How exactly to use/tune a signal combiner? (grabbing OTA in 61874 with two antennas)

That combiner is used to combine the output of a VCR (or similar device) on VHF Channel 4, with other channels. It's designed to "patch in" the RF modulated output of the VCR, so you can tune it along side of the other channels.
It has some strong bandpass filtering, so it won't pass anything else.....I think WAND is on UHF channel 17.

You might find a converter box that could be used to tune to WAND, and gives an NTSC analog output on channel 4, and mix that in, but you'd be watching the analog down-conversion of WAND on your TV set.

Electronically at the audio cathode level the two antennas with two receivers would be hooked together and you could realize a 3 db gain but they must be in perfect phase and the antennas must be placed about 50 feet apart with one antenna 50 ft ahead of the other.(Dual diversity) This is basically cost prohibitve unless you are Bill Gates. There are some industrial applications that combine the receivers at the IF level but you must remember that the AGC circuits control which receiver is sharing the most or whether both have equal signals. Or you could use a parametric amplifier for the front end of one receiver which would only cost about 100K. Your 20 db quieting point will be somewhere around -99dbm for the received signal at approximately 900 mhz. I believe (I am speculating and not sure) that you normal TV receiver 20 db quieting spec is around -20 dbm.
Some people have said that using a splitter backwards will combine the two antennas and that it works most of the time ($3.00) but this is really too simple a solution.
JJK
Edited by JJkizak - 9/3/12 at 10:28am

Like JJK says, a backwards splitter MAY work in many instances.
All you can do is try.....

Best way to do it is, try one antenna at a time, and see how all your signals look. Pay close attention to whether the station that you want on one antenna looks on the "wrong" antenna. If the "wrong" antenna gets much signal on that station, it means the two antennas might interfere with each other...."multi-path", in other words. If they seem to be isolated from each other, go ahead and try combining the two. This usually works best when the stations are offset from each other by a good bit in azimuth (one is in the main lobe of the antenna, the other is in the null, or "weak" direction).

The unit you have now is only good for the one channel on the "combined" input. A full-blown combiner like a "Join-Tenna" just uses specific filters to separate out the different channels. It's customized to whatever situation you have, and usually needs a "gap" of a couple or three channels between what you want, and what you don't, on each input. The advantage of this is, you don't get addition and subtraction of the signals on the two inputs, which is what causes that evil multi-path (the stuff that makes signals come and go, seemingly randomly).

I think JJK was just teasing us.

The 50 ft. distance is for the UHF spectrum. VHF? Don't know. This stuff I threw at you is based on the old UHF tropo systems that used "Quad" diversity setups on the Dewline. The phasing was accomplished with Tektronic scopes comparing at a reference receiver and adjusting the length of the RG-58 IF cable of the receiver in question under good received signal conditions. between sites. Each receiver had a combiner circuit in the audio band with each combiner hooked together at the cathode point and each receiver having it's own AGC controlling the signal applied. This could mean both receivers combined with the same signal giving you a 3 db reduction in noise level or if one receiver has no signal then it is cut off with the good receiver still operating but no reduction in noise level. This was later redesigned to operate at the IF level. Using this old technology with digital signal would be a problem as when a digital signal fades it looses sync and it's a done deal. The analog stuff is instantaneous so it works. So as long as the digital signals don't fade sync should be maintained as long as you can combine the "Synced Signals".I don't know how you would combine two digitally synced signals.
If in fact you can increase the front end sensitivity of the tv set then there is the problem of one tv station trouncing the other if they are on the same channel with one of them being 100 miles more distant than the other. The new LCDs don't handle this very well and get confused. The dual antennas would not help this at all. It's been 40 years since I maintainened this Tropo stuff so I might have left out some details.
JJK

It's been a while since I thought about Space Diversity reception for DTV. I talked to some of the ATSC guys several years ago, about the possibility of using it for places like Cable head-ends and larger MATV systems, when they are located near airports.

You can also space antennas at specific distances apart, for specific channels, to null-out signals (on that specific frequency) from a partcular direction, or simply combine identical antennas that are closely spaced, to get a narrower beamwidth. That's not Space Diversity, but just a trick to direct the beam, when signals are steady.

There is someone on the Cleveland HDTV section of this forum that uses combiners so you might talk to him. He says it works most of the time.There are several older threads about this subject.
JJK

Using two antennas merged together is complicated because one can pick up unwanted signals that can interfere with desired signals coming from the other. But you can use one all-channel antenna with a second only for the NBC channel. Aim them appropriately, then combine the two antennas together using a 2-way splitter backwards. However, you must use a special filter to notch out (eliminate) the NBC spectrum from the all channel antenna, and a second filter to only pass the NBC spectrum from the NBC channel antenna. Bandpass and notch filters are commercially available. Channel Master once provided an all-in-one solution called the Join-Tenna. Not sure if you can find one available for UHF ch.17 anymore. The filters are necessary to keep undesired signals from the "wrong" antenna from interfering with the desired NBC signal from the other antenna.

According to the FCC website, WAND is transmitting with maximum 1,000 kilowatts on UHF channel 17, 488-494 MHz. from 390-meters above average terrain. This is a blowtorch and you can't get it? WAND has permission to change to UHF ch.18, 494-500 MHz with same power and height. How far from the station are you? Any possibility this station is overloading your receiving antenna system??

Well if you don't want to go to the expense of filtered combining, how about a second downlead from the second antenna to a switch at the back of your TV set so you can select between antennas? Or, have you considered an antenna rotor?? As you have learned, misaiming the antenna or merging two antennas aimed in different directions onto one downlead will lead to reception blackouts.

I have had fair to good luck using a 2 way splitter backwards, and I always use a high frequency splitter too. 2.4GHz (or better) units seems to have far less port to port losses.

Above suggestion about connecting one antenna at a time, and verifying signal strength is a good idea too.

If your antenna pointing at NBC is very directional, it probably won't pick up too much ghosting signal on the channels you are getting on the other antenna.

I have had more luck, if I need an RF amp to drive several TVs, of just using one on the combined feed rather than one on each antenna. I don't know why that is, maybe non-linearity in the amp stages makes for heterodyning and that causes interference on the desired frequencies.

I use the high frequency splitters to add modulator channels too, that always works well.