[EIDXA] Feed line question

[reheinri at rockwellcollins.com]

Jason,

The feedline is one are to explore.  I might consider using a dummy load 
and power meter at the base of the antennas to determine the loss of the 
feedline.

As for antenna performance.  We experience just what you describe at PJ2T 
all the time.  The reason for a stack is to deal with opening and closing 
a band.  In the middle of a band opening a single tribander at 40 feet 
will play as well!  As Rod described it is an issue arrival angles. 
Remember that the ionosphere (the reflecting surface) changes during the 
day and the height of the reflecting layer changes.  This changes the 
arrival angle.  To open a band the top antenna is the best.  During the 
transition periods, the middle antenna is the key contributor.  In the 
middle of the opening the lower and mid antenna are the real contributors.

This is the reason for the stack match with the switch selectable 
antennas.  Also remember that you are power dividing across the three 
antennas so you are already losing 3dB+ per antenna.  While you may not 
see this because you are seeing the composite signal from the three 
antennas in-phase (assuming that they are REALLY in phase) the point is 
that during an early morning opening where the top antenna is the only 
contributor.  But it is likely that you will not hear anything with the 
lower tribander at that point either. 

This is why some of the compromise installations use ring rotors on each 
element of the stack.  Consider this - during a contest - the band opens 
to Europe.  But then there is a long path opening to deep Asia.  One 
strategy says to move the run to Europe to the mid antenna and turn the 
top antenna to the deep opening.  OR consider the late night polar 
openings.  You point the lower antenna south for the South Americans, the 
mid-antenna to the South Pacific, and the top antenna "over the pole" - 
this optimizes the signal into all three areas.  This is something that 
you cannot do with a single tribander and a single height.

At PJ2T we don't see any difference between our stacks an/or high antennas 
and the tribander when the band is open - rates are nearly the same which 
confirms that the signal is real.

I would check for feedline loss first.  Take a measurement of power out of 
the transmitter.  Don't touch anything and then go to the base of the 
tower and measure the power again.  This will give you the loss in the 
feedline.  It gets harder to do the same for the phasing lines to the 
stack match, but all can contribute.  And if you have any splices in the 
hardline, they can be a source of loss over time as well.

So many opportunities - so little time...

Hope this helps.

Rick






Jason Joens <pixelpaint at mchsi.com> 
Sent by: eidxa-bounces at mailman.qth.net
04/02/2013 09:21 PM

To
EIDXA Reflector <eidxa at mailman.qth.net>
cc

Subject
[EIDXA] Feed line question






I have a question that's been bugging me for awhile.  What happens to feed 
line over the years?  Can a feed line go bad over time, increasing losses, 
yet still present a good VSWR at the radio? To the point where reception 
is noticeably weakened? Some of the feed lines on the farm have been in 
place for 20+ years.

I've been comparing a TH7 tribander with the triple stack on 20.  The 
difference is far from impressive, and I'm starting to wonder why there 
isn't more of a difference.  Opinions??

73
Jason
Kd0mnd

Sent from my iPad
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