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On May 29, 9:31*am, "JB" wrote:
The 1/4 wave ground plane has a useless pattern. *Main lobe at about 30 degrees above horizon, but broad enough to be usable and simple to build. The 1/4 GP with sloping radials and the 1/2 wave are so close in performance to almost be unnoticeable if both are mounted in the air say at 30 ft. The two antennas will have almost the same pattern at almost the same exact angle. When modeling both at 30 ft, the GP's maximum is at 2.7 degrees, vs 2.6 degrees for the 1/2 wave. "145 MHZ". The GP's performance will vary a bit due to the number of radials, but in general the difference between the two antennas will amount to about .3 to .5 DB. Not much. This does not take into account common mode currents. In some cases, I bet it's possible for a 1/4 GP to outperform a J-pole if the J pole has no decoupling from the line. Myself, I've never used a J-pole. I prefer a gamma loop type match if I build a base fed 1/2 wave. I don't like the matching device to be parallel with the radiating element. Most of my "simple" 2m verticals are 1/4 wave GP's.. :/ I have one in the attic hung from the rafters. It has either 6 or 8 radials.. I forgot which.. Been a long time since I've been up there. If you use more than 3-4 radials which is the norm, you will see an increase in performance. I think more due to better decoupling of the line rather than less ground loss. At 30 ft, ground loss is not much of an issue as long as the antenna is complete. Just one radial will make for a 1/2 wave vertical dipole of sorts, and ground loss should not be much of an issue at several wavelengths in the air. So... I think the increase in performance is more due to better decoupling from the line. Either type can use further decoupling techniques for improved performance. The GP will usually use a 2nd set of radials 1/4 wave below the main radial set. The same scheme can be used for the 1/2 wave if common mode currents are a problem and skewing the pattern up off the horizon. In many cases, decoupling of the line is more important to gain at a low angle than element length. Does no good to use a longer element if line currents skew the pattern upwards. |
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