer to make exercise fast! The end result delivered an SWR response that was almost identical to the EZNEC model.
Given that we're at the low of the solar cycle, I thought I could get away with running each antenna (the inverted L and the vertical dipole) for a 7 to 10 days and comparing the data. This was the easiest and quickest approach as I already had a ton of data for the inverted L.
Part 3 will summarize the results…