|[AMRadio] Vertical antennas|
hallicrafterssr2000 at k9axn.com
Fri Oct 30 23:52:41 EDT 2015
Back in the early 60’s when I was educated we used Physics and Terman’s 1955 edition. Never read Griffith.
I’ve heard a number of theories regarding vertical antennas and like to hear the conceptual explanations for the ideas.
The first question was: A ground mounted vertical uses buried radials. Is the primary function to provide some sort of reflecting effect or a low resistance counterpoise to the driven element?
Have other questions that build on the first that cannot be asked until the first is answered.
Thanks for the response Rob --- Kindest regards Jim K9AXN
From: Rob Atkinson
Sent: Friday, October 30, 2015 7:12 PM
To: Jim Liles
Subject: Re: [AMRadio] Vertical antennas
On Fri, Oct 30, 2015 at 6:34 PM, Jim Liles
<hallicrafterssr2000 at k9axn.com> wrote:
> Conceptual questions regarding vertical antennas --- like to know why --- not not just because:
> A ground mounted vertical uses buried radials. Is the primary function to provide some sort of reflecting effect or a low resistance counterpoise to the driven element?
the ground system is part of the antenna, the other pole of the
monopole if you will. Earth alone has too much resistance; the ground
system provides a lower resistance return of RF current to the
feedpoint, but since there is unavoidable loss, the current returned
is unequal compared to what you'd have with a dipole so the antenna is
unbalanced. The ground system doesn't have to be at or below grade;
the radials can be elevated, but on 75 and 160 m. it is hard to
accomplish because to function as an efficient counterpoise with only
a few of "resonant" length, they have to be isolated from ground and
the length matters. It's usually easier, especially on 160 m. to
simply bury a lot of radials than it is to string up 4 or 6 at a
height of 1/10 lambda which is the height necessary to isolate them
from earth if you are going to elevate them.
at or below grade, the length isn't as critical because their purpose
is to improve ground conductivity around the base of the excited
vertical element, so you need more, a lot more.
your other questions indicate you could benefit from reading a good
book on this topic and other aspects of AM broadcast service. I
recommend you find a used copy of Radio-Electronic Transmission
Fundamentals by B. Whitfield Griffith, Jr. You may be able to find it
used at abebooks.com it's an excellent book that was originally
written to be a college textbook to introduce EE students to practical
matters of RF and radio broadcasting.
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