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Old 30-04-2014, 22:12   #1
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Antenna Cable Impedance-What's a Good Number?

I have 85 feet of low loss VHF antenna cable. What would be a good number, center conductor end to end, and the wire shield (outer conductor) end to end, in ohms? Just curious if anyone else measures these things.
TIA
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Old 30-04-2014, 22:17   #2
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Re: antenna cable impedance-what's a good number?

A couple of ohms max. for centre and outer.
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Old 30-04-2014, 23:54   #3
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Re: antenna cable impedance-what's a good number?

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Originally Posted by redsky49 View Post
I have 85 feet of low loss VHF antenna cable. What would be a good number, center conductor end to end, and the wire shield (outer conductor) end to end, in ohms? Just curious if anyone else measures these things.
TIA
When you measure the DC resistance of the conductors in a coaxial cable, you will not be measuring its characteristic impedance.

A good number would be about 0.1-Ohm.

The DC resistance of coaxial transmission line is typically not measured, unless you are trying to determine if there is a break in continuity of the conductors.

As an RF transmission line, you really don't care about the DC resistance.
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Old 01-05-2014, 05:49   #4
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Re: antenna cable impedance-what's a good number?

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Originally Posted by redsky49 View Post
I have 85 feet of low loss VHF antenna cable. What would be a good number, center conductor end to end, and the wire shield (outer conductor) end to end, in ohms? Just curious if anyone else measures these things.
TIA
To expand on what others have said, the DC resistance will give an indication of basic condition - like whether the cable has open conductors or a short - but after that, what counts is the impedance and loss at the marine VHF frequencies, and measuring that requires a source of signal (your radio or a test generator) and the appropriate RF wattmeter ,and a VHF VSWR meter to assess match and reflections, which can be indicators of cable, connector or antenna problems.

But seriously, if you have a known-good VHF radio and antenna and you're using new, quality low-loss cable intended for 50 ohm VHF, and all your connections are properly made, and everything's connected securely... It's gonna work. Problems are much, much more likely to be elsewhere than the cable itself.
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Old 01-05-2014, 06:21   #5
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Re: antenna cable impedance-what's a good number?

Quote:
impedance |imˈpēdns| noun

the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.
it can't be measured using DC resistance meter.
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Old 01-05-2014, 07:36   #6
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Re: antenna cable impedance-what's a good number?

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Originally Posted by DotDun View Post
it can't be measured using DC resistance meter.
Well, that's news to me. I thought that resistance was indeed measurable, and important, even in DC systems. I guess I am a victim of a little knowledge being a dangerous thing.

Thanks to everyone for their responses. Learn a little more every day.
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Old 01-05-2014, 08:10   #7
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Re: antenna cable impedance-what's a good number?

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Originally Posted by redsky49 View Post
Well, that's news to me. I thought that resistance was indeed measurable, and important, even in DC systems. I guess I am a victim of a little knowledge being a dangerous thing.

Thanks to everyone for their responses. Learn a little more every day.
Resistance is indeed important and measurable, but in a DC circuit. A quick DC resistance measurement of the coax will tell you the connection is end to end, and assuming a very low resistance (<1 ohm), this is good, but it won't tell you of any defects that will affect impedance. It's safe to assume that an undamaged coax will meet manufacturers impedance specs, it's the poorly installed connectors that cause the majority of problems.

The coax between your radio and antenna carries AC, and the resistance to AC current flow is frequency dependent (impedance), hence it can't be measured with a DC resistance meter. The best way to measure the effects of impedance in your coax/connectors/etc is with a SWR meter using the radio as the source.
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Old 01-05-2014, 11:11   #8
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Re: antenna cable impedance-what's a good number?

If you really want to go at it beg, borrow or steal a TDR and measure your cable. Print out a trace and keep it around for later reference.

A TDR (Time Domain Reflectometer) is more or less a radar for a cable. It sends a pulse out on the cable and records the echos that come back from every change in impedence along the cable.

The pulse will be reflected back at every bend, squish, connector and from the antenna itself. You will be able to see where your cable losses are.

And sometime in the future when something breaks or corrodes you will be able to compare the old and new traces and point to the fault.

A bit overkill for the boat but great if you do cable runs at work.

Regards

Look up TDR on wikipedia for an overview
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Old 01-05-2014, 15:00   #9
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Re: antenna cable impedance-what's a good number?

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Originally Posted by DotDun View Post
Resistance is indeed important and measurable, but in a DC circuit. A quick DC resistance measurement of the coax will tell you the connection is end to end, and assuming a very low resistance (<1 ohm), this is good, but it won't tell you of any defects that will affect impedance. It's safe to assume that an undamaged coax will meet manufacturers impedance specs, it's the poorly installed connectors that cause the majority of problems.

The coax between your radio and antenna carries AC, and the resistance to AC current flow is frequency dependent (impedance), hence it can't be measured with a DC resistance meter. The best way to measure the effects of impedance in your coax/connectors/etc is with a SWR meter using the radio as the source.

