Eric and everybody interested in the discussion,
I would like to see the results of an experiment involving the two types of
antenna carried out in a
manner similar to those carried out in Forensic experiments where to two
items are compared
in a LIKE with LIKE manner. In this context two verticals at the same site
but not interacting,
constructed in the same way, with the same uncluttered foreground, feed
with same feed system.
Always keeping in mind ' Like with Like, i feel sure that a meaningful
outcome could be
obtained where Mr average can decide whether to go with elevated radials or
ground mounted.
Perhaps the biggest problem in doing such an experiment is doing it at
topband it may be easier at 40m with an adjustment in elevated radial
height. By the way I use elevated radials in a 80m
4 Square system and i'm happy with the results but I would sure like
another 6db if thats what a
'quality' amateur band experiment came up with !! Clive GM3POI
----------
> From: Eric Gustafson Courtesy Account <n7cl@sparx.mmsi.com>
> To: km1h@juno.com; w8ji.tom@MCIONE.com; topband@contesting.com
> Subject: Re: TopBand: Elevated GP vs. Vertical Antennas
To: <topband@contesting.com>
> Date: 14 March 1998 00:14
>
>
> NOTICE!
>
> Long boring technical discussion to follow. Do not read if you
> don't care about ground systems.
>
>
> Carl,
>
> I apologize for this but I'm not directly responding to the post
> of yours referred to in the header of this message. I Just
> grabbed it to respond to because it was a convenient way to get
> the subject references correct. I don't believe that the quoted
> insert from Tom is taken from that message.
>
> >In a message dated 98-03-08 19:05:39 EST, W8JI writes:
> >
> >> The 160 meter skin depth (distance where current drops to 37%
> >> of the value traveling in the conductor) is about 30 meters in
> >> poor soil, ten meters in good soil, and about a foot in salt
> >> water. Current effectively goes to zero at about 10 skin
> >> depths.
> >
> >
>
> The above is a very interesting statement. Let me see if I can
> make use of it to illuminate _why_ the full ground screen (either
> on or in the ground surface) provides a system with less loss
> than a system of six or fewer elevated radials.
>
> First, I'd like to separate the discussion to a strict comparison
> of one ground system versus the other when used to provide the
> ground system for (nearly at least) identical vertical
> radiators. Please note that I'm NOT saying that elevated radials
> don't work, or that they don't work well enough to make a useful
> antenna. Only that when used near the earth's surface, they
> cannot provide the same high level of efficiency as the canonical
> full size sufficiently dense ground screen under the same
> radiator. We are talking about ground mounted (less than 1/8
> wave above grade) vertical radiators which are less than or equal
> to 1/4 wave long.
>
> The quantity of DX that can be worked with tuned rain gutters is
> irrelevant to this discussion. The only thing we are talking
> about is how many dB stronger (or weaker) is the same radiator
> with ground system (A) versus the same antenna with ground system
> (B).
>
> Frequently it is possible to work DX effectively even on topband
> with an antenna that is 25 dB weaker than a full size, full
> effeciency vertical. This is because the fade margin for the big
> antenna is 50 or 60 dB and -25 dB doesn't take the link SNR below
> zero. Here we are talking about two systems which are within 4
> to 6 dB of each other.
>
>
> OK Now, from Tom's statement above it is pretty clear that unless
> some means is taken to prevent the antenna's near field zone from
> "seeing" the earth under the antenna, a large volume of earth
> will be involved in the near field interaction. The poorer the
> earth conductivity is, the larger the volume will be that is
> involved. As the apparent surface conductivity of the earth's
> surface increases (or is artificially increased with radials),
> the volume of lossy earth exposed to the near fields decreases.
> Eventually a point is reached where further surface conductivity
> increases do not produce lower losses because the dominant loss
> mechanism for the antenna system is no longer the near field
> interaction loss with the earth material.
>
> The effect of increasing the surface conductivity has been to
> prevent the fields from penetrating into the earth material under
> the antenna. Hence the name "ground screen". The ground system
> when sufficiently dense has "screened" or shielded the earth
> material from the antenna's near fields.
>
> Sufficiently dense means that the greatest distance between
> conductors in the screen is 0.015 wavelengths or less. For 1/4
> wavelength radials, this requirement is met with 104 radials.
