Make a co-channel from the rear weaker?

[lithOTA]

300, you're right, I have overbuilt this mast over the last two years. Part of the reason is that the brackets bolt into the outside wall of our bedroom, so on windy nights there would be squeaks and groans resonating through the wall. It wasn't the kind of noises to be worried about, but it kept us awake, so I have beefed it up to keep it quiet.

VanWinkles, thank you for the compliment. I woke up Sunday and realized that I already had everything I would need, so it was worth a shot.
Even though it didn't work yesterday, there was some strange tropo going on that could've been boosting WXFT. Since it is working very well on my "normal", non-co channel stations, I'll leave it up for the week. It might work better when the tropo juice goes away.

[mrvanwinkles]

Searching the internet at large:

found:

http://www.kyes.com/antenna/stackluge.html

Some interesting points about antenna stacking.

Explanation of why Horizontal or Vertical stacking works.

States that optimum distance between antennas for stacking is around 0.94 of wavelength. ( between 0.6 and 0.94 of wavelength of frequency of interest ). States that more or less spacing has negative effects.

Some good points about how to run your phasing lines so they don't interfere with the reception of the antennas.

[nikiml]

Originally Posted by lithOTA

I believe that. Today, I staggered a couple of C2s (actually a deconstructed C4). 1/4 wave of 689 is 4.28 inches, which was the easy part. Getting the long cable right is another matter.

you need a bit of Y-shift(left-right) as well to put the rear Null at exactly 160(or 200)degree

[300ohm]

States that optimum distance between antennas for stacking is around 0.94 of wavelength.

Keep in mind thats only for Yagis and LPDAs. Ive found that for GHs its more like around 2 times the wavelength minus about 2%.

[rabbit73]

lithOTA:

Your co-channel problem reminds me of what balm is trying to do to get reliable reception of WVNY with a co-channel local signal off the back of his antenna:
http://www.digitalhome.ca/forum/showthread.php?t=128429

The tvfool report that he posted for St-Anicet looks like this:
http://www.tvfool.com/?option=com_wr...03d98eba129270
WVNY is very weak for him at his summer cottage and his requirements are:

1. High gain antenna for CH13
2. Ground the coax of the antenna system and TV to shield them from a strong local analog signal (helpful suggestion from holl_ands because the strong local couldn't be eliminated even with 50dB of attenuation!)
3. Mount antenna at optimum height to max signal enhancement from ground reflection ("ground bounce"). His tvfool report was correct about the best height
4. Design antenna for max F to B ratio
5. Utilize any helpful reflections in front of the antenna without picking up any reflections from the undesired co-channel signal from the rear

Balm tried stagger stacking, but it didn't give enough F to B to eliminate the undesired co-channel signal.

He built a nice long yagi for CH13 but it still wasn't enough.

When he aimed his antenna at his metal boathouse that was slightly off-azimuth for WVNY, it gave some signal enhancement, but it probably also reflected some signal from the undesired co-channel signal from the rear.

He plans to try again this spring, and is considering a VHF 4-bay CH12 antenna in an enclosure shielded at the rear and all 4 sides...a monster antenna. The trick will be to keep it from picking up any reflections from the rear signal without sacrificing any signal enhancement from the front.

He made some tests in the fall by mounting his antenna inside the boathouse to eliminate any signals from the rear and sides. WVNY was very weak, which leads me to believe that the tvfool report was too optimisitic.

Even if you mount your antenna in a well shielded enclosure for the sides and back, you still must be careful not to pick-up any reflections in front of the antenna that come from the undesired co-channel signal.

One serious problem that you have is that WISC is 85 miles away. The curvature of the earth starts to become a problem at about 70 miles:



This is the WVNY profile for Balm:

[stampeder]

Originally Posted by kyes

QUAD_STACKS

Stacking four identical antennas, two vertically and two horizontally in a rectangular or diamond pattern, restricts the vision of this combination in all directions off the axis. Called a quad stack, it "sees" as though it were looking through a tube pointed in the direction of the transmitting antenna.
Gain is increased approximately 4 to 5 dB over that of a single antenna.

Wade used to sell a CATV "diamond stack" made up of 4 UHF channel-cut antennas. One pair was mounted side-by-side, and the other pair was above-and-below, all arranged with the pairs orthogonal and their spars equally spaced. Think of a box stack rotated 45 degrees but with the antenna elements kept horizontal. The beam was incredibly tight so aiming was a job for qualified technicians.

