Hi Ray,
Two primary factors affect sky wave signal strength from Topband
verticals close to salt water:
1. ground loss in the immediate vicinity of the feed point. The feed
point must be much closer than 1/4 wavelength from salt water
or a salt marsh to significantly reduce ground loss.
2. ground reflection loss, especially close to the near edge of the
Fresnel Zone
Perhaps the most practical solution to achieving very low ground loss
is to place the base of the vertical in a salt marsh such as at W1KM.
There is an AM broadcast tower on a pier extending well into San
Francisco Bay but this would be exceptionally difficult to duplicate in
a ham installation. Its not practical to place a vertical closer than 1/4
wavelength of an ocean beachfront except in a temporary installation
such as a DXpedition.
Most ground reflection loss within the Fresnel Zone of a vertical with
the feed point at ground level occurs within one wavelength of the feed
point for low angle sky wave signals. This requires that a Topband
vertical be located within several hundred feet of salt water or a salt
marsh.
For horizontally polarized antennas, the only significant factors are
ground reflection efficiency within the Fresnel zone and blockage
of the horizon by terrain. For very tall towers and very low angles,
the Fresnel Zone extends out at least several miles but much closer
than the horizon except in mountainous terrain that blocks the visual
line of sight to the distant horizon. For horizontally polarized antennas,
salt water isn't significantly more efficient for Fresnel Zone reflection
than average soil on very flat terrain.
Perhaps the biggest improvement for horizontal polarization at an ocean
front location derives from its exceptionally flat Fresnel Zone, especially
for VHF EME where the full 6 dB ground gain is very difficult to achieve
over typical urban and densely populated suburban location. An
exceptionally flat Fresnel Zone can be a significant improvement compared
to locations where the Fresnel Zone encompasses many large buildings or
any location with sharp elevation changes (greater than 1/4 wavelength)
close to the near edge of the Fresnel Zone.
Don't forget to consider the many serious maintenance problems of an
ocean front location, not only antenna maintenance but also resolution of
RFI from power lines for many miles around. K3ZM had serious RFI
very efficiently propagated across the Chesapeake Bay from power lines
nearly 20 miles away. This could be a significant problem in an area with
power lines near the ocean.
73
Frank
W3LPL
----- Original Message -----
From: "HVT" <w2re@hudsonvalleytowers.com>
To: topband@contesting.com
Sent: Wednesday, April 1, 2015 12:47:23 PM
Subject: Topband: Salt-Water Qth!
This is my first post to the Top-Band Reflector, so please excuse my ignorance
if this topic has been discussed in the past.
Is there any scientific data in print to prove the theory that ocean front
property is better than a location inland about a mile or so on a ridge
overlooking salt water for HF. I had this heated debate over the weekend with
two ham friends of mine while we traveled to Maine looking at real-estate along
the coast. I understand the theory that verticals literally in or on the water
have a huge advantage. The debate was about how far away from the water does it
become a diminishing effect. I made the claim that the Ocean-Front property
would be a better location than anything inland including a location on a ridge
within a mile. This heated debate went on for about 500 miles while we were
driving back to NY. It was a very interesting conversation and made the long
drive back much quicker! :)
Additional information about the debate:
In the State of Maine there is a setback regulation on shoreline property
regarding structures including radio towers. To play it safe with the shoreline
protective rules, the proposed array system would be setback minimum 500’ from
the water or as far back as 2000'. At these distances on 80/160 meter will a
vertical antenna system see any positive effects with additional gain from the
salt water?
What about horizontal antennas? Do they see any effects from Salt-Water?
I'm sure I can use HFTA to model the terrain, which I have done in the past
with great accuracy. However, I'm not sure if it calculates Salt-Water. Maybe
it does.
We are anxious to start building in Maine ASAP. Any input would be appreciated.
Thanks,
Ray W2RE
W2RE.com
Sent from my iPhone
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