Wednesday, April 17, 2019

Low Band DX - 4 - Vertical or Dipole?

Vertical or horizontal dipole? Excellent question, since I began my low band DX quest, I had assumed horizontal dipoles are NVIS or short hop only antennas, due to the effort required to elevate them 1/4 wave length or more above the ground.

A few days ago I started looking at phased verticals to make some gain, and had settled on a plan to build two 80/40m trapped verticals based on W8WWV - Hex Array - 80/40 Meter Vertical, spaced 33 feet apart - 1/8 wave on 80, 1/4 wave on 40, and setup phasing for 135 degree phase shift on 80, and 90 degree phase shift on 40 with the option to reverse the phase to switch directions.

While scouring the internet for information, I ran across Verticals: got two? by N4JTE. Ok I thought, Bob had success with this, then I took at look at Bob's QRZ page and discovered that Bob had discovered something even better, the same thing but horizontal!

A while ago I noticed something interesting when comparing radiation patterns for verticals and horizontal dipoles, on first glance one concludes vertical has all the power down low. But when I compared the actual gain figures at 10, 20 and 30 degrees elevation, the dipole, even when under 1/4 wave length high has more gain. I dismissed it at the time thinking, I must be overlooking something.


I started looking into this again and found:

Above from comparing antennas on 15m:
  • A very highly optimized vertical. It uses 16 elevated radials 28 inches off the ground.
  • Horizontal fan dipole (0.3 dB better than regular dipole) at 15 feet above ground, 15ft on 15m band is between 1/4 and 1/2 wave length.
What's eating the vertical's energy? That null up top should be pushing more out at a low angle.. Ground losses against the return currents to the base of the vertical? Only when verticals are over salt water does the low angle lobe "push" right out.

Now what? I'm not tearing down the inverted L just yet, but I'll be building an OCFD based on the ON4AA design with the center loading network for 80m. The balun I'll be using is Model 4116 - 4:1 Hybrid Balun 1.5 - 54MHz 3kW from Balun Designs LLC. I have built OCFDs before with Balun Designs OCF optimized baluns, and had pleasing results.

The fun part is I'll be-able to compare the two antennas for a while.

After taking a ZL-Special model in MMANA and dropping it down 1/8 to 1/4 wave length above ground, it suggests the gain will be several dB better than a vertical at low angles.

Ultimately I'd like build something similar to Bob's double whammy with 40 and 80m capability. From my location in the Pacific North West, orientating it for South West / North East it'll favor the Pacific, ZL/VK, Europe and Africa.

Another advantage of horizontal polarization = less local noise pickup, will the RDF exceed small receiving loops? Since I have a K9AY loop control box, I'll also be-able to compare these too.