Yagis - Questions you always wondered about
by Tom, K1JJ
Ola Guys,
Sit back and enjoy some interesting reading.......
Over the last six months I've been spending some time up on 6M and have been
talking with and helping to "develop" a group of enthusiastic hams. Some are
tech/general newbies who want to improve their stations and antenna systems.
They are all running simple 3 or 5 element Yagis, but now want to lengthen
booms, stack antennas , etc. Some are now talking about BUILDING linears rather
than buying. Good crowd.
Well anyway, I've gotten axed some great questions lately about antennas that I
realized I did not know the answers to off hand. I told them I would check out
things on the modeling program and get back. Here's the results of a few emails
and on-the-air discussions that I thought I'd share with you guys here. Some of
the concepts are simple, but were not obvious, at least to me...
As we know, vertical take-off angle on an antenna is EVERYTHING! It's what makes
a high Yagi pissweak locally and a low dipole loud. And what makes a high Yagi
hear the DX while a low dipole hears static. On the low bands there may not be
much we can do about it because of height limitations, but on the higher
frequencies we can often tailor and optimize our antenna system heights to match
whatever operations we prefer to do.
BTW, many AMers are already on 6M! Consider putting up a Yagi and joining the
fun. We all need a break away from HF once in a while!...
eMails as follows and applies to 6 Meters and is discussing the VERTICAL plane
only (H plane):
---------------------------
Hi Guys,
Ray asked a coupla of questions tonight that I couldn't answer, so I did some
modeling to find the answers....
Ques 1:
Does the LOWEST take-off angle lobe of a Yagi at a given height get lower if the
boom is lengthened? ie, a 6M Yagi at 50' is on a 24' boom and then gets
lengthened to 48'. (Still at 50' high and 2.5 wavelengths high)
The answer is NO, the lowest lobe does not get lower **IF** the Yagi is at
ROUGHLY 3/4 wavelength or higher above the ground.. The lowest main lobe does
not get lower with a longer boom. It DOES get lower *IF* the Yagi is LESS than
roughly 3/4 wavelengtn high. (less than 15' high on 6M)
This LOWER height "cross-over point" is variable and depends upon the boom
length. ie, for a 3el Yagi the cross-over is at about 3/4 wavelengths high. For
a 6el Yagi it is more around 1/2 wavelength above ground. So let's say it varies
from 1/2-3/4 wavelength for average sized beams. Consult your own modeling
program when near this area.
When higher than 3/4 wavelength above ground and the boom is extended, the
higher lobes start to move down and form into the lowest lobe to make it fatter.
But this lowest lobe stays the same take-off angle in degrees, even with a
longer boom.. The higher angle energy is forced towards the horizon all into the
lower lobe as the boom lengthens greatly. So, a 3el Yagi at 50' has MANY lobes
with the lowest is at 6 degrees. A 200' long boom Yagi has only one big lobe at
6 degrees and a small minor one up at about 20 degrees. The majority of the
energy is in the lowest lobe with a very LONG Yagi that is within a wavelength
of the ground. The higher it goes up above the ground, the more minor lobes
start coming back into the picture thru reflections.
Ques: 2) Does the take-off angle halve if the height is doubled?
I took a 24' boom Yagi and put it at heights from 3' to 200' high and
determined the take-off angles below in degrees. You will see that it
starts out non-linear at the lowest heights, but seems to get linear once
1/2 - 1 wave off the ground. Check this out:
6M 24' boom Yagi at:
3' high - 34 degree take-off angle
6'....... 25 degrees
12.5'......17 degrees
25'......12 degrees (1.25 wavelength high)
50'...... 6 degrees (2.5 wavelengths high)
100'..... 3 degrees
200'...... 1.5 degrees
Ray.... your 24' boomed Yagi at 25' has a take-off angle of 12 degrees,
not 6 degrees. I was looking at the wrong plot when I told you. It is still
down about -7db at 3 degrees.
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So, to summerize - as mentioned above......
As it turns out, when lengthening a boom, it DOES make a lower take-off
angle, but ONLY when the Yagi is less than roughly 1/2- 3/4 wavelength above
ground, depending upon starting boom length. The lower the Yagi is below 3/4
wavelength, the more dramatic the longer boom has on the lowest lobe coming down
lower. This is very significant on HF freqs where 130' is 1/2 wavelength on 75M,
etc.
However, on 6M, 3/4 wavelength is only about 15', and most modest ham antennas
are above this height. So in this case a longer boom has little to no effect on
lowering the LOWEST lobe. But it does bring down the higher lobes and mix them
into the lowest, resulting in a fatter lowest lobe as the boom increases length.
Bottom Line: With Yagis > 3/4 wavelength high, the lowest lobe take-off angle is
based almost soley on height above ground. It will be changed only in
"thickness" by vertical stacking, and ground reflections.
