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Author Topic: newbie question: feedline/antenna calculation  (Read 708 times)
VA7MDI
Member

Posts: 76




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« on: January 03, 2002, 01:06:01 PM »

Hi All,

Can someone help clarify the actual power/voltage/current on my feedline to my antenna?

I have a Kenwood TS-2000 HF/VHF/UHF radio.  Its 100W on HF/VHF and 50W on UHF.  I also have an MFJ-934 tuner.  I am using a Comet UHV-6 mobile vertical antenna on the balcony of my condo.  It supports most HF bands as well as 6m, 2m and 70cm.  I am using a 60' RG-8 run from my transceiver to my antenna.

I would like to know what kind of voltage and current is actually on the feed line under a max pwr transmission.  I presume, when I am operating on a tuned band, that antenna looks like a 50Ohm load to the radio, therefore, can I simply use the following:

P=(I^2)R
100=(I^2)50
2=I^2
I=1.414Amps

E=IR
E=(1.414)(50)
E=70Volts

Is that correct?  I am just looking for approximates.

Also, what kind of voltage/current would I see on my counterpoise line which is a single 1/4wavelength insulated radial running along the baseboards of my den?

I am using convential speaker wire but there is no max voltage rating so I am not sure if I am going to exceed the "unstated" voltage rating on the counterpoise and burn it up.

Thanks in advance!

73 de VA7MDI
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WB2WIK
Member

Posts: 20565




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« Reply #1 on: January 03, 2002, 02:24:54 PM »

Your assumptions are correct.

The current maxima, which occurs at the antenna connection itself, in your "radial" should be the same as in the antenna radiator, but the voltage at the radial tip is a maxima based on termination to "nothing" (air).  Most estimate this to be ~2000 Ohms in the real world.  At 100W, that would be ~2.8kV potential -- only occurring at the tip of the wire, which is the voltage maxima.

If you want to be safe, insulate the tip of this wire well.  For similar installations, I've used insulated wire, wrapped back on itself to form a small insulated wire loop at the end (tip), tied in place to hold that form, and then taped over with several layers of PVC electrical tape, some of which is rated 600V per layer -- so it only takes 5 layers to build up a 3000V dielectric withstanding voltage rating, not very much.

Of course, if nothing conductive nor combustible is anywhere near the tip of the radial, and nobody can come in contact with it, then it really doesn't matter.

73 de Steve, WB2WIK/6
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WB6BYU
Member

Posts: 13139




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« Reply #2 on: January 10, 2002, 12:15:35 PM »

Yes, your calculations are correct for the RMS values.  The peak
values will be somewhat higher, but still well below what you will
get from the local 120V electric outlet.

Steve's calculations are correct if you are putting all 100 watts into
your radial wire.  However, if it is just to keep the rig from being
hot with RF instead of being part of the actual antenna system,
then there will be much less power flowing into it, and the voltage
at the end of the wire will be much less.  Most normal AC wiring
has 600 volt insulation, and this is more than enough.  However, if
you are concerned, you can enclose the last few feet in heatshrink
tubing, wrap it in tape, or use high-voltage test prod wire.

Generally, though, if anything is going to arc over, it won't be
through the insulation, but instead will be the end of the wire
sticking out of the insulation.  Taping the end, or using adding a
wire nut, should be all the protection you need.
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