> Now, as a practical matter, relaxing the criteria to 0.03
> wavelengths for a radial system results in only about a 0.5 dB
> reduction in measured field strength. So 60 radials this long
> would produce a system which is very nearly as good as one
> meeting the full density criteria.
>
> For radials only 1/8 wavelength long, the requirement is met with
> only 52 radials (26 if you are willing to give up 0.5 dB). Note
> that these numbers are very consistent with the rule of thumb
> (often repeated) that if the radials must be this short, there
> isn't any point in having more than 30 or so. This does NOT mean
> that 30 or 80 or 300 1/8 wavelength radials will produce an
> antenna system as effecient as one that has 104 or more 1/4
> wavelength radials. If the radiator is near 1/4 wavelength long,
> the near field zone will extend beyond the 1/8 wavelength radius
> of the screen and some efficiency will be lost.
>
> Also note that it does not really matter very much wether these
> radials are on the ground surface or elevated above the ground
> surface. The effect is the same.
>
> When we reduce the number of radials to a small number (four for
> this discussion), the story is quite different. Just to be
> clear, for the rest of this discussion I'm talking about four
> resonant (or nearly resonant) radials spaced at 90 degree
> intervals and elevated 16 feet or less above ground surface.
>
> I'd like to start by stating that an antenna with four elevated
> 1/4 wave radials can be just as efficient as one with 120
> elevated 1/4 wave radials. But this is true ONLY if the radial
> system (and the base of the antenna) is elevated 3/8 wavelength
> or more above the ground surface. On topband this would mean
> elevating the radial system 197 feet! To be fair, 3/8 wave is
> the distance where the effect stops being measurable. Not much
> efficiency is really lost until the elevation is reduced to
> somewhat less than 1/4 wavelength. But even 1/8 wavelength
> elevation (where significant efficiency IS lost) would require
> the feedpoint and radials to be 65 feet in the air.
>
> So why is it that the four radials don't work as well near the
> ground as they do when high up in the air? The answer is that
> they DO. They do work as well at providing the radiator with a
> low impedance something to be driven against, that is.
>
> However, they don't help maintain system losses at the same low
> level that a sufficiently dense screen would. There are two
> reasons for this. (Well, actually, there are two ways to look at
> one reason for it.) First, the four radials do not provide an
> effective screen to keep the near field zone from extending below
> the plane of the radials and therefore interacting with the lossy
> earth below. Second, although the fields generated by the
> current in the radials do (very nearly) cancel in the far field or
> radiation zone, in the near field they do not even come close to
> complete cancellation. So the interaction of these fields due to
> RF current in the radials with the lossy earth induces some loss
> into the system.
>
> This does not mean that four elevated radials won't work. It
> just means that over lossy ground, they wouldn't work _as well
> as_ the full density ground screen. The amount of the additional
> loss will vary depending on the exact amount of elevation and the
> nature of the earth under the antenna. But a good useable
> average number to use for planning purposes is about 4 dB. It
> can be as high as 6 dB.
>
> Ground level (far field) field strength measurements are
> perfectly adequate to verify and compare the performance of these
> systems. The screen density affects only the radiation
> efficiency of the system. It does not significantly modify the
> radiation pattern of the antenna in the elevation plane. So a
> change in the "ground wave" field strength is accompanied by a
> proportional change in the intensity of the radiation in the
> antenna's main lobe which is launching the sky wave.
>
> To significantly modify the elevation pattern of the antenna, the
> ground screen must be dense and extended out to beyond 1/4
> wavelength from the base of the antenna. The region beginning at
> 1/2 wavelength and extending out to 3 wavelengths or so from the
> base exerts the most influence on the antenna elevation pattern
> of interest to amateurs.
>
> I don't personally know of any amateur installations where the
> ground screen was extended out far enough to modify the elevation
> pattern of a 1/4 wave vertical antenna operated on topband. But
> erecting a vertical about 1/2 to 3/4 wavelengths up the beach
> from a large body of salt water would be a really good way to
> achieve a very low takeoff angle (over some range of azimuths).
> This would be true if either ground system were used. However,
> the full density screen would still put about 4 dB more energy
> into the low angle main lobe.
>
> 73 Eric N7CL
>
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