Stacking, Ganging, Combining TV Antennas - Page 3 - Canadian TV, Computing and Home Theatre Forums

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post #31 of 1270 (permalink) Old 2008-11-27, 11:41 AM
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Stacking CM4228s is discussed on hdtvprimer if you look for his 16-bay and 32-bay links.
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post #32 of 1270 (permalink) Old 2008-11-27, 12:20 PM
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Quote:
Originally Posted by mlord
Here's an even better article dealing with the complexities of stacking yagis:

http://www.grantronics.com.au/docs/StkYagis.pdf
The K7MEM yagi calculator seems to use the same (or very similar) stacking formulas as what this article describes, which saves a lot of fuss for me at least.

Also in that article, is the magic formula for designing a feedline of a given impedance, something my earlier educations omitted:
Z = 120 arc cosh(D/d),
or approximately Z = 276 log(2D/d),
where D is the centre to centre spacing,
and d is the diameter of the wires.
Handy, that!

When stacking two yagis vertically, the custom is to connect their two feed points with appropriately sized/spaced metal rods to act as feedlines, thus wiring the array and stiffening the structure at the same time.

The impedance of the feedline has to match that of an individual antenna, so for a folded-dipole (in a yagi) it's usually about 300ohms. Impedance at the middle of the connecting feedline (between two antennas) will be half of that, or about 150ohms.

(post restored to it's proper location by mlord, minus the DBGH reference that caused Stampeder to move it to a new thread).
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post #33 of 1270 (permalink) Old 2008-11-27, 12:25 PM
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I also tried the 300 ohm connection, following the instuctions on stacking. The theory is that there will be less loss with only one balun. Big waste of time and effort that was. I think the problem was an impedence mismatch by joining two 300 ohm leads together with no matching transformer.
My results were much better with 2 baluns, and a splitter/combiner.

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post #34 of 1270 (permalink) Old 2008-11-27, 01:39 PM
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ppauper and Tom.F.1, I think the problem with the twinlead theory is that the "rated" 300ohm impedence of each antenna (as broadband antennas their real world impedence is variable depending on the frequency of each channel) will combine to result in only 150ohms, which will cause a typical 4:1 balun to yield only 37.5ohm impedence on the coax downlead on some channels. This means that some channels may be weaker than the coax splitter technique while some may be stronger.

I'm going to address that in the FAQ asap.
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post #35 of 1270 (permalink) Old 2008-11-27, 08:24 PM
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Quote:
Originally Posted by stampeder View Post
  1. stack your antennas one above the other so that their reflectors touch
  2. combine their output with exactly equal lengths of RG6 into a reversed high quality splitter
  3. before nailing anything down permanently, test on an analogue station (easier to see the immediate results than trying for a digital station) to make sure your antennas are in phase
  4. if they are not, just switch the balun leads on only one of the antennas and test again.
On the advice of my supplier, I stacked my CM4228 so that the bowties were about an inch apart. The reflectors overlap significantly. Is that OK?

One piece of RG6 is about an inch longer than the other, but the supplier said that was insignificant. Is that OK?

I saw absolutely NO difference after stacking. Is that a strong indicator that I have a phase problem? I check the orientation carefully before I connected them, but maybe it's not apparent unless you try it both ways...
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post #36 of 1270 (permalink) Old 2008-11-28, 10:05 AM
 
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Quote:
Originally Posted by 99semaj
On the advice of my supplier, I stacked my CM4228 so that the bowties were about an inch apart. The reflectors overlap significantly. Is that OK?
I'd try varying the spacing and seeing if it's better or worse.
In my case, I found that out of the handful of spacings I tried, what worked best was setting the spacing between the top bowtie on the bottom antenna and the bottom bowtie on the top antenna to be the same as the spacing between the bowties on each antenna (about 20cm). I wouldn't claim that me experimentation was exhaustive (I only tried a handful of options)
I was told on another forum that this was wrong and I need to have at least 3 feet between the antennas to avoid interference.
However, I'm using them indoors and there's not enough space for 3 feet spacing.
Quote:
One piece of RG6 is about an inch longer than the other, but the supplier said that was insignificant. Is that OK?
I'd get them the exact same length otherwise they may be slightly out of phase. One inch won't make much difference, but if you've spent 2 or 3 hundred $$$ buying the antennas, it seems silly to scrimp $5-$10 on the cable.
I got 2 identical lengths (2 feet each) of high quality RG6 from TKO CSS on ebay.
Quote:
I saw absolutely NO difference after stacking. Is that a strong indicator that I have a phase problem? I check the orientation carefully before I connected them, but maybe it's not apparent unless you try it both ways...
as suggested by others, try flipping one of the baluns around.....

