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Terminating an UWB antenna

O

oopere

Jan 1, 1970
0
I am playing with two antennas which are know to be useful for sending
and receiving UWB pulses. Essentially I am reproducing the cone and TEM
horn antennas described in http://www.picosecond.com/objects/AN-14a.pdf
Fig.4.

From the time domain S11 parameter of the cone antenna, it follows that
the antenna exhibits a nice resistive impedance until the pulse hits the
end of the cone. Something similar happens with the TEM horn.

I was wondering what would be a good way to "terminate" the cone or the
horn to avoid (or minimize) the reflections from the antenna end.

If I had "ground" nearby, this would be easier.

Do you think I should solder a number (1 every cm) of 0805 resistors
from the cone end to a dummy cone continuation? Perhaps a dummy ring
would suffice? Or better a dummy flat hat?


\ / <- Dummy cone
\ /
R ... R
\ /
\ / <--- "Active" cone
\/
==============

If so, what value should the resistors be?

Would this also work for the TEM horn?

Would it be a better idea to fold back the TEM horn like this?

__
/ /
/ / <- Horn (inclination is excessive in ASCII art)
/ /
R |
=*========== Ground
| <- Coax input

What should be the terminating resistor value?

If no answers are available, any comments are welcome!

Pere
 
O

oopere

Jan 1, 1970
0
John said:
An ideal UWB antenna is "terminated" by space; it launches all its
input energy. Of course. low frequencies aren't terminated, but you
can adjust the drive waveform so that you don't bang it with low
frequency components.

Why do you want to terminate it?

John

During the time that the cone is indistinguishable from an infinite
cone, it generates an electric field that is proportional to the input
voltage. This means, that an input pulse produces a well defined pulse
of electric (and magnetic) field. However, as soon as the wave reaches
the end of the cone, it reflects back, which disrupts the "nice" pulse
that was generated.

If there is a way to absorb the wave at the end of the antenna, the
reflected pulse could be avoided.

Pere
 
B

Baron

Jan 1, 1970
0
oopere said:
During the time that the cone is indistinguishable from an infinite
cone, it generates an electric field that is proportional to the input
voltage. This means, that an input pulse produces a well defined pulse
of electric (and magnetic) field. However, as soon as the wave reaches
the end of the cone, it reflects back, which disrupts the "nice" pulse
that was generated.

Isn't that because the end of the cone doesn't match free space. I
would have thought that expanding the end of the cone (trumpet Like)
could solve that.
 
O

oopere

Jan 1, 1970
0
Baron said:
Isn't that because the end of the cone doesn't match free space. I
would have thought that expanding the end of the cone (trumpet Like)
could solve that.

Essentially, John's point is that the cone will radiate the whole pulse
if its width is less than the time during which the cone behaves as
infinite. This makes sense and I will try to confirm this. Right now I
am driving the antennas with pulses obtained from a SRD, which certainly
have low frequency components.

By "trumpet like", are you suggesting something like this in an attempt
to match free space impedance?
____ _____
\ /
\ / <--- "Active" cone
\/
==============

I am not sure if this would work. Let me explain a line of thought:

The cone over a ground plane gives an input impedance of 150R, which is
half of the characteristic impedance of a biconical transmission line
made from two of such cones. I could design a biconical dipole having
the input impedance of free space (377R). However, I think I would still
have reflections from the ends of the antenna if the length of the
dipole is not long enough compared to the width of the input pulses.

So, I guess that any approach that does not absorb the energy that has
not been radiated will not work. But this is just my humble opinion...

Pere
 
O

oopere

Jan 1, 1970
0
Wimpie said:
Hello Pere,

The antenna range graph in your reference shows the TEM horn (made out
of flat sheet material). The radiation pattern of such a horn is
strongly dependent on the frequency (it can even generate a near "nul"
in the main direction). It also gives some reflection (as you noted).

The performance of such a TEM horn can be improved by bending the flat
plate upwards (so you get a curved surface). The bending/flaring
guarantees that the surface current converts to radiation before it
reaches the end (so you will have negligible reflection).

The higher the frequency, the faster the surface current wave converts
into radiation (so the antenna becomes shorter for higher
frequencies). This effect also assures that you have a better
radiation pattern versus frequency. So a good design doesn't require
resistive termination. A disadvantage of the flared designs is that
the phase center becomes frequency dependent, so you get pulse
distortion.

One may notice that when you curve the TEM horn you get impedance
mismatch. When the impedance of the TEM structure changes gradually
versus distance, the mismatch is insignificant. Another variety of the
flared TEM horn is the tongue antenna.

Off course all these structures have a lower frequency cut-off (about
the quarter wave frequency for the TEM horn), but this should not be a
problem for UHF/SHF UWB.

The curvature and widening in the horizontal plane is of importance
for both S11 and radiation pattern. You can have good wide band S11
performance, but poor radiation pattern and vice versa.

Hope this helps you a bit.

Best regards,

Wim
PA3DJS
www.tetech.nl
without abc you have a valid mail address

Thanks for this info, Wim. I assume that when you bend the upper plate
you achieve radiation of the lower frequency components "earlier": this
introduces distortion and shortens the horn.

I will also have a look at the tongue antenna you suggested.

OTOH, if I really _wanted_ to add a resistive termination, where and how
would you suggest inserting it?

Pere
 
B

baron

Jan 1, 1970
0
oopere Inscribed thus:
Essentially, John's point is that the cone will radiate the whole
pulse if its width is less than the time during which the cone behaves
as infinite. This makes sense and I will try to confirm this. Right
now I am driving the antennas with pulses obtained from a SRD, which
certainly have low frequency components.

