Topband: More on 160 dipoles
Bill Tippett
btippett@alum.mit.edu
Fri, 10 May 2002 02:39:42 +0100
Thanks to Bob for that great info. This may answer a question
I've wondered about for a long time which is "Why do high dipoles
work well on 80 but not on 160?"
Here are some quick modeling results for my 180' vertical
and a horizontal flat dipole at 120' (which is what I can put up
between two towers). I've assumed the same ground under each
(EZNEC's Good ground which is 5 mS/m). This may be a little high
for my far field and is hopefully low for the radial screen under
my vertical, but it is a quick look.
Rad. Angle Dipole Vertical Difference
(degs) (dBi) (dBi) (Vert better)
3 -14.75 -7.92 6.83
6 -8.32 -3.98 4.34
9 -4.91 -2.26 2.65
12 -2.55 -1.37 1.18
15 -0.77 -0.89 -0.12
18 +0.63 -0.67 -1.30
21 +1.76 -0.63 -2.39
24 +2.70 -0.72 -3.42
27 +3.47 -0.91 -4.38
30 +4.12 -1.19 -5.31
33 +4.66 -1.55 -6.21
36 +5.12 -1.98 -7.10
39 +5.50 -2.47 -7.97
42 +5.82 -3.03 -8.85
45 +6.09 -3.64 -9.73
So the vertical is slightly better up to about 15 degrees
TOA and then the dipole takes over. But this does not agree with my
practical experience in Colorado when the vertical would usually
be 2 S-units (10-12 dB) better. Now, lets add the loss figures Bob
gave for the case of horizontal polarization oriented broadside
North and for vertical polarization also looking North. I wish
the data was to the Northeast since that is the way most of orient
our dipoles, but that probably lies between the values Bob gave
for North and E-W for horizontal polarization.
Horizontal polarization Vertical polarization
Rad. Angle Coupling Coupling Coupling Coupling
(to North) (to E-W) (to North) (to South)
(degs) (dB) (dB) (dB) (dB)
NE-???
3 -14.2 -8.2 -0.2 -1.2
6 -15.3 -8.0 -0.1 -1.3
9 -17.2 -8.0 -0.1 -1.4
12 -20.4 -7.4 -0.1 -1.5
15 -27.0 -7.8 -0.1 -1.6
18 -25.0 -6.6 -0.1 -1.8
21 -24.0 -6.2 -0.1 -1.9
24 -18.2 -5.8 -0.1 -2.0
27 -15.1 -5.4 -0.1 -2.1
30 -12.8 -5.1 -0.2 -2.2
33 -11.1 -4.8 -0.4 -2.3
36 -9.7 -4.5 -0.5 -2.4
39 -8.7 -4.3 -0.6 -2.5
42 -7.8 -4.1 -0.8 -2.6
45 - 7.1 -3.8 -0.9 -2.7
Adding coupling losses (North) to the patterns above we get:
Rad. Angle Dipole Vertical Difference
(degs) (dBi) (dBi) (Vert better)
3 -28.95 -8.12 20.83
6 -23.62 -4.08 19.54
9 -22.11 -2.36 19.75
12 -22.95 -1.47 21.48
15 -27.77 -0.99 26.78
18 -24.37 -0.77 23.60
21 -22.24 -0.73 21.51
24 -20.90 -0.82 20.08
27 -15.50 -1.01 14.49
30 -8.68 -1.39 7.29
33 -6.44 -1.95 4.49
36 -4.58 -2.48 2.10
39 -3.20 -3.07 0.13
42 -1.98 -3.83 -1.85
45 -1.01 -4.54 -3.53
So now the results look closer to my actual experience and
the breakpoint between the dipole and vertical is at 39 deg. If I
extrapolate between the horizontal loss from North to East using
Bob's figures, I estimate you could add 5-10 dB for a NE dipole
at low angles which would make the difference more in line with
what I've experienced. I don't guarantee that the above doesn't have
a few mistakes in it but it gives you the general idea. To me it goes
a long way in explaining why dipoles don't seem to work as well
on 160 as they do on higher bands. I believe Bob is calculating
coupling losses for 80 but I understand the effect is lower which
would account for the experience I've had with high dipoles working
better on 80. I hope Bob can take a look at the case for 45 degree
azimuths for NA since that is the way we usually orient our 160
horizontal antennas.
Thanks again to Bob for providing some very useful data!
Hopefully this helps explain the effect a little better.
73, Bill W4ZV
P.S. Long thread but I think I've learned something...hope the rest
of you did!