[TowerTalk] Method of calculating phase delay variation when stacking two...

Mon Sep 2 16:09:31 EDT 2013

Thanks for taking the time to reply, Gerald. 

Thank you for the reply, Pete. 

I played with the antennas a little more over the weekend after adding in the 5ft cable (well, actually 4 and change after the VF is factored in).  All tests with the two antennas at the same azimuth.   I am seeing about 2 s-units on receive at times, other times the upper and Both In Phase are about the same.   This is on 17 and 20m.  On 20m I seem to see the most benefit. The BOP setting really seems to drop down more so on 20.  I actually did see some advantage to the BOP setting when pointing towards EU and copying a VE1 station.  So the angle change using the BOP can have an advangage as opposed to turning the antennas at times. 

Overall, t h ere seems to be a slight improvement with the added delay line. But nothing drastic.  Obviously 10m has been dead lately, so no testing there yet. 

  I guess over time I will be able to get a better idea of how well it plays overall. But it would be interesting to see how it models out overall.  Someday I will need to master the modeling software. 

Thanks again for the reply. 

73 

Eric 

K2CB 

----- Original Message -----

From: TexasRF at aol.com 
To: k2cb at comcast.net, towertalk at contesting.com 
Sent: Sunday, September 1, 2013 3:44:20 PM 
Subject: Re: [TowerTalk] Method of calculating phase delay variation when stacking two... 

Eric, I don't see the answer being quite as complicated as all that. 

If one yagi feed point is forward of the second yagi, the number of wavelengths or degrees would be corrected by making the forward yagi feed line longer by the amount of the offset. 

a 5 ft offset would need an increase in line length of 5ft electrically. Physical length in this case would be 5ft times the velocity factor. RG213 has a v.f. of .66 so the actual length increase is 3.3 ft. LMR400 v.f. is about .84 so the actual length would be 4.25 ft. 

No need for calculating degrees or wavelengths is needed. 

This is assuming you want to make the two feed points in phase and the vswr is low. Elevated vswr would introduce some phase shift but that is a whole new subject. 

The spacing you mentioned should work out very well. It is enough to attain near maximum stacking gain on 20m. While more than enough for the higher bands, the only consequence is somewhat higher grating lobes in the vertical plane. The grating lobes are many dB lower in amplitude than the ground reflection lobes and probably not even noticeable. 

73, 
Gerald K5GW 

In a message dated 9/1/2013 1:30:17 P.M. Central Daylight Time, k2cb at comcast.net writes: 

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 

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