Stacking, Ganging, Combining TV Antennas - Page 5 - Canadian TV, Computing and Home Theatre Forums

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post #61 of 1285 (permalink) Old 2008-12-05, 03:19 AM
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The CM4228 (original), as is, can very capably receive VHF-HI signals in a metro-to-near fringe location precisely due to the combned width of its reflectors. This has been known for many years, so essentially the 2 screens act as one. On my own CM4228 they touch.
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post #62 of 1285 (permalink) Old 2008-12-14, 06:54 PM
 
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Just curious if I was stack two of the antennas (as below), should I just butt up the black plastic end caps together then tie with non-metallic clamps of some sort? (I'm still a bit confused because in the case of the 4221's,the metal reflectors are touching when stacked...right?
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post #63 of 1285 (permalink) Old 2008-12-14, 07:08 PM
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The best way to look at the problem is to think of the antenna in the picture as an entire, whole unit. The idea of stacking is to gang two whole units, one on top of the other. In your case, put them on a pole facing the same direction and slide the bottom one up until it touches the top one, where ever that may be on the antennas. Connect the outputs as recommended and bob's your uncle.

If you are enterprising you could tighten the top one solidly but put the lower one up with wingnuts or other easy fasteners so that you could experiment with different gaps to see if reception is aided in any way.
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post #64 of 1285 (permalink) Old 2008-12-14, 07:10 PM
 
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Ok...thanks Stampeder I'll give it a try.
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post #65 of 1285 (permalink) Old 2008-12-14, 07:13 PM
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You could hack-saw about an inch off the top and bottom of the vertical bowtie mount bar in case you want to try having them up closer to eachother. Don't do it unless you can afford to kiss the antennas good bye if the cut ruins them!
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post #66 of 1285 (permalink) Old 2008-12-15, 06:00 PM
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In a "perfect", loss-less combiner, signals in-phase would yield 3 dB gain and
out-of-phase would CANCEL. With unequal signal amplitudes, the output signal
would be a (complex) addition of the inputs.

Stripline (aka Wilkinson) Couplers are very close to this ideal (typ 0.2 to 0.5 dB loss).
The Lindsay UHF Stripline Coupler cost about $100, back when you could still get it...
Various other companies make VHF and UHF Stripline Couplers, but I have yet
to located something "affordable" for the entire UHF band.

However, a cheap RF Spitter/Combiner is more complicated, since it contains
a Hybid Transformer and Mismatch Resistor:
http://www.macom.com/Application%20Notes/pdf/m560.pdf
http://www.macom.com/Application%20Notes/pdf/m561.pdf
http://www.macom.com/Application%20Notes/pdf/m568.pdf

Any phase or amplitude differences between inputs will be lost in the
internal mismatch resistor. With a typical internal loss of 0.3 to 0.5, the
combining gain reaches a max of about 2.5-2.7 dB ONLY when the
two antennas are perfectly matched in GAIN AND PHASE...good luck...

However, stacking antennas ALSO provides the ability for one antenna to maintain
good signal strength while the other suffers a multipath null....which would be
even more effective if a Stripline coupler avoids the Hybrid mismatch loss.

If antennas are pointed in different directions, the loss is closer to 3.3-3.5+ dB
(I've measured up to 4 dB in some splitters).
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post #67 of 1285 (permalink) Old 2008-12-29, 07:11 PM
 
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Another Way To Check Stacked Antennas Phasing ?

I am in the process of figuring out how to stack 2 - 4 bay antennas (I haven;t decided whether I will stack them vertically or horizontally).
I have read different methods of checking to make sure the phasing is correct (since my 2 antennas are identical with the baluns built into them).
Would a simple way to do this be (using an ohmmeter), confirm that the aluminum left and right bows correspond to the other antennas ,equivalent bows by continuity testing?
thanks
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post #68 of 1285 (permalink) Old 2008-12-30, 12:17 AM
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Quote:
I have read different methods of checking to make sure the phasing is correct (since my 2 antennas are identical with the baluns built into them).
Would a simple way to do this be (using an ohmmeter), confirm that the aluminum left and right bows correspond to the other antennas ,equivalent bows by continuity testing?
Nope, you cant check balun phasing with an ohmmeter. The simple way to check is to try it and note the reception, then reverse one, and check again.

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post #69 of 1285 (permalink) Old 2008-12-30, 08:57 AM
 
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300ohm.

I'm sorry to question your response (you obviously know a lot more about OTA antennas than I do),but my electrical theory makes me say .....mmmm to your response.
If phasing is another word for polarity,then you only have two choices (the conductor or the shield). If each of the conductor and shield wires goes to one bow or another (left and right) then by testing the continuity from the shield of the 75 ohm female jack and the continuity of the conductor side of the jack to the bow left or right,then my assumption would be that by sitting the 2 - 4 bay antennas side by side and confirming which bows go to which side of the 75 ohm jack would prove their polarity.
I do realize that all of the 4 and 8 bay bowties have crossover wires,but as long as the crossovers and bow positions match with each other,isn't that the same as confirming the phasing? I also realize that the ohmmeter would be reading through an impedance transformer and stepping up to 300 ohms,but couldn't this increased resistance be taken into account when testing the phasing ?
Or.... am I way off on a tangent ?.....If so,I apologize for ranting a load of BS
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post #70 of 1285 (permalink) Old 2008-12-30, 09:19 AM
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Unfortunately it doesn't seem to be that simple. Try checking out a commercial balun/transformer with an ohmmeter -- nothing stands out from any way of arranging the two probes.

Internally, they probably use double wire coils or something (like a power transformer), so nothing favours one side over the other when measuring it.

But yes, there is a polarity (or, rather, a phase) when connected to an antenna.

Cheers
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post #71 of 1285 (permalink) Old 2008-12-30, 10:31 AM
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Quote:
I also realize that the ohmmeter would be reading through an impedance transformer and stepping up to 300 ohms,but couldn't this increased resistance be taken into account when testing the phasing ?
Or.... am I way off on a tangent ?.....If so,I apologize for ranting a load of BS
Ohmeters use direct current to measure resistance. DC generally doesn't pass through a balun, and even if it did, it would not tell you anything about phasing.
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post #72 of 1285 (permalink) Old 2008-12-30, 11:53 AM
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Also, you cant measure the imaginary impedance component part of the total impedance. (unless maybe you use your imagination, heh)

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post #73 of 1285 (permalink) Old 2009-01-08, 10:44 PM
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2 Antenna's?

I currently have one OTA antenna & would like to get a 2nd identical one to use together.

I know they have to be identical & spaced far enough apart & aiming in the same direction with two equal lengths used.

I have been reading up on this and have just one question:

Can I just connect the outputs of both these antenna's into a splitter, then to my preamp then to the TV?

Any help would be greatly appreciated.

Thanks in Advance.
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post #74 of 1285 (permalink) Old 2009-01-09, 11:58 AM
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Originally Posted by umaguma
Can I just connect the outputs of both these antenna's into a splitter, then to my preamp then to the TV?
Yes, that's the typical way to combine them into one preamp, or another is to buy a preamp with 2 inputs. You should be fine.
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post #75 of 1285 (permalink) Old 2009-01-10, 12:05 AM
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Thanks Stampeder.
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