I am experimeting with 2.4 GHz pyramidal hornantennas like this one:

http://seattlewireless.net/index.cgi/CardboardHorn

But I ask myself: what is the optimal hight of the waveguide part?

Of course the hight of the waveguide has to be more than lambda/4 in
order for the lambda/4 antenna rod to fit in it. But how much bigger
should it be?

Jeppe

On Sun, 07 Mar 2004 23:44:00 +0100, Jeppe wrote:

I am experimeting with 2.4 GHz pyramidal hornantennas like this one:

http://seattlewireless.net/index.cgi/CardboardHorn

But I ask myself: what is the optimal hight of the waveguide part?

Of course the hight of the waveguide has to be more than lambda/4 in
order for the lambda/4 antenna rod to fit in it. But how much bigger
should it be?

Jeppe

Hi Jeppe,

For the unbalanced probe, the throat cross section should show
1 by 0.5 wavelength dimension.
The mouth cross section should show
2 by 2 wavelengths.

The probe is situated on the face of the wider dimension, thus its tip
penetrates to the middle (where two diagonals would cross). The
illustrations are clear to this matter even though the dimensional
description is a bit rocky.

The standard gain of this antenna is 13dB (which is closely confirmed
by the author's measurements).

73's
Richard Clark, KB7QHC

On Mon, 08 Mar 2004 17:02:07 GMT, Richard Clark
wrote:

For the unbalanced probe, the throat cross section should show
1 by 0.5 wavelength dimension.
The mouth cross section should show
2 by 2 wavelengths.

The probe is situated on the face of the wider dimension, thus its tip
penetrates to the middle (where two diagonals would cross). The
illustrations are clear to this matter even though the dimensional
description is a bit rocky.

tnx f ur reply kb7ghc

0.5 wavelenght seems like a intuitively reasonable choice for the
smaller dimension of cross section of the throat / waveguide part. If
the tip of the probe (=1/4 wavelenght antenne rod?) is very close to
the opposite wall it is hard to imagine that it would not affect the
function and the SWR (which, unfortunately, I lack equipment to
measure)

But are you sure that 1 wavelenght is the optimum value for the other
dimension of the cross section? That is more than in the examples I
have seen on the net.

Is there some reason for choosing a 2 : 1 ratio of the two dimensions?

I may add that in the first horn I made, the dimensions were 0.36 x
0.72 wave. It worked, but probably not quite optimallly (I lack real
measuring equipment.) I then tried to put the antenne rod
perpendicular to the SMALLER side instead, but that did not work at
all (very weak signal).

73

Jeppe (former OZ3FV, hav'n't renewed my licence for about 30 years :-(

On Tue, 09 Mar 2004 10:40:10 +0100, Jeppe wrote:

But are you sure that 1 wavelenght is the optimum value for the other
dimension of the cross section? That is more than in the examples I
have seen on the net.

Is there some reason for choosing a 2 : 1 ratio of the two dimensions?

I may add that in the first horn I made, the dimensions were 0.36 x
0.72 wave. It worked, but probably not quite optimallly (I lack real
measuring equipment.) I then tried to put the antenne rod
perpendicular to the SMALLER side instead, but that did not work at
all (very weak signal).

73

Jeppe (former OZ3FV, hav'n't renewed my licence for about 30 years :-(

Hi Jeppe,

The physical/wavelength dimensioning in microwaves is one of the best
ways to visualize transmission concepts. The ratios you observe offer
the means to render conduction and isolation. The best treatment of
this subject will be found in Frederick Terman's work.

The placement issue of the rod is easily answered. In the picture you
have, it aligns with the creation of electrical fields that would be
naturally beneficial to propagation. When you experimented by placing
the probe on the other wall (90° off) you lost what is called mode
coupling. This is because what you have is a voltage probe that used
to couple to the voltage modes of the resonator. With this 90° twist,
your voltage probe was trying to excite the current modality. You
should have, instead, built a coupling loop to create the magnetic
fields that correspond to the cavity's mode. You would have observed
the same level of gain then.

