[Towertalk] Reflection Gain

[Chris Adams]

Jim,

I respectfully disagree with you.

If the statement that reduced attenuation is caused by Seawater and not by signal
reflection from the ground, then take the process to the ideal limits, free space
and a perfectly conducting ground.  The signal from the perfectly conducting
ground case is substantially higher (5.15dbi vs. 1.77 dbi according to EZNEC).
The way this happens is energy that is destined for "below ground" in the free
space case is reflecting by the perfectly conducting ground and stays "above
ground".


 Secondly, the ARRL antenna book has slightly over 2 1/2 pages related to the
Vertical reflection coefficient.  There is no mention of it being valid for only
VHF/UHF frequencies and there is a graph of Pseudo-Brewster angle vs. various
types of ground for frequencies varying from 1.8 to 30 MHz. (18th edition ARRL
antenna book, pages 3-5 to 3-8).

 Another "ham"related reference is "Horizontal Antennas and the Compound
Reflection Coefficient" in the ARRL Antenna compendium Vol 3.  Although this
article is primarily about the Horizontal Reflection coefficent, there are a few
paragraphs devoted to the vertical reflection coefficient.


chris adams/ n4vi


jljarvis wrote:

> With due respect, Yuri....reflection gain exists for
> grazing incidence moonbounce signals.  There is an
> additive behavior here, which recovers part of the
> signal which would be below the horizon, but reflect back
> up to add into the beam.  But this is NOT at HF...it's at VHF/UHF.
>
> It does NOT exist for vertically polarized HF signals over
> earth.  All you're seeing over seawater is reduced attenuation
> from imperfect earth...when you get to highly conductive ocean.
>
> Yes, you're right that highly conductive ground also reduces
> IR losses at the drive point.
>
> But at low angles, you do NOT get gain from a good ground plane.
> All you get is reduced attenuation.  Result is the same...the
> process is different.
>
> n2ea
>
> -0-
>
> >  Any language suggesting 'seawater gain', or 'reflection gain',
> >  is imprecise, at best.  It IS true that verticals work better over highly
> >  conductive earth.  But there ain't no gain there.
>
> There IS a gain somewhere between 10 - 15 dB next to, or over salt water with
> vertical antennas as compared to similar antennas installed inland. Jump in
> the mobile and drive around you WILL see it! Even modeling programs show it.
>
> Numerous factors come into a play with salt water front: better "ground"
> improves efficiency of the antennas, huge "ground plane" brings gain at very
> low angles (Brewster), unobstructed plane horizon and no noise generators out
> there (except sun).
>
> Jim Jarvis
> Keithley Instruments
> Essex Vermont
> 802 872 5830 voice
> 802 872 5831 fax
>
> _______________________________________________
> Towertalk mailing list
> Towertalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk