[TenTec] Sloper

Sat Jan 3 03:11:41 EST 2004

Sinsa "Hi-hard to respond" your answer was very long and covered
many things.

Sir - theory and practice....

First, in theory, how could I disagree?

Second, Ken wants to run multiple bands (which you only barely
touched on at the end)... which generally means you must accept
compromise or spend lots of money and land space.

Sinsa, to be honest, you said nothing which I disagree with
(except one minor point - later).

The problem is, many (perhaps most) people searching for a simple
solution get scared away easily when things get too complex and
your answer sounded too complex (although it wasn't really).

I will be so bold as to propose that we both agree that we can
only help Ken when he supplies us with more information about his
available space and possibly info about his ground (is it dry
ground - like desert, or normal ground, etc.).

In addition, except for top band, I can ONLY recommend a
symmetrical dipole fed with openwire feedline if one wishes to
run multiple bands with good efficiency and with the least amount
of cost and effort.  Actually this should be true for top band
too, but often the common matchboxes simply don't manage to match
the extremely shortened antenna on 160m.

My gut feeling is that Ken should separate his antenna demands
into two - top band, plus 80 - 30m.  I would suggest a center-fed
dipole with 2x 13.5 meter (about 41 ft.) legs and 450 ohm feeder.
This would be best fed through a true symmetrical matchbox, but
in practice, fed thru a 4:1 current balun into any standard
external matchbox (e.g., MFJ anything).  For top band, bypass the
balun.  Tie the two wires (of the openwire feeder) together and
feed them into the asymmetrical input of the matchbox.  This now
works as a top-loaded vertical.  Only problem is, you need
radials that should be one quarter wavelength long (i.e., about
120 ft. long) and you should have at least two.  They may be
zig-zag and bend as necessary around the yard.  Run them anyway
you can.

This is my "practical" answer.  Again, I "know" it works because
I've done this at least at 50 or more QTH's.

The advantage of the half sloper which Ken wishes to buy is that
it will work many bands (efficiency ?), but again I highly
suspect that it will ONLY work if the vertical leg is grounded,
Sinsa!  I'm not commenting about what is good or bad or worse,
but rather that this thing he proposed to buy will only work
properly on all bands without modification "if" the vertical leg
is grounded!  I fully believe that.  It's probably a bad antenna
but that's the only way it will work!

I'm not saying that I agree that he should do this.  In fact I
would not do it.  I would build my own better solution (as
described above).

Still, if Ken wants to buy something and use it "as is", then I
propose that I am right and you are "less right" Sinsa (hi).

NOW, WHERE I DISAGREE is that a balun is NOT always the right
thing to use.  First of all, in general, NEVER use a balun as
such (meaning wires wrapped around some ferrite core) but rather
use something which many people call a W2DU balun but I refer to
as simply an RF choke.  This may be several toroids around the
outside of a coax or it may be a commercial version thereof.  It
may also be as simple as 6 or 8 turns of coax wrapped in a coil
at the feedpoint.

BUT this is NOT what I meant.  A balun as described above should
be used when feeding symmetrical antennas, but if Ken (or anyone
else) feeds a half sloper where the tower is grounded, I would
NOT use any balun at all.  Also, I once had a dipole where one
half was free and in the clear and the other half was not only
over the roof of a house, but very close to it.  To get it to
exhibit a low swr, I actually had to shorten the leg over the
house a bit (it was capacitive loaded).  I intentionally did not
insert a balun so that the leg in the free could get (and
radiate) more RF than the leg over the roof of the house.

The cases described above are "exceptions".  Otherwise I believe
you should certainly use a balun - just not these mostly
worthless baluns that are sold for far too much money!  It's so
easy to D-I-Y.  For instance, see the W6SAI coax balun.  Works
great for low cost!

KEN, I hope Sinsa and I have not completely confused you.

Sinsa, I bet we actually agree on the theory - it's just that I
always try to think "practical", not theoretically!

73
Rick

-----Original Message-----
From: tentec-bounces at contesting.com
[mailto:tentec-bounces at contesting.com]On Behalf Of Sinisa Hristov
Sent: Saturday, January 03, 2004 1:41 AM
To: tentec at contesting.com
Cc: BalkanDX at yahoogroups.com
Subject: Re: [TenTec] Sloper

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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