[TowerTalk] 2 element phased vertical
Jim Lux
jimlux at earthlink.net
Thu Feb 3 14:04:49 EST 2005
At 10:40 AM 2/3/2005, Van Fair wrote:
>I just put up a Hy Gain Hy Tower which is 53 feet tall and is liner loaded
>somehow to be a 1/4 wave on 75 meters. It works quite well with a modest
>1000ft ground field. I am considering adding a second vertical and would
>use a simple Christman feed system to phase the two.
>
>My question is: Do I have to have another very similar short vertical
>such as a Hy Tower or could I use a full size 1/4 wave vertical for the
>second antenna spaced 1/4 wave away with a second ground field just like
>the hy tower has. I have some Rohn 25 and long mast which could get me to
>62 feet high and use fiberglass rod insulators at the base of the Rohn.
You can make a Christman type feed "work" with almost any radiating
elements. It's "easier" if they're identical and spaced just the right
distance apart.
There are several steps to the process (ON4UN's book describes one way, and
I've got some Excel spreadsheets to help you do it, as well).
Step 1) Know the mutual impedances beween the elements. If you're at the
planning stages, you can use NEC to model it. Gnat's eyelash isn't
important at the planning stage, because all you want to establish is that
it's feasible.
Step 2) Once you've decided what the element currents and phases should be
(the classic strategy is 90 degrees apart, equal magnitudes, but others can
work), calculate the "driving point" (or "feed point") impedances that will
appear with those currents. This is simple matrix math using the mutual
impedance matrix.
This will give you a "desired voltage and current" at each antenna.
Step 3) Use a transmission line calculator to figure out how long to make
the two feedlines. As you move back along the transmission line, the
voltages and currents change. You pick two lengths where the voltages are
equal (mag and phase).
The Excel Solver can do this pretty quickly.
Step 4) Verify that the lengths chosen are "reasonable". You can get
solutions where the sum of the two feedline lengths is less than the
distance between elements). If not, run the analysis again with some
constraints (like a minimum feedline length).
Step 5) Do a quick senstivity analysis. How much is the optimized length
changed if frequency changes a bit or if the mutual impedances are
different? You don't want to build a system that only works if things are
controlled to 0.001%.
Step 6) Optional, you can put the transmission lines into a NEC model and
look at the patterns to see if they're what you want.
At this point, you've assured yourself that if you DO build it, it will
probably work.
Step 7) Build the antennas.
Step 8) Measure the actual mutual Z
Step 9) Recalculate the coax lengths (hope they aren't too different from
your original estimates)
Step 10) Cut the coax
Step 11) See if it works.
Now.. for spreadsheets to make life easier...
http://home.earthlink.net/~w6rmk/antenna/phased/software/drive2.xls
This one will do mutual impedance estimates (for two monopoles) or reduce
actual measurements. And, it will do step 2, above, calculating the
driving point voltage and current.
http://home.earthlink.net/~w6rmk/antenna/phased/software/christ2d.xls
Does the calculations for the transmission lines. If you need to install
the "solver" for your copy of Excel, click "Tools","Add Ins...", then check
the box for "Solver Add-in".
Feedback on the spreadsheets would be appreciated..
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