nbr wrote:
Yes I have considered this. However my original post says that I wish
to be able to run legal limit. No such SGC matching network.

You can probably switch stubs in and out to accomplish the loading.
Start with one band, get it right, and go on to the next.
--
73, Cecil, W5DXP

On Mon, 28 Jul 2003 11:21:12 -0700, W5DXP
wrote:

nbr wrote:
W5DXP wrote:
The center of the "phasing line" on a half-square is a
maximum voltage point but there is nothing to keep that
horizontal wire from radiating. At a TOA of about 65 degrees
the broadside horizontal radiation and vertical radiation of
a half-square are about equal at about -9 dBi.

I don't think you're suggesting the horizontal component is cancelled
out??? So in truth the 1/2-square may perform DX best at low angle TOA
broadside to the two verticals, but may also have high angle lobes
from the horizontal wire (effective close-in cloud-warmer)?

Yes, above a TOA of about 65 degrees, the radiation is primarily
horizontally polarized and there is enough to make some NVIS
contacts on the lower bands. You get an interesting pattern if
you feed it 1/3 of the way down the horizontal wire. You get some
fairly good high angle radiation that can help fill in the nulls
in the half-square patternm i,e, the coverage doughnut gets bigger.

Interesting, I think this could be what happened during our Field Day
ops. We worked virtually NO DX, but did decent with this antenna to
most parts of the USA. The design of the 1/2-square was "wrong" by
conventional design notes, ie the verticals were too long (approx 45'
and 65' each) and were separated by WAY too much horizontal (about
130'). If we were radiating low vertical TOA, the propagation wasn't
supporting it (or the noise level was too high). However the decent
stateside performance suggests we were warming the clouds. Our QTH was
mid-USA (Missouri) and broadside to the horizontal wire was E/W; we
worked about 40 states.
73
Dan (K0DAN)

nbr wrote:
Why is the horizontal sengment of a 1/2-square
considered a "phasing line" and not a radiator?

The center of the "phasing line" on a half-square is a
maximum voltage point but there is nothing to keep that
horizontal wire from radiating. At a TOA of about 65 degrees
the broadside horizontal radiation and vertical radiation of
a half-square are about equal at about -9 dBi.
--
73, Cecil, W5DXP

nbr wrote:
W5DXP wrote:
The center of the "phasing line" on a half-square is a
maximum voltage point but there is nothing to keep that
horizontal wire from radiating. At a TOA of about 65 degrees
the broadside horizontal radiation and vertical radiation of
a half-square are about equal at about -9 dBi.

I don't think you're suggesting the horizontal component is cancelled
out??? So in truth the 1/2-square may perform DX best at low angle TOA
broadside to the two verticals, but may also have high angle lobes
from the horizontal wire (effective close-in cloud-warmer)?

Yes, above a TOA of about 65 degrees, the radiation is primarily
horizontally polarized and there is enough to make some NVIS
contacts on the lower bands. You get an interesting pattern if
you feed it 1/3 of the way down the horizontal wire. You get some
fairly good high angle radiation that can help fill in the nulls
in the half-square patternm i,e, the coverage doughnut gets bigger.
--
73, Cecil, W5DXP

This isn't tough. You have the makings of a fine 160-10 meter antenna.

Step 1 - add wire to run the vertical leg back up toward the top to make it
a total of 135 feet long. (linear loading)

Step 2 - feed it at the top center with 450 ohm ladder line (no radials or
ground plane needed now) Run the ladder line away at an angle to be
equidistant from the legs of the antenna and put a twist in the line about
once every two feet.

Step 3 - run the ladder line back to a 1:1 current balun as close to the
shack as you can (Centaur had some nice big ones)

Step 4 - coax to the tuner in the shack. You may not even need the tuner on
160.

Step 5 - enjoy. I do.

--
Radio K4ia
Craig "Buck"
Fredericksburg, VA USA
FISTS 6702 cc 788 Diamond 64

nbr wrote in message . ..
On Sun, 27 Jul 2003 19:19:26 -0500, nbr
wrote:

I've put up an inverted-L, which consists of approx. 135' horizontal
leg, and approx. 70' vertical leg. I can shorten the overall length of
the antenna, but cannot lengthen it. There are about 4 ground rods
within about 10' of the base ofthe vertical element, plus about 25
square feet of chickenn wire to serve as a ground plane. I can feed
the antenna right at ground level, or can arrange to feed it up to
6-10' above ground. There is about 120' of buried coax to the shack,
which must feed this antenna. I'd like to use this inverted-L on
160-10M (will settle for 80-10M).
1) How to feed the antenna and be able to run legal limit, all bands?
Current or voltage balun? Won't a balun disspiate power and decrease
efficiency? WIll a balun at ground level increase ground losses?
2) How is the inverted-L said to be a vertically polarized antenna,
when a major portion of its radiating element is horizontal?
3) For Field Day we added another vertical leg to this antenna to make
it into a half-square, and had decent results on 40M/20M. How is the
half-square described as "two verticals in phase", when again, there
is a major part of the antenna (the so-called "phasing element") which
is horizontal?
Thanks and 73
Dan (K0DAN)


Thanks for the recent comments on my previous post. They have been
interesting and informative.

The "antenna voodoo" is still bothering me about the theory of some of
these antennas (e.g. inverted-L, half-square, etc.). I understand that
the horizontal leg of the "L" is considered an "inductor at the top of
the vertical" element, but why not the reverse?

Myself, I tend to consider it capacitance rather than inductance. IE:
one leg of a top hat.

