Yeah, shielded foam core twin lead existed, I used to use it.

Twin lead can act as an antenna. Twisting it, say 3 to 4 times a foot can minimize that, as well as keeping it vertical.


I went to buy some twinleads yesterday, they are harder to find than coax and more expensive too.


For short sections, youre probably better off brewing your own low loss open wire. Guide here : http://www.qsl.net/co8tw/openline.htm

The commercial super low loss open wire 300 ohm twin lead, that I still have a little bit left of on the spool, varies from that chart a little.

The latest lab results posted on this site show pretty respectable numbers for the CM 4221HD. When two identical antennas are stacked, it has been suggested that an improvement of 2.5 dbi would result. To further benefit the two stacked antennas, how would the use of a CM 7777 pre-amp (gain of 26, noise of 2.0) fare with the stacked 4221 HDs? The use of the pre-amp seems compatible since there are two inputs in the mast pre-amp. Two exact equal lengths of coax from each antenna would feed into the pre-amp then a single exit/output coax would run to the tv or distribution amp. How's that sound?

How's that sound?


Models 7777 and 7778 may be configured for either a single UHF/VHF input or separate UHF and VHF inputs. It is factory set to "single UHF/VHF input".

It doesnt sound like it has two separate uhf inputs.

the local source by circuit city was clearing out 300ohm twinlead a few months back and I managed to get a pack for $2

300ohm:

I stand corrected, it's 1 input for vhf and 1 for uhf. So, where would the pre-amp be best placed when combining two antennas- before or after the splitter, which I assume would have power pass?

So, where would the pre-amp be best placed when combining two antennas- before or after the splitter, which I assume would have power pass?

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. If after, then youll need two CM7777s really.

Ive never really looked for one, so there might be a preamp with two 75 ohm uhf inputs and one out somewhere.

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. If after, then youll need two CM7777s really.

Ive never really looked for one, so there might be a preamp with two 75 ohm uhf inputs and one out somewhere.
I might be better off just pre-amping one antenna then?

Also, since the splitter/coupler gives signal loss, then how can stacking antennas give more signal?

then how can stacking antennas give more signal?

Stacking using custom phasing lines, making in essense one antenna. But that requires careful modeling.

Stacking using custom phasing lines, making in essense one antenna. But that requires careful modeling.
so, I just can't stack two antennas together, facing the exact same direction, exact lengths of coax into a quality splitter to get the 2.5 dbi increase in signal?

so, I just can't stack two antennas together, facing the exact same direction, exact lengths of coax into a quality splitter to get the 2.5 dbi increase in signal?
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.

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.

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.

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?

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

Today, 12:10 PM #502
mlord


Join Date: Nov 2006
Location: Ottawa
Posts: 569

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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.

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.

Cheers

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?

Amplify before splitting it, and then there are no real concerns.

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.


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

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.

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 ?