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FCC grants WBBM a CP to move transmitter site
- Thread starter CADXER
- Start date
Is the harmonic radiated or receiver induced?
Radiated from the transmitter.
A receiver induced image would usually have been 910 KHz below the main frequency which is impossible on 780 and problematic below 1440.
At the new site, mixtures should be hearable at 560 and 890, but both will be covered by strong local stations, 1450, and of course, the Score on 1340 and BBM on 1560
With WBBM operating at 35 tx power (daytime) and its received 2nd harmonic (1560 kHz) was at a level of -80 dBc, that harmonic would have a groundwave field intensity of about 25 µV/m at a distance of 1 km from the WSCR/WBBM tower.
Probably the greatest daytime field at/near the WSCR tower site from other 1560 kHz stations should be produced by WNWN in Portage, MI, which is a DA-D with a cardioid pattern pointing NW. The WNWN tx site is about 128 miles from WSCR at a bearing of N81°E, and radiates about 7 kW toward WBBM. The groundwave field of WNWN at/near the WSCR site is about 29 µV/m.
But even if WBBM radiated zero r-f energy on 1560 kHz, it could be still be produced within the receiver itself, and/or rectification by poorly bonded conductors in the vicinity of the tower and the receiver.
Probably the greatest daytime field at/near the WSCR tower site from other 1560 kHz stations should be produced by WNWN in Portage, MI, which is a DA-D with a cardioid pattern pointing NW. The WNWN tx site is about 128 miles from WSCR at a bearing of N81°E, and radiates about 7 kW toward WBBM. The groundwave field of WNWN at/near the WSCR site is about 29 µV/m.
But even if WBBM radiated zero r-f energy on 1560 kHz, it could be still be produced within the receiver itself, and/or rectification by poorly bonded conductors in the vicinity of the tower and the receiver.
A front end could generate a second harmonic on 1560. There is one we talked about along I-75 receivable between Saginaw and Bay City from WSGW 790 on 1580 that is most likely receiver induced, according to calculations rfry did. There's a harmonic in the 50 kW DA Major Lobe of WTCM 580 on 1160. That one could actually possibly be legal and radiated 80 dB down from 580, for a mile or so.
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There was an AM RITOIE or radiated third order product that appeared in one instance on 1670, before there was an expanded band. My 1930s table radio picked it up 4 miles away. It had the famed old 1600-1720 kHz Police Band, made famous again by George Carlin. An LC external antenna stage peaked at 1670, not at the strong signal frequencies that produced it. That favored the theory that it was radiated. It was strong on 1670 near one of the station towers.
Well, around here there is WKAR 870, WILL 580, and WOSU 820, which was nondirectional at the time you were probably there. A lot of Daytimers probably had harmonics that never became apparent until they went to full-time or PSSAs. Oh, WBAA 920 and WSUI 910 are outliers you sometimes hear around here on Day facilities. Would have been nice if it had been a station on 100 kHz multiple.
In the early days of radio, harmonics were more prevalent. A lot of stations had harmonics in the old MW Marine Band. This presented a problem for Marine Communications, particularly those used for emergency calls.
I actually remember hearing a harmonic of an AM BC station once, but I don't remember the details. Seems like it was the third harmonic.
I actually remember hearing a harmonic of an AM BC station once, but I don't remember the details. Seems like it was the third harmonic.
In the 1960s, WJR had a 2nd harmonic notch filter comprised of a length of 1-5/8" OD rigid transmission line integrated with the matching network at the base of their 700' tower. The electrical length of the notch filter was 1/4 wavelength at 1520 kHz, and its inner/outer conductors were shorted together at the far end.
This puts a very large impedance (almost infinite) to 1520 kHz energy at the input end, and reduced any possible radiation of 1520 kHz energy practically to zero. The notch filter was additional to the harmonic filtering in the transmitter itself, which was -80 dBc or more.
I haven't seen the WJR tx facilities since then, but I suspect that the notch filter is still in use there.
This puts a very large impedance (almost infinite) to 1520 kHz energy at the input end, and reduced any possible radiation of 1520 kHz energy practically to zero. The notch filter was additional to the harmonic filtering in the transmitter itself, which was -80 dBc or more.
I haven't seen the WJR tx facilities since then, but I suspect that the notch filter is still in use there.
Compilation video of WBBM towers coming down: https://youtu.be/2zFCmdHpyX4
Over here in SE Michigan, WGN and WBBM used to be about the same signal strength Days. Now WGN is much stronger, and WBBM is hardly heard at all. If it weren't for WCFS 105.9, their ratings would be down. It has to be weaker on the "Coast", from distance and reduced power. Industry people who thought this was so great are shooting themselves in the foot. What advantage do they get from HD? The call letter display appearing on a few hundred radios?
As someone else said, they have the equipment so they might as well use it. It doesn't make sense that WBBM would use it since they simulcast on FM.
In the Detroit area, the only station which has turned theirs off permanently is WJR. That was probably because of the mutual CH and Night interference with WABC. Even the Class IV station still has it on. The frequencies from about 730 to 760 are like an Oasis at Night. WABC only gets interference from WBBM, so it's less prevalent.
WBBM's DX signal is weaker. I realize that most listeners and the people that run the station don't care about this. The fact is that on remote SDR's from Hawaii to Europe WBBM is significantly weaker or missing from receivers where it used to be heard. Obviously the money they made from the sale of the land where the old tower was standing was most important. FM is where most of their listeners in the Chicago area are now.
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