[TowerTalk] Method of calculating phase delay variation when stacking two dissimilar yagi antennas?

[k2cb at comcast.net]

Good Morning TT, 



I have a pair of SteppIR yagis that I am attempting to run in phase. A DB36 at 91ft, and a 4E at 55ft.   



The main reason for the two antennas is versatility to hop quickly between bands, or point in two directions at any given time. Having the ability to phase them together is and added bonus.... I understand that the stacking distance may not be optimal, but it "is what it is" , dictated by the tower.    



I originally ran equal lengths of cable between the feed point of each antenna and a two port stack match box at the base of the tower, and ran with it. But I recently did some more reading and came across a Powerpoint doc that WX0B presented some years ago: 



http://www.pvrc.org/Powerpoint/wx0b_files/WX0B%20Stack%20Presentation%202000.ppt 



In the presentation, he states that if there is a difference in the driven element locations, you need to account for this and add a "delay line" of sorts to the leading antenna.  So based on his document, I measured a 5ft difference in the position of the DB36 and 4E driven elements.  I ran the following calculation: 



Variation of driven elements / wavelength x 360 deg.  = degrees of delay needed, s o in my case the variation in driven element distance from the center of the tower results in phase shifts as follows : 

10m - 5/32.96 x 360 = 54.6deg 

15m - 5/43.94 x 360 = 41.0deg 

20m - 5/65.92 x 360 = 27.3deg 



So based on the document, I  inserted a delay stub of coax, as follows: 

5ft x VF(0.87 for LMR400) = 4.35ft 



I then also took my MFJ269 analzer (for what it is worth) and measured the electrical length (not the physical length) of both cables using the VF value of 1.00 per the manual , and they measured out with the 5ft difference, with the DB36 measuring 5ft longer electrically. So I thought I had it accounted for properly...... 





Fast forward to this morning, and I now ran across the following page by WN9O, 



http://www.qsl.net/wn9o/Phaspat.htm#Figure 4   



in which he describes stacking different Force12 tribanders.   His calculation method for his delay line lengths seems to vary greatly from what I see in the WX0B presentation document.  Whereas the WX0B document simply has you account for the spacing variable of the driven element for both yagis, and convert the physical distance to the equivalent coaxial electrical difference, and insert this length into the leading antenna feedline, the WN9O document seems to rely on detailed modeling to first determine the phasing variation in degrees, then use the following formula to calculate the required coaxial delay line: 



Coax length = Wavelength  / 360   *   degree value from modeling calcs * VF of coax 

In his case, for 10m (since 10m is worse of all bands) = (984/28.4) / 360  * 114deg  * 0.78 = 8.55ft 



I noticed that WN9O used 984/F for the wavelength calc, whereas the WX0B doc used 936/F based on his 21MHz system.  I tried to apply the WX0B formula against the numbers used in WN9O's antenna system to see if I could come up with the same 8.55ft for the delay line , but I came up with a value of 3.5ft based on his 4.5ft of difference between driven elements and a 0.78VF coax. And a phase delay of 49.15 degrees.  See the WX0B link, page 36 for reference. 





So my question at this moment is - which is the proper method for me to calculate the proper delay line length to properly phase the two dissimilar SteppIR yagis? 



Use the simple WX0B method of just measuring the variable between driven elements in the vertical plane, and multiply that by the velocity factor of the coax I am using, or do I need to have the two antennas modeled and then u se the WN9O calculation method?     



Unfortunately, the NEC and modelling software is somewhat above my knowledge level at this time. I have tried to play with Ez -Nec a few times , but I just could not get my hands around it.   Please don't bash me for not trying...  I have too many other irons in the fire to attempt to master modeling software at this stage in life.  And my skills in other areas of the hobby more than make up for my lack of modelling skills. I can troubleshoot and repair  radios and  amplifiers to component level , just don't ask me to model an antenna! 



If the latter is the case, would anyone care to offer some assistance and model the DB36 vs. the 4E in my case, and enlighten me on how much of a delay line length I actually should be using  (electrically)? 





Looking for comments or suggestions.... 



Thanks 

Eric 

K2CB