Why is this not a
"bent horizontal" with segments which radiate both in the horizontal
and vertical planes?

In a way it is, but the vertical radiation overshadows the horizontal
when using a 1/4 wave long inv L. Maximum current is at the base if
you are feeding it at the base. The thing with your antenna is it's
longer than a 1/4 wave on even 160 meters. So you will have more
radiation from the horizontal wire than you would a shorter antenna.
But on some bands, this may not be a bad thing. To run on 160m with
power, it's fairly simple. Use a big variable cap in series at the
base to tune out the reactance. The match should be usable. And the
antenna will work well for mid range use. Better close in than the 1/4
wave L. The max current point is well up the vertical part from the
base, and ground losses are reduced. But it might not be as simple on
the other bands. You almost need a tuner at the base. A simple L
network should do the job. It would be much simpler for all band use
if you had a 135 ft center fed dipole fed with ladder line to a tuner
in the shack. The overall performance on most bands would be better.
If your run of coax to the antenna were real short, and you used good
fat coax, you could tune the L with a good tuner at the shack and have
a usable signal. But I think you said you had 120 ft. Thats a bit long
for very high SWR use. And being it's buried, the expected likely
common mode currents that could be usefully radiated, will be wasted
heating earthworms...:/ MK

nbr wrote:
When I get into these antenna mysteries, the thing for me to remember
is to focus on where the likely voltage and current points are on the
element(s), and from there I have a better chance of prediciting the
characteristics of the antenna at a given frequency. Otherwise it's
pure voodoo. (I need to tattoo a couple graphs on my hand.)

You could probably learn a lot by using a modeling program like EZNEC,
which is available in a free demo version at http://www.eznec.com
--
73, Cecil http://www.qsl.net/w5dxp



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On Tue, 29 Jul 2003 10:12:16 -0500, W5DXP
wrote:

nbr wrote:
When I get into these antenna mysteries, the thing for me to remember
is to focus on where the likely voltage and current points are on the
element(s), and from there I have a better chance of prediciting the
characteristics of the antenna at a given frequency. Otherwise it's
pure voodoo. (I need to tattoo a couple graphs on my hand.)

You could probably learn a lot by using a modeling program like EZNEC,
which is available in a free demo version at http://www.eznec.com

You know, I've been meaning to do exactly that. A few years ago tried
a free demo (might have been early version of EZNEC?) and was scared
away from it because I found it extremely difficult to input the
antenna data....a simple dipole in free space was no big deal, but the
oddball antennas I like to experiment with (goofy lengths, bends and
angles,(in real world environment...near tower, metal barn etc.!) were
practically impossible to input.Garbage in, garbage out. Will give it
another try.
Tnx & 73
Dan (K0DAN)

I see that most of your questions about inverted L antennas have been
answered.

The questions you asked (quoted below) illustrate the danger of trying
to oversimplify antenna operation. For example, the problem with the
"inductor at the top of the vertical" is simply that the horizontal leg
isn't just an "inductor at the top of the vertical". If you consider it
one, you end up with dilemmas like you've encountered. Likewise,
dividing wires into two separate and distinct classes of "phasing lines"
and "radiators" gives you no room for wires which do both simultaneously
(let alone deal with "phasing lines" whose phase shift doesn't equal the
electrical length of the line, which often happens).

You'll have to develop a more basic understanding of antenna operation,
and avoid trying to pigeonhole antenna characteristics into a convenient
handful of categories, if you're ever to have antennas make sense and
cease being "voodoo". The EZNEC demo is a way many people have found to
help learn what's going on. If you do download the demo, be sure to go
through the "Test Drive" tutorial to get started. If you don't find
EZNEC to be your cup of tea, there are now a number of free antenna
analysis programs available, and you'll probably find one that's to your
liking.

Roy Lewallen, W7EL

nbr wrote:

Thanks for the recent comments on my previous post. They have been
interesting and informative.

The "antenna voodoo" is still bothering me about the theory of some of
these antennas (e.g. inverted-L, half-square, etc.). I understand that
the horizontal leg of the "L" is considered an "inductor at the top of
the vertical" element, but why not the reverse? Why is this not a
"bent horizontal" with segments which radiate both in the horizontal
and vertical planes? Why is the horizontal sengment of a 1/2-square
considered a "phasing line" and not a radiator?

73
Dan (K0DAN)

As to this particular project, I've received several interesting
suggestions, some modifying the shape/size, some adding a tuner at the
base. I'm probably gonna try both. I have a line on an extra tuner,
which I'm gonna try to weatherproof and install (single band)

Dan, I homebrewed a remote controlled tuner that would work with your antenna,
and you could change bands without ever leaving the shack. It uses a motor
driven roller inductor and variable capacitor in an "L" network. I also have a
relay switching scheme to reconfigure the "L" network. With this tuner at the
base of a vertical or inv. L you can match a wide range of impedances. The L
and C in this tuner are rated at legal limit.
Obviously the only thing standing in the way of you using something like this
is obtaining the parts and building it. The inductor is surplus, about 30 uH.
MFJ sells one that they use in their 1 kw tuner, around $65. The capacitor is
a Ten Tec 500pf variable cap. kit, around $45. The drive motors came out of a
kids battery operated car, 12vdc around .5 to 1 revs/sec that can be changed
with the supply voltage. The hard part is coupling the motors to the L and C.
The control cable is 100' of 8 cond.#20 shielded industrial control cable,
about $45. Forward and reversing was done with relay logic.
This was not a weekend project, but it wasn't that bad either. If you want
some more details, email.


73 Gary N4AST