Looking at post 16 of the FAQ, I tend to disagree with the advice that when ganging antennas they must be identical. For stacking I definitely agree, but once the antennas are pointing in different directions, enough differences occur in the reception patterns that it becomes impossible to match the phases and thus any gains in having identical antennas are lost.

I have said this before, but a far more important factor is choosing antennas with a narrow beam-width to ensure that you don't pick up any signal with the wrong antenna, as this will cause phase interference. For that reason, bow-tie antennas (such as a CM422x or DB-x) are a very poor choice for ganging. Some might argue that the 8 bay antennas have narrow beam-widths, but they also have side lobes with high gain, so unless you know that the other stations are in a deep null, you could run into the same problem.

An alternative to ganging antennas is to horizontally stack the antennas, and space them so as to create side lobes in desired directions. You can either have the antennas in phase, creating large lobe straight ahead and two smaller lobes on either side, or have them out of phase, creating a null straight ahead and two side lobes that are closer together and larger. This process takes a bit of math to determine the optimal spacing, but should result in great results. Since you are stacking the antennas, they need to be identical and having a wide beam-width antenna will help give you a bit more forgiveness in your calculations.

HDTV primer has some interesting information about how to do this (you should probably read both links), which you can find at the following links (but be sure and come back here for further discussion):

The Two Antenna Trick (indoor version) (http://www.hdtvprimer.com/ANTENNAS/silver.html#TAT)
The Two-Antenna Trick (outdoor version) (http://www.hdtvprimer.com/ANTENNAS/ganging.html#TAT)

I've finally gotten around to doing some updating and rewording in Post #16. :)

I am trying to get stations ~180 degress apart (~170 precisely). Ganging the antennas back to back is very close to achieve my objectives. The gain pattern should be the same as a reflecotless bow-tie with slightly higher gain (?). This is only based on the limited experiments I have done with the 4221HD and the particular M4 I made. I am speculating ganging two 4221HD in exact opposite directions would achieve similar results. Comment?

May be stacking two reflectorless M4s or a reflectorless M8 can be better?

I think I am only a couple of dBs away from perfect for my particular situation and needs. Any suggestion is welcome.

I did read all the links you have mentioned.

Cheers!

Definitely have a look at your TVFool report and try to determine if a reflectorless bowtie will do the trick on its own. If the stations are strong from both directions you might want to try a Stealth Hawk (http://www.digitalhome.ca/forum/showthread.php?t=123803) for its almost omnidirectional pattern.

I have an Antenna Craft U800 sitting at the top of a ten foot mast that is chimney mounted. Three feet of mast is strapped to the chimney, and seven feet rises above.

I'd like to attach an FM-Radio Winegard HD6000 to the same mast below the U8000.

Q: will this work, and how far below the U8000 must it locate to avoid interfering with the TV signal?

I get good VHF reception on the U8000, even though it isn't supposed to.

UHF antennas do not like having FM or VHF antennas above them, so if you need to keep them on a single pole you should be fine with keeping the U8000 up top but you'll want to keep the FM antenna about 25" apart, but ideally either a full wave of 88MHz apart from it (11.18 ft) or you could use half-wave spacing (5.59 ft).

I've got some antenna spacing figures listed in Post #16 of the OTA FAQ. :)

Definitely have a look at your TVFool report and try to determine if a reflectorless bowtie will do the trick on its own. If the stations are strong from both directions you might want to try a Stealth Hawk (http://www.digitalhome.ca/forum/showthread.php?t=123803) for its almost omnidirectional pattern.
I have tried the reflectorless M4 by itself. The gain is not enough to pick up all local Vancouver stations - missing 10.1(47) and 2.1(58). Burnaby Mountain is in the way to the TV towers. No station from US side at all (lower VHF Hi gain?).

I quess it simply needs a few dB more gain somehow to be perfect. Hope 2.1 will be stable for me once it moves to a lower frequency before August.

Although particular antennas were different, stacked Hi-VHF and UHF antennas were analyzed here:
http://imageevent.com/holl_ands/stacked/stackuhfhivhf
F/B Ratio is much more sensitive to degradation than Gain. Minimum is about 4 to 5-ft....more is better....

I am trying to get stations ~180 degress apart (~170 precisely). Ganging the antennas back to back is very close to achieve my objectives. The gain pattern should be the same as a reflecotless bow-tie with slightly higher gain (?).

No. Although adding a reflector will increase the antenna's gain by slightly less than 3dB, combining the two antennas will result in losses (primarily caused by reflections inside the combiner) of 3.5 to 4dB depending on the combiner so you will end up with less gain overall.

May be stacking two reflectorless M4s or a reflectorless M8 can be better?

That may indeed be a better option as stacking identical antennas (pointed in the same direction) will not result in reflections within the combiner (since the signals are identical the reflections will cancel each other out).

