N6DX Darrell
Hi Eric,
Congratulations on your eloquent and informative response to Jay's pastural
plea. As you state the "connection" loss can easily be eliminated with as
few as two well balanced radials. However, the near-field loss is not
reduced as it would be with an extensive ground radial system. I agree that
a ground screen below a sparce elevated radial system is effective in
reducing near-field losses in the absence of an extensive radial system.
The size of the ground screen can be reduced considerably by shortening the
elevated radials using a series inductor and/or a bigger tophat. Bandwidth
may be reduced but the benefits of a small counterpoise are usually important
especially on small lots. With a top loaded short vertical and a shortened
counterpoise the radiation resistance is low but the losses can be greatly
reduced by using low ohmic resistance materials. The system is resonated and
may be fed at a convenient point not necessarily at the electrical center.
However, I suggest using a serious balun or choke on the coax at the
feedpoint to assure that all the current stays in the resonant antenna and
not leaked to the lossy earth. By the way, there is no radiation resistance
in a properly balanced elevated radial system.
As far as the far-field losses as you say; "There is very little that the
average ham can really do about it". I suggest choosing your site well. If
you are near saltwater or a marsh in the direction you work DX this loss will
be low. Personally, I chose a mountain top with steep drop-off making the
effective height for the far-field very high. This particularly greatly
improves the very low angle gain.
I agree that the near field losses can be around 4 to 6 db depending on the
earth and surroundings. However, losses in the 10 to 40 db are possible due
to "connection" losses with a poor ground. This is easily eliminated with a
good resonant counterpoise.
I hope this is helpful to the vast majority of topbanders that can't put out
60 or more 300' ground radials.
73, Darrell, N6DX@aol.com
---------------------
W8CAR Dan
Jay, Youhave opened the proverbial can of worms.
I have used elevated radials for 3 years with an inverted L.
Many people will tell you abt losing 6 db and they don't work and they can't
work and programs can't model them (I have Nec-Win which has the Nec algoithm
and seems to run them okay)
Bottom line- I run 1200 watts and if I hear a station and call them I usually
work them. I made 700 plus contacts in CQWW160 contest with abt 25
countries-some of those called me while I was cqing. I was one of the few 8s
to work a Pacific dxepition last year-first call.
So, if it works go for it! Besides, the ground stself determines how your
antenna radiates-radials, unless very long (wavelengths) only determine loss in
radiation resistance. I live on a small bump (10ft above avearage terrain and
the soil is sandy and loam. My3 elevated radials seem to like it cause tehy
grow lots of dx for me!
73 Dan
W8CAR
--------------------
K9ER Roy
Eric, Tom, Darrell,
Thank you gentlemen. I think I finally understand the general nature of
earth losses regarding ground mounted full height and shortened 1/4 wave
resonant vertical radiators (and how to reduce these losses).
But, alas, I must ask for further enlightenment.
Unless I missed something, Eric limited his ground loss explanation to 1/4
wave vertical linear radiators (or shortened ones, i.e. series inductances or
capacitive top hats).
I experiment some with helically wound nonresonant verticals of greater than
1/4 wave electrical length - currently with 5/8 wave for 75 meters wound with
12ga THHN on 30ft of 4in PVC drainage pipe.
My question is, do the "connection", "near field", and "far field"
explanations for earth losses apply equally for nonresonant ground mounted
radiators of greater than 1/4 electrical wave length? And, if different, in
what manner?
For instance, I recall from some research I have done that 1/2 wave and 5/8
wave verticals are supposedly less ground dependent than 1/4 wave verticals
because the current maxima along their lengths do not occur at ground level.
Is this a factual statement? And, if true, how are the requirements for
radials affected - both in number and in length, if at all? Does the process
of physical shortening by the practice of helically winding the radiator have
any specific effect as regards ground losses?
