Hi, Roger
It sure seems like a lot of confusion arises when folks attempt concise
electrical and mathematical thoughts and calculations using words with very
broad and fuzzy definitions. Words like radial, vertical, topload, etc.
can mean different things and can create remarkably confusing sentences and
descriptions.
Like “vertical with one elevated radial”. If one is thinking that radials
are a specific kind of wire group used as a counterpoise that effectively
minimizes radiation, then “with one radial” can’t be called a radial. It
simply is one end of a bent dipole, with one wire very close to and
parallel to the ground. Less lossy than some constructions, e.g. the
horizontal wire actually laying on the ground or buried, but decidedly
lossy. But there will be disagreement about a bent, ground-adjacent dipole,
quite more than one way of talking about that.
Being accused of not having a radial will undoubtedly be defended with "I
worked VK9ABCD at noon long path with a vertical over one radial." But
that comeback does not pay any attention to whether one can include "one
radial" in a precise definition framework for discussion about how radials
work and why the commercial gold standard radials work so well. And of
course the worst of antennas can sometimes make QSOs in the best of
conditions, while the best of antennas can often barely make QSOs in the
worst of conditions. How can one possibly have an academic grade discussion
with all that flak flying around under the same list-serve thread title?
Since it’s not possible to referee a precise dictionary of such terms that
everyone will agree on, the answer to questions can’t be precisely
formulated with those terms IF what we’re looking for is precise answers or
at least answers good enough to risk precious hours, sore backs and
monetary expenditures.
In the end the answer to what wires and aluminum tubes do is what the
antenna modeling says they do, assuming that attempting the actual antenna
doesn’t expose a gotcha that requires additional work to produce in the
model what the wires and tubes actually individually do. The antenna model
is the only device we have that can break down the problem into small
pieces, calculate all the micro-interactions, and then add them up into
patterns and gain figures.
But even the high-priced pro antenna modelers get tricky to do right with
conductors very close to or in the ground, especially an issue on 160m.
Losses are the two ton elephant in the room on 160. Conversations with
fuzzy terms and concepts don't have a chance at accurately telling you
about RF losses, other than to warn they need to be dealt with.
We estimate efficiency by constructing the idea in a model, comparing
average and worst case ground results, and then doing a near field run set
to the ground surface. The former tells you how badly ground could affect
the performance and the latter shows if the design has created hotspot RF
field zones that can excessively induce ground losses, possibly pointing to
design improvements without the hotspots.
Verbal discussion is good for airing general ideas and concepts, providing
mental constructs for at least basically understanding involved
principles, if you can get consensus on definitions of terms. But as soon
as you want to know dB's, whether A is better than B, or not, you have to
do the work to put the idea up in models, avoiding all the gotchas, so all
the interactions between conductors, between conductors and ground, can be
calculated, added up and presented as loss figures and radiation patterns.
As to your mention of a typical VHF ground plane with counterpoise members
in a plane perpendicular to the radiating member, far field radiation from
those four will be minimized. When those four are "drooped", the four now
have a vertical component to their fields which modifies the pattern of the
main radiating member.
Frankly I think that the VHF "coaxial dipole" has long since replaced that
design. Consider the Celwave Stationmaster, etc, since even barely high
angle VHF radiation is lost altogether. One of their improvements in those
fiberglass encapsulated collinear antennas is to produce patterns that max
out three or even six degrees below perpendicular to the vertical to aim
max power "down" to the horizon or nearer service area from their high
mounting points. I don't see radials on those things. Still on some simple
low VHF antennas.
Using "down-angled" elevated radials in a 160 antenna has mild advantages
seen on a model. But one's physical construction has to have something
handling a loss avoidance need to keep RF off the tower beneath the
feedpoint. That seems to discourage that idea from becoming
popular, because without defeating RF on the tower beneath, that added loss
more than cancels the mild advantage seen in simple models without the
tower-to-ground treatment.
There actually is a way to accomplish tower loading, without tower base
radials using one or two FCP's, a vertical wire parallel to the tower, and
a horizontal from top of the vertical wire, or a top FCP, a REQUIRED
isolation transformer, and a positioned short between the vertical wire and
the tower which MUST be the only copper connection between the L/FCP and
the rest of the world. The version with an L is described on k2av.com,
click on "65+ Tower L Bend". The version with a top FCP is experimental
and as yet without the mountain of NEC4 runs that determined the
positioning of the short between vertical and tower.
73, Guy K2AV
On Tue, Mar 23, 2021 at 6:34 PM Roger Kennedy <roger@wessexproductions.co.uk>
wrote:
>
> Guy (K2AV) - I really liked your explanation about the function of a radial
> field on a 160m vertical . . . and how the radials don't actually radiate.
>
> But I've often thought about the other extreme . . . I have a homebrew 2m
> Ground Plane on top of my mast . . . just a quarter wave vertical . . . and
> 3 quarter wave radials sloping down about 45 degrees.
>
> Now if those radials were vertical, it would really be a vertical Dipole .
> .
> . i.e. the radials would be radiating. Whereas if they were horizontal, I
> guess they wouldn't be.
>
> You get my question . . . what really is the difference between the 3
> different radial situations?
>
> And here's another question . . . rather than all the issues of radials and
> matching, has anyone ever used a Vertical Dipole on 160m ?
>
> Sure, you're not going to have a 260 ft vertical . . . but suppose you had
> a
> 100ft support, so that you could have 50 ft vertical legs either side of
> the
> coax feeder, and then just bend the legs at right angles at the top and
> bottom to make up the length (linear loading) - maybe running in opposite
> directions.
>
> I would have though that would be quite an efficient antenna? And
> presumably (just like any dipole) it would work even better if the ground
> underneath it was very poor (in my opinion)
>
> Roger G3YRO
>
>
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