Originally Posted by mrvanwinkles

States that optimum distance between antennas for stacking is around 0.94 of wavelength. ( between 0.6 and 0.94 of wavelength of frequency of interest ). States that more or less spacing has negative effects.

Applicable only to channel-cut (single channel specific) antennas or arrays. For broadband antennas you start with a separation of one full wavelength of the lowest frequency channel desired and start tweaking from there.

Originally Posted by 300ohm

Ive found that for GHs its more like around 2 times the wavelength minus about 2%.

Once again the GH breaks the rules...

[lithOTA]

Originally Posted by rabbit73

lithOTA:
One serious problem that you have is that WISC is 85 miles away. The curvature of the earth starts to become a problem at about 70 miles:

True, but take a look at WKOW and WMSN- they're on the same tower, but at lower height, than WISC. WMSN is a tough get, but I have it almost 24/7. WKOW has about 3 points more NM than WISC, and it's pretty much dead-solid all of the time- even from the attic.
Last night, I did get some decode on WISC- no more than 15:1 SNR- so I took a peek outside to verify whether it was pointed at 290 (directly opposite WXFT) or 315 (directly at WISC). It was pointed at 315, and the signal went away when I turned left to 290. This leads me to believe that I was getting it because of a little tropo enhancement, and not because I was phase-cancelling WXFT.
Regarding the accuracy of the cable lengths when doing a stagger stack- how accurate do those cuts need to be? In other words, if one cable is off by 1/4", is that enough to prevent cancellation? Would a good strategy be to start with a way-too-long cable and trim it by 1/4", try it, trim it another 1/4", and try it again, until I hit the magic number?
On the other side of the coin, if my cable lengths were wrong, wouldn't that make my forward gain suffer (because the signals wouldn't add together properly?)

[300ohm]

It was pointed at 315, and the signal went away when I turned left to 290. This leads me to believe that I was getting it because of a little tropo enhancement,

Or that youre getting a strong signal boost from a reflected object at 315 degrees. Youll need to repeat the experiment over many more days to know for sure.

On a lot of tropo days, I can get a station that is 130 miles away and that is pretty close to my DBGHs null zone. I am also pretty close to being in the transmitter antennas null zone also. The point is, the direction the tropo signal comes from can be highly unpredictable.

Would a good strategy be to start with a way-too-long cable and trim it by 1/4", try it, trim it another 1/4", and try it again, until I hit the magic number?

Yes, but unfortunately you wont know when to stop, which would mean eventually cutting another cable at the best length.
Another option is, if you can find some some, is to use 450ohm twin lead to join the two antennas and use a small sliding piece of aluminum foil over it to adjust for the best signal.

[Stereocraig]

lithOTA:

Is WISC football programming different than WBBM out of Chicago?

I've thought about pointing that way to see what I can pick up.

[mrvanwinkles]

Originally Posted by nikiml

you need a bit of Y-shift(left-right) as well to put the rear Null at exactly 160(or 200)degree

So is that true?

That on a Vertical, 2 antenna stagger stack, that shifting one antenna a little to one side could move the rear deep null also ... a little to one side?

If true ... then mounting the whole rig on some sort of adjustible mounts for experimentation / and final setup ... might be worth the effort.

ex. threaded rods w/adjustment nuts. So long as their placement does not interfere with the incoming signals.

or, ex. short slots in support members (rather than just holes), where the antenna frame could be slid a little each way, nuts tightened, and tried.

[Dave Loudin]

Just a friendly reminder - the affect of a physical layout on the reception pattern is frequency dependent. Therefore, a design that achieves deep nulls in a particular direction is inherently a narrowband, even single-channel, design.

[lithOTA]

Originally Posted by Stereocraig

lithOTA:

Is WISC football programming different than WBBM out of Chicago?

I've thought about pointing that way to see what I can pick up.

Different enough. All 4 of my markets (Chicago, Rockford, Milwaukee, Madison) will show the Bears or Packers first, and the other second. If neither of those is available, they usually go to the other 2 teams in the division, the Lions and Vikings. Chicago, Rockford, and Milwaukee will usually choose the Lions (especially now that they are good), but Madison (WMSN FOX-47 and WISC CBS-3) will always choose the Vikings beacuse the market abuts Minnesota. It's probably worth between 2 and 4 extra games per season.
It's not that I'm a Vikings fan, I'm just greedy and want as many games as possible. After all, people pay Direct TV big bucks for NFL Sunday Ticket, and I get a reasonable facsimile for FREE. I love stickin' it to the man.

