Re: Terminating an UWB antenna

On 20 nov, 09:53, oopere <m...@xxxxxxxxxxxxx> wrote:
Wimpie wrote:
On 19 nov, 13:12, oopere <m...@xxxxxxxxxxxxx> wrote:
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 inhttp://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

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

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 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).

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
.

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