Hi Rick,
> Sinisa, I do not agree with you.
OK, let's see if you would agree with physics :-)
> I believe it will work better if grounded.
It is a simple fact that pushing RF power into soil
does not produce useful radiation, although some other
goals may be achieved, like "perfect" SWR.
> However, to be honest, I am not 100% sure and if it turns out you
> are right, then I shall learn something useful.
Or vice versa :-)
> As I see it, if that piece is not grounded, then it needs to be
> resonant (which it is not).
But it should and certainly could be made resonant.
There are many ways to obtain resonance and "perfect SWR".
Pushing RF into soil is not a solution.
And, as we all know, resonance has nothing to do with radiation,
although it has very much to do with simplicity and efficiency
of feeding.
> That's why I insist that the vertical wire must be grounded ( I
> know it works ).
I have yet to see an antenna that doesn't "work" :-)
It is as difficult to make a 0 % efficient antenna,
as it is to make a 100 % efficient one.
So, they all "work", and what separates them
is not whether they "work" or do not "work",
but "how efficiently they work".
Amateurs are frequently willing to accept 1 % or 5 % efficient antennas
if there are no better means. "Half slopper" is intended for unlucky ones.
Having no tower, Ken N2XY has better prospects.
Armed with understanding of basic physics and a bit of imagination
he can produce an antenna fitting his environment and operating
with optimum efficiency. With less expense and more fun.
* * *
So, let's turn to basic physics first.
1. All HF antennas are transmission lines.
Of course, they cannot simultaneously be good antennas AND good lines.
Good lines do not radiate much, so antennas must be poor lines,
but lines they are.
2. A line will radiate well whenever radiation from one wire
is not cancelled by opposite radiation from another wire.
Radiation itself is a natural process occuring wherever
a changing current exists, and cannot be avoided.
3. Any form of line in which both wires do not run too close
to one another, can be used as an antenna. There is nothing
sacred in usual forms of so-called "dipoles", "quads",
"deltas", or "half-sloppers". Any form will do, provided
that wires do not run too close. And wires do not have
to run straight, for heaven's sake.
4. There are only two general families of HF antennas:
those made of open-ended transmission lines, and
those made of short-circuited transmission lines.
"Dipoles", "monopoles", "GP's" are made of open-ended
transmission lines. One can always identify two conductors
starting from the feedpoint and diverging. They do not meet
anymore.
"Quads", "deltas", etc. are made of short-circuited
transmission lines. One can always identify two conductors
starting from the feedpoint and diverging for a while,
but meeting again at the end.
5. Input impedance of antenna is made of two parts:
active and reactive.
The active part is caused by transfer of energy.
It represents radiation and losses (in the antenna
and in surrounding objects).
The reactive part is caused by wave reflection at the end
of the transmission line. This is unwanted as it does
nothing for radiation. Yes, your "perfect SWR"
HF dipole runs with SWR of approximately 1 : 15
on the antenna itself (not on the feeder).
6. For ease of feeding we prefer having a resonant antenna,
with reactive part being zero. (This is not mandatory,
just convenient.) We achieve this by making transmission
line length equal to a multiple of lambda / 4.
7. For ease of feeding we also prefer having a low impedance
at feedpoint. We achieve this in two ways:
* make open-ended transmission line length = lambda / 4 (or an odd
multiple);
high impedance on the line end is transformed into low impedance
at feedpoint; this is a general property of lambda / 4 lines;
(note that a "half-wave dipole" is actually a quarter-wave
transmission line).
* make short-circuited transmission line length = lambda / 2 (or a
multiple);
low impedance on the line end is repeated at feedpoint;
this is a general property of lambda / 2 lines;
(note that a "full-wave loop" is actually a half-wave
transmission line).
8. Antenna must be fed properly. The feeder must not be allowed
to become a part of the antenna. Run the feeder away from antenna.
Use a good balun. This was discussed at length few days ago.
* * *
I would not advise Ken to rush into buying any kind of antenna.
Instead, I'd suggest reading and understanding the basic physics,
which is simple and easy anyway.
Then I'd suggest careful and creative inspection of his premises,
searching for a way to fit an adequately deformed quarter-wave
open-ended transmission line. That's our old friend, half-wave dipole.
Run one leg as far from the other as possible. Do not worry about
both of them being on the same axis, or even being straight.
Just run them as high and as clear as possible. Examples can be found
in G6XN's and ON4UN's books. If you need an amount of vertical polarisation,
make at least one leg [almost] vertical.
Be prepared to erect your antenna twice or so, in order to adjust resonance.
And place a good balun at the feedpoint. Run the feeder away from the antenna.
But, let me see... Ken intends to operate on 160-30 meters.
Well, that makes things much worse. I'd guess that he'd have
to accept rather low efficiency on topband. But even then,
it would make no sense to push RF energy into soil. If the
manufacturer insists that the point of highest impedance
must be grounded, tell him that you need an antenna, not a dummy load.
Then be your own manufacturer, it's easy and fun. You wouldn't dare
to attempt "magic antennas", but they do not exist anyway.
73,
Sinisa YT1NT, VA3TTN
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