1/4 VS 1/2 WAVE VERTICALS
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| Jun 21st 2012, 22:46 | |
N2GVOJoined: Jun 20th 2011, 23:22Total Topics: 0 Total Posts: 0 |
WHAT DO YOU GIVE UP OR GET BY USING 1/4 WAVE VERTICALS VS 1/2 WAVE. ASIDE FROM BURYING DOZENS OF RADIALS. THANKS, JERRY N2GVO |
| Jun 22nd 2012, 17:29 | |
AA6ETotal Topics: 0 Total Posts: 0 |
I'm not an antenna authority, but here's my little list: the feed point impedance is lower (by 50%) for the 1/4 wave, making it a little harder to match. The 1/2 wave wants to be fed in the center, and the feed line wants to run perpendicular to the vertical (as much as possible), making things mechanically tricky. The radiation pattern might be good at low angles for the 1/2 wave in practice, depending on height above ground, etc. What you get with the 1/4 wave depends on the quality of your radials. Gain could be better than the 1/2 wave, since (for a perfect ground plane) there is no power dissipated in the earth. [A quarter wave vertical above a perfect ground plane is effectively a half wave vertical in free space. It's as if there is an "image" antenna below the ground plane.] 73 Martin AA6E |
| Jun 25th 2012, 13:54 | |
W1VTJoined: Apr 4th 1998, 00:00Total Topics: 0 Total Posts: 0 |
If you run low power or QRP a highly effective option can be the end fed half wave antenna. http://www.aa5tb.com/efha.html If isn't as useful at higher powers because the voltage across the tuning capacitor becomes excessive. Also, shield currents that aren't an issue at low power can become significant as you up the power. Zack Lau W1VT ARRL Senior Lab Engineer |
| Jul 27th 2012, 04:39 | |
dksac2Joined: May 24th 2012, 23:18Total Topics: 0 Total Posts: 0 |
I have seen times where a 1/4 wave works better on VHF/UHF in canyons vs. a higher gain antenna just because of the pattern differences the antennas put out. I use a high gain antenna on my moble, but carry a 1/4 wave for those few times when I'm in a place that VHF/UHF signals don't like to get into or out very well. It has helped more than once. 73's John |
| Jul 29th 2012, 22:17 | |
K0BGJoined: Apr 4th 1998, 00:00Total Topics: 0 Total Posts: 0 |
The issue is common mode current. Remember, RF MUST return to its source. In a dipole antenna, that is the opposite leg. In a vertical, that is the ground plane (typically radials). If the current flow between the elements is imbalanced, the imbalance flows on the outside of the coaxial feed as common mode. At low power levels, there isn't too much worry, but there can be. At high power levels (>20 watts), all sorts of maladies can, and do, occur. Half-wave, loaded verticals work fairly well, and long as they're not subject to heavy loading from near by structures (trees, buildings, even fences). This is the reason their owner's manuals state that they must be mounted free and clear. Mounted close to the ground, or near other structures, conducting or not, they're difficult to tune, if not impossible. The bottom line truth is, it is very difficult to surpass the performance of a simple dipole, even one which is electrically shortened. You just have to decide how much effort you're willing to put into your antenna installation. While you're thinking about that issue, remember too, any antenna is the most important, single piece of hardware, you'll ever purchase! |
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