vivaldi antenna

7/16/12 Vivaldi Antenna

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Vivaldi Antennas

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Vivaldi antennas are simple planar antennas that are very broadband. The polarization is linear, and the basicantenna 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 theleft of the feed is is a short-circuit. However, antennas are RF-type devices and therefore the short-circuit actsmore like a parallel inductor. To the right of the feed is the radiating element. It can be considered a taperedslot 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 ofbuilding 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 apiece of FR4:

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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 about5-10mm 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 SMAconnection 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 antennastructure. This keeps the cable itself from being a separate radiator - since it is part of the antenna structurethe 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 theantenna from 500 MHz to 6 GHz. This is plotted in Figure 5:

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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 sayresonance here, I mean a region where the VSWR dips and then goes back up. The reason I call this is aresonance, 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:

around 1 GHzFrom 2.5-4 GHzAt about 5.8 GHz

This is interesting in my opinion. All we really did was feed a metallic structure, and we get a bunch ofradiation. 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'scool.

Our antenna is a little bit like an IFA at this point, a little bit like a slot antenna, and also a bit like a dipoleantenna. But never mind too much analysis right now. Let's say we shortened the slot of our antenna, so thatthe 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 isnow 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 theresonances. However, only the 3 GHz resonance increased in frequency. This tells me that the slot mode ofradiation 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 betterimpedance match with a shorter slot. Shortening the slot has the effect of decreasing the shunt inductance thatis the short circuit to the left of the feed. In Figure 6, the antenna resembles a dipole antenna that has the shortcircuit to the left (the inductive path). Hence, by shortening the slot, I basically improved the impedancematching 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 thevery large inductance to the left of the feed basically doesn't matter at 6 GHz (this is because the impedanceof 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 wouldspend all year trying to think up a crappy equation - and they would get it wrong. Not only that, these peoplehave never put an antenna in a product, so don't have a clue. If you can make a change to an antenna, andobserve the changes in resonances and explain them, then you understand the antenna. If not you might aswell 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 capacitancekills 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 thingshappened - 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 seefrom the lower and broader VSWR. This means our tapering of the slot made things much better from aradiation 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



Figure 11 shows that the overall bandwidth of the Vivaldi antenna increased (the impedance matching wasbetter over a wider range). We also see the lowband resonance increase in frequency slightly (i.e. the 1 GHzresonance shifts up). This is due to less capacitive loading (capacitive loading, or capacitance in the antennastructure 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 higherfrequency resonance. We suspect very strongly that this radiates (since there is no other losses in thestructure). However, to be sure, we should measure the antenna efficiency in an anechoic chamber. This isdone 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 andvery 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 thatthe antenna acted as a dipole type antenna. In this case, the direction of peak radiation is directly into or out ofthe page as viewed in Figure 10. For the higher frequency band of radiation (3.5-6 GHz), the antenna is usingthe aperture to radiate. In this case, the direction of peak radiation is to the left 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 inFigure 13:



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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