Because usually when we're analyzing a coax cable the current we care
about is flowing within the cable, on the outer surface of the inner
conductor and the inner surface of the outer conductor. The physical
dimensions and materials of the cable components and the dielectric
constant (relative permittivity) of the material between the conductors
defines the velocity factor.
This case is different since we're analyzing currents flowing on the
outside of the shield.
--
Patrick Shoemaker
Vector Data Systems LLC
shoemak...@vectordatasystems.com
office: (301) 358-1690 x36
http://www.vectordatasystems.com
On 4/13/2011 23:00, Greg Ihnen wrote:
If the dielectric of coax cables causes them to have a velocity factor
of .6, why would the jacket insulation be different?
Greg
On Apr 13, 2011, at 9:28 PM, Patrick Shoemaker wrote:
Except in this case, assuming the shield is intact and good quality,
we are dealing with undesired currents flowing on the surface of the
cable shield only. At 100 MHz and assuming an aluminum foil shield in
the cat5, the shield metal is thicker than a few skin depths (about
8 um skin depth). So we essentially have a solid conductor in open
air, and if the insulation’s relative permittivity is close enough to
1 to neglect (should be), the velocity factor is close to 1.
--
Patrick Shoemaker
*From:*wireless-boun...@wispa.org
<mailto:wireless-boun...@wispa.org>[mailto:wireless-boun...@wispa.org]*On
Behalf Of*Greg Ihnen
*Sent:*Wednesday, April 13, 2011 21:32
*To:*WISPA General List
*Subject:*Re: [WISPA] Ethernet Cable for FM Tower Install
In space (vacuum). Cables have a velocity factor.
On Apr 13, 2011, at 8:42 PM, Patrick Shoemaker wrote:
And speed of light (c) = 300,000,000 m/s
--
Patrick Shoemaker
*From:*wireless-boun...@wispa.org
<mailto:wireless-boun...@wispa.org>[mailto:wireless-boun...@wispa.org]*On
Behalf Of*Patrick Shoemaker
*Sent:*Wednesday, April 13, 2011 21:10
*To:*sc...@brevardwireless.com <mailto:sc...@brevardwireless.com>;
WISPA General List
*Subject:*Re: [WISPA] Ethernet Cable for FM Tower Install
Wavelength (m) = speed of light (m/s) / frequency (Hz)
Avoid ¼ wavelength multiples.
--
Patrick Shoemaker
*From:*wireless-boun...@wispa.org
<mailto:wireless-boun...@wispa.org>[mailto:wireless-boun...@wispa.org]*On
Behalf Of*Scott Carullo
*Sent:*Wednesday, April 13, 2011 14:54
*To:*can...@believewireless.net <mailto:can...@believewireless.net>;
WISPA General List
*Subject:*Re: [WISPA] Ethernet Cable for FM Tower Install
Get with someone who can tell you exactly how long to make the
cables. You don't want them any whole fraction of the FM wavelength
freq or it will compound your problem. Make them as de-tuned length
as possible.
I've had a coax in my hand not hooked to anything in the vacinity of
a high-power FM station and it was a resonant length and it got so
hot I had to drop it.
Scott Carullo
Technical Operations
855-FLSPEED x102
------------------------------------------------------------------------
*From*: "can...@believewireless.net
<mailto:can...@believewireless.net>" <p...@believewireless.net
<mailto:p...@believewireless.net>>
*Sent*: Wednesday, April 13, 2011 9:39 AM
*To*:wireless@wispa.org <mailto:wireless@wispa.org>
*Subject*: [WISPA] Ethernet Cable for FM Tower Install
We are mounting close to a 50kW FM antenna and want to use heavy,
double shielded cable
for the runs to the APs since we've seen issues in the past. Fiber up
the tower but will need
3-4 ft jumpers to the APs.
Any recommendations?
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