I have 2 UHF antennae of the same model installed on the same mast with rotors and amplifiers and I would like to combined the output signal in order to get of up to 3 dB of the signal gain. I understand that the signal must be in phase at the combining point.
That isn't an easy proposition at UHF frequencies due to the short wave length. I may combine the 2 signals at the antennae location or inside of the house but in this case I must consider the electrical length or the
30' cable. This will feed a High Definition TV set but before I start the work I would like your opinion on the subject.
I have 2 UHF antennae of the same model installed on the same mast with rotors and amplifiers and I would like to combined the output signal in order to get of up to 3 dB of the signal gain. I understand that the signal must be in phase at the combining point.
That isn't an easy proposition at UHF frequencies due to the short wave length. I may combine the 2 signals at the antennae location or inside of the house but in this case I must consider the electrical length or the
30' cable. This will feed a High Definition TV set but before I start the work I would like your opinion on the subject.
It's also not an easy proposition if your various local TV stations are broadcasting from different directions: If the two antennas are in-line with each other, there's some finite delay between the two received signals (measured right at the antenna's feedpoint) if they're "looking" at a transmitter that's also in-line with the antennas, whereas there's no delay when they're looking at a transmitter broadside to the line.
Hence you need to be able to adjust the phasing based on the frequency (channel number) as well as the direction the transmitter is in. While this is certainly doable, in most cases I think it's beyond the the point where it's easier to instead just get one 3dB higher-gain antenna. (Also keep in mind that if you get your antennas too close to one another -- within a few wavelengths -- they may start coupling enough to significantly distort each other's performance.)
If there's only one particular station you want to tune in, though, then I'd say, yeah, go for it!
Connect the two antennas together with a piece of transmission line. Connect your feeder (main) transmission line to the center of the line that connects the two antennas. They will be in phase.
Now the feedpoint of the array will be some impedance which will probably not match the main line which will cause large standing waves and therefore will result in large signal loss. In addition, it will be rather frequency-selective as to where it works best and where it almost doesn't work at all.
Your best bet is to do what Joel said in his last sentence.
You did not state in which country you are living.
If you live in a country with HDTV services distributed with COFDM (DVB-T/DVB-T2) the best results can be obtained by moving the antennas as far away from each other as possible (spatial diversity). Of course, proper diversity receivers should be used (e.g.as in mobile TV reception), but even a simple RF combining will help the situation, it is just like a single frequency network (SFN) with multiple transmitters on the same frequency sending identical data.
However, if you are unfortunate enough to live in a country with 8VSB (ATSC etc.) apparently the nth generation equalizer are finally living up to the expectations with a single antenna, but adding an other multipath path, does not help the equalizers, thus, it is questionable, if the second antenna will actually help anything,
Yes, although note that splitters like this often have insertion losses of a dB or two as well... at which point it's becoming pretty questionable as to whether or not the whole exercise is really worthwhile, IMO.
There won't be an impedance match at the TV receiver (assuming that the TV receiver terminates the line with its characteristic impedance) and thus there will be no reflected wave from that point, and no development of standing waves on the feedline between the receiver and the central joining point.
However, it seem to me that there will be an impedance discontinuity at the joining point, since you may have (e.g.) two 75-ohm feedlines coming in from the antennas, wired in parallel, joining a single
75-ohm feedline down to the TV receiver. This discontinuity will be able to generate a reflection back up towards the antenna, creating standing waves... and if the reflection back towards an antenna is itself partially reflected from the antenna's impedance (if the antenna isn't a perfect match to its feedline) you can end up with the signal ricocheting back and forth between the antenna and the joining point.
This can result in a loss of signal strength (due to cancellation). Possibly more troublesome, the delayed reflections can degrade the intelligibility of the signal... just as multipath reflections from surfaces near the antenna can do.
In analog (e.g. NTSC) TV applications, these short-period reflections within the antenna system may not be delayed enough to cause visible "ghosting", but they can result in some amount of horizontal blurring of the signal (e.g. what should be sharp transitions in the signal brightness can be blurred out).
