Need Reliable Picture On Ch. 7

[Billiam]

I almost forgot. Additional research shows that the Winegard Baluns are not quite up to snuff with others like the CM 3075.

Can anyone verify which Baluns are going to offer lower signal loss than the Winegard and especially on VHF? I may get lucky and pick up another DB of gain this way.

[Tower Guy]

Originally Posted by Billiam

Can anyone verify which Baluns are going to offer lower signal loss than the Winegard and especially on VHF? I may get lucky and pick up another DB of gain this way.

A better balun would also raise the signal strength of the interfering stations. You need to drop the signal strength of the two interfering stations. I would not waste any time on making KQTV stronger until the interference has been reduced.

[Billiam]

Originally Posted by Tower Guy

A better balun would also raise the signal strength of the interfering stations. You need to drop the signal strength of the two interfering stations. I would not waste any time on making KQTV stronger until the interference has been reduced.

That makes sense.

[GroundUrMast]

Originally Posted by Tower Guy

I'd suggest that the problem is a combination of a weak signal and interference from KOAM.

Your pair of Y1713s can be stacked horizontally about 34" apart to null KOAM and still add 2-3 db to KQTV. http://www.hdtvprimer.com/ANTENNAS/ganging.html

When combined with a CC7870 they will add by about 2 db. If this arrangement works your problem is interference. If not, you'll need more antenna gain.

When I crunch the numbers, I come up with 31 1/8" boom to parallel boom to null KOAM. Logarithmic center frequency of CH-7 = 176.975 MHz so 1/2 wave length = 33.34618". Relative angle of KOAM vs KQTV = 111°... 33.34618" * COS(111-90) = about 31 1/8". If you are building a Horizontal stack, just be sure you can adjust the boom to boom dimension a bit either way.

If you want to field build a balanced open wire line combining harness, here is a table for various wire gauges.

[Billiam]

GUM. Thank you for taking the time to do these calculations. That spacing distance is considerably better than the 38 inch option.

Won't those elements be awful close together on the back of the antenna?

I assume this distance will work for any antenna that receives CH. 7 and not just the YA 1713?

[Tower Guy]

Originally Posted by GroundUrMast

When I crunch the numbers, I come up with 31 1/8" boom to parallel boom to null KOAM.

Your math is correct. I did extrapolate off the right side of the graph. In any event you may have discovered an offset in the HDTV primer graph.

Unfortunately the element length of the reflector is longer than 31 1/8".

[GroundUrMast]

Originally Posted by Billiam

GUM. Thank you for taking the time to do these calculations. That spacing distance is considerably better than the 38 inch option.

Won't those elements be awful close together on the back of the antenna?

I assume this distance will work for any antenna that receives CH. 7 and not just the YA 1713?

Originally Posted by Tower Guy

Your math is correct. I did extrapolate off the right side of the graph. In any event you may have discovered an offset in the HDTV primer graph.

Unfortunately the element length of the reflector is longer than 31 1/8".

Yes, that's a problem. One option is to use 3/2 wave length on the vector toward KOAM which would make the parallel boom spacing 93 3/8". (Do I hear an 'OUCH'?)

[Tower Guy]

Originally Posted by GroundUrMast

Yes, that's a problem. One option is to use 3/2 wave length on the vector toward KOAM which would make the parallel boom spacing 93 3/8". (Do I hear an 'OUCH'?)

Another plan would be to have the antennas about 2-4' apart vertically and also offset a total of 31 1/8" left/right. (Or one antenna would be 15.0625 to the left of center and the other would be 15.0625 right of center.)

[GroundUrMast]

Originally Posted by Tower Guy

Another plan would be to have the antennas about 2-4' apart vertically and also offset a total of 31 1/8" left/right. (Or one antenna would be 15.0625 to the left of center and the other would be 15.0625 right of center.)

That solution would add a bit of vertical beam width adjustment as well. A plus when trying to reject the signal from a tropospheric path. If the signal from the offending source is arriving at an angle from above, the simple 2D trigonometry starts to fail. Again, we can estimate the spacing but it may need to be adjusted in the field.

Now I'm wondering if a Rhombic would be an easier solution to fabricate... http://forum.tvfool.com/showthread.php?t=291 It can certainly be less visible than the erector set of hardware needed for an adjustable horizontal stack. At 40 to 70 feet long, front to back, it would be tough to fit on many city lots.

Or a huge 20+ foot home-brew Yagi. (See attached) (Which will not offer the deep null that is possible with the horizontal stack.)

[Billiam]

Originally Posted by GroundUrMast

That solution would add a bit of vertical beam width adjustment as well. A plus when trying to reject the signal from a tropospheric path. If the signal from the offending source is arriving at an angle from above, the simple 2D trigonometry starts to fail. Again, we can estimate the spacing but it may need to be adjusted in the field.

Now I'm wondering if a Rhombic would be an easier solution to fabricate... http://forum.tvfool.com/showthread.php?t=291 It can certainly be less visible than the erector set of hardware needed for an adjustable horizontal stack. At 40 to 70 feet long, front to back, it would be tough to fit on many city lots.

Or a huge 20+ foot home-brew Yagi. (See attached) (Which will not offer the deep null that is possible with the horizontal stack.)

LOL. A 20 foot long yagi on this house would make it tip over.

My guess is that tropo is responsible for the offending source provided that it is a TV signal. Another possibility was given to me in the form of a FM signal as the culprit of the interference. I'm a bit uncertain about this since the only nearby FM transmitter is 2,500 watts give or take and the tower is 9 miles away. Nothing else close by.

[Billiam]

Here are the results of the installation of the new antennas. I replaced the stack of YA 1713's with a single Antennacraft Y10-7-13. At the 15 foot height off the roof a single Y10-7-13 has better gain and than either a single or stacked and phased YA 1713 setup. I added another five feet of height for a total of 20 feet off the roof which added another half db of gain. The Antennacraft antenna is also rejecting interference better as well.

After installing the Hollands FM filter I am still seeing interference though it is not as frequent as it was before with the YA 1713's stacked. After installing the Y10-7-13 I've found this antenna to be superior to the Winegard antennas. A single YA 1713 could not fully lock onto Ch. 7 while a single Y10-7-13 does and also rejects the offending interference better. I am no longer having any kind of complete dropouts of the signal either like before.

So, I don't know if that FM signal is the problem though or even a co channel 7. I am actually starting to think it might be something in the local area because the interference is generally not long lived and does not occur when skip on VHF channels is present. It would make sense if a co channel 7 was causing problems at a time when other VHF skip is present but that is not the case. It also seems to occur at nearly the same intervals of time during the day. This is not a constant problem. The signal drops a few DB and the interference kicks in and then it vanishes the signal is back to its normal strength here.

At this point I can live with the current quality of the picture. The signal is fine during the time of day when I do watch Ch. 7. The only time the interference is happening now is when is a few minutes here and there during the mid day when I am not going to be watching.