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Old 2009-02-23, 11:06 PM   #91
goforit
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Quote:
Originally Posted by 300ohm View Post
2.5 dbi increase minus 3.5 dbi decrease (from a good marked splitter, the unmarked ones can be about 4.5 dbi or more) leaves minus 1 dbi.
Therefore, the single antenna is better than the stacked with splitter/joiner, because joining two antennas increases gain (+2.5 dbi) but the use of the splitter/joiner nullifies this gain (-3.5 dbi) plus an additional loss of 1dbi.
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Old 2009-02-23, 11:18 PM   #92
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Yep, you got it.

I do have some dimensions for the CM4221HD. I was waiting for the closeups and details of the middle section. Of course, if stacking using phasing lines, the whole middle section would have to be torn out anyway.
If you do have a CM4221HD, post some detailed pictures of it.

Of course, this is not a cheap way to go about getting a 8 bay bowtie. If you want an eight bay bowtie, mclapp has just posted the phasing line pdf of his.
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Old 2009-03-01, 01:13 PM   #93
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Default Build your own joiner/reverse splitter

Is it possible to simply use aluminum wire to connect the elements of two antennas? Where would you do it? Eg on two 4221s- whiskers or reflector? Is this better than using a conventional joiner/reverse splitter for signal gain/loss?
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Old 2009-03-01, 01:22 PM   #94
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Quote:
Originally Posted by 300ohm View Post
Yep, you got it.

I do have some dimensions for the CM4221HD. I was waiting for the closeups and details of the middle section. Of course, if stacking using phasing lines, the whole middle section would have to be torn out anyway.
If you do have a CM4221HD, post some detailed pictures of it.

Of course, this is not a cheap way to go about getting a 8 bay bowtie. If you want an eight bay bowtie, mclapp has just posted the phasing line pdf of his.
300 ohm:

Please comment, as this seems contrary to your comments, thanks. G

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Quote:
Originally Posted by goforit
So..., if you connect two identical antennas (eg 4221s) with a joiner/reverse splitter, the theortical advantage is a gain of 2 to 3 dbi, but the loss of the splitter is .5 dbi, so an overall gain of about 2 dbi..., is this correct?

Right! Overall net gain of 2 - 2.5dB

Assuming the antennas are positioned in a complementary fashion, the cables from each to the splitter are identical type/length, and all of the other bits of fine print are adhered to.

But, yes, that is exactly the idea.
I use a splitter just like that here on my dual-PR8800 array, for example.
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Old 2009-03-01, 01:46 PM   #95
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Quote:
Originally Posted by 300ohm View Post
Thats the tricky part. If before the CM7777, youll get a 3.5 dbi loss from the splitter/coupler on a good one, negating the gain increase from ganging the antennas.
Tsk tsk.. That's not correct, and could be quite confusing to our newer members.
The insertion loss from a combiner (reversed splitter) is more like 0.5dB.

When used as a splitter, it divides the signal into two equal halves (a -3dB reduction for each), which when combined with the 0.5dB insertion loss gives 3.5dB less signal on each output, compared with the input.

When used as a combiner, it adds the two input signals (a +3dB gain), which when combined with the 0.5dB insertion loss gives a net gain of around +2.5dB (in a perfect situation). Other connector losses from the required coax patch cables can reduce this to +2dB in real-life.

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Old 2009-03-01, 05:41 PM   #96
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Tsk tsk.. That's not correct, and could be quite confusing to our newer members.
The insertion loss from a combiner (reversed splitter) is more like 0.5dB.

When used as a splitter, it divides the signal into two equal halves (a -3dB reduction for each), which when combined with the 0.5dB insertion loss gives 3.5dB less signal on each output, compared with the input.

When used as a combiner, it adds the two input signals (a +3dB gain), which when combined with the 0.5dB insertion loss gives a net gain of around +2.5dB (in a perfect situation). Other connector losses from the required coax patch cables can reduce this to +2dB in real-life.

Cheers
So if you use the splitter as a combiner you get an increase of 2.5 dbi- on two identical antennas.
But if you have to split after this- say for two TVs you could use a powered splitter which would have minimal signal loss?
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Old 2009-03-01, 06:25 PM   #97
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Amplify before splitting it, and then there are no real concerns.
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Old 2009-03-01, 07:23 PM   #98
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Quote:
Other connector losses from the required coax patch cables can reduce this to +2dB in real-life.
But even in a best case scenario of combining two CM4221HDs, you have the .5 db min loss from each of the built in baluns and .5 db insertion losses into the splitter, giving 2 db total minimum loss. Since the best you can hope for is 2.5 db gain from combining two antennas in the same direction, that leaves .5 db gain, if youre lucky. Not really worth the expense IMO. Phasing lines are the only way to go.


Quote:
So if you use the splitter as a combiner you get an increase of 2.5 dbi- on two identical antennas.
But if you have to split after this- say for two TVs you could use a powered splitter which would have minimal signal loss?
This may help.
http://www.hdtvprimer.com/ANTENNAS/merging.html
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Old 2009-03-01, 07:43 PM   #99
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I've built home made antennas similar to the cm4221's and I found that using the open wire feed system or the 2 balun and combiner set-up have about the same peak gain which is 1 - 3 db above a single antenna depending on the channel.

The difference in where the peak gain occurs is due to the difference in where the best impedance match occurs between the 2 different feed methods. The reason for the variation in the gain (1-3db) over a single antenna is also due to the fact that there are places where the impedance match of 2 antennas is better than 1 and some places where it's worse.
The open wire feed set-up should be better (less loss) but I didn't really see it in my tests.

