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Vertical antenna longer than 5/8 wave?

Jul 29th 2011, 15:59

SIZE358

Joined: Apr 19th 2009, 19:30
Total Topics: 0
Total Posts: 0
I have read some articles about vertical antennas that state if you make the vertical element taller than 5/8 wave the takeoff angle will drasticly rise making it useless for DX but i have been told this is not true .who is right ?
Jul 29th 2011, 17:19

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
They can both be right!

The fact is that signals can easily be down by an S unit or two if the antenna is too tall. Now, if you are running very low power, or trying to break a really big pileup, it is quite possible that such an antenna may appear useless for your needs. On the other hand, with an adequate amount of power and good operating skills, it is quite possible to make many DX contacts with a compromise antenna--even if your signals are an S unit or two worse than if you were using a full size dipole or ground plane.

As an example, I made 131 contacts with 56 countries in the recent ARRL DX CW contest with 5 watts to a half wave dipole at 30 feet--boosting my power to 100 watts would let me do just as well with an antenna that was 2 S-units worse.
Jul 29th 2011, 17:29

W1RFIAdmin

Joined: Jul 25th 2011, 14:25
Total Topics: 0
Total Posts: 0
If the article said that a 5/8 wave vertical is the length at which the gain at low angles, toward the horizon, is the greatest, it is correct. However, a taller vertical is far from being a useless antenna for DX. Over average ground, EZNEC predicted for me the following for a 3.5 MHz ground-mounted vertical:

    Length, gain, angle, gain at 10 degrees
    1/4-wave, 0.4 dBi, 23.5 degrees, -1.6 dBi
    1/2-wave, 0.6, 17 degees, -0.4 dBi
    5/8-wave, 0.5 dBi, 13.5 degrees, 0.2 dBi
    3/4-wave, 4.7 dBi, 46.5 degrees, -3.0 dBi
    1-wave, 4.7 dBi, 32 degrees, -3.7 dBi

The nature of the ground can influence the results, as you can probably see that this model, for which typical ground characteristics were programmed, is different than the theoretical information that probably appeared in the article.

The 5/8 wavelenght antenna does indeed have the best gain at the low angle of 10 degrees, but it is about 0.6 db better than half-wave vertical and about 2 dB better than a quarter wave. And, at 10 degrees, the longer antennas are only 1 or 2 db worse than a quarter-wave vertical.

Now, as these verticals are raised higher, that also affects the pattern by creating upward lobes.

I ran all this quickly using one of the antenna-modeling programs that are widely available at reasonable cost, or free. See ARRL's antenna modeling page for more information.

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