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Message |
    
Alleycat
Junior Member
Username: Alleycat
Post Number: 31
Registered: 3-2005
| | Posted on Thursday, July 07, 2005 - 2:34 pm: | 
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What is needed to measure the impedance of the antenna coax where it hooks up to the radio ? Can I use a simple volt/ohm meter or do I have to use a fancy (expensive) meter of some type ? What can happen if the impedance is 150 ohms insted of 50 or so ohms and I go ahead and hook up and start using the radio ?
Thanks, Alleycat, KI4KHT |
Hollowpoint445
Advanced Member
Username: Hollowpoint445
Post Number: 511
Registered: 6-2004
| | Posted on Thursday, July 07, 2005 - 7:13 pm: |
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It's a complex measurement and you need a device that is capable of reading it. I use an MFJ259.
Coax shouldn't change impedance unless it's damaged or has degraded over a long period of time. Usually if there is an impedance problem it's an antenna problem and it's matching network needs to be adjusted or the antenna itself needs to be trimmed for a better match.
The VSWR will be high and the radio might not work too well. As long as the radio hasn't been peaked out it shouldn't hurt the radio, but if it has been peaked I wouldn't use it with a poor VSWR. |
Twowatt
New member
Username: Twowatt
Post Number: 8
Registered: 7-2005
| | Posted on Thursday, July 07, 2005 - 8:31 pm: |
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hey Alleycat, autek research out of florida makes a nice little impedance/signal gen also.
in response to your last question, your swr would be 3:1. |
Alleycat
Junior Member
Username: Alleycat
Post Number: 32
Registered: 3-2005
| | Posted on Saturday, July 09, 2005 - 2:00 am: |
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Somewhere in the back of my mind I knew that the SWR would be 3:1. All I needed was a slap on the back of my head. Thanks.
I had intended to build a Yagi antenna and would need to know the impedance for a balun. That and Gamma Match and some of the other things that need to be adjusted are just over my head at this time. I have decided to build a Moxon antenna insted because it should not need anything more than a choke balun.
Thanks, Alleycat, KI4KHT |
Twowatt
Junior Member
Username: Twowatt
Post Number: 12
Registered: 7-2005
| | Posted on Saturday, July 09, 2005 - 1:49 pm: |
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you're welcome. swr is simple and complicated, but just the simple for now - if the impedance is 100 ohms the swr is 2:1 (50 ohms being our desired impedance to match the radio, which was randomly determined around the Big War; if any thought had been put into it they would have chosen 70 ohms to match a halfwave dipole; wishes oh wishes: if wishes were horses, beggars would ride, if wishes were fishes, we would all have some fried).[soapbox time over]. similarly if the impedance (z) is 25 ohms the swr is 2:1. if the z is 200 ohms the swr is 4:1. if the z is 16.67 ohms the swr is 3:1. if the z is 60 ohms the swr is 1.2:1. see i told u it was simple.
put a gamma match on your yagi about 4" from your driven element for low swr. they are very easy to add to a yagi, (another benefit is the gamma match matches your unbalanced coax to the balanced yagi without the need for an additional balun) then all you may need is a rf choke about 9' down the coax from the yagi.
regards |
