Re: [TowerTalk] 2 element phased vertical

[Jim Lux <jimlux@earthlink.net>]

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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