Stacking, Ganging, Combining TV Antennas

[stampeder]

Just a reminder that this thread is a discussion of the antenna theory behind combining antennas and the effects people are seeing in the real world & with computer modeling.

Originally Posted by ppauper

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

My specific answer is... "that all depends". Combining antennas is only something you do under special situations, so see Posts #5 and #16 in the OTA FAQ to understand the situations and options that some people might have:

http://www.digitalhome.ca/forum/show...3&postcount=16

If you have further questions after reading through that post we can continue discussing your specific situation in the Reception Results thread for your area.

cheers

[stampeder]

From the UK, an excellent resource from a HAM with essential theory that also works for stacking TV yagis:

http://www.ifwtech.co.uk/g3sek/stacking/stacking2.htm#1.

[300ohm]

I was going to post that site too, heh.

Note the "shape" of the yagi aperature, its a horizontal oval that extends beyond the reflectors.

Ive read that the aperature shape of a parabola is a circle that is LESS than the diameter of the parabolic reflector, go figure.

IMO, the aperature shape of the Grey Hoverman or Bow Tie antennas is a vertical oval that extends a little beyond the reflector.

[weehaggis]

I read through the FAQ's but didn't see any mention of this (unless I missed that part).
I have 2 identical 4 Bay antennas (I wont mention the name for fear of ridicule !
I am planning to stack them facing the same direction.
In the FAQ's it talks about wiring the 2 antennas with equal lengths of 300 Ohm twin lead. My antennas have the baluns built into the back. Does this mean that I simply install 2 equal lengths of RG6 through a slitter ?
Because I am using 75 ohm cable, is there no phasing issue ?
Thanks

[stampeder]

Post #16 in the FAQ is the one about combining antennas. I've edited that FAQ post to include the following:

To answer your questions, in your case you would:

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.

It should be fine after that, and Bob's your uncle.

[weehaggis]

Thanks Stampeder.
Since the balun box on each antenna appears to be sealed,I'm not sure how I could reverse the wiring to fix the phasing issue.
When you say "test on an analog station", are you suggesting that I try and tune in the strongest analog signal (then adjust the antenna)until I get the sharpest image....and if I cant get a reasonable picture they may be out of phase ?
With regards to this particular style of antenna,would the connection not be phased the same for each antenna at the factory?
Thanks

[stampeder]

If you crack open those balun boxes you can always tape or glue them back together, or you could always just replace them with normal baluns. In your case you're right that the antennas are probably already in phase since they're duplicates. Just by habit I never assume that.

The reason for testing phasing on an analogue station is that you will see something immediately so that you have something to work with. It is not an aiming test in any way, except that they have to be oriented in such a way that an analogue signal will show up.

If you try to scan for digital stations right away, what if your antennas are out of phase? With no way of locking stations you might not realize what the problem is. I threw that in as a potential "gotcha" because someone might pull their hair out thinking the problem is something else.

As I say, with an analogue station you'll get it right almost immediately.

[mlord]

Here's an even better article dealing with the complexities of stacking yagis:

http://www.grantronics.com.au/docs/StkYagis.pdf

[weehaggis]

OK Stampeder...that sounds good,thanks again.
FAQ #16 is indeed very helpful

[Tom.F.1]

good idea on the phase test, but hows that going to work once all the analogs go away?

I measured signal strength on each antenna separately, then measured after they were conbined. When i obviously had more signal strength, i knew they were in phase.

[stampeder]

Ya I've been thinking the same thing, figuring I'd have a few years before having to change that!

A signal meter is of course a great tool like you say.

[boumbo]

1. when u stack 2 4-bay like a 4221 one above each other, does this setup have the same or better performance as a 8-bay like 4228? what is the reason behind stacking?

2. Can you place them next to each other instead of one above each other? does it make a difference? because a 8 bay 4228 is like two 4221 next to each other. am i right?

3. when u say the 2 reflectors need to be touched each other? does it need to be taped from top to bottom?

[stampeder]

1. Stacking is a way of increasing the gain usually by about 2 or 3 dB over just the single antenna because the receiving patterns of the antennas join into one, and their focus is sharpened more to the horizontal than the vertical plane.
2. Side-by-side ganging is also a way of (hopefully) increasing gain, but it is very hard to get working properly. The CM4228 did it perfectly but many, many people have tried it with other antennas and failed.
3. You don't need to connect the reflectors, but if you want to you need to make sure it is with non-metallic material and something that will not degrade in the light and the weather.

[donnyjaguar]

Stacking four antennas is very easy. Just use identical lengths of 300Ω twinlead and observe correct phasing. Parallel them all together and attach to the 75Ω coaxial cable. You'll need to put a few ferrite rings at the end of the coax to keep the RF off the shield, but it should work perfectly.

If you do the same with only two antennas it won't work as well because there will be an impedance mismatch between the 2x 300Ω feeds (150Ω) and the coaxial cable (75Ω). This will represent a 2:1 VSWR which is lossy, albeit not 3dB lossy.

[ppauper]

I tried stacking 2 identical 4228's vertically:

in my particular case, I got better results joining them with identical (2 feet) lengths of RG6 than I did with identical lengths of 300 Ohm wire (foam-insulated from the source by circuit city)

your mileage may vary