By "trumpet like", are you suggesting something like this in an
attempt to match free space impedance?
____ _____
\ /
\ / <--- "Active" cone
\/
==============

Yes. If the end of the cone is flared there should be a point at which
a match to free space occurs.
I am not sure if this would work. Let me explain a line of thought:

The cone over a ground plane gives an input impedance of 150R, which
is half of the characteristic impedance of a biconical transmission
line made from two of such cones. I could design a biconical dipole
having the input impedance of free space (377R). However, I think I
would still have reflections from the ends of the antenna if the
length of the dipole is not long enough compared to the width of the
input pulses.

I can see your point and agree.
So, I guess that any approach that does not absorb the energy that has
not been radiated will not work. But this is just my humble opinion...

Pere

So would a restive foam plug of the right thickness have the desired
effect ? Or would it simply adsorb the whole pulse before any
radiation took place.
 
O

oopere

Jan 1, 1970
0
baron said:
oopere Inscribed thus:


Yes. If the end of the cone is flared there should be a point at which
a match to free space occurs.


I can see your point and agree.


So would a restive foam plug of the right thickness have the desired
effect ? Or would it simply adsorb the whole pulse before any
radiation took place.

Once the signal has propagated along the cone or the horn, it has
already done it's job: it has radiated the wave. In the case of the
horn, rather than a piece of foam, I could imagine inserting an
attenuator along the transmission line that is formed by the upper plate
and the ground plane, or even a termination:

+--------==========================[ ]====
Gen [ 20 dB ]
---------==========================[ ]====
<--radiating portion -->

+--------==========================[ ]
Gen [ Zo ]
---------==========================[ ]
<--radiating portion -->

I could attach two 2xZo resistors at the edges of the mouth to not block
the radiation but they have to be physically long and still behave as
resistors. I am not sure if a long string of 0805 resistors will behave
as a resistor at the required frequencies (which are the lower ones,
since the fast ones have already been radiated)

However, I can not imagine a way to achieve this termination to ground
in the case of the cone. Perhaps in this case your suggestion might work
wrapping the upper part in absorbent foam...


Pere
 
B

Baron

Jan 1, 1970
0
oopere said:
baron said:
oopere Inscribed thus:


Yes. If the end of the cone is flared there should be a point at
which a match to free space occurs.


I can see your point and agree.


So would a restive foam plug of the right thickness have the desired
effect ? Or would it simply adsorb the whole pulse before any
radiation took place.

Once the signal has propagated along the cone or the horn, it has
already done it's job: it has radiated the wave. In the case of the
horn, rather than a piece of foam, I could imagine inserting an
attenuator along the transmission line that is formed by the upper
plate and the ground plane, or even a termination:

+--------==========================[ ]====
Gen [ 20 dB ]
---------==========================[ ]====
<--radiating portion -->

+--------==========================[ ]
Gen [ Zo ]
---------==========================[ ]
<--radiating portion -->

I can see your logic with regard to a lossy transmission line.
But I have difficulty in seeing how this translates to a horn as a
radiating structure. In fact I belive that if you could actually do
this the device may fail to radiate anything.
I could attach two 2xZo resistors at the edges of the mouth to not
block the radiation but they have to be physically long and still
behave as resistors. I am not sure if a long string of 0805 resistors
will behave as a resistor at the required frequencies (which are the
lower ones, since the fast ones have already been radiated)

However, I can not imagine a way to achieve this termination to ground
in the case of the cone. Perhaps in this case your suggestion might
work wrapping the upper part in absorbent foam...


Pere

I was thinking a plug of some lossy material across the horn where the
thickness was appropriate for the pulse period. However I belive a
controlled flare of the horn exit would have the desired effect.
 
B

Baron

Jan 1, 1970
0
Wimpie said:
Hello Pere,

It is the other way around; low frequencies use the whole structure,
high frequencies use a part only. By selecting correct flaring in the
horizontal plane and in the vertical plane (the upward bending of the
sheet material), you can have reasonable radiation pattern and wide
Band |S11| < 10 dB (VSWR < 2).

I agree. Flared horns tend to be wider bandwidth structures.
I suggest that you try to get access to an EM field simulation program
that supports full 3D metallic structures. It will enable you to see
the travelling wave present in the TEM horn (or derived
construction).

It would be interesting to be able to see what it would actually look
like.
Regarding the resistive termination. Assuming nice radiaton pattern
and radiation efficiency, I don't know where to connect resistive
termination. One thing you may consider is the absorbing foam as
suggested by others. This however doesn't help for the lower
frequencies.

With kind regards,

The danger with using resistive adsorbers might be that they themselves
become resonant elements.
 
O

oopere

Jan 1, 1970
0
Wimpie wrote:
Hello Pere,

It is the other way around; low frequencies use the whole structure,
high frequencies use a part only.

Hi, Wim. I think we mean the same. Placing the connector at the origin
x=0, a straight upper plate would have radiated frequencies [fa..inf) at
x=xa. If the upper plate is bent, frequencies [fa..inf) would already be
radiated at x<xa (what I meant saying "earlier").
By selecting correct flaring in the
horizontal plane and in the vertical plane (the upward bending of the
sheet material), you can have reasonable radiation pattern and wide
Band |S11| < 10 dB (VSWR < 2).

There is however still the issue of dispersion due to different apparent
points of generation.
I suggest that you try to get access to an EM field simulation program
that supports full 3D metallic structures. It will enable you to see
the travelling wave present in the TEM horn (or derived
construction).

I will try to have a look at that. This will be interesting!
Regarding the resistive termination. Assuming nice radiaton pattern
and radiation efficiency, I don't know where to connect resistive
termination. One thing you may consider is the absorbing foam as
suggested by others. This however doesn't help for the lower
frequencies.

With kind regards,

Wim
PA3DJS
www.tetech.nl
Please remove abc in case of PM

Pere
 
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