Hereford (Tucson) AZ: Co-Channel Interference from Mexico - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 15 Old 05-13-2019, 09:25 AM - Thread Starter
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Hereford (Tucson) AZ: Co-Channel Interference from Mexico

I moved from my hill location in Northern California to a valley location in Hereford, AZ. I still don't want to pay for satellite so I set up my antennas in my new location. This is another fringe area as shown by TV Fool:

http://www.tvfool.com/?option=com_wr...903806eefa1495

TV Fool is way out of date as the repack is complete here. The only station above RF 36 is a Mexican station on RF 43.

My reception of the Tucson stations on Mt. Bigelow is reliable with two exceptions. KMSB VC 11 / RF 25 and KUVE VC 46 / RF 34 are co-channel with Mexican stations XHCAN and XHCNS respectively. Reception is reliable even on the low power stations. I'm using my 37 element homemade log periodic on UHF. It tested 1-2 dB better than the pair of 91XGs to my surprise.

I cannot receive KMSB because XHCAN is nearly 20 dB stronger on the same channel. See attached spectrum analyzer image of the two signals. In order to decode KMSB I need at least 35 dB of rejection. I know of no antenna with that sort of rejection. I tried putting the antenna behind the house where it was shielded to XHCAN and open to KMSB but the KMSB signal dropped a lot from being near the ground. Too much vegetation to look through.

I know this is a tough problem and I don't expect any miracles but I wonder if anyone has any ideas that I haven't thought of? I have not tried constructing a shielded box with one end open pointed to Mt. Bigelow. That's a significant project.

I subscribed to Hulu so I can stream FOX programs. This is okay but it doesn't include live programs. KMSB live streaming is available for $45 per month which I won't pay.

Trip told me that my area is not in the Tucson service area so no objections were raised before XHCAN and XHCNS went on the air.

Chuck
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post #2 of 15 Old 05-13-2019, 11:41 AM
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Quote:
Originally Posted by Calaveras View Post
I moved from my hill location in Northern California to a valley location in Hereford, AZ.
Well, that's a surprise, Chuck. I keep thinking of you as a dedicated member of the NCDXF.
Quote:
I still don't want to pay for satellite so I set up my antennas in my new location. This is another fringe area as shown by TV Fool:

http://www.tvfool.com/?option=com_wr...903806eefa1495

TV Fool is way out of date as the repack is complete here. The only station above RF 36 is a Mexican station on RF 43.
Here is an image of your new report:



and an image of a rabbitears.info report:



The distance and azimuth of the rabbitears.info reports don't always match those of the TVFool report because the location accuracy of the FCC TVStudy software isn't as good as the TVFool's; it's only to the nearest 0.5 km.

Quote:
I'm using my 37 element homemade log periodic on UHF. It tested 1-2 dB better than the pair of 91XGs to my surprise.
That is a surprise. How does the F/B and F/R compare with the 91XGs?
Quote:
I have not tried constructing a shielded box with one end open pointed to Mt. Bigelow. That's a significant project.
That would be a big project, and you would still have reflections of the Mexican stations from objects in front of the antenna.



Quote:
I know this is a tough problem and I don't expect any miracles but I wonder if anyone has any ideas that I haven't thought of?
Several thoughts come to mind to improve the F/B:
Stagger stacking
Tower Guy (retired TV station engineer) mentioned a 2x2 array of yagis on the TVFool forum. Two of the yagis were to increase the gain and create a rear null, and two were for stagger stacking to increase the F/B.
http://forum.tvfool.com/showpost.php...76&postcount=8
Quote:
Originally Posted by rabbit73
Ah, so; a null is also created at the back.
Quote:
Originally Posted by Tower Guy
Yes. So here's a true story. In the late 70's I installed a translator in Utica, NY. It retransmitted WNYT on channel 13 from Albany. There was also a channel 13 in Rochester, NY that was 14 degrees off the back of the antenna. The terrain toward Rochester was more favorable than the terrain toward Albany. When testing reception with a 10 element yagi Albany was noisy and showed cochannel interference. When aimed at Rochester the audio was fine but the picture was not there. By ganging 4 yagis; two with horizontal stacking and two offset using stagger stacking, the picture from Albany was perfect.
Low Noise Yagi
It's a Yagi that uses a horizontal loop for the DE; very quiet on the rear.
https://www.g0ksc.co.uk/intro-lfa.html

