|Originally posted by tvantennaman
In order for the signals from the two TV antennas in the picture to arrive at the combiner at the same exact time, equal lengths of coax cable must be used between the antennas and the combiner. Without equal lengths of cable between both antennas ghosting will occur unless bandpass filters and channel traps are used. And for the application shown in the picture (I wrote the tutorial and installed this antenna system in Greenwood, Indiana) this antenna system is receiving channels 4, 42, and 63 from the south and channels 6, 8, 9 (DTV), 13, 20, 23, 25 (DTV), 40, 45 (DTV), 46 (DTV), and 59 from the north. There are equal lengths of coax cable between the two antennas and the UHF/VHF band separators that separate the signals so I can attenuate channel 4 by 20dB.
Then everything gets combined back together using another UHF/VHF combiner and a backwards 2-way splitter before the 19dB UHF preamplifier. Measurements were taken with a Tektronix spectrum analyzer and each of the channels were as flat as a pancake. I like to use a simple backwards 2-way splitter to combine broadband signals together so that as many stations (analog and digital) can be received from both directions. And if everything is carefully measured out this setup works great.
You are again taking theory and advice for combining stacked antennas pointed in the same direction and applying it to unstacked antennas pointed in different directions. With two stacked antennas pointed in the same direction and mounted on the same mast, the signals arrive at the dipoles exactly in phase with each other and that phasing is maintained by keeping the coax cables to the coupler the exact same lengths.
In the situation in your photograph, however, the antennas are pointed in opposite directions. Lets analyze this for channel 2. Lets assume that the antennas are physically identical yagis. You have furnished a half wavelength figure for channel 2 of 8.64 feet. If these two dipoles were fortuitously each exactly 4.32 feet from the mast, then the channel 2 wave would hit the further dipole 180 degrees out-of-phase from the way it hit the nearer one. And since the antennas are facing opposite direction, the phase relationship of the dipoles to the coax is reversed, so if the coax lengths for two antennas were exactly equal, then the waves would be in perfect phase and would be added to each other. This would not be the addition of equal signals, however, since the gain on the antenna pointed the wrong way would be about twenty dB less than the gain of the antenna pointed at the channel 2 signal source.
Now lets suppose that a channel 6 is being broadcast from the same tower as the channel 2. Lets say that its half wavelength in the atmosphere is 5.73 feet (8.64 x55.25/83.25). It isn't going to hit the far dipole at exactly 180 degrees out-of-phase from the way it hits the first one, because it is located about three feet (about 90 degrees) beyond the phase inversion point, which demonstrates that even if you are lucky enough to have these two antennas phase matched at one frequency (in this example, 55.25Mz), they will only be phase matched at harmonics of that frequency (110.5Mz, 221Mz, 442Mz), which means that they will NEVER be phased matched for any other TV broadcast frequency because the three broadcast bands are deliberately staggered to avoid harmonics.
Your combined output was flat for two reasons:
1) because your antennas are pointed in exact opposite directions, you have the maximum front-to-back effect mitigating the undesired signal, and
2) Even if you add a significant-strength out-of-phase digital signal to a desired signal, the additional signal would be flat in this case as long as the dipole receiving it is perpendicular to the transmission path, which, in this case, it is. A flat signal does not reliably indicate a quality, "ghost-free" digital signal,
|So what do you do if you live in Frederick, MD, between two major cities, like Washington and Baltimore and you have many channels from each direction that you would like to receive without using a rotator? Use JOIN-TENNA combiners? I don't think so. You'd have to use a ton of them and the ghosting would be horrible with all of those, which would affect the digital channels. Your only solution would be to combine two broadband antennas together using a simple combiner...such as a backwards 2-way splitter. Good yagi antennas do a great job of nulling out back and sidelobes, unlike most consumer antennas. That's why it's so important to look at the antenna specs and understand them before buying an antenna.
If you live in Frederick, MD, where the angle between the two antennas is probably a little less than 90 degrees, then you cannot use jointennas because there are too many frequency conflicts between the two markets, including 36Washington/38Baltimore/39Washington and 51Washington/52Baltimore. You can't reliably count on the directionality of your antennas to conveniently null out the off axis signals. There is very little difference between the sidelobe rejection of a residential grade Winegard or Channelmaster antenna and a commercial broadband antenna. Sure, UHF yagis are tighter than VHF, but what if the Washington signals have to be boosted by, say, 15dB to 20dB, whereas the Baltimore ones might not need amplification? That gain on the out-of-phase Baltimore UHF signals will bring them up close to the level of the desired signals from the Baltimore antenna. And unlike the two antennas in your picture, the elements of the Washington and Baltimore antennas will not be perpendicular to the unintended signals, so those waves will hit one end of the dipole before it hits the other and develop unintended signals that will degrade the intended signals.
Most antenna installers today, including some commercial antenna installers, either don't understand the basics of how radio waves work and the proper techniques of installing antennas or they simply don't care.
Fortunately, I'm not among them!
According to the 2002 Blonder Tongue Broadband Reference Guide in chapter 11, "D) The minimum spacing between antennas of different channels and is the figure given for the antenna with the lowest frequency".
This refers to the chart on the page which has the "D" column showing the spacing for 1/2 wavelength on each of the VHF TV channels and is true for the UHF channels as well. And unless you are cantilevering the antennas off of a tower, I would agree that you would definitely want to mount two combined VHF-LO antennas on separate masts.
Purists recommend one wavelength spacing. Blonder-Tongue, which sells VHF broadband antennas that retail for about $500, recommends a minimum of one-half a wavelength. Most residential customers who cannot afford $500 antennas also cannot afford enough masting to space two VHF lowband antennas even eight feet apart and with the lower antenna six feet about the roof. While I agree with tvantennaman that this problem can best be addressed by putting the VHF lowband antennas on different masts, he didn't in his picture, and I can tell you from extensive experience that spacing them at a quarter wavelength, if you have no practical alternative, will almost assuredly get you acceptable performance.
The only way to successfully combine signals from two antennas pointed in different directions without bandpass filtering is to get lucky. Anyone doing a residential rotor-free antenna installation that includes channels from different directions that are too close together for filtered coupling might as well try the passive coupling, because they might get lucky, and if they don't, the splitter and pads only cost a few dollars. Those who are not lucky enough to be in a situation in which simple passive coupling will work for them will be best served by running two cables to each TV reception area and using an A/B switch to select the correct antenna for each program. The RCA DTC-100 has two off air antenna inputs that "seamlessly" integrate the signals from two off-air antennas into the guide. Otherwise, a remote controlled A/B switch for that purpose can be had for under $40.