I'll take a shot at some "qualitative" answers, based entirely on
theory...Others will pitch in with practical experience, which ALWAYS varies
from theory.
----- Original Message -----
From: "Stephen Reichlyn" <Stephen.Reichlyn@ryansci.com>
>
> We have all read that vertical antennas work especially well on (or near)
> the beach. I have first hand experience
> with this and certainly agree that this is the case. I often wondered
> whether radials (even simple 2 wire systems)
> are really needed next to salt water.
>
> 1.The ultimate question is 'what effect does the salt water have on
> horizontally polarized HF antennas, if qny'?
Not nearly as much as for verticals. For horizontally polarized waves, the
reflection coefficient is pretty high for almost any incidence angle,
regardless of soil properties. For vertically polarized waves, though, this
isn't the case.
>
> 2. Do you think that tidal changes (for instance +/- 6 feet) in the near
> field have an effect on angle of radiation
> of horizontally polarized antennas, such as yagis.
Kind of depends on what you mean by "near field"...
The effect would be of the same nature as raising or lowering the antenna by
that amount... for a moderately high antenna, that may not be all that
significant (if even measurable)..
There would also be an effect from resistive losses in the near field
changing.
>
> 3. How far away from the ocean (in miles) do you think the near field salt
> water effect disappears?
The near field ends (by convention) where the amount of energy radiated away
(per unit area) is equal to the amount of energy kept in the antenna).. By
convention (especially for simple antennas) the boundary is at
2*pi*wavelengths away...For high gain antennas with lots of coupling among
elements, this may not hold.
By the way, the polarization of the E or H field in the near field at a
given point doesn't necessarily correspond to the far field polarization.
You might have an antenna with currents in vertical members where they
cancel in the far field, but in the near field, they aren't. Think of two
vertically stacked horizontal 1/2 wavelength dipoles connected at the ends
by 1 wavelength long wires, with only the bottom element fed. Close to one
or the other end, there's a significant vertical component, but in the far
field, the vertical components from the two ends will cancel.
The pattern effects (from reflection coefficient and polarization) will be
affected many wavelengths away, particularly at low angles. For instance,
at 6 degrees (about 1/10 radian), the "spot" where the ray is reflected is
about 10 antenna heights away. (tan(6 deg) = 0.1). These are (in antenna
analysis) far field effects, because the reflection coefficient at that
point does not affect the current distribution in the antenna, and are more
properly described as "propagation effects".
But hey, in practice, they're both important in antenna siting (that's why
it's the IEEE Transactions on Antennas AND Propagation)
>
> 4. Or do you think that salt water near field has NO effect on
horizontally
> polarized antennas?
>
> 73,
> Steve AA4V
>
> Stephen Reichlyn
> Ryan Scientific, Inc.
> CALL toll-free in North America
> 888-884-4911
> 843-884-4911 ext 302
> 843-884-5568 (fax)
> www.ryansci.com
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|