Im planning on combining two antennas

Ant#1 : Cm3010
Ant#2 : CM4221

What's the best splitter to use to join the signal

Ant1 for VHF and FM
Ant2 for UHF

join them to bring them to one receiver

hsantos, since they are on different bands, don't use a splitter. You are better off using a band combiner/separator (such as a UVSJ). Some pre-amps have one built in, so if you have one, you would be best to use that.

Some time ago I successfully stacked a pair of 4221HD's (reflectors touching, matching cables into a backwards splitter and then into a CM7777 pre-amp). It improved the overall signal slightly, and really helped solidify the weaker channels. Earlier this year I added an Antennas Direct C5 for VHF reception, on a separate mast about 8 to 10 feet from the stacked 4221HD's, fed into the same pre-amp's VHF input.

Recently I purchased a second C5 for another install, but ended up not using it. Rather than return it, I figured to try stacking the C5's. I ran the experiment twice, once during the good tropo a couple of weeks ago and again this last weekend, with puzzling results. I realize stacking can be hit or miss, but it just feels like I'm missing something obvious. My setup is easy to get at, so each change took 5 or 10 minutes at most.

1. Obtained baseline figures for a single C5, both visually and with the TV's strength meter, for all channels 2-13.

2. Added the second C5 to the mast immediately below the first, reflectors touching. There was an immediate drop in signal on all channels. Strong analogs became much snowier, weaker analogs all but disappeared and the digital signals for 7, 10 and 13 dropped by 15-20%.

3. Thinking one of the new cables might be wonky, I connected each C5 separately in turn (with both antenna still on the mast). The signal strengths for each C5 matched the baseline figures from a single antenna.

4. Separated the 2 C5's on the mast by about 25 inches (any more and the lower antenna is blocked) and repeated the test, with the same results. Each C5 by itself was clearly stronger than when combined.

5. Repeated the test with new cables and 2 or 3 different splitters, always with the same result. I even tried the UVSJ that comes with the C5, which obviously failed as well.


Questions: Is there something inherent to the C5, or VHF in general, that makes stacking more difficult? Would an actual "combiner" work any better than a reversed splitter? Shouldn't the second antenna, right underneath the first, have had some effect on the signal even if not connected? Could experimenting with separation (1/2 wave, 1/4 wave, etc.) produce radically different results? Is there something in my sad tale which makes you think "Hey moron, you forgot to..."

Thanks for reading this far. I'm open to any comments or suggestions you may have.

Steve

My first reaction is that I would examine the effects of separation between your C5s.VHF-HI (7-13) antennas: channel 7 λ = 5.65 feet (1.72 metres) apart, but 1/2 can be doneHopefully you can experiment easily with moving the C5s up and down, checking performance each time, but certainly the ideal is the distance I've quoted ± any slight variation unique to your own location found after testing. If the baluns/connection points are identical on the two C5s the odds of your antennas being out of phase are nil so you are good to go.

Stampeder:

I figured that would be where to go next, but most of the stacking reports here have the reflectors touching or fairly close. I guess I assumed a full wave apart was more relevant to ganging as opposed to stacking.

I'll experiment with various distances. The C5 has a loop as opposed to bow ties. Would that be 1.72 metres reflector to reflector, center to center, or the bottom of one loop to the top of the other? (Yes, yes, I know. Try them all)

The baluns on the C5 are sealed pcb's(?) so I can only assume they're not out of phase. If they are, swapping one around is probably beyond my expertise.

Thanks for the help (although I was hoping the moron theory would produce an easier fix).:D

Actually the idea of touching reflectors is relevant only to certain (but not all) Bowtie Reflector UHF antennas because the quarter wave rule pretty well applies under those conditions. :) As always, test test test!

Try turning the bottom C5 upside down...

You might also try removing the factory balun boards, constructing a pair of parallel feed lines as a harness, then connecting one of the baluns to the parallel feed lines. While the impedance won't be right, you'll be effectively making a "C2" for high-VHF instead of a "C1".

You're thinking that they're out of phase?

Yep. Try duplicating the geometry of the C2. Keep in mind that the C5 is essentially a C1 "on steroids", rescaled for high-VHF.

Thank you for the suggestions. I'll read up some more and give them a try.

Pardon my ignorance, but I was under the impression that out of phase meant little or no signal got through. I tried this while receiving digital channel 7 (67.1), and 10.1 and 13.1 from Rochester. Combining the two antennas weakened the signal on the strength meter, but all three channels remained watchable.

Actually the idea of touching reflectors is relevant only to certain (but not all) Bowtie Reflector UHF antennas because the quarter wave rule pretty well applies under those conditions.

Just to be clear, UHF signals have a much shorter wavelength, so with many antennas the antennas can be close enough for the reflectors to touch. Since VHF signals have much larger wavelengths, this typically isn't possible.