Topband
[Top] [All Lists]

TopBand: Antenna Info -REPLY DE K0FF

To: <topband@contesting.com>
Subject: TopBand: Antenna Info -REPLY DE K0FF
From: K0FF@nemonet.com (K0FF)
Date: Sun, 09 Nov 97 10:38:28 CST
Too many words.
I am not a smart guy. Still keep my logs on paper with a pencil. Don't run 
computer models. I do build and compare antennas, and watch others who also do 
so. In other words I'm a nuts-and - bolts kinda guy.
My reply to this issue of elevated radials is short and simple:

A) K9FD runs elevated raials (4 at 10 feet)
b) K9FD worked Heard Island on 160 meters from westen IL.
c) Elevated raials must be a good thing.

Thanks for listening- GEO (K0FF)

----------
>
> Hello 160M fanatics. The following, very lengthy, was copied from another
> reflector. I found it very interesting.
> 73  Carl  KM1H
>
>
>
>
> >There seems to be a lot of confusion over the issue of elevated radials
> and
> >vertical antenna ground system losses in general.  To compound the
> problem,
> >none of the most popular antenna modeling programs handle any ground
> >system other than an infinite perfectly conducting plane very well.
> >
> >My experience during testing of antennas for military use suggests the
> >following:
> >
> >1.  There are a number of components to what we routinely group together
> as
> >    "ground losses".  I think the understanding of what is going on may
> be
> >    enhanced when we start talking about these loss components
> separately.
> >    I would like to apologize in advance for a somewhat oversimplified
> >    but nonetheless long winded treatment of these topics.
> >
> >
> >    First, there is what I like to call the "connection" loss.  This is
> the
> >    effective resistance of the earth terminal connection of the antenna
> >    system to the flow of whatever RF current the antenna system is
> >    attempting to pump into and out of the earth on the wire that
> connects
> >    it to earth.  This is most easily visualized when the ground system
> >    consists of a single ground rod at the base of a 1/4 wave (or
> shorter)
> >    antenna.
> >
> >    Second, there is the loss due to the interaction of the near-field
> >    energy storage fields of the antenna with nearby lossy ground,
> >    vegetation and structures.  This loss component behaves slightly
> >    differently depending on wheter the antenna in question does most of
> >    its near field storage in the magnetic field or in the electric
> field.
> >    For our usual amateur discussion of short linear verticals and
> >    horizontal dipoles, the near field storage is predominantly
> electric.
> >    Lets refer to this as "near-field" loss.
> >
> >    Third, there is the RF radiation far-field interaction with the
> >    (somewhat less nearby) ground around the antenna.  This is the beam
> >    forming or elevation pattern affecting interaction of the RF field
> with
> >    the surrounding ground.  This interaction is very difficult to
> >    completely describe since the nature of it changes with the distance
> >    from (and therefore the angle to) the antenna's phase center and the
> >    plane of the "RF earth".  The frequency involved is another variable
> >    factor here.  As someone earlier pointed out, the "skin depth" of
> the
> >    earth is significant at 1.8 MHz.  But as the grazing angle is
> decreased
> >    to approach and exceed the critical angle, very little penetration
> >    occurs (this is not to say that loss is completely eliminated).
> >
> >
> >
> >2.  Short (1/4 wave or less) base fed vertical antennas require
> significant
> >    RF current to flow in the ground return terminal of the feedpoint.
> >
> >    If the base is at ground level and no metallic ground "screen" (to
> mean
> >    either radial system or actual mesh screen covering a large enough
> >    area) is provided, then the losses are dominated by the first two
> kinds
> >    ("connection" and "near-field") for obvious reasons.  The third kind
> >    (radiation field losses) are also present but are swamped in
> magnitude
> >    by the first two.  I am assuming a single ground rod attachment for
> >    ground return currents here.
> >
> >    If we add just two slightly elevated resonant 1/4 wave radials (one
> at
> >    0 and the other at 180 degrees), we can reduce the "connection" loss
> >    component to a very small value.  The RF ground return current can
> be
> >    made to flow almost exclusively in the resonant radials with very
> >    little loss.  Radiation fields from these radial wires are very
> small
> >    due to nearly complete cancellation in the far field.  However, we
> are
> >    left with a very significant amount of "near-field" interaction
> loss,
> >    and the radiation field interaction loss.
> >
> >    How significant is the "near-field" component?  Usually between 4
> and 6
> >    dB depending on the exact nature of the local earth and
> surroundings.
> >    The farther we raise the base (and the two radials) from the earth,
> the
> >    more we can reduce the effect of the "near-field" losses.  How far
> must
> >    we elevate the antenna to eliminate the "near-field" losses?  Our
> work