This is my housewarming present to you.
73,
rabbit
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post #3 of 15 Old 05-13-2019, 06:49 PM
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I actually ran Chuck's location in TVStudy directly when we chatted privately, rather than from the web interface. The results are basically the same though, and TVStudy is generally accurate. I think you might be recalling what I said about the FCC's DTV Maps page, which are not point-to-point the way TVFool or the RabbitEars TVStudy mode is, but rather goes to a nearby precalculated point.

In any case, the issue is that Longley-Rice doesn't handle knife-edge diffraction well, so the predictions for the Bigelow stations are pessimistic by 15-20 dB.

- Trip
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Comments are my own and not that of the FCC (my employer) or anyone else.

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post #4 of 15 Old 05-14-2019, 06:01 AM
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Quote:
Originally Posted by Trip in VA View Post
I actually ran Chuck's location in TVStudy directly when we chatted privately, rather than from the web interface. The results are basically the same though, and TVStudy is generally accurate. I think you might be recalling what I said about the FCC's DTV Maps page, which are not point-to-point the way TVFool or the RabbitEars TVStudy mode is, but rather goes to a nearby precalculated point.
Thank you for the clarification, Trip. My memory is faulty. I thought TVStudy for FCC DTV maps and rabbitears.info was precalculated for a more rapid result and to reduce server load.

But now I am wondering why azimuth and distance don't match. Perhaps it's because I diidn't have his exact coordinates.

Quote:
In any case, the issue is that Longley-Rice doesn't handle knife-edge diffraction well, so the predictions for the Bigelow stations are pessimistic by 15-20 dB.

- Trip
Ah, yes; I've seen that before in TVFool reports with 2Edge signals that are way off.

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post #5 of 15 Old 05-14-2019, 07:20 AM - Thread Starter
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Quote:
Originally Posted by rabbit73 View Post
Ah, yes; I've seen that before in TVFool reports with 2Edge signals that are way off.

I ran a Noise Margin test on KOLD which is one of the weaker UHF stations and the NM was 12 dB. When I take system noise figure, antenna gain and misc losses into account that make it 9 dB stronger than predicted. That's not as much as I expected.

The good thing here is that there's very little influence from temperature inversions. For the most part signals don't vary by more than a couple dB over 24 hours.

Chuck
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post #6 of 15 Old 05-14-2019, 08:07 AM - Thread Starter
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Quote:
Originally Posted by rabbit73 View Post
That is a surprise. How does the F/B and F/R compare with the 91XGs?
That would be a big project, and you would still have reflections of the Mexican stations from objects in front of the antenna.

Several thoughts come to mind to improve the F/B:
Stagger stacking
Tower Guy (retired TV station engineer) mentioned a 2x2 array of yagis on the TVFool forum. Two of the yagis were to increase the gain and create a rear null, and two were for stagger stacking to increase the F/B.
http://forum.tvfool.com/showpost.php...76&postcount=8
Low Noise Yagi
It's a Yagi that uses a horizontal loop for the DE; very quiet on the rear.
https://www.g0ksc.co.uk/intro-lfa.html

Stagger stacking is something I could try since I have two 91XGs on the ground. But if there are reflections in front of the antenna then that would negate anything you'd get from stagger stacking.

Even though I've seen models showing over 30 dB F/R, in practice it never seems to be much better than the 25 dB and often only 20 dB.

Now that my TV/FM installation is complete, I can try to optimize the house blocking the Mexican station.

I had to use the big high VHF LPDA because KGUN on RF 9 is far weaker than predicted. This goes against my previous experiences with high VHF. I have managed to get the SNR up to 22-24 dB.

