On Wed, 11 Jan 2012 08:07:47 -0800 (PST), klem kedidelhopper
wrote:
I live in a small town in New Hampshire. About twenty years ago,
(before we got cable into town) I suddenly started to get numerous
calls from customers on one side of town complaining that TV channel 5
had just simply disappeared.
Many such TV's had AFC (automagic frequency control). Give it a
strong nearby carrier, and it will lock on the carrier instead of the
TV signal carrier.
So I drove around town visiting different homes and asking if they
were also affected by this. Eventually I was able to draw a sort of
"lobe" of the pattern.
Nicely done. That sounds like quite a bit of work.
Among these services was a
radio data link to Massachusetts which operated on a frequency 200KHZ
below channel five's video carrier.
It had to be more than 200Khz. Channel 5 video is at 77.25MHz. The
75MHz "telemetry and radio control" band is roughly from 75 to 76MHz.
It would need to be more like 2MHz below the CH5 carrier. Still, for
a 6MHz wide TV signal, that's quite close if running high power.
However, as I recall (and am too lazy to lookup), the highest power
allowed at 75MHz is something like 1 watt. I don't think it was a
filter, but rather far too much power.
While in the shack I noticed a
bandpass filter sitting on the shelf which was marked with his
operating frequency. I asked if that shouldn't have been in line with
the antenna and at that point the meeting became adversarial.
Nice detective work. I once gave a visiting FCC inspector an
"unofficial" tour of our mountain top radio site. I explained
everything and answered many questions. In gratitude, he sent about
100 "failure to post licenses" and other administrivia violations, not
to the service company, but directly to the customers. That didn't
cost much in fines, but as a result, we lost a few customers. I don't
give tours any more.
I often
wondered if perhaps the filter was tuned incorrectly and it's
insertion into the line caused problems, so that was why he removed
it, or perhaps he was overmodulating, creating excessive sidebands
which poked into channel five.
My guess is that he was running too much power trying to span the
distance between NH and Boston. Adding the filter probably increased
the loss to the point where the link failed. My guess is that it was
taken off the air when the site owner discovered what was happening.
When I was fifteen I built my first kit, an Eico CB radio. The
receiver was as wide as a barn door but it had an excellent
transmitter section. We lived in an apartment building in the Bronx
and most television sets of the day were built with 21MHZ IF strips.
So when I keyed that transmitter no one for blocks around was able to
watch channel two on their TV sets. I installed a low pass fiIter on
my rig and I lost track of how many high pass filters I installed for
my neighbors.
Ummm... you have it somewhat wrong. The hi pass filter goes on the 27
MHz xmitter, in order to remove any spurious rubbish from the transmit
signal at 21.4MHz. A low pass filter would NOT work on the TV set as
the frequency range is 54 to 800MHz and a low pass filter would block
all of it. However, a 21MHz notch filter on the antenna would work
wonders.
Incidentally, I helped an older friend build the same EICO CB radio.
The tunable receiver was horrible, but at the time, I didn't know
quality when I saw it.
http://www.radiomuseum.org/r/eico_770770.html
I would also think about your radio's IF frequency, although on second
thought that would not be limited to only certain channels.
I'd really like to know if you find this thing. Please keep us
informed. Lenny
Unless the front end is broadband and crude, the IF rejection of
modern receivers is quite good.
For 2.4GHz, the last device that used an IF frequency was the ancient
Lucent chipset and possible the early IBM wireless oddities. Literally
everything for the last 10 years or so has been direct conversion,
with no IF frequency. Google for "802.11 direct conversion receiver"
for hundreds of examples. IF feedthrough is unlikely if there's no
IF.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558