Terminating an UWB antenna

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

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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?

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Reply to
oopere
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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

Reply to
John Larkin

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

Reply to
oopere

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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

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Reply to
Wimpie

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.

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Best Regards:
                     Baron.
Reply to
Baron

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? ____ _____ \\ / \\ / > If there is a way to absorb the wave at the end of the antenna, the

Reply to
oopere

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

Reply to
oopere

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.

--
Best Regards:
                Baron.
Reply to
baron

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 ]

---------==========================[ ]====

+--------==========================[ ] Gen [ Zo ]

---------==========================[ ]

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

Reply to
oopere

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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

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Reply to
Wimpie

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 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.

--
Best Regards:
                     Baron.
Reply to
Baron

I agree. Flared horns tend to be wider bandwidth structures.

It would be interesting to be able to see what it would actually look like.

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

--
Best Regards:
                     Baron.
Reply to
Baron

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 By selecting correct flaring in the

There is however still the issue of dispersion due to different apparent points of generation.

I will try to have a look at that. This will be interesting!

Pere

Reply to
oopere

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Hello Pere,

Regarding "earlier", I agree.

The dispersion is an issue, but I don't know the impact on your proposed system. I think it would be difficult to design an antenna with reasonable radiation efficiency that both provide good radiation pattern versus frequency and no dispersion. Maybe somebody can confirm whether this is a physical rather then a technical issue.

Kraus has some notes on UWB antennas. I have the third edition, chapter 21-26, 21-28. chapter 3-3 and 3-4 shows some examples of narrow band and intrinsically wide band structures.

I don't know your situation regarding simulation. I do have limited access to IE3D (MoM 3D package) and did some flat plate multiple/ single feed designs and one curved design to get wide bandwidth. For your case, the disadvantage of MoM packages is that they are frequency domain simulators. A time domain solver will probably give you the results in a more convenient way.

Best Regards,

Wim PA3DJS

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Reply to
Wimpie

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