Hi Don,
Let me take a crack at your questions.
You asked why use equal length coax vs odd multiples of 1/4 wave lengths of
line. The
biggest reason is that you can measure equal lengths (and I should add we are
talking about
equal electrical lengths in this discussion, not physical lengths) but you
cannot conveniently measure
1/4 wave lengths on a line that's, say, 50 to 200 feet long running to the
antenna. There are other
reasons, see below.
The reason is that calculations using the velocity factor, frequency
(wavelength), and a mathematical constant
(in other words, a formula) does not work. It gets you close, but not close
enough for stacking antennas.
You actually have to measure the line using frequency measurement techniques
and cut and measure again, especially
if you're after several of the same/different 1/4 multiple lengths. Cutting
one line equal to one 1/4 wavelength
on 20 meters by frequency measurement techniques is ok, but measuring a 180
foot piece and cutting it at exactly
11*1/4 wavelengths is fraught with error, due mostly to measuremant accuracy,
unless you measure it with
frequency measurement techniques, not cable distance measurements. BTW, how
do you account for the
electrical length variation due to series lightning protectors using the 1/4
wave requirement?
On the other hand, cutting any length cables to exact (negligible error) equal
electrical lengths by using a TDR is EASY/SIMPLE.
I can describe the measurement techniques for each if anyone is interested.
BTW, I use equal electrical length cables (made up of varing lengths of Andrew
LDF-4, RG-213, and RG-8 and with PolyPhaser
lightning protectors all on the same run) running back to the shack from each
antenna in the stack. Then I use the TDR to prune the
cable in the shack until the TDR oscilloscope patterns from each antenna are
identical when superimposed.
I then have equal phase delay to each antenna and can feed them through a BUL
switch (which uses 1/4 wave 50 and 70 Ohm lines for
feeding the antennas simultaneously and separately)--all per the schematic in
Lawson's book. Further, there is no
additional cable loss by doing this vs using a WX0B Stackmatch at the tower,
as some say there is. (Sorry, Jay.) I can prove this
mathematically, if anyone is interested.
Also, the system I use is more robust in that if one cable run fails, I can
always use the
other antenna alone (important in a contest). Also, there is no mechanical
relay failure point at the tower.
Lastly, the scheme I have described was taught to me by Frank, W3LPL, and is
the method he uses on all of his stacks.
I should add, further, that all my and Frank's antennas are monobanders, not
tribanders. Further confusion?--Ask me to elaborate.
Bill, N3RR
http://www.erols.com/n3rr
http://www.erols.com/n3rr/n3ringrotor/index.htm
Don Moman wrote:
> I see comments on two methods of connecting antennas in a stack.
> Most frequently, using equal lengths of cable from all antennas is
> mentioned. The other way mentioned is using odd multiples of 1/4 wave
> lengths of 50 ohm.
>
> In my three high stack on 20, I went with the odd multiples. The upper and
> lower antennas use 80' (7x) of RG 213, and the middle uses 34.3' (3x) and
> all reach the WX0B stackmatch box near the center antenna. This approach
> "wastes" very little coax. Using equal lengths would require an extra
> 45.7' to be used at the center beam.
>
> I am in the process of planning the 15m stack, 4 high with .75 wave
> spacing, and again plan to use the 1/4 wave approach which will minimize
> wasted coax (and loss). A stack match type of approach will be used to
> select various combinations. All antennas will the 6 el 48' boom OWA
> design, which has a very flat and broad SWR curve. However I see frequent
> comments about using equal lengths, which makes me wonder why. I am not
> stacking tribanders, which is about the only reason I see for using equal
> lengths. I am willing to believe both methods are fine, and perhaps the
> equal length approach is simpler (no math) and that is why it is suggested
> more frequently. Is one method better than the other? and why?
>
> On a related matter, does anyone know if the DX Engineering 6 way box uses
> equal length routing? I was planning to use one for the 15 and eventually
> 10m stacks, but not if I have to allow for different electrical lengths.
>
> Thanks, 73
> Don
> VE6JY
>
> VE6JY Don Moman email:ve6jy@freenet.edmonton.ab.ca
> Box 127 Lamont, Alberta email forwarding: ve6jy@rac.ca
> T0B 2R0 (403) 895-2925
>
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