> >    (mostly between 9 and 18 MHz) showed diminishing returns setting in
> >    around 3/8 wavelength above earth surface and loss of measurability
> >    somewhere just beyond 1/2 wavelength.  On top band, even the 3/8
> >    wavelength number translates into a very difficult support
> structure.
> >    Imagine a 1/4 wave vertical with its BASE at almost 200 feet!
> >
> >    If instead of raising the structure, we begin adding radials to
> >    "screen" the "near-fields" from "seeing" the underlying lossy earth,
> we
> >    can also reduce the effect of the "near-field" losses.
> >
> >    Did someone ask how many radials does it take to eliminate the loss?
>  I
> >    thought so.  Well, in a nearly ideal flat, large enough, open field
> >    without any vegetation we found that in terms of length (assuming
> ideal
> >    screen density), returns again diminished in the 3/8 to 1/2
> wavelength
> >    range (for a full size 1/4 wavelength radiator).  In terms of screen
> >    density (with various lengths of radial), diminishing returns began
> >    when the distance between the open ends of the radials was less than
> >    0.03 wavelength.  Loss of measurability occurred at around half of
> that
> >    or about 0.015 wavelength.  Note that there is nothing resonant in
> this
> >    ground screen.  It can be replaced (or large areas of its central
> zone
> >    can be) with square welded intersection mesh.  Using the mesh has no
> >    measurable effect so long as the comparison is between mesh with
> 0.015
> >    wavelength or less openings and an identical (size and shape) radial
> >    screen with no more than 0.015 wavelength spacing between the radial
> >    ends.
> >
> >So, does this answer the question "Do elevated radials work?"  Yes. The
> >answer is that they are effective at reducing the "connection" loss. 
> And
> >if they are "elevated" far enough (along with the base of the antenna),
> >they work as well as a full density on-the-ground screen in terms of
> >radiation efficiency (ignoring changes in the shape of the resulting
> >elevation pattern).
> >
> >So it is probably the case that both the guy who says "I added 4
> elevated
> >radials to my vertical antenna and the performance improved greatly" and
> >the guy who says "I and others have evaluated 4 elevated radials against
> a
> >full ground screen and the elevated radials loose by 4 - 6 dB" are
> correct.
> >
> >Interestingly, our work also showed that physically short (less than 1/8
> >wavelength) top loaded (hat + inductor) verticals required _less_ ground
> >screen in terms of radial length to get to diminishing returns.
> Required
> >screen density was the same.  Unfortunately, as the radiator gets
> shorter,
> >the losses from the ratio of radiation resistance to ohmic losses in the
> >radiator and loading and matching system component losses overcome the
> >benefit of needing a smaller ground screen area.
> >
> >Anyone notice that I didn't talk about radiation field interaction
> losses
> >very much?  There are a number of reasons for that. First, I'm not
> >confident that I can quantify it very well.  Second, there is very
> little
> >that the average ham (or on top band, even the rich, obsessed, landed
> gonzo
> >contester) can really do about it.  This is because the distances from
> the
> >base of the antenna which are important for DX (low angle beam forming)
> >range from about 0.75 to 3 wavelengths.  The exact boundaries are
> somewhat
> >dependent on the height of the antenna's effective phase center above
> the
> >plane of the surrounding "RF earth".  Those numbers are approximately
> >correct for a ground mounted 1/4 wave vertical.  So the "perfect" RF
> >radiation field ground plane requires enough 1580 foot long radials to
> keep
> >the tip to tip distance to 0.015 wavelength.  That is 630 1580 foot long
> >radials!  Even Uncle Sugar declined to do that.
> >
> >The bottom line is that any individual amateur has to be limited to what
> he
> >_can_ do at his QTH.  To that end, my recommendation for anyone driving
> >a ground mounted short vertical antenna is to use as many on the surface
> >radials as he can reasonably achieve.  But no more than are required to
> >limit the tip to tip distance to 0.03 wavelengths or a bit less.  Note
> that
> >the less space you have for radial length, the smaller the number of
> >radials you need to get into diminishing returns.  And, yes, DX can be
> >worked with numbers of radials ranging from zero through "enough".
> >However, with "enough" you will be louder.
> >
> >73,  Eric  N7CL
>
>
> ------
> Submissions: antennas@qth.net
>
>
> --------- End forwarded message ----------
>
> --
> FAQ on WWW:               http://www.contesting.com/topband.html
> Submissions:              topband@contesting.com
> Administrative requests:  topband-REQUEST@contesting.com
> Problems:                 owner-topband@contesting.com



--
FAQ on WWW:               http://www.contesting.com/topband.html
Submissions:              topband@contesting.com
Administrative requests:  topband-REQUEST@contesting.com
Problems:                 owner-topband@contesting.com

<Prev in Thread] Current Thread [Next in Thread>