Vivaldi Antennas
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Vivaldi antennas are simple planar antennas that are very broadband. The polarization is linear, and the basic antenna structure is shown in Figure 1:
 Figure 1. Basic Geometry of a Vivaldi Antenna. In Figure 1, we have the antenna feed connecting two symmetric sides of a planar metallic antenna. To the left of the feed is is a short-circuit. However, antennas are RF-type devices and therefore the short-circuit acts more like a parallel inductor. To the right of the feed is the radiating element. It can be considered a tapered slot antenna or an aperture antenna. At this stage we could go through some moronic and complicated equations to understand what is going on. But that is boring, so we won't do that. I'd like to explain the vivaldi antenna by going through the process of building one, so we can see that is the evolution of a slot or IFA (Inverted-F Antenna). To start, let's just take a square area of copper tape, as shown in Figure 2. The length (horizontal dimension) is 84 mm, and the height (vertical dimension) is 54 mm. This is just a flat sheet of copper tape sitting on a piece of FR4:
 Figure 2. A Rectangular Slab of Copper.
Now, to start, I'm going to cut a slot out of the slab in Figure 2. The slot will be about 80mm long, and about 5-10mm wide, as shown in Figure 3:  Figure 3. Cutting a Slot out of a Rectangular Slab of Copper.
This slot is not an antenna yet, because there is no feed. So I grab a standard coaxial cable with an SMA connection and solder it about 38mm from the end of the slot, as shown in Figure 4:
 Figure 4. Adding the Feed to Our Antenna.
In Figure 4, I've soldered the center conductor of the coaxial cable to one side of the slot, and the ground (shield or outside) of the cable to the other side. I also solder the cable along the length to the antenna structure. This keeps the cable itself from being a separate radiator - since it is part of the antenna structure the electric currents don't care if they are flowing on the cable or the antenna. This is similar to a balun. I hooked the antenna of Figure 4 to a Vector Network Analyzer (VNA) and measured the VSWR of the antenna from 500 MHz to 6 GHz. This is plotted in Figure 5:
 Figure 5. The VSWR for the Antenna of Figure 4.
In Figure 5, we see that our antenna, which is pretty simple, already has a few resonances. When I say resonance here, I mean a region where the VSWR dips and then goes back up. The reason I call this is a resonance, is that there is no loss in my circuit - no matching components, resistive devices, loss materials, etc. Hence, if the VSWR drops, then energy is probably being radiated away (it is, but I'll get to that later). From Figure 5, we see that we have 3 frequency bands where our antenna acts somewhat like an antenna:
This is interesting in my opinion. All we really did was feed a metallic structure, and we get a bunch of radiation. This is pretty cool, and it shows that nature wants things to radiate. From Maxwell's Equations, we know that if we can just get electric currents or voltage to add in phase, we will have radiation. And that's cool. Our antenna is a little bit like an IFA (Inverted-F Antenna) at this point, a little bit like a slot antenna, and also a bit like a dipole antenna. But never mind too much analysis right now. Let's say we shortened the slot of our antenna, so that the feed is now 18mm from the left edge, as shown in Figure 6:
 Figure 6. The same antenna, but with a shorter slot.
The resulting VSWR of our antenna is now shifted up in frequency - we should expect this since our slot is now shorter. This is shown in Figure 7:
 Figure 7. The VSWR curves for the Original Antenna (black) and the Antenna in Figure 6 (blue).
We can learn something by looking at Figure 7. I expected the slot shortening to increase the frequency of the resonances. However, only the 3 GHz resonance increased in frequency. This tells me that the slot mode of radiation is responsible for the 3.5 GHz resonance. The 1 GHz resonance did not shift - however, it did get deeper, which indicates the antenna has a better impedance match with a shorter slot. Shortening the slot has the effect of decreasing the shunt inductance that is the short circuit to the left of the feed. In Figure 6, the antenna resembles a dipole antenna that has the short circuit to the left (the inductive path). Hence, by shortening the slot, I basically improved the impedance matching of the dipole antenna mode - and I also now know that the dipole antenna mode occurs at 1 GHz, and the slot/ifa antenna mode occurs at about 3.5 GHz. The higher resonance had a slight downshift, but really it appears that the resonance got broader as well. Hence I suspect this is also a slot antenna mode, but this relies more on the slot to the right of the feed, as the very large inductance to the left of the feed basically doesn't matter at 6 GHz (this is because the impedance of an inductor is very high and basically an open circuit for high frequencies). The preceeding 3 paragraphs is exactly how antenna engineers think. Now, the morons in the university would spend all year trying to think up a crappy equation - and they would get it wrong. Not only that, these people have never put an antenna in a product, so don't have a clue. If you can make a change to an antenna, and observe the changes in resonances and explain them, then you understand the antenna. If not you might as well work in the defense industry doing nothing with your life. Now, back to the design. We know that more volume for antennas generally means more bandwidth. However, we have a little too much ground plane here - and that means a lot of capacitance. And capacitance kills your bandwidth and isn't good for radiation. So right now, we'll taper the slot, or flare the aperture as shown in Figure 8:
 Figure 8. A Tapered Slot Antenna.
The VSWR of this antenna is measured and plotted with the other curves in Figure 9:
 Figure 9. VSWR of the Tapered Slot Antenna (in Fig 8).
In Figure 9, the green curve is the VSWR of the tapered slot antenna of Figure 8. We see some nice things happened - the two resonances at 3.5 GHz and 5.8 GHz start to blend together, giving a very large bandwidth. In addition, the lowband (1 GHz) resonance also became broader band and better matched, as you can see from the lower and broader VSWR. This means our tapering of the slot made things much better from a radiation perspective. Since this was a pleasant change, let's taper both sides as shown in Figure 10:
 Figure 10. Dual Tapered Slot - The Vivaldi Antenna.
The VSWR of this antenna is plotted in Figure 11, with the others:
 Figure 11. VSWR of the Vivaldi Antenna (in red).
Figure 11 shows that the overall bandwidth of the Vivaldi antenna increased (the impedance matching was better over a wider range). We also see the lowband resonance increase in frequency slightly (i.e. the 1 GHz resonance shifts up). This is due to less capacitive loading (capacitive loading, or capacitance in the antenna structure tends to shift resonances down in frequency). We have now constructed a Vivaldi Antenna. We see that is has a low resonance and a very broad higher frequency resonance. We suspect very strongly that this radiates (since there is no other losses in the structure). However, to be sure, we should measure the antenna efficiency in an anechoic chamber. This is done and is plotted in Figure 12:
 Figure 12. Antenna Efficiency for the Vivaldi.
From Figure 12, we see that the Vivaldi antenna I threw together in Figure 10 has very broad bandwidth and very high efficiency. This shows that our resonances in the VSWR plots are indeed due to radiation resistance - and not loss. The direction of peak radiation is frequency dependent. For the low frequency band of radiation, I stated that the antenna acted as a dipole type antenna. In this case, the direction of peak radiation is directly into or out of the page as viewed in Figure 10. For the higher frequency band of radiation (3.5-6 GHz), the antenna is using the aperture to radiate. In this case, the direction of peak radiation is towards the right as seen in Figure 10. Finally, you may also see Vivaldi antenna with odd shaped slots cut out of the left side of the feed as shown in Figure 13:
 Figure 13. Vivaldi with a Loop to the Left.
The primary purpose of altering the short-circuit path to the left of the feed is for antenna tuning purposes (i.e. impedance matching). There is no real radiation that is added via adding the circular cut shown in Figure 13, but it does give the antenna designer more freedom in optimizing the VSWR or tuning of the antenna.
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