Two DB2s, 90° of Separation

[CowboyDren]

I'm using a hand-built clone of a DB4 without the reflector inside my attic. It's okay, but doesn't get channel 42, which is about 90° off of "center mass" for the other transmitters. I built a 5-element YAGI for channel 42, and just used a splitter to combine the signals, and it didn't seem to hurt anything, but I'm not sure that it's roof-worthy.

I'm thinking about putting up a mast or a pair on the roof, and buying a couple of DB2s or CM-4220s to point perfectly off-axis. The Channelmaster Jointenna costs every penny as much as a DB2 does, while the Winegard CC-7870 clearly notes a 3.5dB loss when connecting two antennas. I've been all over the HDTV Primer, KYES Antennadex, and other sites strongly cautioning me to not plug two antennas into each other, but never saw an example that fits my problem.

The question is the theoretical effect of just combining the signals without an inserter. What happens to the polar diagram of a DB2 when you electrically connect it to another that's perpendicular? What would happen with if one was a DB2 and the other was a DB4? What if I put them 3' or 4' apart from each other?

 

[Don_M]

The experts you quote urge people to buy a better single antenna, rather than pointing two antennas in the same direction, in hopes of getting a stronger signal. That's different from what you propose.

You're in trial-and-error territory. This works well in some situations; in others, not so well. Results from your experiment with the two DIY antennas seem to be encouraging (did you get channel 42 on the combination without losing other stations?). As you discovered, a splitter can be used in reverse for off-axis signals. You can do this with dissimilar antennas and at varying separations (but always at least two feet apart, so as not to create interactions between the antennas themselves). Every reception situation is different, and what's best for yours is what works best through experimentation. A couple of pointers:

* Keep the screens on "panel" antennas, or replace them if they've been removed. Reflectors make bowtie antennas more directional; better directionality reduces the chances of reception-robbing phase issues.

* Might as well use a good quality splitter rather than the CC-7870. Insertion loss won't be any worse, and a splitter is a whole lot less expensive.

* When combining on a splitter, try switching the balun leads around on one or both antennas if initial results aren't encouraging. This will sometimes reduce phasing problems.

* If none of the above seems to do the trick, that's the point at which buying a Join-Tenna tuned for channel 42 makes sense.

You can't damage a TV tuner with any of these experiments, so have at it, enjoy, and best of luck!

 

[CowboyDren]

Plugging the yagi into the whiskers did improve reception on 42, and nothing else was dropped. As a matter of fact, channels 18 and 47 improved as well, which I can't explain with my eyeballs. I'd think that being 90° off axis from each other would make my situation easier to deal with, but reading about the two antenna trick on the HDTV Primer made me realize that there's a lot of stuff you can't just dead reckon your way through when radio signals are involved.

How do you spot a "good quality splitter" in the mass of junk that's out there? Radio Shack is pretty proud of theirs, but "pride" and "value" don't always overlap. The price alone of the CC-7870 channel inserter told me that it was a poor investment for the insertion loss reason you quoted.

I think the next thing to try is to put a reflector on the whiskers; I built it in November and finding grill gaurds for outdoor cooking isn't exactly easy that time of year. At that time I can plug the yagi back in and see what happens. Ultimately I do want a factory-built solution, and I want the best signal I can get, but price is an object!

Thanks for the encouragement. In the chart below, I'm really only interested in channels 34 (292°) through 47 (334°), and 42/41 from 235°. Everything else on the chart is either shopping or religious programming.

 

[Don_M]

Most splitters are simple commodity products. Look for a 2-way model that covers all frequencies from at least 50 MHz to 900 MHz, and which states an insertion loss of 3.5 dB at each output. (Sometimes they say "-3.5 dB," which is the same thing.) Splitters don't get better than this, really. The RS splitter is probably fine; some of the big-box home centers also sell them. You're all set if the specs are on the splitter itself, or at least printed on the packaging/instructions.

I also built a 4-bay DB4 clone that I use for UHF reception from the attic. For reflectors, I spent $4 on a couple of cookie-cooling racks at Big Lots. Check out the dollar stores, too. The racks can be screwed or stapled horizontally to the back of the antenna board. A couple of cautions: First, make sure the racks are at least 16 inches wide, and mount them a minimum of 3.5 inches behind the whiskers. (The reflectors on the real DB4 and CM 4221 are more like 4.5 inches away.) The width of the reflector helps with reception below roughly channel 30; if the reflector is too close to the whiskers, it hurts gain on all channels.

Whatever you do, don't ever use any kind of amplifier with your antennas. The stations are so close and signals so strong that reception-robbing amp or tuner overload is very likely by the looks of that TVFool report!

 

[CowboyDren]

Just got off the phone with Winegard tech support who recommended sticking with the in-attic location. He suggested erecting a fence of chicken wire between them, and grounding said fence. It just might work! Naturally, he strongly recommended his brand of antenna combiner, and if a splitter doesn't work, I certainly will.

 

[Don_M]

Got my doubts about the CC-7870 working better than a splitter. W-G quotes insertion loss of 3.5 dB, which is the same. I suspect it's nothing more than a splitter at five times the cost. The only real difference is its weather-resistant enclosure, which would be much more durable on an outdoor mast.

 

[CowboyDren]

The tech support guy told me that the CC-7870 was a "true balanced combiner," leading me to believe that it has some sort of winding or capacitance inside. The rugged case would definitely be a plus, but only if it were being mounted outside. I will definitely try a $4 splitter first.

 

[CowboyDren]

I got my hands on an Eagle Aspen DTV2B-UHF, and it seems to work okay on it's own. By peaking it at 87% on channel 42 (the one transmitter on it's own), I still get acceptable signals on everything else, even analog channel 9, which is where the antenna should theoretically point. This is encouraging, because I'm just using an old wing dish mount, and the antenna is less than a foot from my roof hips; not electrically ideal. Worse, it has to look through a notch between my roof and my neighbors', 15' away, to see the 42 tower. I will move it to the top of a 3' mast section in the middle of the house to give it some more distance from reflective surfaces, and give it a broader line of sight than it currently has. If I can stay over 80% on 42, it's going to work out beautifully.

Thanks, Don, for being my sounding board.

 

[Don_M]

Glad to help, and it's good to hear things worked out well on one antenna. Those 2-bays can be amazing little antennas! Your willingness to experiment should keep you in strong signals for the long haul.

 

[CowboyDren]

It's done now. I moved to the other mast I was talking about, got 88% on channel 42, the southernmost, and still have acceptable (72%) signal on channel 18 ,the northernmost. If I try to swing the antenna clockwise to gain on 18, 42 falls off much too rapidly. I get down below 80% on 42 well before 18 breaks over 80, and that's not good (after all, "Chuck" is more important than "Between the Lions"). I settled on an orientation that's at 82% on 42, still at 76% on 18, but hopefully it's a stable 76%. When the DTV transition comes, I just have to hope for a power output bump in 64133.

There's a fresh 30' drop of 3GHz RG6, pro-spec compression fittings, a full service loop at both ends (on the mast and inside the wall), plenty of slack in the attic, and I'm knackered. But my TV experience is AWESOME; no signal losses at all, despite awful weather in KC for the last two days.

Now if I could just get Dish to send me a 722K instead of my 722 receiver so that I could do two concurrent OTA recordings...