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@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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