You have all given me some very good things to consider -
sure appreciate
the input!
I will get together with my local Ham friend who is much
better at these
things than am I and see what we can whip together. I have
copied this
to him and printed it so that I may read it a few times to
let it sink in then
will visit the suggested links and make some notes.
I have tried to summarize the suggestions below ...
----------------------------------------------------------------------------------------------
Excellent suggestion, Jim. Microwave Filters, Inc. produces
such a filter
with a HPF function and can be ordered with a knee at the
lower edge of Ch.
7. Their model 3378 would work well between the antenna and
DTV receiver --
or between antenna and a mast-head preamp.
http://www.microwavefilter.com/lowhighpass.htm
Wingard makes a reasonably good preamp with DTV in mind
http://www.winegard.com/kbase/upload/WC-809%20_HDP-269.pdf
> > "Non-linearity will be a potential problem in digital
TV preamps. If
you are out in the weak-signal sticks, overload and
non-linear response
aren't as likely to be a problem, but in areas close to the
transmitters it
could be. You can have too much of a good thing with
digital TV."
Agreed. I ended up removing my preamp from a small
mast-mounted bat-wing
antenna where the DTV stations are 10 miles away. I would
first attempt
signal capture without the preamp, and only add it if one or
more stations
cannot be decoded. Many DTV sets have built-in signal
strength meters. Not
that they are lab-quality instruments by any means, but they
do a good job
of letting you know if the received signal level is within
an acceptable
signal-strength window.
Also, in my case, the stations I am trying to decode are not
all in the same
geographic area. Accordingly, one may find that a
compromise must be struck
when aligning a directional antenna . I ended up pointing
the antenna
mid-way between two antenna farms. In the analog world,
that probably would
have resulted in visual problems, but the industry is now on
6th-generation
8-VSB chip-sets and they do a remarkable job of decoding,
even in the
presence multipath and low signal strength.
Paul, W9AC
Two things to consider:
1. After the digital transition next week, there will be
very few
stations left on channels 2-6. If there are none on these
channels in your area,
you can get by with a smaller antenna (element length),
which will reduce
pickup at HF and 6 meters. Likewise, a HIGH-PASS filter
cutting off just
below channel 7 (174 MHz) will be the ideal situation.
This would also
reject two meter ham signals. I don't know if such filters
are readily
available in the marketplace.
2. Non-linearity will be a potential problem in digital TV
preamps. If
you are out in the weak-signal sticks, overload and
non-linear response
aren't as likely to be a problem, but in areas close to the
transmitters it
could be. You can have too much of a good thing with
digital TV.
73 - Jim K8MR
1. Google up "Winegard" and check out their line of mast
mount pre-amps.
Look very carefully at the spec sheets, which can be
downloaded from their
web site. The AP-series of pre-amps are available in a
couple of
different gain and noise figure configurations, and they
also publish the
overload capabilities. I have been VERY pleased with my
Winegard pre-amp;
it really is resistant to overload. One more thing: it's
actually made
in USA!!! (The wall wart power supply and bias-T are made
in China, but
that's expected.) If you like what you see, follow the
click links to
their on-line retailers. I found that Solid Signal has very
good prices
on the Winegard products.
2. Next, keep in mind the following: NO such broadband
device as a
VHF/UHF pre-amp is going to have brick wall filtering
characteristics at
the bottom and top ends of its passband. If you are
concerned about HF
overload, put a good, in-line high pass filter at the INPUT
to the pre-amp
to minimize any effects from your HF operations. That is
very important
if you are going to operate 6m. I have had no problem with
2m, but when I
transmit on the 222 MHz band with 30 watts, I do get some
problems on a
couple of channels. I'll live with that for now, but I do
plan to put a
222 suck-out trap on the input to the pre-amp one of these
days to see if
it will reduce or eliminate the interference. I've had no
problems at all
when using 70 cm band, any mode, at 100 watts.
3. I placed my pre-amp about half-way down the tower so as
to keep it out
of the near field of my VHF and UHF antennas. That seems to
work OK. The
unit is in a plastic case (rats!). I can also tell you that
if your tower
takes a lightning hit, as mine did in April, the pre-amp
will be toast. I
am trying a new idea to see if I can keep the replacement
pre-amp from
heading to that big silicon pile in the sky: I am providing
a physical
grounding and bonding link between the IN and OUT ports of
the pre-amp and
tying that link to the tower itself. The idea is to not
have any
lightning current pass thru the pre-amp (which is probably
what killed
it), and to reduce the chances of arcing by bonding it to
the tower
structure. The IN and OUT type F connectors are the ONLY
means to make
this connection. If that idea doesn't work, the next try
will be to
totally repackage the pre-amp into an all-metal case.
4. Besides the aforementioned high pass filter on the
pre-amp input, you
may also wish to insert an in-line surge suppressor. These
are available
with type F connectors at MCM Electronics, and perhaps other
places. You
really have to be careful what you place on the output of
the pre-amp
because DC operating power for the pre-amp is fed to it via
the supplied
bias-T and into the "OUT" connector. That method saves
having to run a
separate power cable to the pre-amp.
Good luck.
73, Dale, WA9ENA
I would also use one or more common mode chokes near the
preamp to prevent
fundamental overload by VHF and HF transmitters that could
excite the
downlead as a long wire antenna. Use the guidelines in my
Cookbook for the
transmit frequencies in use.
http://audiosystemsgroup.com/RFI-Ham.pdf
At 6M, you want 2 turns for most cores; at 2M and above you
want a string
of beads. See either my data or Fair-Rite data, and aim for
at least 1K -
2K ohms with the resonance near the transmit frequency. Note
that Fair-Rite
is now publishing data for 1, 2, and 3 turns for many of
their suppression
beads. To find it, go to the pdf catalog, select the part
number to get the
data sheet for that part, then click on the data at the
bottom of the page
to see the multi-turn data.
For 440 MHz you'll want #61 cores. For 6M, 2M, and 220 MHz,
#43 or #31.
Note also that #43 is cheaper than #31, and #43 is
equivalent above 5 MHz
and slightly better above 20 MHz. The advantage of #31 cores
shows up below
5 MHz.
73, Jim K9YC
--
Thanks! & 73, doc, KD4E
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