[CQ-Contest] Two Towers, Two Tribanders - Coax Length for Stack Match?

[Bill Hider]

Rich,

Let me respond to your questions and I'll also comment on several of the
comments you have received from others, so far.

First of all, let me state my understanding of your 2nd tower:
3 tribanders (C31XR's, per your QRZ.com page, right?)
StackMatch box located on that new 55G rotating tower (again, per your
QRZ.com page)
Equal length coax from that StackMatch to each of the 3 tribanders
One feedline back to the shack from that StackMatch (not clear which length,
140' or 278' - makes no difference though... read on.)

Your other tower is quite nice, too, as pictured on your QRZ.com page.

Also, you say you are thinking about adding another StackMatch in your shack
to select:  Tower 1 - Tower2  or  Both Towers.

My comments, in no particular order:

1. Equal length feedlines from each tower to the shack:
No need to do that now.  If you ever want to do that for whatever reason,
you can simply add (insert) the additional coax at/in the shack at the coax
connector going to the tower with the 
shorter length coax going to it.  Use a TDR to find the length you need to
add so that the coax lengths are equal.
I have separate feedline runs from each of my 12 antennas and I used the TDR
with my stacked antennas.  Just coil up the additional coax locally in the
shack or elsewhere that's convenient.
I suggest you model (EZNEC+ v5, or other software) what you are thinking of
so you may make an engineering decision based on that model
before you actually add the coax.

2.  I agree with KK9A, subject to more modelling. I have two towers,
different heights, 125' apart, almost exactly North - South from each other,
each with one 10M beam & one 15M beam (and other antennas not pertinent to
this
discussion).  One 10M beam is at 141', the other is at 52' on the other
tower.  One 15M beam is at 109' and the other is at 67' on the other tower.

I have separate coax feedlines cables coming back to the shack from each of
my 12 antennas (total) on both towers.
The coax feedline lengths for each of the two 10M antennas are different.  
The coax feedline lengths for each of the two 15M antennas are different.
I have one, two-antenna StackMatch (Upper, Lower, Both) in my shack where
the two 10M antenna feedlines connect.
I have another two-antenna StackMatch (upper, lower, both) in my shack where
the two 15M antenna feedlines connect
I have modeled my 10M and my 15M configuration using EZNEC+.  

Now, the pertinent info:
The model shows that on 10M OR 15M:
A pattern NULL is "obtained" if I operate either the 10M or 15M  StackMatch
in the BOTH position IF the two same-band antennas are pointing within
plus-or-minus 20 Degrees of North or South.
That is, with both antennas pointed at the SAME heading (within
plus-or-minus 20 Degrees) AND that heading causes  one antenna to be
pointing at the other antenna within plus or minus 20 Degrees, then the
transmitted and received pattern is a NULL.
At that heading I can operate either beam separately by selecting the
StackMatch in the Upper or Lower position.  I just cannot transmit or
receive at those headings in the BOTH position of the StackMatch (!)
You need to model your situation based on the azimuth orientation of your
two towers and separation distance, antenna heights & feedline lengths to
determine what you have.
BTW, there may be another feedline length for one antenna that would
alleviate a bad situation, per the model.  If so, just add the needed coax
length to the needed antenna at the shack coax connector as I described
above.

3.  To K4VUD's comments re: ERP.
My comments pertain to contesting, not necessarily to DXing or casual
operating: 
When you have contesting-type antennas that have high gain, say  > 8dB to >
12 dB gain, when you split your 1.5KW power to, say four antennas, yes you
provide 375W to each antenna, but if each antenna has 9dB gain, your ERP
will make up for the power splitting (all numbers less transmission line
losses). Yes, that's less power in any single direction than you could get
from one 9dB gain antenna driven by 1.5KW, but that's the trade-off to spray
in many directions with a competitive contest-type ERP. 
That's why stations with high gain antennas can 'afford' to split their
power to achieve different azimuth coverage.  

4.  Related to #3 - I totally agree with Dave, K8CC, as his comments mirror
mine in #3.

73,

Bill N3RR

-----Original Message-----
From: CQ-Contest [mailto:cq-contest-bounces at contesting.com] On Behalf Of
Richard Thorne
Sent: Sunday, July 12, 2015 8:14 AM
To: CQ-Contest at contesting.com
Subject: [CQ-Contest] Two Towers, Two Tribanders - Coax Length for Stack
Match?

I'm in the process of putting up a second tower which will have stacked
tribanders.  These 3 tribanders will be fed via a Stack Match with equal
lengths of coax.

My current tower also has a tribander.

I plan on installing a stackmatch II switch between the two towers for quick
direction changing or spraying my signal in 2 directions.

Is it worth the trouble to use equal length feed lines?  If I use what I
currently have one feed line will be 278' and the other will be 140'.

Thanks

Rich - N5ZC



_______________________________________________
CQ-Contest mailing list
CQ-Contest at contesting.com
http://lists.contesting.com/mailman/listinfo/cq-contest


-----
No virus found in this message.
Checked by AVG - www.avg.com
Version: 2015.0.6081 / Virus Database: 4392/10215 - Release Date: 07/12/15