Tom, I know from reading some of your previous posts that you hold hellically
wound radiators in low esteem due to their narrower bandwidths and low
radiation efficiencies. Please humor me on this as I am limited at my site in
both real estate and the type and height of structures I can erect (unlike my
previous QTH where I had 15 acres and deep, conductive corn belt soils of
which to take advantage).
Darrell, you made some interesting statements regarding shortened radials and
reducing ohmic losses. Are you refering to multiple series inductors for
each radial? Or is there a central inductance at the base of the radiator to
which several shortened radials are connected? As regards ohmic losses, what
would you suggest are materials of low ohmic characteristics for 75 meters or
top band? Or is the frequency irrelevant? Specifically, would you consider
10ga copper wire to be satisfactory? 1/4in copper tubing?
Eric, thanks for the very clear and relatively nontechnical explanation of
the various factors affecting ground losses. I finally was able to see the
overall picture of the obstacles to be overcome in reducing ground losses.
Finally, gentlemen, I beg your indulgence. If your replies, assuming there
will be any, of course, take a highly technical direction, I will probably be
lost. I have the utmost respect for the work of James Clerk Maxwell and a
general understanding of the concepts he put forth. But, sadly, his
equations are beyond my meager understanding.
Thanks and regards,
Roy K9ER
---------------------------
N7CL Eric
>From: RDTALBERT@aol.com
To: <topband@contesting.com>
>Date: Sun, 13 Oct 1996 21:30:32 -0400
>Cc: topband@contesting.com, n7cl@mmsi.com
>
>But, alas, I must ask for further enlightenment.
>
>Unless I missed something, Eric limited his ground loss explanation to 1/4
>wave vertical linear radiators (or shortened ones, i.e. series inductances or
>capacitive top hats).
The discussion applies equally well to inverted "L" types, etc.
>
>I experiment some with helically wound nonresonant verticals of greater than
>1/4 wave electrical length - currently with 5/8 wave for 75 meters wound with
>12ga THHN on 30ft of 4in PVC drainage pipe.
>
>My question is, do the "connection", "near field", and "far field"
>explanations for earth losses apply equally for nonresonant ground mounted
>radiators of greater than 1/4 electrical wave length? And, if different, in
>what manner?
>
Yes, the loss categories are all still the same. To the extent that
increased electrical length raises the feedpoint impedance, the
"connection" loss is reduced. The higher (physically off the ground) the
antenna's phase center is, the less significant the "near-field" component.
The relative distances I referred to previously were for full size 1/4 wave
antennas. Antennas physically shorter than this, will have their phase
center (probably) closer to the earth (assuming that the base is not
elevated). Top loading raises the apparent or "effective" phase center
(not relative to 1/4 wave radiator but relative to where it would otherwise
be on the physically shortened radiator). Center, linear, distributed, and
bottom loading tends to lower it. The physically shortened radiator will
require less radial length for the ground radial screen to minimize the
"near-field" losses than a full size radiator would.
73, Eric
------------------------
N7CL Eric
>From: W8JITom@aol.com
To: <topband@contesting.com>
>Date: Sat, 12 Oct 1996 21:52:19 -0400
>
>All three fields exist very near the antenna.
>
Yes, indeed, they do! Right down to the surface of it, actually.
>There's no such thing as a Magnetic radiator and noise is electromagnetic,
>JUST like a signal.
Yes, in the far field, a radiator is a radiator. Out there you can't tell
if the radiation came from a small loop or a big tall vertical. And yes,
there are radiation fields in the zone we refer to as the near field. But
in the near field area, there are also fields that do _not_ radiate. These
are the energy storage fields.
The antenna usually referred to as a "magnetic radiator" is not really a
magnetic radiator but an antenna which has the characteristic that the
energy storage field which predominates in the near field vicinity of the
antenna is the magnetic field. The magnitude of the energy storage fields
is typically much larger than the component of the "fields" in the same
zone due to radiation. In a lumped element resonant circuit, we would be
talking about the difference between "circulating currents" and the the
real power producing current which is in phase with voltage and brings
energy into and out of the network. This is possibly a poor analogy but the
germ of the idea is in there.
