Feeding and matching a monopole
时间：04-04
整理：3721RD
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I'm learning to design antennas, and I'm working on a monopole for 2.4Ghz. I have a solid circular ground plane of about 5" (that's as big as I can go). I drilled a hole in the middle and solder a tiny coax wire's gnd shield to the plane (like what you find in laptop antennas) . The coax is 50 Ohms and about 4 inches long. Then I solder a piece of copper wire to the center conductor of the coax. Finally I measure 1/4 wave which is about 1.23" and snip off the top of the antenna. It worked much better than any off the shelf antennas I bought :)
Oh and the one nice thing I have going for me is I have a pretty nice VNA on hand.
Now though I'm trying to figure out how to match it to my radio. The ideal monopole is something like 36.5 + j21.25. My knowledge of transmission lines tells me I should match at the end of the transmission line to the antenna structure otherwise there will be a reflection back down the cable. How do people usually do that? I understand the concept of matching with a smith chart, but should I put a little board there with an inductor and capacitor? Or maybe as someone suggested to me I should vary the size of the gnd plane vs the length of the wire? I feel like that would reduce the antennas radiation efficiency? Or maybe I adjust the diameter of the center wire?
Sorry I'm new at this and I don't understand the tradeoffs yet. I made two identical antennas but the feed lines were about 1/4" different. I got pretty different measurements on the VNA for each one. Someone else told that the feed line difference can have a big impact when your antenna doesn't match it's impedance and I can understand that. So I'm trying to learn the proper approach to matching it to the feed line.
Oh and the one nice thing I have going for me is I have a pretty nice VNA on hand.
Now though I'm trying to figure out how to match it to my radio. The ideal monopole is something like 36.5 + j21.25. My knowledge of transmission lines tells me I should match at the end of the transmission line to the antenna structure otherwise there will be a reflection back down the cable. How do people usually do that? I understand the concept of matching with a smith chart, but should I put a little board there with an inductor and capacitor? Or maybe as someone suggested to me I should vary the size of the gnd plane vs the length of the wire? I feel like that would reduce the antennas radiation efficiency? Or maybe I adjust the diameter of the center wire?
Sorry I'm new at this and I don't understand the tradeoffs yet. I made two identical antennas but the feed lines were about 1/4" different. I got pretty different measurements on the VNA for each one. Someone else told that the feed line difference can have a big impact when your antenna doesn't match it's impedance and I can understand that. So I'm trying to learn the proper approach to matching it to the feed line.