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Well, actually, I'm not sure I really understand what is the meaning of matching impedance, even for an ordinary antenna, but I can figure out the following: if you send a signal to the antenna, then there is no reflection whenever the impedance is matched (say to 50 Ohm). I can also figure out that AT A GIVEN FREQUENCY, you can match the impedance by adding (or cutting) some length of coaxial cable, or with a suitable RF transformer.

But now comes my real problem: I did an ultra-wideband self-grounded bowtie antenna, (400MHz - 3GHZ), and I am even less sure that I understand what is the meaning of "matching impedance" for these antennas.

Assuming it is, as previously that there are no reflections, how to do such a matching ? all the matching methods I know depend upon the frequency of the signal, but here, you have no defined frequency.

EDIT: It is tempting to think (and probably to answer) that it is not possible to do an ultra-wide band matching. Yet, let take a very expensive double ridge horn antenna. They claim that the antenna is roughly flat in the huge range of 500MHz - 18 GHz (say). So, how do they match their antenna ?

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    \$\begingroup\$ With cutting or adding coax you do wrongly - you match coax+antenna that becomes an antenna, so even the coax. You need a remote matchbox/transformer: send -> measure SWR -> adjust. \$\endgroup\$ – Marko Buršič Oct 26 '18 at 6:54
  • \$\begingroup\$ But still, this seems to be frequency dependent no ? \$\endgroup\$ – MikeTeX Oct 26 '18 at 7:21
  • \$\begingroup\$ Sure it is, since the antenna has no flat SWR vs. frequency. \$\endgroup\$ – Marko Buršič Oct 26 '18 at 7:30
  • \$\begingroup\$ Some antennas have a low and flat SWR, use these if you need UWB. Fat conical monopoles and self-similar antennas like Vivaldi, ridged horns, sinuous, spirals. You can do some matching with discrete components and transmission lines, but not turn a narrowband antenna like a dipole into a wideband antenna. \$\endgroup\$ – tomnexus Oct 26 '18 at 8:47
  • \$\begingroup\$ Do you mean that a UWBA need no matching ? \$\endgroup\$ – MikeTeX Oct 26 '18 at 8:53
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... I'm not sure I really understand what is the meaning of matching impedance, even for an ordinary antenna

Let's go here first. Impedance matching means that the internal resistance/impedance of the source equals the resistance/impedance of the load. When that is true, the maximum energy is transferred from the source to the load.

That is what we are looking for in an antenna. We want maximum energy transferred from the transmitter to the antenna. So the transmitter impedance should match the line and antenna for maximum transmitted energy.

I did an ultra-wideband self-grounded bowtie antenna, (400MHz - 3GHZ),

By definition, "wideband" means that the antenna matches the transmitter and feed line, over the range of frequencies that you provide (400 MHz to 3 GHz). So, if you say that your antenna is "wideband" then you are saying that it matches over the band.

The ultimate "wideband" antenna is a dummy load. It matches at any frequency. Of course, it doesn't radiate. (Y0u can't have everything!) But it certainly absorbs the maximum amount of energy from the transmitter and feed line.

What is the precise meaning of “matching impedance” for an ultra-wide band antenna, and how to do that?

So, the answer to your FIRST question is as I stated in the beginning. And as to the second question "how to do that", the answer is "nothing". The information and conditions of the antenna that you provide already says that you don't need to do anything to match the antenna impedance to the transmitter and line. As you say, the antenna is "wideband".

If you want, you can test the antenna, measuring the SWR for all of the frequencies in the "wideband". That test is up to you to verify that your antenna is really "wideband".

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  • \$\begingroup\$ Very nice and informative answer Baruch Atta - Baruch Hachem ;-). Thank you. \$\endgroup\$ – MikeTeX Dec 16 '18 at 18:06

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