This is interesting. I didn't know that the VHF signal was an alternating current. I had always assumed (rather, I hadn't really given the matter much thought), that the broadcast signal was based on the same power as was supplied to the radio - direct current. I follow about the impedance being dependent on frequency though.

For the record, I measured about 0.25 ohms through the center pins, about 0.4 ohms through the outer conductor/shield. I installed the connectors myself - PL-259s - and was mostly concerned about any shorts. None detected.

I guess that the best test is (other than with a TDR), will I be able to broadcast to the moon, or maybe the TOWBOATUS station about 35 miles distant from me.

Thanks again!
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Old 01-05-2014, 15:18   #10
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Re: antenna cable impedance-what's a good number?

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Originally Posted by redsky49 View Post

...I guess that the best test is (other than with a TDR), will I be able to broadcast to the moon, or maybe the TOWBOATUS station about 35 miles distant from me.
There is a reasonable presumption that signals from your VHF Marine Band radio will not reach the moon at a level that could be detected by an ordinary receiver. The path to the moon, although a line-of-sight path, is a long distance. The path loss will be rather high, about -188-dB.

An ordinary receiver will have a usable sensitivity of about -110-dBm. Since your transmitted signal power is 20-watts or about +43-dBm, we need a net gain from antennas (including transmission line loss) of about 35-dB. You're going to need a bigger antenna. That's if you want to use voice telephony. You can improve your minimum receive signal if you try other modes.
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Old 01-05-2014, 15:28   #11
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Re: antenna cable impedance-what's a good number?

To get back to your initial inquiry about the characteristic impedance of a coaxial transmission line to your antenna, the best impedance is hard to say with certainty, as it depends on the antenna impedance and the transmitter output impedance. Typically in radio an antenna is expected to have or to be transformed to an impedance of about 50-Ohms, and transmission line cable with a matching impedance, about 50-Ohms, will be used. Most modern transmitters that do not have operator adjustment of the output matching are set to transmit into a 50-Ohm impedance as their nominal load.

The construction of coaxial transmission lines is a compromise among various parameters. If you are not planning on transmitting power through the line, just a low level signal, an impedance of 75-Ohms is used as it gives the least loss. If you are concerned about the capacitance per foot, and how it might roll off the frequency response, an impedance of 93-Ohms is used.

If you want to transmit a lot of power and keep the peak voltages low, an impedance around 25-Ohms is used.

The 50-Ohm or 51.5-Ohm or 52-Ohm coaxial transmission lines are mostly used for radio transmitters. They are a good compromise of power handling and loss parameters.

UPDATE: Here is a good explanation of coaxial transmission line impedance:

http://www.belden.com/blog/broadcast...-Impedance.cfm
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Old 01-05-2014, 16:02   #12
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Re: Antenna Cable Impedance-What's a Good Number?

One more comment:

A transmission line does not really have its own impedance. A transmission line has some load on its terminal end, and the impedance of that load is reflected back to the input end. When you measure the input impedance of a coaxial transmission line, you see the collective effects of:

--the impedance of the load attached

--the length of the transmission line

--loss in the transmission line

But the transmission line does have a characteristic surge impedance. And this also affects the input impedance, so we add that to the list:

--characteristic surge impedance of the transmission line

Someone mentioned earlier about using VSWR to measure impedance. Not really possible. VSWR is related to the mismatch between the terminating impedance and the characteristic impedance. If you have a mismatch, a VSWR measurement gives only the magnitude of the mismatch and not any sort of vector or direction, so you can't know the impedance.
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Old 01-05-2014, 16:19   #13
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Re: Antenna Cable Impedance-What's a Good Number?

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Originally Posted by continuouswave View Post
Someone mentioned earlier about using VSWR to measure impedance. Not really possible. VSWR is related to the mismatch between the terminating impedance and the characteristic impedance. If you have a mismatch, a VSWR measurement gives only the magnitude of the mismatch and not any sort of vector or direction, so you can't know the impedance.
No, that's not what I said! I stated you can measure the effects of impedance using SWR. But you are correct, SWR will show the mismatch. Again, it's safe to assume the antenna and coax meet manufacturer specs (i.e. no impedance mismatch with those components), hence most mismatches are caused by improperly installed connectors or physical damage to the coax/antenna.

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Originally Posted by DotDun View Post
The best way to measure the effects of impedance in your coax/connectors/etc is with a SWR meter using the radio as the source.
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Old 01-05-2014, 16:52   #14
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Re: Antenna Cable Impedance-What's a Good Number?

Hey fellas, slip over to Long wire HF antenna length - Cruisers & Sailing Forums and help out 40 South next
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Old 01-05-2014, 16:54   #15
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Re: Antenna Cable Impedance-What's a Good Number?

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Originally Posted by redsky49 View Post
I have 85 feet of low loss VHF antenna cable. What would be a good number, center conductor end to end, and the wire shield (outer conductor) end to end, in ohms? Just curious if anyone else measures these things.
TIA
Are you sorry you asked
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