Re: [TowerTalk] 2 element phased vertical

[Jim Lux <jimlux@earthlink.net>]

At 02:50 PM 2/3/2005, Van Fair wrote:
> Thanks Jim. So If it was Up and I measured the impediance, all I would need
> is someone to help me with the calculations to determine the length of  the
> 3 pieces of coax in degrees. Do you measure each antenna independetly with
> them 1/4 wavelength apart.

The standard technique is as follows:

Open circuit the feedpoint of antenna 2
Measure feedpoint impedance of antenna 1  (call this Z1open)
Short the feedpoint of antenna 2
Measure feedpoint impedance of antenna 1 (call this Z1shorted)

Open circuit feedpoint of antenna 1
Measure feedpoint impedance of antenna 2 (Z2open)
short feedpoing of antenna 1
Measure Feedpoint impedance of antenna 2 (Z2shorted)

Now.. there's some math involved to get to 3 numbers:
Z11 - self impedance of antenna 1
Z22 - self impedance of antenna 2
Z12 - mutual impedance between 1 and 2

Say I make the following measurements:
Z1open 60+j10
Z1short 32+j39
Z2open 45-j20
Z2short 39+j12

After grinding a bit, you'll wind up with
Z11 60+j10
Z22 45-j20
Z12 36.5-j25.5

Now, lets say I want to phase it 90 degrees apart..
I want element #1 current to be 1+j0 amps, and element #2 current to be 
0+j1 amps

A bit of complex math, and I find that the voltages at the feeds are:
1) 85.6+j46.5
2) 56.5+j19.5

(The feedpoint impedances are 85.6 + j46.5 ohms and 19.5-j56.5 ohms 
respectively)

Restricting the search for lengths to those longer than 75 ft, and using 75 
ohm coax with 0.5 dB of loss/100ft with velocity factor 0.66, I get that 
the feedline to #1 has to be 24.66 meters and the one to #2 has to be 23.28 
meters. (about 11.75 and 2.71 degrees at 3.6 MHz).

THis is somewhat different from what you'd expect in the example, mostly 
because the impedances are a bit weird.

It turns out that if you want to phase it the opposite way (i.e. -90), you 
can't find a solution.  It gets close, but there isn't a pair of cable 
lengths that makes it work.  I suspect you'd need to tune out some of the 
reactive component at the feeds to make it work.






> Would the lengths be 84 ,84 and 71 degrees as
> shown on 11-33 or would this be a part of the calculation based on the
> measured impediance to make the length turn out to be in phase for broadside
> and 90 degrees for off the end. Or am I completely off base.  Once again
> thanks for your help and I promise to quit bothering you.
>
> ----- Original Message -----
> From: "Jim Lux" <jimlux@earthlink.net>
> To: "Van Fair" <giw@bellsouth.net>
> Cc: "towertalk" <towertalk@contesting.com>
> Sent: Thursday, February 03, 2005 4:46 PM
> Subject: Re: [TowerTalk] 2 element phased vertical


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