Lest some of my treatment lead to confusion, voltage probes (the rod)
couple to electric fields; current loops (the alternative for that
experimental position) couple to magnetic fields. The electric fields
and the magnetic fields are at 90° angle to one another in what is
called a "mode." The mode is generally fixed in one orientation (we
can add complexities of many modalities, but it wouldn't add any
significant instruction). Thus with this fixed geometry, you shift
the position of the coupling to suit all considerations.
Additionally, you must take care to orient the loop correctly to work.

73's
Richard Clark, KB7QHC

Jeppe wrote:
"Of course the height of the waveguide has to be more than lambda / 4 in
order for the lambda / 4 rod to fit in. But how much bigger should it
be?"

Not critical.

Circular waveguides work but have a disadvantage of allowing multiple
modes. Square waveguides share this disadvantage.

To restrict formation of multiple modes of propagation, elliptical and
rectangular waveguides are used.

We are familiar with the illustrative and instructive diagram of
formation of a rectangular waveguide from countless 1/4-wave
short-circuit stubs attached to a parallel-wire transmission line. That
would require one dimension of a waveguide to be at least
1/2-wavelength. That computes and it has less attenuation if the wave
has just a tiny bit more elbow room.

There is no requirement on how small the spacing of a transmission line
can be as long as it doesn`t short where you don`t want it to short.
There is no abrupt minimum spacing for the small dimension of a
waveguide either. As in the longer dimension of the waveguide
cross-section, the attenuation rises as the guide narrows, but it does
not cut off propagation down the guide at 1/4 wavelength as the longer
dimension does at 1/2-wavelength.

I don`t have a catalog at hand but it seems to me that rectangular
guides are about twice as wide as they are tall. This ratio has proved
to work very well.

See page 265 of "Transmission Lines, Antennas, and Wave Guides" by King,
Mimno, and Wing for confirmation and more details.

Best regards, Richard Harrison, KB5WZI

On Tue, 9 Mar 2004 14:46:46 -0600 (CST),
(Richard Harrison) wrote:
There is no requirement on how small the spacing of a transmission line
can be as long as it doesn`t short where you don`t want it to short.
There is no abrupt minimum spacing for the small dimension of a
waveguide either. As in the longer dimension of the waveguide
cross-section, the attenuation rises as the guide narrows, but it does
not cut off propagation down the guide at 1/4 wavelength as the longer
dimension does at 1/2-wavelength.

I don`t have a catalog at hand but it seems to me that rectangular
guides are about twice as wide as they are tall. This ratio has proved
to work very well.

Tnx a lot for your reply Richard

I have no in-depth knowledge of the physics/mathematics involved but I
think I have gathered by now a meaningfull understanding of how wave
guides work and the importance of their dimensions.

So my problem is really mostly that I am trying to guess how close the
tip of the 1/4 wavelength probe / antenne rod can be to the opposite
wall of the wave guide - without "detuning" the probe / affecting the
SWR in a detrimental way.

As I wrote I have no measuring equipment. I just disassembled the
dipole antenna of a 30-Euro Belkin Wireless USB adapter and inserted
the 1/4 stub into a cardboard/aluminium-foil horn antenna. That way I
do not have to solder anything and there is no extra transmission wire
loss. Crude electronics? I dare say it is. But it helps me share an
internet connection with a friend living half a block away. And it is,
of course, kind of interesting for an old radio amateur...

Jeppe

Jeppe wrote:
"So my problem is really mostly that I am trying to guess how close the
tip of the 1/4 wavelength probe antenna rod can be to the opposite wall
of the wave guide - without "detuning" the probe / affecting the SWR in
a detrimental way."

I don`t know, so I`m glad you asked the question. This is a good
newsgroup and I`d wager that someone will provide the answer. It`s been
a long time since I worked with microwaves and then it was with
commercial equipment.