The extent to which the energy storage fields interact with the
surroundings to convert power from imaginary to real (heat) determines the
additional loss due to these fields. Both kinds of energy storage field
can produce losses when interacting with the antenna's surroundings but
they do so using different mechanisms of interaction. Therefore they have
slightly different characteristics in practice depending on the mature of
the lossy material in the vicinity of the antenna.
>Even a small "Magnetic" loop antenna is greatly affected by ground loss
>near the antenna.
Very true, see above. In fact, I think a significant part of the loss
reduction attributed to "magnetic" antennas is due to the much smaller size
of their near field zone rather than the difference in loss due to
interaction with the predominant field type in the near field zone. It is
much easier to keep the smaller energy storage zone off of lossy material.
The tradeoff, if effeciency is preserved, is bandwidth. On top band, a
"magnetic" antenna which was as radiation effecient as a full size 1/4 wave
vertical would not have enough bandwidth to pass a SSB signal through.
>Trying to sort out these fields and blame earth loss on one or the other
>gets us into all sorts of confusion.
>
I wasn't trying to blame losses on one or the other of them but on the
combination of all of them. I still think that understanding that there
are several components of this loss is necessary to formulate a plan to
deal rationally with minimizing the ones that we can minimize without doing
more than is really necessary.
>>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.
>
>It's pretty difficult to measure 6 dB by over the air tests. QSB alone can
>mask the change, plus the antennas would have to be over identical soil. In
>an A-B test here with a FSM, that's exactly the change I measured. About 5 dB
>going from 4 elevated radials to 60 1/4 wl radials.
>
Somewhere between pretty difficult and impossible. Even what we were doing
was what I would call difficult and had probably about +/- 1dB of
cumulative error. We were measuring the field strength in the area around
the antenna out to a few miles using a field probe (small aperture
reference antenna) hanging below a helicopter. We would take enough data
at a particular range to verify the shape of the main lobe and then we
could estimate the losses from the difference between field strengths
predicetd by theory (no losses) and what we were actually measuring. We
would _not_ have been able to make these measurements with sky wave
propagated signals.
But when signals are as marginal as they usually are for working DX on top
band, 6 dB is usually easily noticeable if not easily quantifiable. If it
wasn't, then why would people so consistently spend money on only 11 dB
worth of amplifier? If a ham has 20 dB of "connection" loss and eliminates
(most of) that with a few elevated radials, even though he still has
significant near field losses, he will perceive a great improvement in his
station performance. It is surprisingly easy to get 20 dB of "connection"
loss. Simply base feeding a short (~30ish feet) vertical against a single
ground rod driven into dry sand will get you easily into the 20 dB of
"connection" loss ballpark on top band.
73, Eric
------------------------
W8JIT Tom
Hi Eric,
This may be the wrong place for a continuance, but I wanted to re-enforce
points you made that are negelected in most antenna articles.
In a message dated 96-10-14 17:01:03 EDT, you write:
>Yes, in the far field, a radiator is a radiator. Out there you can't tell
>if the radiation came from a small loop or a big tall vertical. And yes,
>there are radiation fields in the zone we refer to as the near field. But
>in the near field area, there are also fields that do _not_ radiate. These
>are the energy storage fields.
An important point is folklore has it that a loop is quieter by gender, or
that a loop (or other antennas) can be shielded from electric fields. That is
impossible, unless the antenna is made to not radiate or receive at all. We
can only change the characteristics *very* near the antenna under some
special cases.
>energy into and out of the network. This is possibly a poor analogy but the
>germ of the idea is in there.
It's a very worthwhile analogy, because it gives a good picture of many of
the near field storage effects, like Q and bandwidth.
> >Even a small "Magnetic" loop antenna is greatly affected by ground loss
> >near the antenna.
