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I’m about to do something I really hate to do,
but it’s been a very hectic month as I write this in late December in a
motel in Fordyce, Arkansas. I’m going to write about an antenna I have not
had a chance to prototype. It is, however, a pretty low-risk project.
RadioShack sells a “Universal Antenna” (catalog No. 17-345; photo A) for
analog, TDMA, CDMA, and GSM cell phones, with even an implied reference
for using it with 915-MHz spread-spectrum phones. That’s a pretty big
range of frequencies.
I purchased two and split open the first one before I even left the
store’s parking lot. A circle and a rectangle? I’d never seen anything
like this and could hardly wait to get it home and fire up the network
analyzer.
In photo B you can see the SWR is less than 2 to 1 from just under 800 MHz
to 2000 MHz. I didn’t take a photo of my later sweeps, but it kept that
same pattern of humps just under 2 to 1 SWR up to over 6000 MHz. That’s a
pretty big range of frequencies!
Scaling up the 3-inch diameter disk to a 17-inch disk would bring it down
to 140 MHz. What a construction project!An antenna that would work on 146,
222, 440, 900, and 1290 MHz (figure 1)! Then throw in the VHF, UHF low,
and UHF high scanner bands as well. Don’t be afraid to use something
bigger than 17 inches in diameter. It’s not a particularly critical
dimension as long as it’s at least 17 inches across. Five ham bands in one
antenna is a pretty versatile antenna.
Note how the coax is run down the center of the rectangular element (photo
C). That coax routing down the center was very important for a broad
frequency response.
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