One reason may be that it's very difficult to run the open wire feeders between the horizontally stacked antennas with out putting some funny kinks in the feeder to get around the frame work and other wires.
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Old 2009-03-02, 01:25 AM   #100
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But mclapp, doesnt your 8 bay work much better with the phasing lines than it would have if you had used two baluns and a splitter to combine the 4 bays ?
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Old 2009-03-02, 09:41 AM   #101
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Originally Posted by 300ohm View Post
But even in a best case scenario of combining two CM4221HDs, you have the .5 db min loss from each of the built in baluns
Even a single antenna usually has a balun, with the same loss. But only one of them. A second balun adds only 0.5dB loss (or whatever), not 1dB. And this still does not change the math that gives +2.5dB or so from combining two identical antennas with or without baluns.

In practice (rather than armchair theory), the two combined PR8800s work spectacularly well here, whereas a single PR8800 does not have nearly the reach.

This indicates a real-life gain of probably 2dB or more.

But I do still plan to go for that extra 1dB or more, by getting rid of *both* store-bought baluns, in favour of a direct wire connection between the two antennas, with a single coax-loop balun in the middle.

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Old 2009-03-02, 10:04 AM   #102
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Originally Posted by 300ohm View Post
But mclapp, doesnt your 8 bay work much better with the phasing lines than it would have if you had used two baluns and a splitter to combine the 4 bays ?
I tested it last fall both ways using a spectrum analyzer the main difference was where the peak occured, the open wire feeder peaked around 580mhz and the 2 balun set up peaked a little higher (610mhz).

There were spots where the open wire feeder was 1 db better than the 2 balun and places where the 2 balun was .5db better and other spots where they were even. In both cases the the 8 bays were 1 -3db better than the 4 bay except above 700mhz.

The average gain across the ch 14-50 range my have been .5 db better for the open feeder. I was using a pair of phillips $1 baluns and it's been noted that balun quality can make a difference as well.

These were the results from my particular test someone using different lengths of coax, baluns, or a different type or length of open feed line will get something different for sure.
http://www.frontiernet.net/~mclapp/A...der%20test.gif
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Old 2009-03-02, 11:01 AM   #103
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Quote:
But only one of them. A second balun adds only 0.5dB loss (or whatever), not 1dB.
Thats what Im saying, .5 db loss from each balun, giving 1 db total. But Im also using the often quoted .5 db insertion loss per connection, which is probably too high. Someone needs to do a loss analysis of the splitter using it as a combiner, because that loss cant be zero either.

Yeah, Im conducting my own real life testing today. It seems to me that snow is better for reception than rain, heh.

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Old 2009-03-02, 11:10 AM   #104
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Great photo!

We get tons of that white stuff here, too, but it never builds up on the antennas, thank goodness.

Cheers
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Old 2009-03-02, 12:48 PM   #105
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Default Re: Build your own joiner/reverse splitter

Quote:
Is it possible to simply use aluminum wire to connect the elements of two antennas? Where would you do it? Eg on two 4221s- whiskers or reflector? Is this better than using a conventional joiner/reverse splitter for signal gain/loss?
I know this might be more about the new 4221HD model, but several years ago, I wanted to put a 4228 in my attic on a rotor (didn't really want anything outside if I could avoid it). But the opening to my attic was small and would only allow a 4221 to fit through. So I bought two 4221s (the old kind, of course) and attached them together side-by-side with two long horizontal metal brackets attachd to the backs of the reflectors. I then mounted them on a mast (the mast and brackets clamped to the back side of the reflectors right at the vertical seam between the two), with the U-bolt clamps holding the edges of the reflector screens together right at their edges and against the mast.

I then followed some online close-up photos of the 4228's connection setup, and I fashioned stiff parallel aluminum wires to connect the two 4221s, joined at the center in front of the antennas just like the 4228. I used a small rectangular piece of wood and drilled two holes in it for the attachment bolts. At the time, I tried to orient and wrap the aluminum wires to look identical to the 4228 picture I was following. I used whatever 300-75 ohm balun I had sitting around at the connection bolts and ran coax down to the tv from there.

This setup has worked well over the years. Analog signals were receivable out to about 60-65 miles with a signal strength of about 7 out of 10 (maybe 8 out of 10 at night). In digital, it is right on the threshold of 65-mile reception (somewhat stable at night, not stable or nonexistent during the day).

This question is quite timely for me now, as I've learned a lot about antennas by lurking on this board for a while. From reading the recent balun thread comments, I plan to replace the existing balun with a CM 0089/94444 that I just bought, to see if that helps any. Also, (now that I know much more than when I connected these 4221s) I now suspect that perhaps the lengths of the two aluminum wire connector pieces are supposed to be exactly identical. Could someone please confirm that for me? They might be identical (or close), but I'm not sure how exact I was, so I plan to check that soon also. Although the aluminum wires I fashioned are parallel, I knew nothing at the time about their wrapping at, or angling away from, the attachment points, nor did I know anything about distance for these wires in front of the antenna or proper spacing for the attachment bolts at the center wood block. I'd very much appreciate any comments on which of these might be critical dimensions, and I'll adjust/improve accordingly.

I also seem to remember reading some recent comments about wood pieces/blocks not being that great at connection points for SBGH/DBGH antennas (PVC or other plastic was recommended instead). Wondering if I should replace that wood piece connecting my 4221s with some PVC/plastic.

As a side note, inspired by DogT, 300ohm and others, I've nearly completed a 30"x75" DBGH (original screen version) that very closely resembles DogT's PVC design. I plan to compare the results to my twin 4221 setup, and I'll of course report my results here.

Thanks to all for this very informative and helpful forum...
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