In his classic book "TV and other Receiving Antennas", Arnold Bailey discusses the effect of these sorts of in-antenna and in-feedline reflections at some length, and suggests some limits on how much delay and reflection can be tolerated without unacceptable degradation of an NTSC picture.
The effects are different on digital TV signals, of course... you don't see horizontal blurring or ghosting... but you can see an increase in the error rate due to inter-symbol interference. Whether it's severe enough to result in a loss of reception (or undo the benefits of the signal-strength increase that the OP was trying to achieve), I don't know... probably depends a lot on the details of the installation.
There is, I believe, an effective way around this problem. Don't just hard-connect the feedline segments together. Instead, use a good
2-way signal splitter (the sort you'd use to connect a single antenna feed to two different televisions), in reverse. Hook the "out" ports to the antennas (via identical lengths of the same batch of feedline), and the single "to antenna" port to the feedline going down to your TV set. These splitters contain a "hybrid" which has the effect of isolating the two "output" ports from one another and maintaining an acceptable impedance, minimizing reflections.
I agree with others, though, that it's probably less trouble, and more reliable, to replace a single TV antenna with a larger one having more gain, rather than try to use two separate antennas and combine the signals.
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I think your first sentence "There won't be an impedance match at the TV receiver (assuming that the TV receiver terminates the line with its characteristic impedance)" is correct if you replace match with mismatch. I'd bet that's what you meant.
Hams, especially the moonbouncers, stack antennas all the time. I'm not really sure how they get around the loss of the Wilkenson combiner.
I'd just get a bigger antenna. The results would be more predicatable since that larger antenna in theory has some test specifications as is, while you would be hacking with your stack. If there is a particular station that is an issue, you could get a yagi cut for that station. Log periodic antennas have less gain than a yagi for the same size.
This is a good place to discuss gain versus aperture. Take a half wave antenna. It has a specific gain. As it is scaled for higher frequency, the size of the antenna is reduced, but the gain stays the same. Now field strength is measured is uV/m. The smaller antenna has less aperture, so the amount of signal captured is less. That is why UHF antennas tend to be multibay, i.e. get more aperture to keep the signal strength the equivalent of a VHF station.
Most preamps are crap. The good news is they are cheap crap. For the preamp to be very useful, it needs a noise figure better than your TV.
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I have a number of Chip's preamps. Not cheap and you have to wait for delivery, but good stuff. He builds customs if there is something close to what he sells but not exactly what you need.
If you have a lot of cable loss, you might consider one of those Silicon Dust networked receivers. You could put the box right by the antenna, though indoors unless you put it in a NEMA box.
Such reception is only "true diversity" if it feeds a "diversity receiver". Otherwise, there is no discrimination done by the receiver.
True diversity would choose the best signal from whatever array of antennas it has. Not necessarily the strongest signal. A good diversity receiver chooses the "best" signal, and strength is not the only factor.
Where are you relative to the stations? Do you have Line Of Sight or are you hidden behind buildings or hills? What antenna(s) are you using? You say you have a rotator but that can be a big PITA if you want to watch the east station but record the north station. Look up 'jointenna' which are channel selective combiners. Not cheap but you end up with a system that all channels are available simultaneously like a cable company.
Also check into DVRs built from PCs. I have 3 of those sometimes all recording at once.
I reckon changing the cable would be his best bet unless he *really* wants to play around with a twin antenna interferometer just for fun.
The lobe pattern of combined sensitivity will peak along the perpendicular to the spar length D separating the two aerials and in the direction that they point. The fringe pattern has a pitch proportional to lambda/D. This always makes pointing more difficult.
And if the cable lengths are even slightly different then the main lobe will shift with frequency which could be somewhat annoying.
Placing the aerials too close together also allows additional local interactions that will be hard to predict but should be modest for Yagis pointed at the transmitter. A vertical separation might be helpful if you have dynamic multipath problems from low flying aircraft, but it would require some experimentation.
I think the first question for the OP is why does he want to do this?
I doubt it is worth the effort for optimistically a measly 3dB gain (when 2dB can be had reliably with a better cable run).
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