[50mhz] Re: ground boost
Lance Collister, W7GJ (DN27ub)
[email protected]
Wed, 06 Mar 2002 09:43:06 -0700
[email protected] wrote:
>I have offered to give my lecture on 'Ground gain and radiation angle at VHF' in Prague, but no answer yet from the organisers. The lecture might be a starter for the discussion, as I do not know too much about the Brewster angle. See www.oz1rh.com , please read the Word-document as the html version lacks some formatting.
>
>Quote from Chapter 5 Polarization and ground effects:
>"For DX-ers on HF it is well known that the radiation angle of a horizontal polarized beam is a function of its height over the ground and ground conductivity has little influence on low angle of radiation. Things are different for vertical polarization. A vertical antenna needs perfect ground to give low angle of radiation. In fact is does not radiate at all below an angle called the Brewster angle. This angle is dependent of the conductivity of the reflecting ground. Check an antenna book on verticals for 80 m, the 80 m DX'ers know all about this. Does low angle of radiation interest us for VHF? Yes ground gain is happening in the range of 0-10 degrees.
>
>Horizontal polarization
>Ground gain for one reflection 5-6 dB
>Ground conductivity Little influence at low angles
>Lossy ground Little less ground gain and not so deep nulls in pattern
>Good ground conductivity (salt water) Deep nulls in pattern, but peak almost uninfluenced
>
>Vertical polarization
>Ground gain for one reflection Perhaps 3 dB
>Ground conductivity No radiation below Brewster angle 5-20 degrees
>Lossy ground High Brewster angle 20 degrees?
>Good ground conductivity (salt water) Low Brewster angle < 5 degrees?
>
>It seems a good idea to use horizontal polarization if one wants low angle of radiation. Vertical polarization has less ground gain. A circular polarized signal contains two components: a horizontal polarized and a vertical polarized one. Ground reflection influence the two components differently as describes above and after ground reflection the signal is no longer circular but somewhat elliptical polarized at low radiation angles."
>
>73, Palle, OZ1RH.
>
>Original Message:
>-----------------
>From: Conrad G0RUZ [email protected]
>Date: Wed, 6 Mar 2002 10:58:26 -0000
>To: [email protected]
>Subject: RE: ground boost
>
>- snip
>
>Well the good news folks is that it can all be resolved in Prague whilst
>propping up the bar! Another excellent reason to come along. "The great
>Ground gain debate" I can't wait as all 3 of the above involved gentleman
>will be in attendance.
>
>Place your bets now :-)~
>
>Conrad G0RUZ
>
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Hello Palle,
I am certainly no expert in any of this, but I encourage you to spread
the information about ground gain. Many folks seem to be confused about
it. All I can tell you, is that on 6m EME, everything is so marginal to
begin with, that at least one station MUST have ground gain, and of
course, if both stations have ground gain, it is all the better.
I still receive inquiries from 6m stations, for example, who ask for
suggestions about elevating their one or two small yagi arrays. Their
single yagi pointed at the horizon acts like 4 yagis, and 75% of the
time cndx are not favorable enough for a contact even then. Such
stations certainly cannot afford to "throw away" such ground gain and
elevate their antennas! In most cases, a single 9 element yagi pointed
at the horizon has the effective gain of my 4 yagi 6m array, and since I
hear my own echoes over half the time while pointed up at the moon, it
should be just a matter of time until I can contact anybody who can hear
their own echoes on the horizon.
Ground gain with a single yagi can be a wonderful thing! As so nicely
illustrated by the charts on your website, a single yagi has muliple
ground gain lobes, providing muliple chances at EME contacts as the moon
moves in front of the different lobes. At least on 6m, it seems we need
as many chances as we can get - HI! And I have been simply AMAZED at the
difference it makes when an antenna points at a salt water horizon! The
smallest stations I have worked on both 2m and 6m EME have had single
yagis looking over a salt water horizon. The books may say that there
are only a couple dB to be gained from salt water compared to average
ground....but I am here to tell you that every last dB makes a HUGE
difference when you are involved with a very marginal EME situation.
I found on 6m EME with my 21m long single yagi 21m above ground, that my
third lobe up at 18 degrees was the same gain as my free space gain
(when it was elevated). By leaving the antenna pointed at the horizon, I
had significantly more gain in my first (4 degrees) and second (11
degrees) ground gain lobes, and I was able to make a number of contacts
using the second lobe. I might also point out that I have noticed that,
especially on 6m, there are many factors that can "interfere" with the
propagation of the signal to the moon and back. Most of these factors
(ionospheric scattering and refraction, tropospheric ducting, etc.) are
more of a factor the lower the angle of radiation (and the more
atmosphere/ionosphere the signal has to pass through). On a number of
occassions I have noticed good signals from horizon stations when the
moon was high enough to be up in front of their second ground gain lobe,
but I have heard NOTHING when it passed in front of their main antenna
lobe.
Long live the big single yagi pointed on the horizon, with its higher
ground gain lobes! Good luck in spreading more information about this
very important subject! MNI TNX and VY 73, Lance
--
Lance Collister, W7GJ (ex: WN3GPL, WA3GPL, WA1JXN, WA1JXN/C6A, ZF2OC/ZF8)
P.O. Box 73
Frenchtown, MT 59834 USA
QTH: DN27UB
TEL: (406) 626-5728 FAX:(406) 728-6320
URL: http://www.bigskyspaces.com/w7gj