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dnyberg2
Antenna-Theory.com Newbie
Joined: 07 Mar 2012
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Location: United States
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 Posted: Thu Mar 08, 2012 5:06 pm Post subject: Open Transmission Line... |
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| I had an occasion to hook up a piece of open ended coax to a network analyzer. I know the wire has some pF per foot but I expected it to be linear over inches but it wasn't. In other words instead of 5 inches = 5X the pF per foot, the capacitance rises sharply near the end of the coax. Any idea why? |
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bigSteve
Antenna Wizard
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| Posted: Thu Mar 08, 2012 6:24 pm Post subject: Transmission Lines, Antennas and Capacitance |
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How did you determine the capacitance rises sharply near the end of the coax?
My guess: Remember an open circuited transmission line (at the end) is two separated conductors. This is a capacitor. It is also why the impedance of a very small dipole is strongly capacitive (a short dipole antenna is two separated pieces of metal - a capacitor). |
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dnyberg2
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| Posted: Thu Mar 08, 2012 8:33 pm Post subject: Re: Transmission Lines, Antennas and Capacitance |
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| bigSteve wrote: | How did you determine the capacitance rises sharply near the end of the coax?
My guess: Remember an open circuited transmission line (at the end) is two separated conductors. This is a capacitor. It is also why the impedance of a very small dipole is strongly capacitive (a short dipole antenna is two separated pieces of metal - a capacitor). |
The coax under inspection came at a length of 42". I set the VNA to 49 MHz, Freq of OP, and made measurements as I cut 1" pieces off the end. The coax does indeed flip from capacitance to inductive at ~36" BUT, 36" is NOT a λ/4 of 49 MHz so what else am I missing? Thanks! |
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bigSteve
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| Posted: Fri Mar 09, 2012 1:02 am Post subject: |
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| Are you sure the phase is set correctly on your VNA? That is, when nothing is connected, do you see an open circuit (infinite impedance, zero capacitance or very high inductance)? |
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dnyberg2
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| Posted: Fri Mar 09, 2012 2:12 pm Post subject: |
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| bigSteve wrote: | | Are you sure the phase is set correctly on your VNA? That is, when nothing is connected, do you see an open circuit (infinite impedance, zero capacitance or very high inductance)? |
I calibrated the VNA with a 3.5 mm (SMA) STD before the measurement, using the smith chart display, and adjusted the electrical length so that the marker was teetering just between capacitive and inductive on the open side of the chart, then plugged the coax in. |
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bigSteve
Antenna Wizard
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| Posted: Fri Mar 09, 2012 10:48 pm Post subject: |
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| I think it is ok. If the coax has some dielectric in there (say epsilon [permittivity] is 2 or whatever), the results would line up with what you are seeing. |
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