My first thought was that the open-circuit end of a probe has an energy
about-face at its end which produces a null. Then out of curiosity I
went to my 3rd edition of the RSGB VHF-UHF Manual where I found tuning
adjustments made by adjusting depth of insertion of probes into cavities
and waveguides. There is much practical information in the RSGB Manual
but the microwave is mostly centered on 10 GHz. That`s OK because most
of it scales to your frequency of interest.

Best regards, Richard Harrison, KB5WZI

On Wed, 10 Mar 2004 08:47:27 -0600 (CST),
(Richard Harrison) wrote:

Jeppe wrote:
"So my problem is really mostly that I am trying to guess how close the
tip of the 1/4 wavelength probe antenna rod can be to the opposite wall
of the wave guide - without "detuning" the probe / affecting the SWR in
a detrimental way."

I don`t know, so I`m glad you asked the question. This is a good
newsgroup and I`d wager that someone will provide the answer. It`s been
a long time since I worked with microwaves and then it was with
commercial equipment.


Hi Guys,

I thought that was pretty well evident by the photograph alone. It
should also be quite obvious by the sheer ratio metrics of wavelength
dimensions: A quarter wave element penetrating into a half wave
dimension finds the tip of the element one quarter wave from the
opposite wall. When you reduce the fractions, the answer is simply
half way across. It is like a key, if it doesn't fit, you've done
something very wrong.

You can observe that the tip resides at a spot defined by the
intersection of two lines drawn from the four corners of the interior
box dimension of the throat.

The physics of wavelength, the size of scale for microwaves, the
regularity of surfaces, and common sense combine naturally to allow
anyone to manipulate and visualize all interactions with more ease
than attempting to perform the same operations at HF.

Adding inline lumped reactances is also visually elegant. You erect
short walls on opposite faces of the throat to build inductors or
capacitors (capacitive and inductive elements are 90° from each other;
inductors are erected across the shorter face, capacitors across the
longer face - all should note the correlation to e-field and h-field
geometries) and if you want to build a resonant trap (LC) you
construct a window frame (the combination of the four short walls
built) or iris occupying the interior of the throat.

One dimension that has lacked discussion is how far from the back of
the throat (the back wall) should this probe be placed. The link
offered reveals that this, too, maintains the standard ratio metric of
a quarter wavelength. However, other sources of mine suggest this is
variable for matching issues and may be as great as 3/8ths wavelength
or as short as 1/5th wavelength. Such adjustments add inductance or
capacitance to the rod to balance out its reactance (if present).
This is of less importance to receiving and the recommended quarter
wave placement is sufficient.

73's
Richard Clark, KB7QHC

On Tue, 09 Mar 2004 10:40:10 +0100, Jeppe wrote:
0.5 wavelenght seems like a intuitively reasonable choice for the
smaller dimension of cross section of the throat / waveguide part. If
the tip of the probe (=1/4 wavelenght antenne rod?) is very close to
the opposite wall it is hard to imagine that it would not affect the
function and the SWR (which, unfortunately, I lack equipment to
measure)

But are you sure that 1 wavelenght is the optimum value for the other
dimension of the cross section? That is more than in the examples I
have seen on the net.

Is there some reason for choosing a 2 : 1 ratio of the two dimensions?

I may add that in the first horn I made, the dimensions were 0.36 x
0.72 wave. It worked, but probably not quite optimallly (I lack real
measuring equipment.) I then tried to put the antenne rod
perpendicular to the SMALLER side instead, but that did not work at
all (very weak signal).

73

Jeppe (former OZ3FV, hav'n't renewed my licence for about 30 years :-(

Hi Jeppe,

Sorry for the delay in response, this message of yours has arrived
only today - very late.

The 2:1 ratio follows from the basic ratio metrics of microwaves, but
are not absolute. Rather than go into a difficult description that
begs graphics, visit:
http://www.fnrf.science.cmu.ac.th/th...heory%202.html

As to your question about the dimensions of 0.36 x 0.72 wave (you
will no doubt note the aspect ratio again). The answer to this is
that such components tend to work over an octave of frequency (another
2:1 ratio, pretty common that).

73's
Richard Clark, KB7QHC