[TowerTalk] Strong FM Station Impact on Antenna Analyzer Use in VHF

[Jim Brown]

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 at 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