[TowerTalk] Coax Lengths for Stacked Yagis

john at kk9a.com john at kk9a.com
Thu May 3 20:04:08 EDT 2018

I missed seeing your Bonaire tower construction project in March but I heard
that you made positive construction progress with one tower. It's definitely
not easy building a station away from home.

It is a wise choice to put the antenna relays at the base of the tower. It
is really convenient when trouble shooting and relays are probably the most
problematic electrical part of the antenna system. 

My phasing relays are also at the tower bases. I run equal lengths of Heliax
up the tower and coil the excess.  Shortening one coax 1/2 wl will give you
a very high radiation angle.  Shortening a coax by one wavelength seems like
it will give you the correct radiation pattern but is the one antenna a
cycle off? Equal length feedlines is so easy.

John KK9A  (PJ4R in 2018 WPX Phone)

Richard Smith n6kt wrote:

I am part of a team that is building a station that will incorporate stacked
yagis on 20 thru 10 meters. We plan to put all of the RF switching at the
base of the towers, for ease of troubleshooting and maintenance. My question
deals with the lengths of the coax runs between the yagis and the switching
box at the base of each tower. 

Example - 15 meter stack:

Upper Yagi at 150'Middle Yagi at 120'Lower Yagi at 90'
One option is to run equal lengths of coax from each yagi down to the switch
box, and coil up the excess coax from the middle and lower yagis. But does
it make more sense to use shorter runs of coax from the lower antennas to
the switchbox? Specifically, if the coax runs from the lower yagis are
shorter by multiples of one-half wavelength compared to the upper yagi
(using the velocity factor of the coax in the calculation), would all of the
yagis still be in phase?
Thanks in advance,
73, Rich, N6KT

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