[BC] Wireless Internet Installation on AM Tower

[RichardBJohnson at comcast.net]

Probing for the correct place to short the conduit to the tower can be done at low power (or any power you are not afraid of) by shorting the conduit to the tower with a screw-driver. You probe for the smallest spark. At the correct place, there won't be enough current through the short to light a number 47 pilot light (if you could find one anymore).

Ultimately, this means that the voltage induced in the conduit is identical to the voltage at that point on the tower so no current will flow. This allows one to connect these equal-potential elements together, decoupling any cat-5 cable being fed through the conduit. However, in the process of inducing this voltage, a mutual impedance exists, which will be non-zero. Therefore, the drive-point impedance of the tower could change a small amount. If you were to find the shorting-point at which there was no change in drive-point impedance (a possibility with some tower heights), there would be a non-zero current flowing through the conduit, which is not the optimum condition because that current will create resistive losses in the steel.

Hanging an antenna on the tower, especially near the top where the RF voltage is highest, therefore the effect of this additional capacity greater, will certainly change the drive-point impedance anyway, so I would probe for the lowest parasitic current, not the lowest delta. 

In my younger days, I used to do all kinds of stuff like this. That was part of the "old-time" radio engineering as we used to know it.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: towers at mre.com

I didn't state conduit would be needed all the way up...which I should
have.  The conduit then acts as the farraday shield for the cable.

The caviat is the CAT5 cable.  Recall, you need to decouple the cable's
shield just like any other cable on an AM tower and the tie point for the
short. The problem really is the cable's quality as well as the
installation practices by the installer.  Unlike Heliax, et. al., the
shield on the CAT5 cable is rather small and delicate.  So conduit is the
preferred choice of carriage up the tower through the isocoil and that's
easily coupled to reduce circulating currents.

Otherwise, everyone else has echoed my comments about reproofing.

MM