Combining antennas for the same channel

Tim58hsv

DTVUSA Member
#1
No need to keep them seperated by x number of feet, right? I always read about how they have to be at least three or four feet apart to keep them from interfering with each other but when you buy an 8 bay it's usually made up of either two 4 bays side by side or four 2 bays grouped together.

What's the low down on combining bow tie antennas for the same channel?
 

Tim58hsv

DTVUSA Member
#3
Be more specific. Are you wanting to combine 2 fourbay antennas? side by side or one on top of the other?
I'm wanting to combine two Antennacraft U4000's side by side. Thing is they'll need to have a gap of 3" to 6" between them in order to fit them in the attic. Is that doable?
 

Piggie

Super Moderator
#4
I'm wanting to combine two Antennacraft U4000's side by side. Thing is they'll need to have a gap of 3" to 6" between them in order to fit them in the attic. Is that doable?
I know the exact distance for the 4221A which the U4000 are nearly clones.

You space them 20 inches apart measuring from the center of each antenna to the other. The center being where the antenna by itself would bolt to the boom.
If there is any way you can experiment with changing this an inch or two in either direction, you may find an even better distance.

The distance between them determines how the lobes will look from the antenna. Two close they interfere with each other, and farther apart the gain goes down from what ever you find is the optimum distance but 20 inches should be darn close to correct.

Remember I am not talking about 20 inches from the tip of one antenna's elements to the other, but the distance between the center of each antenna.


This distance should result in about a 2.5 db gain increase using a broadband combiner.

Then make two identically long pieces of feedline from the same piece of cable. You can't just use 2 pieces close you have laying around. They MUST be exactly the same length and from the same roll or piece of longer coax.

Then put them into a very high quality combiner or splitter run backwards. Here you want a name brand like Winegard or Channel Master. If you are interested Ken's site shows the loss in cheap Radio Shack versions.

If the screen overlap between then you can either tied them together or just leave them overlapping. Two 4221As don't overlap screens at 20 inches, so I doubt the Antenna Crafts will over lap either.

Be sure to point them in the exact same direction. Even 5 degrees off will ruin all your work.

Both antennas must also lie within the same plane. Not one behind the other.

===================

All that said there are interesting things you can do with some phasing tricks to change the pattern out of two side by side antennas, even staggering one behind the other with proper coax phasing to enhance front to back ratio.

Stacking multiple antennas
The whole page is valuable and probably need to understand most of it but the section labeled:
The Two-Antenna Trick
shows how you can make other antenna patterns with two side by side antenna just by changing the distance and phase between them.

This applies to vertical stacking which you are not doing but interesting if you understand phase combining of RF waves.
http://www.anarc.org/wtfda/stagger.pdf

Something of an advanced primer on what happens stacking antennas.
Stacking TV Antennas
 
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Tim58hsv

DTVUSA Member
#5
Thanks a million Piggie! I do appreciate all the help you give to everyone on the board.

What you say about the distance between the antennas and that it could vary some seems spot on. I measured one of my U4000's and it's exactly 20" wide from reflector element tip to reflector element tip. Then I looked up the U8000 and it's 40.5" wide, so that model has a half inch cap between the two antennas.

Looking up Antenna Directs DB4, it's 19" wide while the AD DB8 is 42" wide so it has a 4" gap between the antennas.

I'm also going to try to hook them together like they do the U8000 so as to avoid using two baluns, two cables, and a combiner. Should get less signal loss if that works. If not then it's two baluns, two cables, and a combiner.

And Thanks Again. You're the bestest :)
 

Thomas G

Contributor
#6
All that said there are interesting things you can do with some phasing tricks to change the pattern out of two side by side antennas, even staggering one behind the other with proper coax phasing to enhance front to back ratio.

Stacking multiple antennas
The whole page is valuable and probably need to understand most of it but the section labeled:
The Two-Antenna Trick
shows how you can make other antenna patterns with two side by side antenna just by changing the distance and phase between them.

This applies to vertical stacking which you are not doing but interesting if you understand phase combining of RF waves.
http://www.anarc.org/wtfda/stagger.pdf

Something of an advanced primer on what happens stacking antennas.
Stacking TV Antennas
Another quality post there Piggie, great links too. I often see OTA posts that talk about "out of phase" problems with dual antenna setups. Would something like that apply here?
 

Piggie

Super Moderator
#7
I'm also going to try to hook them together like they do the U8000 so as to avoid using two baluns, two cables, and a combiner. Should get less signal loss if that works. If not then it's two baluns, two cables, and a combiner.

And Thanks Again. You're the bestest :)
The Two-Antenna Trick (outdoor version)
Stacking multiple antennas

If you can make sense of that chart, it hold the answers.

Note at 20 inches you get the first null about 30 degrees at channel 35. Then look at the second lobe, which is off the chart past 60 degrees. Remember the black CH 60 line no longer matters. So Channel 35 is about the middle of the new band. This means there is a one forward lobe and no grating or side lobes at 20 inches. As you get farther apart you start getting side lobes.

Just opening up the gap between the center of the two antennas to 25 inches, will create a significant second lobe 53 degrees either side of the main lobe. This could be good if there is a station there you want or very bad if your worst mulitpath comes in at that angle.

Moreover if all your stations are not in a central location, like my UHF are spread across 53 degrees, you can run on of the antennas out of phase by reversing the wires on the balun and create a 2 strong lobe pattern, without the 3 db loss of just combining two antennas pointed in different directions.

If you have any kind of spread on your stations up to about 60 degrees you could look into running one out of phase.

The Two Antenna Trick (indoor version)
Silver Sensor
Notice here Ken does stack them side by side staggered to kill mulitpath.

-----------------------------

Ken talks about building a phasing harness here. It's non-trivial.

You gain 0.5 db for all that work, if and ONLY if you do it right compared to 2 baluns and a combiner. If you build it right, you will still end up with a 300 ohm feed point and still have a balun. Chances are any design you try will not be worth the work and not be as good as 2 baluns and a combiner.
 

Piggie

Super Moderator
#8
Another quality post there Piggie, great links too. I often see OTA posts that talk about "out of phase" problems with dual antenna setups. Would something like that apply here?
If you look below to the Two Antenna Trick, that answers your question. Yes it makes a HUGE difference if the two baluns are not in phase. Yours truly hosted up two yagis once with a rain storm coming (lame excuse), went inside and the results were dismal. After the rain I walked back out and let out a huge oh dang it!! (worse but it will get censored). I put one of the baluns on backwards
! So it's not too hard to screw up, and that was with built in baluns you can look at and tell (in case if anyone wonders if I totally screw up!).

What is harder are the standard baluns. They look the same on both wires, so you pretty much have to test them by rotating the antenna looking for your peaks. If pointing at the station is weak, but 20 to 40 degrees off the station on either side is a peak, you have the baluns backwards. This applies to two identical antennas pointed in the same direction horizontally stacked. If you vertically stack two, all you can do is change it and see which is stronger, where stronger is correct.

That said if you point two antennas in different directions, in particular two different models it is nearly impossible to guess at the resulting patterns. Of course the reason to go this would be to not use a rotor but have strong enough signals to suffer 3.5 db of loss. You can model the result on good software, just put them up and see what happens. Here too, you can try reversing one of the baluns to see what effect it has. Normally doing this one is trying to generate two lobes so rotating the antenna checking your nulls and peaks would be a very good idea, with different baluns wiring and orientation of one antenna to the other.
 
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