> >Trying to sort out these fields and blame earth loss on one or the other
> >gets us into all sorts of confusion.
> >
>I wasn't trying to blame losses on one or the other of them but on the
>combination of all of them. I still think that understanding that there
>are several components of this loss is necessary to formulate a plan to
>deal rationally with minimizing the ones that we can minimize without doing
>more than is really necessary.
I did mean to imply disagreement with you at all. I was alluding to articles
that draw capacitors from the antenna to earth, and speculate that moving a
130 foot long radial up a few feet or breaking a connection suddenly makes
losses "go away". The coupling doesn't follow the normal distance vs field
strength rules so close to the wire. It's a geometry problem, not a distance
problem, until the wire and earth are far field relationship.
> >It's pretty difficult to measure 6 dB by over the air tests. QSB alone can
> >mask the change, plus the antennas would have to be over identical soil.
In
> >an A-B test here with a FSM, that's exactly the change I measured. About 5
>dB
> >going from 4 elevated radials to 60 1/4 wl radials.
My point was the evidence offered supporting elevated radials has always been
indirect comparisons (like measurements compared to FCC estimated ground
conductivity and field strength charts), or gut instinct. Neither of which is
accurate.
>Somewhere between pretty difficult and impossible. Even what we were doing
>was what I would call difficult and had probably about +/- 1dB of
>cumulative error. We were measuring the field strength in the area around
>the antenna out to a few miles using a field probe (small aperture
>reference antenna) hanging below a helicopter. We would take enough data
>at a particular range to verify the shape of the main lobe and then we
>could estimate the losses from the difference between field strengths
>predicetd by theory (no losses) and what we were actually measuring. We
>would _not_ have been able to make these measurements with sky wave
>propagated signals.
Sounds better than my "on the ground" measurements, but I'm not surprised
they agree so well. Wave angle changes very little with ground system
changes, unless the ground system radiates. Again, it is interesting several
different measurements at different locations by different methods agree.
73 Tom
-------------------
K6STI Brian
W8JI:
>An important point is folklore has it that a loop is quieter by gender...
K6STI:
In general, the folklore is wrong. However, there does exist a special kind
of loop that greatly minimizes response to local noise while maintaining
strong response to skywave signals. See the September 1995 issue of QST for
a description of it and a technical explanation of how it works.
Brian Beezley, K6STI
------------------------
N7CL Eric
>From: W8JITom@aol.com
To: <topband@contesting.com>
>Date: Mon, 14 Oct 1996 18:34:25 -0400
>
>Hi Eric,
>
>
>An important point is folklore has it that a loop is quieter by gender, or
>that a loop (or other antennas) can be shielded from electric fields. That is
>impossible, unless the antenna is made to not radiate or receive at all. We
>can only change the characteristics *very* near the antenna under some
>special cases.
>
Yes, they can be shielded fron electroSTATIC fields but not from
electroMAGNETIC fields. Only in the case where the noise field is NOT a
radiated field but is primarily due to an electrostatic field in the
vicinity of the antenna (rare) can the electrostatic shielding (or
balancing) be effective at reducing noise.
>
>I did (not) mean to imply disagreement with you at all. I was alluding to
>articles that draw capacitors from the antenna to earth, and speculate
>that moving a 130 foot long radial up a few feet or breaking a connection
>suddenly makes losses "go away". The coupling doesn't follow the normal
>distance vs field strength rules so close to the wire. It's a geometry
>problem, not a distance problem, until the wire and earth are far field
>relationship.
I didn't think you were taking issue with me. I wasn't trying to take
issue with you either. I just thought I need to restate things a bit
more clearly than I had originally. As you say there is a _lot_ of
folklore associated with this area.
73, Eric
------------------------
That's all she wrote,
73, Jay WC7M
73
Jay WC7M in Gillette, Wyoming
On the 1AB Ranch in Campbell County
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