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Antenna Wire Inside Grounded EMT

Mar 10th, 17:03

AC8UR

Joined: Oct 22nd 2015, 03:23
Total Topics: 0
Total Posts: 0
Two antenna wires are connected to RG8X inside a jbox mounted to the side of an aluminum sided house. The jbox is 6ft above the ground. A vertical 10ft EMT pipe is connected to the jbox. The pipe is grounded because it is adjacent to a power line running vertically to the 2nd floor and put a lot of noise in the antenna. The wires exit the EMT 16ft above ground and are tied off to two trees in the yard unfortunately at only about 75 deg apart, a sort of lateral V. HOA issues so can't be real obtrusive.

The question is how to treat the 10ft sections of wire antenna inside the grounded EMT. The antenna wires are connected together inside the jbox, tie to RG8 at that point, and probably cross more than once inside the EMT. I assume it is to be treated as a type of plain feeder line and not part of the antenna. Better results with direct connect to RG8 rather than via balun. Any advice here?

RLP AC8UR
Mar 10th, 20:30

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The two 10 ft sections of wire form a parallel transmission line that is shielded by the EMT. The EMT should also function as a sleeve balun for the 12 meter band.

Kenneth L. Nist, KQ6QV has free software for calculating arbitrary transmission line impedance, as well as transmission line equations for Mathcad 11.
http://www.hdtvprimer.com/KQ6QV/HomePage.html

atlc - Arbitrary Transmission Line Calculator (for transmission lines and directional couplers) by Dr. David Kirkby (G8WRB), who works at the department of Medical Physics, University College London.
http://atlc.sourceforge.net/

It may be easier to just measure it with a antenna analyzer. A quarter wavelength will act as an impedance transformer. I'd attach 50 and 100 ohm loads to the far end and measure the impedance at the other end.

The characteristic impedance of the line is the square root of Zinput*Zoutput, the impedance at either end of a quarter wavelenth of transmission line. A shorted quarter wavelength of transmission line acts as an open circuit.

Zak W1VT
ARRL Senior Lab Engineer
Apr 11th, 15:17

WA8NVW

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Any chance you could add a plastic outdoor electrical box at the top of the vertical EMT and mount a 1:1 balun inside the box? Then you can run coax all the way up, which will look more 'official' and easier to hide.

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