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BTW, here's the goods - what each wavelength above ground produces for a takeoff
angle - notice all but one are all above 3/4 wavelength high, so a longer boom
will have little effect on the LOWEST lobe. This works for any frequency. Shown
for 6M in feet. The degree readings are approximate as limited by the polar
resolution on my computer screen:
48' boom 9 el Yagi Yagi
HEIGHT FT WAVELENGTH DEGREES
9.8'.... 1/2 wavelength....... 17 degrees (lowest lobe take-off angle)
19.6'.... 1 wavelength ........ 12 degrees
39.2'.... 2 wavelengths........ 7 degrees
58.8'.... 3 .......................... 5 degrees
78.4'.... 4 ........................... 4 degrees
98'.........5.......................... 3 degrees
117'...... 6............................... 2 degrees
137'..... 7................................ 1.8? degrees
156' ..... 8................................. 1.6? degrees
6' boom 3el Yagi
HEIGHT FT WAVELENGTH DEGREES
9.8'.... 1/2 wavelength....... 28 degrees (lowest lobe take-off angle)
19.6'.... 1 wavelength ........ 15 degrees
39.2'.... 2 wavelengths........ 7 degrees
Notice the huge difference boom length makes at 1/2 wavelength high comparing a
6' to a 48' boom. (17 vs: 28 degrees) However, the take-off angle gets real
close when they are both at 2 wavelengths above ground.
Bear in mind this is an extreme example comparing a 6' to 48' boom, but you get
the idea... the cross-over point is not digital, it's a a little rough...
-----------------------------
HORIZONTAL vs: VERTICAL STACKING of YAGIs - (side by side vs: one over the
other)
Question: You have two Yagis. Is it better to vertically or horizonally stack
them?
Like everything, it all depends upon what you want to do with them. DX only,
local ground wave and tropo, sporadic E skip, combination of all ... , etc.
Here's a comparsion I did of a pair of 48' boom, 9el Yagis on 6M.
In the first listing is the pair vertically stacked at 100' and 64'. (36'
spacing) They are shown in-phase and 180 degrees out-of-phase.
The second is with the two Yagis horizontally stacked, 36' apart at the
booms. Shown only in-phase.
I show the major lobe amplitudes in db at the various angles. You will see
that EVEN when the vertical stack is switched 0/0 or 0/180 to get more high
angles, the horiz stack is always stronger in amplitude, though has thinner
lobes at the horizon. (3 degrees) The same relative differences will apply to
your 24' pair, Ray. Very interesting. Print it out to be able to look at all
three colums to see the big picture. Take each angle at a time and compare the
gain to the others.
This is OVER GROUND, so there is ground reflection gain... just look at the
differences between the three to get the idea.
***** Vertical take-off in degrees vs: gain: (Fed in phase) *******
2 Yagis Collinear (Horiz stacked):
3 degrees....23dbi
9.............. 22 dbi
15............ 21 dbi
20............ 19 dbi
27............ 15 dbi
---------------------------------------
Vertical Stacked - both in phase:
3 degrees......... 22.57 dbi
10.................... 16.5 dbi
20................... 6.5 dbi
25................... 14.5 dbi
Vertical stacked - out of phase:
8 degrees ............... 18.8 dbi
13........................... 20.8
20.......................... 16.8
The main difference here and not shown without the polar plot is the fact that
the vertically stacked lobe at 3, 8 and 13 degrees are MUCH fatter (and fewer)
than the collinear horiz stacked Yagis. This is an advantage assuming you are
interested in putting the majority of your power near the horizon at the expense
of the other higher angles. Thats why vert stacking is good for F2 propagation,
etc. But with the MANY incoming angles we deal with on 6M it's better to have
thinner lobes that are more distributed as shown with higher gain. At least
that's what I plan to do with my new experimental 4 X 48' boom Yagis in a
horizontal line - 98' cross boom - yikes!
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Question: How would you modify an M2 5 el 18' boom Yagi for more gain?
I would lengthen the boom to 24' and retune the elements and spacing to gain an
extra 1.5 db. A 6M 5el Yagi "wants" to be on a longer 24' boom if you want max
forward gain with a compromised 22db f-b ratio and 12.6 ohms for a feedpoint to
match the 4:1 match that M2 uses now.
Here's the dimensions to build from scratch or modify an existing 5el Yagi.
(Gary/ INR, here's the antenna we talked about for you to build)
5 el 6M Yagi (designed for 1/2" aluminum tubing elements and a single 2"
diameter .125" thick wall, 24' boom.)
Gain: 12.46 dbi (free space)
Two horiz stacked at 29.4' spacing: 15.58dbi
F-B: 22.16 db
Position on boom Inches....Length (1/2) (half lengths)
Ref: 0".............................57.480" X2
Driven: 43.152".................55.000
D1: 106.245.....................53.206
D2: 194.222.....................52.183
D3: 270.832.....................52.150
This is optimized for a compromise 22db f-b so that the boom is now 22' 7" long.
This allows room for the end element boom to element plates.
Input impedance is 12.6 ohms +J2
If used with the boom above, "YagiStress" mechanical modeling program says no
overhead trussing support is needed. I have one up now and it has survived ice
storms and wind with no overhead supports.
The overall design looks very good.
Rock and Roll, baby!
73,
Tom, K1JJ