Difference between revisions of "Talk:Edison Fong DB-1 Antenna"

Latest revision as of 23:56, 1 July 2016

J-Pole Gain

Steve Katz: Many omni vertical antennas have a gain advantage over a j-pole, which is a "zero" gain antenna.

The longer colinear omni VHF antennas usually don't meet their advertised gain claims, but most do have obvious gain over a j-pole. They are *NOT* just a long vertical with some radials at the bottom! The trick to the colinears is they use stacked vertical elements in phase to compress the radiation angle down very close to the ground (near zero degrees elevation above horizon), thus creating gain at that low angle, while of course sacrificing signal strength at higher angles (the only way to produce "gain" is to produce equivalent loss in some other area -- that's how all gain antennas work).

The way they achieve the in-phase sum gain is by careful design and construction, using phase shift networks between the radiating elements. If you just make a "long" vertical, its radiation angle can be close to "straight up" towards the sky, and the only gain you'll have is when working airplanes flying overhead. The objective is to produce gain down close to the horizon, where you want to work people. And the only way to do that is to properly phase the radiators: You never use one long radiating element -- even though from the outside, a lot of these antennas appear to be that (they're not).

That's what makes homebrewing gain colinears "tricky." If you don't have a way to measure the phase shift of a network, you can screw it up easily and produce loss instead of gain.

However, one way that isn't so tricky is to make phase shift networks from transmission line, which can be accurately measured with just a ruler. That is, for example, what Cushcraft uses in their "Ringo Ranger" series of VHF omni gain antennas: You can see the phase shift network, it looks like a shorted stub of open wire line halfway up the antenna. The famous "Stationmaster" designs developed by Communications Products Corporaton (then Phelps Dodge, then Celwave, then Radio Frequency Systems -- and over 50 years, the design never changed!) uses double shielded coaxial cable or hardline, precisely cut, to accomplish the phase shifts required.

Lots of these plans are on line at the Repeater Builder website.

WB2WIK/6