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ANTENNAS: Slot Antennas
antennas
BY KENT BRITAIN, WA5VJB
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SlotAntennas
Photo A. 2.4-GHz slot antenna
In many ways a slot antenna is the exact opposite of what most hams think of as an antenna. With a dipole, you have a half wave of metal surrounded by a lot of insulator. With a slot, you have a half wave of insulator surrounded by a lot of metal!
And even polarization is the opposite. Generally speaking, a vertical antenna puts out a vertically polarized radio wave. A horizontal slot also puts out a vertically polarized radio wave.
In Photo A, we have a 2.4-GHz slot antenna. As shown, it is vertically polarized and has a pattern similar to a 6element collinear antenna.
In Photos B1and B2, we have crossed slots which behave much like crossed dipoles or a turnstile antenna. On the back side you can see the two PCB traces that excite the slots. Note that one trace is a bit longer, a quarter wave longer to be precise. This gives the 90° phase shift needed for the slots to put out a circularly polarized wave.
With a center-fed dipole, the impedance is 72 ohms. (To defend myself before getting a full mailbox, yes, 72 ohms when the wire is very thin and the antenna is in free space.) Now, as many HF antenna builders have learned in recent years, that 72 ohms is when the dipole is fed in the middle, but if you slide the feed off to the side a bit, the impedance goes up, and up, and up, approaching infinite impedance as you reach one end of that dipole. Well, a similar situation occurs with slot antennas. But again, in the reverse. If that line feeding the slot is near one end, the impedance is very low. The PCB versions can be fed in the middle with a 50-ohm trace, but the length sticking out the other side can be important.
Unintentional Slot Antennas
In Photo C, we show the back of a typical desktop computer. Dozens of slot antennas back there. Some years back, I would do an EMI demonstration with Photo B2. Circularly polarized slot antenna, rear view
a computer case. Inside the case I had a broadband antenna to which I would connect the tracking generator port from a spectrum analyzer. Then with another broadband antenna connected to the spectrum analyzer’s RF port, I would move it around the outside of the computer. Wow, some of those slots would become amazingly efficient antennas. If the computer just happened to have a clock oscillator on that frequency, it could badly flunk FCC testing.
Now you understand why commercial RF modules have so many screws. In Photo D, you can see a module from a mobile phone cell site. Lots of screws ensure that any slots are too high in fre-
Photo C. Lots of slot antennas
Photo D. Closely spaced screws break up possible accidental slots Photo E. Aircraft radar slot array
quency for any oscillators inside the box to think it’s an antenna.
Slot or Dish
Let’s see if I can keep this part fairly simple since there are tons of material as possible feeds for a dish antenna. But in simple terms, it is hard to get the aperture efficiency to a prime focus dish better than 40% or so. The feed tends to send more of the signal into the center of the dish. Also, the center of the dish is much closer to the feed and the edge of the dish. Again, the center of the dish is RF hot and the edges of the dish barely used. In Photo E, you see the slot array from an aircraft nose radar. Lots of evenly-spaced slots with a low-loss waveguide power divider on the back of the antenna. These antennas are evenly distributing the RF energy over most of the surface of this antenna. Aperture efficiencies approaching 90% are possible. Oh, do the aerodynamic guys like these! Now that nose cone of their supersonic airplane can have about half the aerodynamic drag of a nose cone hiding a dish. Again, trying to get ahead of all those emails, yes, an offset feed dish like the one in Photo F can also approach 90% aperture efficiency, but let’s see you squeeze the dish and feed in Photo F into the nose of an F-16.
Quack!
Not sure exactly what we will talk about next time, but the subject of rubber duck antennas keeps coming up. I really have trouble thinking of a few inches of rubber being a proper antenna, but it may be more like “how to make the best of a bad situation. ”
If you have any antenna question or a possible column topic, you can use snail mail to my QRZ.COM address. For email use <wa5vjb@cq-amateur-radio.com>. For many additional antenna projects have a look at <www.wa5vjb.com>.
