> I was looking for more of a "generic" answer along the lines
> of, "As you
> move down in frequency, electrical downtilt ____." (Enter INCREASES or
> DECREASES here as necessary -- if this is the case.)
I know you wanted a short answer, but this got kind of long-winded...
A series-fed antenna will have more downtilt as you go down in operating
frequency, and likewise more uptilt as you go up in operating frequency. I
vaguely recall seeing a plot of uptilt/downtilt in an antenna catalog back
in the early 80's (maybe Antenna Specialists) that showed a little less than
2 degrees of beamtilt per 10 MHz of frequency change for a collinear omni at
UHF. Considering a 10 dB stick has a half-power beamwidth of only 7
degrees, the tilt can become very noticible very quickly as you get out of
band.
In contrast, a 6 dB stick has a much wider beamwidth (around 15 degrees), so
the uptilt/downtilt is less of an issue. If your antenna has a fairly wide
beamwidth, unless your antenna height is extremely high and the target
coverage area is very close to the tower site, purposely adding downtilt is
rarely necessary for the purposes of covering the target area (for the
purposes of putting less power on the horizon for interference reduction,
that's a different story). Antennas that are up that high tend to have
clear line-of-sight to the close-in coverage areas, and like they say up on
24 GHz, "if you can see 'em, you can work 'em".
As a trig refresher, you can determine how far out from the tower site the
bottom of the main lobe (-3 dB point) hits the ground using the following
equation (assumes flat earth, which is good enough for what we're doing):
d = h / tan(b/2 + t)
where
h = antenna height
b = half-power beamwidth in degrees
t = downtilt in degrees, uptilt being a negative number
d = radial distance from tower base
So for a 10 dB stick (7 degree beamwidth), with no downtilt, at 1000 feet:
d = 1000 / tan (7/2 + 0)
d = 1000 / tan (3.5)
d = 0.06116
d = 16,350 (3.1 miles)
If you add 3 degrees of downtilt, d becomes 1.7 miles. The trade-off,
however, is that now your gain on the horizon is reduced to something closer
to 7 dB instead of the full 10 dB, so although you've improved your close-in
coverage, the long-distance performance suffers.
So in this example, at distances less than 3.1 miles from the tower, the
users aren't in the main lobe. This may or may not pose a problem. If
they're mobile, or even outdoor handheld, they probably won't have problems
because the repeater antenna probably has clear line of sight to them (since
they're less than 3 miles away, and the repeater antenna is 1000' up), so
even if they're somewhere in the nulls and minor lobes, they'll probably do
OK. Building penetration is whole different issue, as nulls may be on the
order of 30 dB deep or more (some may be "perfect nulls" in theory, but in
the real world that doesn't happen due to both near-field and far-field
reflections). It would be perfectly understandable to learn that you have
better indoor handheld coverage 4 miles out than you do 2 miles out in this
example.
Keep in mind when you do these kind of beamwidth touchdown analyses that you
need to leave some wiggle room to account for antenna flexing, mounting
imperfections, and other real-world limitations that will cause the antenna
to be something other than perfectly plumb.
It should also be obvious that if there is too much uptilt the main lobe
never hits the ground, which is fine if you want to talk to space aliens...
I have a repeater (440) with an antenna (PD1151, 12 degree beamwidth) at
700' above ground level. I have a warehouse about 3/4 of a mile away from
the repeater at roughly the same ground elevation as the repeater (within 50
feet or so). The warehouse is cinderblock with steel beams and a metal
roof, no windows. If I stand in the doorway to the warehouse I can see the
repeater antenna, yet there are spots inside the warehouse where people tell
me I'm noisy running a 4-watt handheld. In this example, I'm below the main
lobe which hits the ground at about 1.25 miles. However, with that same
handheld, I can sit in my living room 20 miles away and work the repeater
fine.
In ham radio, maximizing total coverage area is often the goal when the
population density is fairly uniform (and when you think about population
density, you can't limit it to just where people live, you have to consider
where they work, where they go on weekends, the travel routes they use in
their communits as well as interstates and other highways used by
non-residents, etc. - it's not as simple as it might seem, but I
digress...). Since area varies with the radial distance squared, giving up
some close-in coverage perfection is usually an acceptable trade-off if it
gets you another few miles out on the horizon. Let's say, for example, you
have a nice high site and your repeater has a 50 mile coverage radius with a
zero-downtilt antenna. That's about 7854 square miles. We'll assume a
truly worst-case scenario where everybody within 3 miles of the repeater
can't work it because the antenna has no downtilt, so we subtract off that
area (28 square miles), leaving us with 7826 square miles. Now let's swap
out the antenna for one with 3 degrees of downtilt to improve the close-in
coverage to make the guys near the site using an HT in their basement happy.
In doing so, the long-distance coverage is reduced to, let's say, 45 miles
because the gain on the horizon has been reduced. We'll assume that now
everybody within 45 miles can work the repeater by virtue of the magical
downtilt antenna, so we have 6362 square miles of coverage. We've picked up
28 square miles (less than 0.4%) close-in, but we've given up 1464 square
miles (18.7%) out on the fringes in doing so.
> I am also wondering if 20MHz on the receive is far enough off
> to cause a
> problem. Remember, this stick is within 1 MHz of the bottom
> of its range on
> TRANSMIT, and well below it on Receive. So this is why I ask
> about adverse
> effects.
What is the advertised gain and beamwidth of the antenna?
--- Jeff WN3A
