Topband: 5/8 wavelength vertical is mo betta than shorter versions??

[Jim Brown]

On 9/7/2013 6:59 PM, Tom W8JI wrote:
> This would also apply to an extended double zepp, where the second 
> antenna half makes the image unnecessary. Each half of the double zepp 
> is the image of the other side, so we don't need earth. 

False premise alert.  There are two different mechanisms involved. One 
is the pattern distortion (gain) in the horizontal plane produced by 
current distribution on the double zepp, and amounts to a couple of dB 
but a narrower pattern as compared to a dipole. But the earth reflection 
is still there, because it's a horizontal antenna at some height above 
earth.

A few additional fundamental concept helps understand some of this.

1) Current distribution in antennas is determined by where the open 
circuit is -- that is, current MUST be near zero at the end, will be at 
a maximum a quarter wave along the wire (if the wire is long enough), 
and will be near zero again a half wave along the wire, if the wire is 
long enough.

2) As Tom has noted, radiation is produced by current, and the vertical 
pattern is formed by how that radiation interacts with the earth at a 
distance from the antenna.

3) N6BV and I have been doing some modeling of simple quarter wave 
monopoles (what we would call a "ground plane" antenna) and half wave 
dipoles at various heights above ground, and comparing the patterns with 
the same antennas with their base at ground level, with a good radial 
field.  We've done this for a rather varied set of ground conditions. 
The premise is, "If I could mount my multi-band HF vertical on my roof, 
should I?"  We specifically looked at 40M - 10M. For 40M, we looked at 
30 ft and 45 ft mounting heights, and for 20-10, modeled at 20 ft and 33 
ft.

4) The executive summary is that raising an antenna above ground by 1/8 
wavelength to 1/4 wavelength is nearly always beneficial, provided that, 
if it's an antenna that needs radials, at least four radials are 
provided, the advantage of roof-mounting varies from about 1-2dB for 
very good soil to 6-7 dB for really lousy soil. "Works better" was 
defined by more radiation at low angles (below about 20 degrees).

5) The higher mounting location "works better" both because near field  
ground losses are reduced, and because the high current part of the 
antenna is higher.

6) Sea water is an exception -- lobes (peaks and nulls in the vertical 
pattern) begin developing with even small elevation of mounting height, 
and lower is better.

7) While half wave verticals work without radials, a radial field under 
a half wave vertical dipole with its base just above ground, not 
connected to the antenna, increases field strength by a dB or so.

73, Jim K9YC