April 2001 - Co-Channel Protection

The co-channel season is here. The sun will be at the stormiest phase of its 12-year cycle. Whether your system has been hit by co-channel brownouts and rolling blackouts, or the co-channel season has not reached the area, Biro Engineering offers a three-step co-channel interfer-ence protection plan for your consideration.

STEP ONE.

STEP TWO.

STEP THREE.

THE COMPUTER-AIDED CO-CHANNEL STUDY

Before conducting any meaningful co-channel verification testing, the CATV technician should know in advance which are the potential offenders. These days, computer generated and printed studies list with incredible speed and accuracy all potential co-channel interference sources within 400 miles. The accuracy is better than 1/100 mile for distance and 0.1 degree for direction. The design of interference rejecting phased-arrays require that level of accuracy. Some suppliers still use "pioneer" methods. The results are often less than satisfactory.

CHAN	CALL	LOCATION	STATE	NTWK	DTV	POL	POWER kw	OFST	HAAT FEET	DISTANCE	AZIMUTH DEGS.
DESIRED STATION
2	WFMY	GREENSBORO	N.C.	CBS		H	100.0	-	1840.	64.43 MI	38.0
CO-CHANNEL CALCULATION
2	WCBD	CHARLESTON	S.C.	NBC		C	100.0	+	1950.	159.29 MI	161.9
2	WSJK	SNEEDVILLE	TN.	ED		H	100.0	+	1760.	170.63 MI	301.0
2	WSB	ATLANTA	GA.	ABC		H	100.0	0	1033.	236.85 MI	247.5
2	WUND	COLUMBIA	N.C.	ED		H	100.0	0	990.	241.87 MI	76.2
2	WKRN	NASHVILLE	TN.	ABC		H	100.0	-	135.	358.19 MI	281.9
2	WMAR	BALTIMORE	MD.	ABC		H	100.0	+	1074.	360.05 MI	35.3
2	KDKA	PITTSBURGH	PA.	CBS		H	100.0	-	968.	370.50 MI	4.3
2	WDTN	DAYTON	OH.	ABC		H	100.0	0	1000.	375.58 MI	328.4

A frequent misconception is the distance of the undesired TV stations. It should not be estimated with the aid of a road atlas. Electromagnetic waves propagate in great circle mode, the shortest possible path between two points on the globe. The distances calculated for the Fairview, N.C. antenna site are exact to 1/100 of the mile. The Azimuth Angles, their direction from the true North, to 1/10 of a degree. Compare our information accuracy with those of the "free" signal sur-veys.

ON-SITE INTERFERENCE VERIFICATION TESTING

The fact that 33% of the US TV stations are offset by +10 kHz, and 33% of the stations operate with -10 kHz frequency offset, makes co-channel offender identification possible and practical. Namely, the offset created ±10 kHz and ±20 kHz beats are clearly visible on the screen of the spectrum analyzer. The rotation of the search antenna will change the amplitude of the beats.

Note the -10 kHz beat (two divisions to the left of center) and the +10 kHz beat (two divisions to the right) on the screen of the analyzer. The +10 kHz beat was consistently 5 to 8 dB higher, resulting in positive interference source identification.

THE NEED FOR PRECISION ANTENNA ORIENTATION

The Computer Aided Co-Channel Study was completed for the antenna site, followed by an on-site Verification Survey, and culminating in the proper antenna array design. Unfortunately, there is still a possibility for poor interference rejection. Observe the computer generated phased-array radiation patterns below. The nulls are narrow and deep, meaning that the orientation of the array can make all the difference between moderate (20 dB) or excellent (40 dB) interference rejection.