I'd like o be able to work some low-band DX.

I'm trying to figure out how to feed a proposed delta loop antenna.In the
documentation, such as in "Practical Wire Antennas" from the RSGB it is
suggested that there is a lot of difference in the main angle of radiation
from the antenna depending on where one puts the feedpoint.

I don't argue with that, but I'd like to know whether that is because the
effective mean height of the antenna varies according to the shape, or
because of where the antenna is fed.

Because I can support the antenna at about 100 feet, no matter how I orient
the antenna, the effective mean height will be more than a half wave above
ground, at least on forty and at twenty metres and higher, it will be more
than a wavelength.

Because it will be simpler to orient the antenna as an equilateral triangle
with the base along the top, and fed at the apex (pointing down) does the
feed location actually matter?

Diagram A - Easiest to feed.
--------------------------------------- 100 feet above ground
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
| |


Diagram B Harder to Feed, and Feedline is 120 feet longer.


--------------------------------------- 100 feet above ground
\ -----
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\/


Comments on how well either of these is likely to work are welcome.

Bob, VE7HS

On Wed, 21 Nov 2007 12:09:31 -0800, Robert Smts wrote:

I'd like to know whether that is because the
effective mean height of the antenna varies according to the shape, or
because of where the antenna is fed.

Hi Robert,

You can model this, and all variations easily with EZNEC available
online (and is within the limitations of its free use).

As for feeding it at the top, as you illustrate. That would tend to
be unbalanced with the transmission line dropping in close proximity.
Without special attention to choking this line, it will become a
radiator for some of the bands (it'll be a guess). This may not be
all that bad, but you might find the antenna tunes oddly.

You could as easily put up a dipole and skip the complexity (hint,
loops are NOT low noise).

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On Wed, 21 Nov 2007 12:09:31 -0800, Robert Smts wrote:

I'd like to know whether that is because the
effective mean height of the antenna varies according to the shape, or
because of where the antenna is fed.

Hi Robert,

You can model this, and all variations easily with EZNEC available
online (and is within the limitations of its free use).

I'll look at it and see. I tried the free version but it wouldn't install
with Crossover Pro (I don't use Windows) and I may have to install VMWare
first.

As for feeding it at the top, as you illustrate. That would tend to
be unbalanced with the transmission line dropping in close proximity.
Without special attention to choking this line, it will become a
radiator for some of the bands (it'll be a guess). This may not be
all that bad, but you might find the antenna tunes oddly.

Well, I can pull the antenna coax over about 20 feet from the edge of the
corner of the loop before it goes down the tree. What concerns me is that I
need to add an extra 120 feet of coax to feed it this way. The shack
conveniently is quite near where the antenna cable would come down if I fed
it at the bottom corner. That's why I was curious about the radiation angle
effect of feeding it from the bottom, as long as I had the loop up high
enough.


You could as easily put up a dipole and skip the complexity (hint,
loops are NOT low noise).

Hmmm. I had an 80 metre loop some years ago, in a noisy urban environment,
and had the opposite experience, which is why i was interested in trying
it. Oh well, wire's cheap. I do already have a dipole at 100 feet.

Thanks, Richard.


Bob, VE7HS

Bob, VE7HS wrote:
"I had an 80 metre loop some years ago, in a noisy urban enviroment, and
had the opposite experience, which is why I was interested in trying
it."

According to Bill Orr, W6SAI in "All About Cubical Quad Antennas" the
full-wave loop produces a power gain of about 1.4 dB over the 1/2-wave
dipole and its feedpoint impedance is about 125 ohms when the center of
the loop is about 0.65 wavelengths above the earth.

Take off paterns depend on soil, polarization, and height. ON4UN does a
good job with charts, graphs, and illustrations in his chapter on "Large
Loop Antennas" in "Low-Band DXing".

When the loop is vertically polarized over good soil, and the center of
the loop is 0.17 wavelengths above the earth, the elevation angle peaks
about 21 degrees above the horizon.

Best regards, Richard Harrison, KB5WZI

On Wed, 21 Nov 2007 21:28:48 -0600, (Richard
Harrison) wrote:

When the loop is vertically polarized over good soil, and the center of
the loop is 0.17 wavelengths above the earth, the elevation angle peaks
about 21 degrees above the horizon.

Hi Bob,

What is left out of this, is that a loop is sensitive to where it is
excited, and what major polarization it will present. Again, given
all the variables you have, modeling would solve this far faster.
[Hint: you are probably wanting vertical polarization, but with the
heights you have advantage of, you need to test all options.]

73's
Richard Clark, KB7QHC

You can model this, and all variations easily with EZNEC available
online (and is within the limitations of its free use).

I'll look at it and see. I tried the free version but it wouldn't install
with Crossover Pro (I don't use Windows) and I may have to install VMWare
first.
=======================
Probably another Linux user . Roy ,please note ,there might be an
emerging market for a Linux version of EZNEC.


Frank GM0CSZ / KN6WH

Richard Harrison wrote:

Bob, VE7HS wrote:
"I had an 80 metre loop some years ago, in a noisy urban enviroment, and
had the opposite experience, which is why I was interested in trying
it."

According to Bill Orr, W6SAI in "All About Cubical Quad Antennas" the
full-wave loop produces a power gain of about 1.4 dB over the 1/2-wave
dipole and its feedpoint impedance is about 125 ohms when the center of
the loop is about 0.65 wavelengths above the earth.

Take off paterns depend on soil, polarization, and height. ON4UN does a
good job with charts, graphs, and illustrations in his chapter on "Large
Loop Antennas" in "Low-Band DXing".

I have copies of both, but the problem is that all the material refers to
loops at low heights above ground, which is not what I'm looking at.

When the loop is vertically polarized over good soil, and the center of
the loop is 0.17 wavelengths above the earth, the elevation angle peaks
about 21 degrees above the horizon.

Yes, apparently depending on where you feed it.

Thanks, Richard.

Highland Ham wrote:

You can model this, and all variations easily with EZNEC available
online (and is within the limitations of its free use).

I'll look at it and see. I tried the free version but it wouldn't install
with Crossover Pro (I don't use Windows) and I may have to install VMWare
first.

Probably another Linux user . Roy ,please note ,there might be an
emerging market for a Linux version of EZNEC.

Yes, I use Linux. I'd be especially interested in an open-source antenna
analyzer program. So far all I see is NEC2++ in a debian version.

Bob VE7HS

Highland Ham wrote:
=======================
Probably another Linux user . Roy ,please note ,there might be an
emerging market for a Linux version of EZNEC.

I'm very aware that there is. All it has to do is emerge by another
factor of ten or so to make it worthwhile to develop a version for it.

Roy Lewallen, W7EL

On 2007-11-22, Robert Smts wrote:
Highland Ham wrote:

analyzer program. So far all I see is NEC2++ in a debian version.

The xnecview graphic front end is somewhat helpful as an analyzer program and
uses the nec2 engine. It's rather generic for Linux and I'm using it on
Slackware 10.1.

....Edwin
--
__________________________________________________ __________
"Once you have flown, you will walk the earth with your eyes
turned skyward, for there you have been, there you long to
return."-da Vinci http://bellsouthpwp2.net/e/d/edwinljohnson