Stacking, Ganging, Combining TV Antennas

[roger1818]

Originally Posted by Cottager

I understand how adding a second antenna pointed in the same direction doubles the gain - that is a gain of 3 dB. What I don't understand is where the second 3 dB of gain comes from. How can a splitter become more efficient and go from introducing loss to adding gain?

When combining the signal from 2 antennas, there are signal reflections (both back to the originating antenna and to the other antenna) that result in a 3dB loss (plus any additional loss due to inefficiencies of the combiner). However, for reasons I don't totally understand, when the two antennas are pointing in the same direction these reflections end up canceling each other out and the 3dB loss ends up disappearing completely.

[mclapp]

This is kind of a simplification of what happens and alot of stuff is left out but here goes.

When 2 signals meet together totally in phase from identical antennas you have 2 times the signal. For every doubling of the signal you gain 3db but due to cabling and combiner losses it will be somewhat less (2 - 2.5db typical).

If the signals meet out of phase there will be some sort of signal cancellation which could cut the signal by as much as half. Half the signal will be minus 3db and the combiner and cable losses still apply so it could be more like minus 3.5 - 4db. The max negative effect almost never happens with directional antennas because it would be difficult to get 2 identical signals coming from different directions with equal strength.

There is also the impedance mismatch to consider which could work for you or against you. When you combine 2 antennas the combined antenna impedance will be about half of what the antennas would be alone. Combining (2) 300 ohm antennas will give you 150 ohms where they combine not counting the interaction of the antennas due to their close proximity to each other. Most antennas are not a true 300 ohms across the whole TV band so there could be spots where the antennas are actually closer to 600 ohms which when combined together would give more than 3 db gain net and other places where it may be 100 ohms which it could make it almost nothing. That's one of the reasons if you look at the net gain curves of a 4221 and 4228 (which is just (2) 4221's stacked side by side) you will see there are spots where the 4228 isn't any better than a 4221 and other places where it's 3db or more better.

[roger1818]

If the signals meet out of phase there will be some sort of signal cancellation which could cut the signal by as much as half.

There are a few flaws with this explanation. First of all, phase cancellation can cut the signal by more than one half. If the signals are equal strength and 180 degrees out of phase the signal would theoretically be cut completely. That is why when ganging two antennas together pointed in the same direction you need to make sure that the two antennas are in phase, otherwise you won't get any signal.

Having said that, phase canceling is not a significant factor when the antennas are pointing in different directions since the signal strengths from the two antennas for a given station will be significantly different. Assuming the signal from the desired antenna is at least 10dB stronger than the one on the other antenna (likely the case), the maximum possible signal loss due to phase canceling (i.e. 180 degrees out of phase) is about 0.5 dB. If the signals are less than 90 degrees out of phase, you will get positive interference and the signal will actually be stronger (though you may still have phase distortion, which could cause other problems).

[mclapp]

Originally Posted by roger1818

There are a few flaws with this explanation. First of all, phase cancellation can cut the signal by more than one half. If the signals are equal strength and 180 degrees out of phase the signal would theoretically be cut completely. That is why when ganging two antennas together pointed in the same direction you need to make sure that the two antennas are in phase, otherwise you won't get any signal.

Not true ganging 2 identical antennas 180 degrees out of phase will result in a 2 lobe pattern with a gain near the same as a single antenna. If they are vertically stacked you will probably get some sort of weird radiation pattern but the signals would still not cancel.

If you were talking purely signals not antennas maybe they could cancel but there is way more that goes into it than that.

http://www.hdtvprimer.com/ANTENNAS/ganging.html

I've experimented with this and it works.

[roger1818]

Originally Posted by mclapp

Not true ganging 2 identical antennas 180 degrees out of phase will result in a 2 lobe pattern with a gain near the same as a single antenna.

You are forgetting that the path from the broadcast to receive antennas is part of the signal path that contributes to phase of the signal at the combiner. The reason for the 2 lobe pattern is in part due to the 2 antennas receiving the signal at slightly different times and resulting in a phase shift. Thus at those lobes, the signals are no longer out of phase. The exact position of the lobes is dependent on (among other things) the distance between the antennas and the wavelength of the signals received.

If you were talking purely signals not antennas maybe they could cancel but there is way more that goes into it than that.

I agree. Antenna theory is very complicated, and anything I am saying on here is a gross simplification.

[mclapp]

Originally Posted by roger1818

You are forgetting that the path from the broadcast to receive antennas is part of the signal path that contributes to phase of the signal at the combiner. The reason for the 2 lobe pattern is in part due to the 2 antennas receiving the signal at slightly different times and resulting in a phase shift. Thus at those lobes, the signals are no longer out of phase. The exact position of the lobes is dependent on (among other things) the distance between the antennas and the wavelength of the signals received.

I know what you're saying since the antennas are not aimed directly at the transmitter the wave will strike one antenna before the other which creates a phase shift between antennas then the feed line phase shift will put them back in phase depending on distance between antennas and frequency (simply put).