Chuck
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post #7 of 15 Old 05-14-2019, 10:13 AM
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It looks like you will need F/B & F/R Ratios of about 30+ dB [and a bit more since not exactly opposite each other]. Lets look at the alternatives....

1) I was NOT impressed by the Rear Suppression of a simple UHF 8-El FLF-Yagi (with just ONE Reflector)...or course adding more Reflectors would help:
https://imageevent.com/holl_ands/yag...8flfyagi300ohm

2) OTOH, a SINGLE CHANNEL (Ch22 example) 9-El FLF-Yagi (Flat Loop Fed, with just ONE Reflector) provided MORE THAN 40 dB Rear Rejection...but you would presumably need one for Ch25 and another for Ch34....plus a Full-Band UHF Antenna for the remainder....PLUS a pair of Single Channel Insertion Filters:
https://imageevent.com/holl_ands/yag...9flfyagi300ohm
PS: I call these FLF = Flat Loop Fed Yagi's, which is a generic name for something invented many decades ago, as explained in link.

3) I SHOULD be able to Re-Optimize 9-El FLF-Yagi....or one with even more elements, esp more Reflectors, so that Rear Rejection is Optimized for BOTH Ch25 AND Ch34....with whatever is left for remainder of UHF Band. Let me know if you are interested....

4) Sigma-Tau Twin-Boom LPDA: I conducted a study of Optimized LPDA's to determine the "Best" combination of Sigma & Tau Design Parameters for a wide range of Element Pairs. IF you chose this alternative, I would recommend building a "Layered" Twin-Boom LPDA with 19-Element Pairs, which should provide close to 40 dB Rear Rejection. Also note that there is only minimal Gain increase by using even MORE Element Pairs. I can look at the Spread Sheet to find Dimensions if you are interested [PS: Spread Sheet Graphs are for ALL Sizes, Results shown are only for 7-El Pair LPDA's]:
https://imageevent.com/holl_ands/zig...dawedgelayered
Note that 40+ dB of Rear Rejection is maintained across the ENTIRE Rear Hemisphere, so you will have superior Rear Rejection even when Pointed towards the Desired Signals, even though Mexican Stations are NOT exactly opposite Desired Direction.


5) Sigma-Tau FLAT LPDA: Cut out of two pieces of Sheet Metal (similar to Silver Sensor and Terk HDTVi). Here are results for 15-El Pair FLAT LPDA...19-El Pair would be a bit Longer. Let me know if you are interested in this alternative:
https://imageevent.com/holl_ands/zig...5elflatlpdaopt


6) Your 37-Element LPDA: If you provide details, I can analyze and let you know how it is REALLY working.....

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post #8 of 15 Old 05-14-2019, 11:16 AM - Thread Starter
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Originally Posted by holl_ands View Post
6) Your 37-Element LPDA: If you provide details, I can analyze and let you know how it is REALLY working.....
W8IO analyzed it for me when I built it. The results are attached. Do you think any of your suggestions would be better than this?

http://www.w8io.com/LPCAD.htm

I don't care about channel 34 because it's a Spanish station with just a couple of low interest sub channels. I'd be happy with just channel 25.

This morning I ran the test of using the house to block Mexico. I put a 91XG at 12' behind the house but where it was unblocked to Mt. Bigelow. The SNR went up a few dB but not enough to decode. The main problem was that the KMSB signal went down about 10dB. Despite what TV Fool says, higher is better. I need the signal of having the antenna at 27' but the Mexico rejection of behind the house. :-(

Chuck
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post #9 of 15 Old 05-14-2019, 04:52 PM
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Beautiful pattern, Chuck.

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Originally Posted by Calaveras View Post
Stagger stacking is something I could try since I have two 91XGs on the ground. But if there are reflections in front of the antenna then that would negate anything you'd get from stagger stacking.
No more so than the reflections reducing the performance of your very impressive UHF LP even if it was shielded by a building behind it.

