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100 Ohm Twin Lead
Sonny Hood wrote:
I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? Let's take a look at how the 450 ohm feedline "messed up the match". The SWR on the 450 ohm feedline will be 450/100 = 4.5:1. A feedline length equal to a multiple of 1/4WLs will transform the 100 ohms to ~2000 ohms or a 50 ohm SWR of 40:1 which is probably out of the matching range for built-in autotuners, for instance. You can change the impedance seen at the tuner back to 100 ohms by adding or subtracting 1/4WL of 450 ohm line which will make it a multiple of 1/2WL. As someone else said, one of the most common ways to match a 100 ohm antenna is to use 1/4WL of 75 ohm coax and then 50 ohm coax the rest of the way. Instead of "messing up the match", it fixes up the match. The SWR on 75 ohm coax would be 100/75 = 1.33:1. The impedance at the end of the 1/4WL matching section would be 75/1.33 = ~56 ohms, a good match to 50 ohm coax. If you are wanting to use the antenna for multi-band operation, change the length of the 450 ohm line until the match is not "messed up". -- 73, Cecil http://www.qsl.net/w5dxp |
#2
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100 Ohm Twin Lead
Now, however did I know this thread would go on 'n on, 'n on 'n on...... Cecil Moore wrote: Sonny Hood wrote: I have an antenna that needs 100 ohm twin lead to match the impedance of the feed point. The 450 ohm lead has messed up the match and screwed up the resonance, does anone know of such an animal? Let's take a look at how the 450 ohm feedline "messed up the match". The SWR on the 450 ohm feedline will be 450/100 = 4.5:1. A feedline length equal to a multiple of 1/4WLs will transform the 100 ohms to ~2000 ohms or a 50 ohm SWR of 40:1 which is probably out of the matching range for built-in autotuners, for instance. You can change the impedance seen at the tuner back to 100 ohms by adding or subtracting 1/4WL of 450 ohm line which will make it a multiple of 1/2WL. As someone else said, one of the most common ways to match a 100 ohm antenna is to use 1/4WL of 75 ohm coax and then 50 ohm coax the rest of the way. Instead of "messing up the match", it fixes up the match. The SWR on 75 ohm coax would be 100/75 = 1.33:1. The impedance at the end of the 1/4WL matching section would be 75/1.33 = ~56 ohms, a good match to 50 ohm coax. If you are wanting to use the antenna for multi-band operation, change the length of the 450 ohm line until the match is not "messed up". |
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