To say that combining 2 antennas out of phase will result in no signal is only true in that there will be no signal somewhere in the reception pattern at a certain frequency.

To say that the signal could be cut by nearly half as I said is also not true in all situations either and as pointed out could be much more. I guess that's what happens when you try to simplify a complex situation

[roger1818]

Originally Posted by mclapp

I know what you're saying since the antennas are not aimed directly at the transmitter the wave will strike one antenna before the other which creates a phase shift between antennas then the feed line phase shift will put them back in phase depending on distance between antennas and frequency (simply put).

That is true. The net result of what I was originally trying to say is that if two identical antennas, pointing in the same direction are connected 180 degrees out of phase, there will be a null directly in front of the antennas that will theoretically completely cancel the signal (in practice there will be some leakage as nothing in real life is perfect).

There are some practical applications for this where you have two stations on the same channel interfering with each other, you can horizontally stack two antennas (in phase) to double the signal from the desired station and cancel out the undesired station. All you have to do is use some simple trigonometry to figure out how far apart the antennas need to be spaced to put the signals received from the undesired station 180 degrees out of phase.

[300ohm]

you can horizontally stack two antennas (in phase) to double the signal from the desired station

Horizontal stacks present their own kinds of problems. A phasing line is the most efficient way of getting the most gain. But a horizontal phasing line acts like part of the antenna, thereby reducing the additional gain.

Excerpts from the June 1975 Issue Of ELECTRONIC TECHNICIAN/DEALER:

Stacking TV Antennas
By James E. Kluge
The author is a technical editor in the Engineering & Research Division of the Winegard Company.


VERTICAL STACKING

Stacking two identical antennas on a common vertical mast significantly narrows the vertical beam-width angle. That is, vertically stacked antennas more effectively reject those interfering signals arriving from above or below their horizontal plane than does a single antenna. It’s as though they were looking through a horizontal venetian blind. Because there’s nothing mounted to the side of either antenna, their side-to-side vision is virtually uneffected. In the process, gain increases about 2.5 dB over that of a single antenna.

Vertical stacking improves both gain and vertical directivity. This helps reduce airplane flutter and attendant picture roll, and certain types of ground noise and ground reflections.


HORIZONTAL STACKING

Stacking two identical antennas side by side in a horizontal plane (Fig. 3) significantly narrows the horizontal beam-width angle, as shown in Fig. 4. That is, the antenna combination, like a horse wearing blinders, "sees" fewer interfering signals arriving from the sides while its vision up and down (in a vertical plane) is virtually unaffected. In the process, gain increases approximately 1.2 dB over that of a single antenna.

The up and down (vertical) "vision" of a horizontal stack is comparable to that of a single antenna, but its side-to-side "vision" is more restricted.

[99gecko]

300ohm, thanks for that quote.
Stacking, horizontally or vertically, also presents the problem to the DIY/recreational OTA'ers that the aim of the antenna that was once "good enough" with a single antenna, is now not nearly good enough.

[roger1818]

300ohm, that is true and is why a CM4228 has a much narrower beam width than a CM4221. Of course the reason for this is that the two antennas are receiving the signals out of phase when not directly in front of the antenna, as I explained earlier. The side lobes on the 4228 (shown below) are caused in part by the signals getting back into phase again.





(pictures from HDTV Primer)

[Tom.F.1]

Interesting discussions. I stacked 4228's pointed at buffalo and added a 3rd pointed at Toronto.
When i fist connected the lower one, it interfered with the upper ones and I had some wierd effects. I had some channels too strong, and lost others. I played with aiming for a long time, before I realized all i had to do was lower the toronto antenna away from the others.

[stampeder]

Yep, if you want to have 2 or more antennas that are not ganged or stacked and you need to reduce or eliminate effects of the antennas upon each other here's the formula (I used the term "identical" to respond to a question but this formula applies to non-identical antennas too that are not combined in any way):

http://www.digitalhome.ca/forum/show...&postcount=477

[300ohm]

Interesting discussions. I stacked 4228's pointed at buffalo and added a 3rd pointed at Toronto.
When i fist connected the lower one, it interfered with the upper ones and I had some wierd effects. I had some channels too strong, and lost others. I played with aiming for a long time, before I realized all i had to do was lower the toronto antenna away from the others

Im confused. You tried to gang 3 4228's with one pointed in a different direction ?

[Tom.F.1]

Originally Posted by 300ohm

Im confused. You tried to gang 3 4228's with one pointed in a different direction ?

yup, thats what works. Works good.

Maybe you're not familiar with my location. Toronto/hamilton stations are 23 & 45 miles, buffalo, 57 & 86 miles.

16 bowties face buffalo, 8 face toronto/hamilton. Somewhere around 40 degrees apart.

[ppauper]

specifically, what do folks recommend for combining 2 4228s ?