ANY antenna aimed at KMSB Fox 25 (335 deg), will pick up front reflections from XHCAN 25 (197 deg).

At my former location I had an antenna aimed south (5 ft AGL) with a building behind it. It picked up reflections from the front of a channel from the NW. The channel could not be decoded because it was only multipath reflections, but it would have been strong enough to cause co-channel interference:



If mounting an antenna with a high F/R in the best location you can find doesn't give you enough SNR to solve a co-channel problem, the only way to reduce the reflections from the front is to put the antenna in an enclosure with an opening in the end just large enough to allow entry of the desired signal.
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post #10 of 15 Old 05-15-2019, 01:00 PM - Thread Starter
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Quote:
Originally Posted by rabbit73 View Post
If mounting an antenna with a high F/R in the best location you can find doesn't give you enough SNR to solve a co-channel problem, the only way to reduce the reflections from the front is to put the antenna in an enclosure with an opening in the end just large enough to allow entry of the desired signal.
I tend to think you're right about this. When you need >35 dB of rejection even weak reflections will cause a problem. I may try the enclosure at some point but right now I'm too busy settling in with many higher priority projects.

Chuck
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post #11 of 15 Old 05-15-2019, 01:26 PM - Thread Starter
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Originally Posted by rabbit73 View Post
At my former location I had an antenna aimed south (5 ft AGL) with a building behind it. It picked up reflections from the front of a channel from the NW. The channel could not be decoded because it was only multipath reflections, but it would have been strong enough to cause co-channel interference:


The spectrum analyzer is an unreliable tool to determine multipath because high amplitude variation / high multipath and high amplitude variation / low mulitpath can look the same. In the attached images both KHRR and XHAPT can be decoded with an SNR of 17 dB. I was very surprised the first time KHRR decoded knowing how ugly the spectrum analyzer display was. As long as no part of the signal was below -86dBm on the analyzer it would decode. The path to KHRR is really bad.

Chuck
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post #12 of 15 Old 05-15-2019, 05:48 PM
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Originally Posted by Calaveras View Post
The spectrum analyzer is an unreliable tool to determine multipath because high amplitude variation / high multipath and high amplitude variation / low mulitpath can look the same.
That is true; the display is only amplitude vs frequency. I have also had signals with a bad looking scan decode. A bad looking scan doesn't have to mean there are multipath reflections. In my case with CH42 I can say multipath reflections because there was no direct signal at all. And in the case of CH13 in my avatar, I can also say multipath reflections because my antenna was an indoor folded dipole without a reflector. When I added a reflector the scan improved and the signal decoded.

The converse is also true: when the scan looks good, the signal might not decode. That happened to Trip where he lived just before moving to his new FCC job. He didn't understand what was happening at first, but when he put his antenna in a can, the local signals decoded.
https://www.avsforum.com/forum/25-hd...l#post22156611
Quote:
Originally Posted by Trip in VA View Post
What's strange is that the multipath here doesn't really show on the spectrum analyzer I have. The signals all look flat, they just don't decode. And I've moved my outdoor antenna all over the deck in every possible position.

http://www.rabbitears.info/specan/chatt/

I did a side-by-side on WSB and WDSI because WSB decodes and WDSI does not.

- Trip
In one of his papers, Dr. Oded Bendov explains that the reflections can fill in the dips in a scan, making it look good.

DTV Coverage and Service Prediction, Measurement and Performance Indices
by Dr. O. Bendov

page 4

VIII. SNR AND “FIELD STRENGTH” MEASUREMENT VIA
SPECTRUM INTEGRATION

Quote:
Defining the Signal as the total received power and theNoise as AWGN (Additive White Gaussian Noise) leads to the conclusion that the SNR at the input to the receiver increases with increased multipath.

In urban and indoor situations, there may not even be a main signal, only reflections, some of which are of equal magnitude.

If all multipath signals are part of the signal power, then the SNR margin may not be an indicative figure of merit of reception robustness. In any case, even accurate measurement of the total received power may not be trivial.

