At one QTH where I lived in Chicago, I was about 2.5 miles from the
Hancock building and 3.5 miles from Sears Tower, where virtually ALL of
the FM and TV transmitters in Chicago were located. I had a good-sized
FM/TV VHF/UHF Yagi on the roof pointed midway between those two sites,
with passive splitters to an excellent Technics FM RX with 6-section
variable capacitor front end. Over a period of about 5 years, I heard
intermod only once, and it occurred during a late night maintenance
period. It was there for perhaps an hour and then gone. And I heard that
intermod on the FM RX, not in my ham rigs. In those days, my only VHF
operation was 2M via repeaters.
Because these transmitters were all on tall buildings >1,200 ft and east
of the Mississippi River, ERPs of the FMs and low-band VHF TVs were in
the range of 6kW, high-band VHF TV in the range of 100 kW, and UHF TV in
the range of 300 kW.
I was on the tech committee maintaining 2M, 220 MHz, and 440 MHz
repeaters in the Chicago loop. The 2M repeater was at about 1,000 ft, in
the shadow of Sears and a mile or so from Hancock. We could not use an
on-site RX on 2M because the intermod and phase noise from 50+ broadcast
transmitters and a thousand or so 2-way sites was so bad that it
couldn't hear a talkie less than a mile away. Instead, we used voted RX
sites 30 miles to the S and 15 miles to the N. The 440 MHz system used
only on-site RX, and intermod was low enough that it worked quite well.
As to AM band -- I was within a few miles of a couple of low power
(250-1,000W) stations, and roughly 15 miles from five 50KW clear channel
stations. I often heard WBBM's second harmonic, but not very loud (780
kHz, 50 kW), and had intermittent intermods on 160M from several
stations that I never identified. These came and went, so I always
suspected they were generated by flaky connections in structure or
wiring and re-radiated. NONE of this would have been helped by a BCB
filter, because it occurred outside my station.
A BCB filter only helps when the signal is overloading the receiver in
YOUR station. If you're using bandpass filter sets, it's likely that
they provide at least as much suppression in the BCB as do low cost BCB
filters, and for many filter sets, a lot more suppression. I DID use BCB
filters when I lived in Chicago, but I wasn't convinced that they did
much. :) I was mostly using pretty decent rigs -- K2, TS850, OmniV,
which also helps.
73, Jim K9YC
I later moved about 3 miles N,
On Fri,7/29/2016 10:42 AM, charlie@thegallos.com wrote:
Speaking of Broadcast band interference
I always see that you should run a BCB filter if you are "Close" to a
transmitter - but define "close"? I'm 7.8 miles from the highest power AM
Stick in the nation (two 50KW stations on one stick), Roughly 4 miles from
a few FM stations, and of course, 11 miles from all the transmitters in
Manhattan
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|