I saw absolutely NO difference after stacking. Is that a strong indicator that I have a phase problem? I check the orientation carefully before I connected them, but maybe it's not apparent unless you try it both ways...

Last edited by stampeder; 2008-11-28 at 11:32 AM. Reason: Please use quote tags - fixed
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post #37 of 1270 (permalink) Old 2008-11-28, 10:07 AM
 
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Quote:
Originally Posted by stampeder View Post
ppauper and Tom.F.1, I think the problem with the twinlead theory is that the "rated" 300ohm impedence of each antenna (as broadband antennas their real world impedence is variable depending on the frequency of each channel) will combine to result in only 150ohms, which will cause a typical 4:1 balun to yield only 37.5ohm impedence on the coax downlead on some channels. This means that some channels may be weaker than the coax splitter technique while some may be stronger.

I'm going to address that in the FAQ asap.
we'll look forward to that...

presumably that would mean that we'd need
a) 150ohms wire instead of 300, if such a thing exists
b) a 2:1 balun instead of 4:1
??
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post #38 of 1270 (permalink) Old 2008-11-28, 11:33 AM
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ppauper, yes a 2:1 balun would be a solution or so would stacking 4 identical antennas!
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post #39 of 1270 (permalink) Old 2008-11-28, 11:38 AM
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99semaj, I think your supplier had some misconceptions - the goal of stacking is not to make 2 antennas into 1, but rather to get both of the antennas working as a team.

That's a very important difference. The info you were given is guaranteed to cause headaches for stacking. If you can fix it to go with the stacking steps in the FAQ we'll see if stacking is a good option for you.

I'm not saying that stacking is a solution for everyone, but the only way to test is the right way.
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post #40 of 1270 (permalink) Old 2008-11-29, 10:45 AM
 
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Quote:
Originally Posted by stampeder
yes a 2:1 balun would be a solution or so would stacking 4 identical antennas!
this is probably a silly question, but since a 4228 is 2 x 4221's (connected by those horizonal metal wires), why do we use the same balun for a 4228 as we do for a 4221 ?

Last edited by stampeder; 2008-12-02 at 12:44 PM.
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post #41 of 1270 (permalink) Old 2008-11-29, 11:55 AM
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Those are broadband antennas so their impedence is actually not the same on every channel.
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post #42 of 1270 (permalink) Old 2008-11-29, 12:08 PM
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Well, I cut a 1.25" off one of the cables, making them identical length, but it made no difference.

I also reversed the phase on one of them, but it also made no difference.

It's as if the second antenna isn't even there. Very perplexing. I'm using FOX29 analog as a test station, and my on-screen meter shows peak strength of 49/100 , but it keeps toggling from locked/unlocked. Not sure why an analog station would do that...
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post #43 of 1270 (permalink) Old 2008-11-29, 12:29 PM
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Hmmm... just to confirm, you did all the steps in Post #2 of this thread?
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post #44 of 1270 (permalink) Old 2008-11-29, 03:22 PM
 
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I noticed in the FAQ that there is some sort of quirk in the CM4228 design that allows them to pick up some high band VHF channels.
Will this same"quirk" occur with vertically stacked 4221's ?.......oh,and another question. Is the low number channels based on the displayed channel or the actual channel ?(or are low number just analogue)
thanks
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post #45 of 1270 (permalink) Old 2008-11-29, 04:09 PM
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No, it relates to the width of the CM4228's reflector screen causing the higher than expected gain on VHF-HI channels.
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