[AK-VHF] Fw: [FFMA] Salt water gain

[Paul Kiesel]

 Folks, I don't usually forward information like this, but this email sent by W7GJ explains a lot about how things like ground gain versus height of antenna versus polarization of the antenna affect antenna performance. Folks interested in weak signal DX pay close attention to these factors. For your reading pleasure.
73,Paul, K7CW / VE7IBAVG Contest Guy

   ----- Forwarded Message ----- From: 'Lance Collister, W7GJ' w7gj at q.com [FFMA] <FFMA at yahoogroups.com>To: "FFMA at yahoogroups.com" <FFMA at yahoogroups.com>Sent: Wednesday, July 10, 2019, 4:52:14 PM UTCSubject: Re: [FFMA] Salt water gain
 Hi Dwight,

There is some introductory information about 6m ground gain on my web page here:

http://www.bigskyspaces.com/w7gj/6mTable.htm

A hortizontally polarized yagi looking out over calm salt water has ideal ground 
gain, as much as 6 dB at certain elevations.  The elevations of the ground gain lobes 
depends on the height above the water. If the yagi is one wavelenght above he water, 
the lobes are broader and higher. If it is far above the water, the lobes will be 
more frequent and lower. The more distant terrain will determine the existence/gain 
of lower ground gain lobes, and the closer terrain is responsible for higher lobes.

On my 6m EME DXpeditions, where the antenna has directly looked looked out over 
unobstructed water on the distant horizon from heights greater than 4 wavelengths, I 
have seen ground gain lobes down to negative 1.5 degrees due to the curvature of the 
earth. If there is land on the horizon, you don't get those lobes around 0 degrees 
elevation, but you can still benefit from good ground gain on the higher lobes.

I have found that vertically polarized yagis have higher ground gain lobes compared 
to horizontally polarized yagis at the same elevation.  The ground gain of vertically 
polarized yagis is typically around 3 dB less than horizontally polarized yagis. For 
strong signal ionospheric propagation (which rotates the polarity around anyway), 
this doesn't matter much. For very weak signal propagation (such as EME with a 
marginal system), it can make quite a difference when you are aimed on the horizon.  
Of course if your antenna is elevated high at the moon, there is no ground gain, so 
it really doesn't matter - you are then at the mercy of Faraday Rotation to properly 
line up the polarity of the incoming signals with your antenna polarization.

For terrestrial propagation, if the signals are cross polarized, you can lose around 
22 dB by not having your antenna properly oriented. Since most 6 yagis are 
horizontally polarized (to try to take advantage of ground gain, and avoid 
interference from vertically installed masts, feedline, etc.), you will be more 
successful with them if you are also horizontally polarized.

If you are trying to activate a rare grid, you probably want a horizontally polarized 
yagi with as much gain as you can get. If you are trying to work the farthest, then 
height above flat terrain is important. I have found that operation from uneven 
mountainous terrain is questionable in terms of ground gain, due to the fact that the 
incoming signal's ground reflections are often dispersed rather than being directed 
to the antenna. Of course, if you have a smooth downward sloping open field without 
ground clutter in front of the antenna down to the sea, you can have very good and 
low ground gain lobes.

If all you are using is a whip on your car, the gain should be better if you park by 
the sea (compared to parking in a parking lot or a development), but you can't expect 
much gain when you start with such a small antenna.

GL and VY 73, Lance


On 7/10/2019 11:45:20, dwightjones at outlook.com [FFMA] wrote:
>
>
> We know that signals are stronger when operating next to salt water, but I don't 
> know how close I need to be to have acceptable angles for reflection. Would a 
> person get the same effect a half mile from the ocean while standing on a hill that 
> is several hundred feet high? What about ten miles away and 5000 feet? What about 
> sitting next to the water with land a mile away on the other side of a bay?
>
> VHF operators say you must use horizontal polarization, but HF ops say it must be 
> vertical. Are we talking about two different kinds of reflection?
>
> None of these questions are hypothetical for me. I'll get a mobile whip and head to 
> Crescent City harbor if that will give me an honest 3-6 dB.
>
>
> Dwight
>
> KO6FE
>
> CN71
>
>
>
> 


-- 
Lance Collister, W7GJ(ex WA3GPL, WA1JXN, WA1JXN/C6A, ZF2OC/ZF8, E51SIX, 3D2LR, 5W0GJ, E6M, TX5K, KH8/W7GJ, V6M, T8GJ, VK9CGJ, VK9XGJ, C21GJ, CP1GJ, S79GJ)
P.O. Box 73
Frenchtown, MT  59834-0073
USA
TEL: (406) 626-5728
QTH: DN27ub
URL: http://www.bigskyspaces.com/w7gj
Skype: lanceW7GJ
2m DXCC #11/6m DXCC #815

Interested in 6m EME?  Ask me about subscribing to the Magic Band EME
email group, or just fill in the request box at the bottom of my web
page (above)!



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