The integrated signal power is not just the Desired Signal power. It includes, Man-made, Galactic, and thermal noises and residual transmitter generated in-band noise. It also includes some but not necessarily all multipath signals. For example, pairs of identical and asymmetric echoes, one of positive amplitude and positive delay relative to the main signal and one of negative amplitude and negative delay relative to the main signal, will cause only a second-order distortion of the displayed power spectrum. They will create group delay. Thus, in a multipath channel, a pair of such echoes would measure high SNR when using the spectrum integration technique whereas in reality, the true SNR would be much lower.
There may be other combinations of echoes that would yield essentially flat spectrum display.




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post #13 of 15 Old 05-16-2019, 05:06 PM
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Originally Posted by Calaveras View Post
W8IO analyzed it for me when I built it. The results are attached. Do you think any of your suggestions would be better than this?

http://www.w8io.com/LPCAD.htm

I don't care about channel 34 because it's a Spanish station with just a couple of low interest sub channels. I'd be happy with just channel 25.
You didn't post a link to the Design for YOUR "37-Element" LPDA....just the generic LPCAD Design Program.
So I had to eyeball performance from your posted Charts:
Ch25 (~520 MHz): Gain ~14.8 dBi, F/B & F/R Ratio ~32 dB.
Ch34 (~590 MHz): Gain ~14.2 dBi, F/B & F/R Ratio ~30 dB.
But there is considerable variation with Frequency, so ACTUAL performance may be a bit BETTER.....or WORSE....

My recommendations 2), 3), 4) and 5) should all provide a (more or less guaranteed) MINIMUM of 40 dB F/B & F/R Ratio....so theoretically they should be significantly "better". Let me know if you have a preference(s) so I can fire up the Re-Optimizer so F/B & F/R are Optimized MOSTLY for Ch25.


I also intend to analyze the "Terk-In-A-Trashcan" Antenna shown in photos in preceding post to see if there is any significant improvement....and/or degradation to the Terk's Antenna Pattern....and perhaps for different Trash Can Sizes.....

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post #14 of 15 Old 05-16-2019, 07:52 PM - Thread Starter
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Originally Posted by holl_ands View Post
You didn't post a link to the Design for YOUR "37-Element" LPDA....just the generic LPCAD Design Program.
So I had to eyeball performance from your posted Charts:
Ch25 (~520 MHz): Gain ~14.8 dBi, F/B & F/R Ratio ~32 dB.
Ch34 (~590 MHz): Gain ~14.2 dBi, F/B & F/R Ratio ~30 dB.
But there is considerable variation with Frequency, so ACTUAL performance may be a bit BETTER.....or WORSE....

My recommendations 2), 3), 4) and 5) should all provide a (more or less guaranteed) MINIMUM of 40 dB F/B & F/R Ratio....so theoretically they should be significantly "better". Let me know if you have a preference(s) so I can fire up the Re-Optimizer so F/B & F/R are Optimized MOSTLY for Ch25.


I also intend to analyze the "Terk-In-A-Trashcan" Antenna shown in photos in preceding post to see if there is any significant improvement....and/or degradation to the Terk's Antenna Pattern....and perhaps for different Trash Can Sizes.....
I didn't understand what you were asking for. Attached is the design info for the antenna. Ignore the "1/2 el length + .25in." That's the length I needed to make each element half to insert into the boom to make the total length come out right. The boom material is 1/2" square aluminum.

Your Single Channel 9-El FLF-Yagi sounds like it might be the easiest to build. I'd be interested to see the analysis of the Terk in a Trashcan even though that's a lot of wind load.

Chuck
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post #15 of 15 Old 05-17-2019, 03:07 PM
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I started the FIRST Run for UHF 9-El FLF-Yagi (with just 1 Reflector Rod)....Re-Optimized for Full-Band Gain and 40+ dB F/B & F/R Ratio for JUST Ch24-26 and 17 dB elsewhere....stand-by for a few days/weeks, depending on what I have to do next. Also started UHF 110El FLF-Yagi with 3 Reflector Rods....
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