[nikiml]

Originally Posted by mrvanwinkles

So is that true?

That on a Vertical, 2 antenna stagger stack, that shifting one antenna a little to one side could move the rear deep null also ... a little to one side?

If true ... then mounting the whole rig on some sort of adjustable mounts for experimentation / and final setup ... might be worth the effort.

The whole idea of the stagger stacking is that the distances the rear signal travels from the transmitter to the connection point of the two antenna's feed lines should differ by wavelength/2, and for the front signal there should be no difference.

For exactly opposing transmitters it seems the only way is to shift the antennas in X by wavelength/4 and then use feed lines that differ be wavelength/4(corrected by the feed line velocity factor).

For signals at an angle, one has many more options. This is because shift in Y changes the distances the rear signal travels, but not the distances the front signal travels.

Therefore the stagger effect can be achieved in many ways:
1) just Y shift such that the distance from the rear transmitter to the two antennas is wavelength/2 + equal feed lines.
2)shift in X by wavelength/4 and some shift in Y such that the distances from the rear transmitter to the both antennas differ by wavelength/4 + feed lines that differ by wavelength/4
3) any combination of shifts and feed line lengths that ends up with the desired length differences. (here is where an optimizer would be useful)

Since the X shift of the antennas tilts their frontal lobe up or down
I imagine it would be better to use less [b]X[/] shift and use more Y shift.

Now to the question about an adjustable mount - the problem with it is that every time you adjust it you'll have to re-cut the feed lines as well.

[rabbit73]

Originally Posted by lithOTA

True, but take a look at WKOW and WMSN- they're on the same tower, but at lower height, than WISC. WMSN is a tough get, but I have it almost 24/7. WKOW has about 3 points more NM than WISC, and it's pretty much dead-solid all of the time- even from the attic.

Yes, they are on the same tower (43.055829 -89.535122), but they don't have to contend with an undesired co-channel signal that is 38.5 dB stronger.

The F to B of your WISC antenna needs to be greater than that, because you must add 15.5 dB to avoid co-channel interference from WXFT.

Take a look at
ATSC Recommended Practice: Receiver Performance Guidelines
http://www.atsc.org/cms/standards/a_74-2010.pdf2.72 MB

Scroll down to 5.4.1 Co-Channel Rejection page 13 and 14 and you will see Table 5.1 Co-Channel Rejection Thresholds

The receiver should meet or exceed the following thresholds for rejection of co-channel interference at the following desired signal levels (Table 5.1).

So you see, you need at least 54 dB F to B because the top of the WXFT signal must be below the SNR floor for WISC.

I have ignored the forward gain of the WISC antenna on purpose, because you will need that as fade margin for consistent reception of the 2Edge WISC signal that acts like Tropo.

On the other side of the coin, if my cable lengths were wrong, wouldn't that make my forward gain suffer (because the signals wouldn't add together properly?)

Correct

I certainly don't want to discourage anyone from doing antenna experiments because I always learn a lot from them even when the results are not what I hoped they would be. But, I don't think stagger-stacking will give you enough F to B/R, Mike.

However, you might get lucky when WISC is coming in strong.

[lithOTA]

Originally Posted by rabbit73

But, I don't think stagger-stacking will give you enough F to B/R, Mike.

I think I've arrived at that realization. Perhaps, with a couple of strong Yagis like the 91XG, 9032, or MXU59, I might be able to get that F/B up that high...but that's a whole lot of metal for just one station.
Last night, I took the cables off and re-measured them, and made sure I had a good pair for a velocity factor of .85 (as quoted by the cable manufacturer). I ended up trimming about 1/4 inch off of the longer cable, but when I re-installed, the results were just the same. Considering that the front-side signals are doing great, and that I do see a big null to the rear on RFs 47, 50, & 51, I have to assume that my phased array is working properly- it's just that a pair of C2s are not gonna get me near that F/B ratio that I need.
But still, experimenting with this cost me nothing, and it advanced my understanding very much, so it was well worth the work. Thank you all for the great ideas.