Topband: [Bulk] Re: Salt-Water Qth!

[Grant Saviers]

Since this thread continues, I thought I would share some EZNEC Pro/4 
modeling results I have submitted for publication to QEX, with a focus 
on "verticals on the beach" for DXpeditions.  EZNEC Pro4 can segment 
ground along a line into two arbitrary ground properties, in my modeling 
4 S/m, 80 for salt water and 0.005 S/m,13 for land. W3LPL's and other's 
guidance is roughly consistent my modeling of a vertical at various 
separations from the tide line.

The brief summary of modeling results is there is significant benefit at 
elevation angles <20 degrees towards the salt water IF the antenna is 
less than 0.7 wavelengths (WL) from the tide line. The pattern is 
asymmetrical in azimuth as a result, favoring the salt water. The -2db 
azimuth pattern at 5 degrees elevation is 140 degrees wide at peak 
4.5dbi gain towards the water when the vertical is 0.3 WL from the tide 
line, with 2 radials elevated 0.025 WL.  In this case the F/B is 11 db.  
Around 0.45 WL from the tide line the elevation gain -2db point starts 
to fall below 20 degrees and continues to fall as separation is 
increased.  Since my objective was to better understand the tradeoffs 
for DXpeditions, only 1 or 2 elevated radials were modeled and 
additional radials did not enhance seaward performance.   In this case, 
elevating the radials helps the peak gain, about 0.15 WL is optimal.    
Further than 1 WL from the tide line, there is essentially no low angle 
gain benefit from the sea and the vertical pattern is whatever you have 
as ground + radials.   The results for azimuth and elevation gain and 
pattern showed no fractional wavelength peaking, the values all smoothly 
trend out to more than 1 wavelength from the tideline.

These are only gain results, so the seaward path may have much lower 
skip and/or ground wave losses.

Grant KZ1W



On 4/2/2015 9:51 AM, Kenneth Silverman wrote:
> Below, KK9A took a snip from my web page.  When I wrote that, I missed one
> very important point: simple trigonometry.
>
> Let's imagine a perfectly straight coastline, and we place a 160 vertical
> 1/2 WL back(520' for this discussion) to try and get that potential of 3 dB
> of gain.  That "gain" is only seen for the point directly in front of the
> vertical - i.e. 520' away.  Let's call this azimuth 0 (zero)
>
> If your target destination is 45 degrees off zero azimuth, the distance
> over land is 735'.  Based on or tests, that could be in the -3 dB zone! If
> the destination is 75 degrees off zero, the distance over land is +2000'.
>
> One quickly realizes that the distance over land varies, meaning the first
> reflection point over water is farther away at any azimuth other than
> zero.  And the enhancement of the salt water reflecting plane is quickly
> diminished.
>
> In hindsight, having the vertical as close to the water as possible (or
> over it), will give you better overall performance in multiple directions.
> And that's been the secret to our success.
>
> One time in Jamaica I ran a test between two 10m verticals:  one right on
> the sea wall, and another 100' back and spaced about 200' apart (enough
> that they shouldn't have much interaction at 10m).  The vertical on the sea
> wall was at least 2 S-units better than the one 100' back, and often much
> more.
>
> Going back to W3LPL';s comments, he was right on.  When placing a vertical
> near the water, the goal is to minimize the near field loss (maximize
> current return), while maximizing the far field reflecting surface in
> multiple directions.
>
> Going to an elevated site 1 mile from the shore line will have unknown
> benefits.  But the vertical is likely to work better than a horizontal
> antenna regardless.
>
> 73, Kenny K2KW
> Team Vertical
>
>
> -----------------------------------------------------------------------------------
>
> I did not recall seeing tests for verticals a wavelength or more way from
> the sea so I checked the team vertical website and found the following:
>
> John KK9A
>
> While field testing the verticals this past summer, we decided to test the
> effect of the land-water boundary on the pseudo Brewster angle. Since our
> receive site was elevated less than 1 degree across the bay, we could see
> any change in the low angle energy. To our knowledge, there has not been
> any published tests of this kind. The goal was to see how far from the
> water the vertical would loose the benefit of the salt water on the pseudo
> Brewster angle. The tests were done with a 20m ZR vertical, and we moved
> the antenna away from the water in 5' steps. The water's edge was
> considered the reference point. As the vertical was moved back from the
> water, there was little change until we came close to 1/4 wavelength from
> the water. At that point there was a 3 dB increase in signal level! Moving
> farther, the received signal level dropped, indicating a loss of low angle
> energy. This was most significant at 1/2 wavelength from the boundary,
> being down about 3dB from the waters edge. Moving farther back to 3/4
> wavelength, the signal picked up again, to more than 2dB enhancement from
> the water's edge. We could not move the antenna farther due to
> obstructions. During the tests, we did not believe the data, and reran the
> test. We also observed the same results on the second test. At the time we
> only had 20m antennas, so we could not confirm that enhancement was truly
> frequency dependent. But based on these results, more testing is
> warranted.
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>