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Author Topic: Using RG-58 as a DC Line  (Read 701 times)
W6AKF
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« on: March 10, 2009, 05:06:23 PM »

I have an Icom IC-8000 that I'm going to install in to a 2002 Honda. I have found a path for the DC power line to go through on in to the interior. A ham friend said he used RG-58 for the DC line and he said it is working fine for his application for many years and never had a problem, even on high power. He running a rig of about the power out as the 8000.

Has anyone ever done this? Have you ever heard of RG-58 being used as a 12V DC line?

My plan is to go directly to the battery with the power leads and using 30amp Powerpoles and supplied fuses. I'm sure the Powerpoles are rated for the high current that the 8000 has at around 15amps. Not sure about the RG-58.

Any thoughts?

Contact me at the address below.

Stephen, W6AKF
stephenmcd1@comcast.net


 
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KE4DRN
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« Reply #1 on: March 10, 2009, 05:25:07 PM »

Hi Stephen,

Perhaps your friend used RG-8 ?

RG-8  has a #10 AWG STRANDED center conductor

RG-58 has a #20 AWG SOLID center conductor

How long a run are you going to have ?

Easier to run some # 6 THHN stranded wire,
smaller voltage drop with this larger cable.

you can buy at home depot, lowes, menards, etc.

73 james
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WX7G
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« Reply #2 on: March 10, 2009, 05:55:02 PM »

The DC resistance of RG-58 is 0.015 ohms per foot. The limit is a bit less than 10 amps before the inner insulation melts.

RG-8 is 0.0015 ohms per foot. A 10' run to a rig drawing 20 amps and the voltage drop is 0.03 volts. Not bad at all. And 20 amps will not overheat RG-8.
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K0BG
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« Reply #3 on: March 10, 2009, 06:22:58 PM »

Let's don't give poor advice here. Any coax shouldn't be used as feed line, no matter what the size.

The premise behind using coax is two fold. First, to eliminate alternator whine. It won't. Alternator whine is caused by a ground loop 99% of the time. And that's usually caused by a mag mounted antenna.

Secondly, it is suppose to stop common mode current flowing on the power cable. If folks really understood what common mode currents were, they'd know coax isn't the answer.

Most modern radios come equipped with what appears to be #10. It's actually slightly larger than AWG #10. For runs longer than 10 feet or so, you should use 8 to minimize voltage drop. In any case, limiting voltage drop to less than .5 volts is a worthy objective, especially for SSB radios where you have to worry about low voltage induced IMD.

If you want more information, visit my web site and look under Wiring.

Alan, KØBG
www.k0bg.com
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W5ROY
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« Reply #4 on: March 14, 2009, 03:13:15 PM »

Alan (K0BG) It is a shame that there is so much bad info on the sites. You have a great deal of useful info on your site. Too bad more people do not make use of it.  73  de W5ROY  Clovis,NM.
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K5LXP
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« Reply #5 on: March 15, 2009, 08:18:00 AM »

A long time ago I needed some DC in the trunk and used an already present run of RG-58 by twisting the center conductor and shield together and using it as a single conductor.  Not sure what the equivalent AWG of that would be, but I would guess at least 14, maybe 12.

Mark K5LXP
Albuquerque, NM
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K0BG
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« Reply #6 on: March 17, 2009, 06:17:00 AM »

Dave, WX7G: Run that voltage drop equation by me one more time.

Alan, KØBG
www.k0bg.com
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AA4PB
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« Reply #7 on: March 17, 2009, 06:51:38 AM »

RG-58 by twisting the center conductor and shield together
-----------------------------------------------------------
Two things would concern me here. One is the outer insulation and the possibility of scraping and a short to the chassis. The other is that the the chassis is being used as the only return (I assume).

Better to use coax for what it was intended - RF to the antenna. Use the proper automotive grade heavy guage wire for the power and return.
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W0FM
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« Reply #8 on: March 18, 2009, 11:23:26 AM »

When I read my friend Alan's post above it made me scratch my head.  I have the utmost respect for KØBG's expertise which is why I sit here scratching me noggin.

I agree that using coax to run DC to a mobile transceiver is not something I'd do.

However, I have a commercial 6 meter receive preamp (SBE maybe?) that not only recommends using RG58 to run the required 200ma 12VDC up to the antenna, the power connection provided on the preamp chassis is an SO-239!  This is not a 'biased on the coax' DC run, but a separate run in addition to the transmission line.

The instructions indicate that a PL259 should be connected to the preamp end of the power coax with the center pin positive DC voltage.

Of course, mine is not a 10A application.  But never say "never".  I guess there is an exception to everything.

73 de Terry, WØFM
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AA4PB
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« Reply #9 on: March 18, 2009, 01:17:43 PM »

Alan's last post is because WX7G's voltage drop calculation example is wrong. 0.0015 ohm/ft x 10 feet = 0.015 ohm. 0.015 ohm x 20A = 0.3V drop (not 0.03 as stated).

I don't know why they would want coax for the power feed on the preamp. Maybe they are concerned with RF or noise pickup on the power line getting into the preamp, although that should be easy enough to filter on a DC line. I don't like the idea of +12V on the pin sticking out of a PL259. Anyway, running 200mA via coax is a lot different than 10-20A.

Alan's advise against using coax for mobile radio power feed is good advise. There is mis-information going around that coax somehow "shields" the power line from noise pickup. It's in the same category as the mis-information that you can "shield" your ground connection to get rid of noise.
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K0BG
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« Reply #10 on: March 18, 2009, 02:46:46 PM »

Actually, it's .6 volts, because you have to consider both conductors.

Alan, KOBG
www.k0bg.com
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W0FM
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« Reply #11 on: March 19, 2009, 10:33:25 AM »

AA4PB:  "I don't know why they would want coax for the power feed on the preamp. Maybe they are concerned with RF or noise pickup on the power line getting into the preamp, although that should be easy enough to filter on a DC line."

They don't give a reason, Bob, they just designed it that way.  SSB Electronics model SP-6.

http://gatorradio.org/Manuals/SSB_SP-7000_preamp_manual.pdf

As I stated, I agreed that this was a totally different application of DC on coax than that being discussed earlier in the thread. I just tossed it out there as an example of DC on coax by design.  For whatever reason.

73 de Terry, WØFM
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AA4PB
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« Reply #12 on: March 19, 2009, 12:52:18 PM »

Actually it probably wouldn't be 0.6V because both conductors don't have the same resistance with coax. I went on the assumption that his resistance per foot accounted for both conductors.

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WX7G
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« Reply #13 on: March 30, 2009, 07:38:12 PM »

K0BG. One more time...the voltage drop is 0.3 volts for 20 amps thru 10' of RG-8 type coax.

Coax is used in industry for DC power from low voltages to high voltages. I have used multiple paralleled coaxial cables for very low inductance power routing in IC test.

RG-213 is used to 70 kV and RG-59 to 20 kV. I have seen RG-213 used for 20 amps DC (with PL-259 connectors) in DC plasma chamber feeds. There is no fundamental reason not to use coaxial cable for DC power.
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WX7G
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« Reply #14 on: March 30, 2009, 07:40:08 PM »

K0BG, we generally ignore the resistance of the shield as it is about 1/5 that of the inner conductor. So, 0.6 volts is incorrect, it is 0.3 volts at room temp.
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