Nice explaination,,
I understand it a lot better now, I like your idea of the off centre
advantages especially when the tree distances are off,
should work out ok, unless I cut one of my neighbor's tree,,,, might look
too planned.
Any good ideas for a homebrew omni on the top of the 50ft tower?
For which band or bands? 2 meters? If so, you could install a
homebrew J-pole (these are easily made from copper pipe), or a simple
quarter-wave whip with a few ground radials bent down at a 45-degree
angle (two radials is enough for a very functional ground plane).
There are fancier omni antennas available, depending on what
frequencies you want to work, how much gain you need, and how much you
want to spend. A 1.25-wavelength "extended double zepp" made out of
copper pipe, side-mounted at the top of the tower, would give you a
few dB of gain compared to a J-pole or ground-plane.
I also have a couple hd antennas I must get higher, the dish is on there but
lower.
I would like to put a whip on the top mast, half way down the masting put
the two hd antennas for tv ( I figure one aiming north, one south (my
local's station most direction)) then tee that up then there at the top of
tower tie in a OCF dipole towards two trees.
The chap at the electronic store who sold me the hd antennas said it is
better to do it this way or to have the router. Any body confirm or deny
that? only a hundred bucks worth of parts there so not much but great signal
and high def.
Well, let's see. "HD" antennas aren't fundamentally any different
than decent analog-TV antennas - the RF has no idea whether it's
carrying analog or digital modulations. What you want or need, in an
ATSC (US digital TV) antenna, depends to a significant extent on which
RF frequencies your local stations are using. In many areas, they're
all up in the UHF band these days, and you can get away with a
relatively small antenna... multi-bay bowtie-and-reflector antennas
are popular.
In some areas (mostly urban) there are still stations transmitting
digitally on the VHF bands - quite a few in the VHF "high band" (old
channels 7-13) and a few still down in the VHF low-band (old channels
2-6). In these areas you'll need an antenna which is big enough to do
a decent job on VHF... and the old UHF/VHF log-periodic rooftop
antennas work just fine for this.
In general, you will get the best results with an antenna pointed
fairly much towards the transmitter... you'll get some gain, and the
antenna will be more likely to reject multipath reflections from
nearby buildings and trees. If your local stations are split between
mostly-north and mostly-south, then either using a bidirectional
antenna, or a single antenna with a rotator, or a pair of fixed
antennas with a selector switch, would make sense.
I wouldn't hook up two antennas (north and south) and just wire them
in parallel... the signals will interfere and you could end up with
worse results than you'd have gotten from a simple dipole.
A few HDTVs and set-top boxes now support a "smart antenna" control
system. They can send a control signal (and some DC power) back up
the coax to the antenna, allowing a multi-element antenna to switch a
phasing network and thus "aim" itself electronically... no moving
parts, just some phased beam-shaping. The DTV decoder / TV will
"switch" the antenna around when channel-tuning, to figure out which
beam pattern gives the cleanest signal.
Unfortunately, the control protocol and specs for this system aren't
freely available, so I don't think it's possible to home-brew one.
--
Dave Platt AE6EO
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