Neil VillacortaBuer wrote:
Greetings guys-n-gals, I have an over-the-air television antenna question.
I have a 4-bay bow-tie UHF antenna that has a beam-width of approx. 45-degrees.
About 10-feet in front of it, is a UHF yagi.
Directly in front or vertically offset?
The antennas are 100-degrees offset from each other (e.g., bow-tie faces North-West, yagi faces East) and I was planning to use a Channel Master Join-Tenna to join the two antennas together onto a single 75-ohm coax.
However, I get a BETTER signal level not only the far-away North-West channel (45% vs. 23%) if I don't use the Join-Tenna, leave the yagi in front of the bow-tie, and just connect directly into the bow-tie.
Not familiar with the "Join-Tenna" but it sounds like a plain old two-port splitter connected backwards. If so it will result in a 3-db loss per antenna.
I was assuming it was the loss of the Join-Tenna, however, when I had the yagi face NW to the far-away channel, and the bow-tie face East to the local channels, the results were not better on ANY channel.
I'm thinking that the Yagi pointed at the strong local channels is coupling the strong local channels in front of the bow-tie antenna, thus giving the bow-tie a 90-degree side-lobe. This WITHOUT the loss from the Join-Tenna is giving me the better signals.
Your beamwidth for the bowtie sounds tighter than I would expect. Keep in mind that beamwidth is generally expressed as 3dB down and there are likely some substantial sidelobes and reception off-axis.
Does the make sense ?
Comments ? Ideas ?
In general when connecting two antennas, things can get really complex. If a signal is visible to both antennas, then the distance between them and the length of the transmission lines to the summing point become rather critical. For UHF, this is a matter of inches. If the two antennas happen to be picking up the same desired signal, the effect can be subtractive depending on the distance between the driven elements and feedline lengths. Randomly slightly moving one antenna with respect to the other will give peaks and valleys if you're in this situation. Plating with these factors may allow you yo optimize it. Also if these are coaxial-fed there could be a ground loop in the shields if everything is on a commonly grounded structure.
Other factors to consider are reflections and multipath and possibly fresnel zone if the path is relatively close to a flat surface nearby.
Two separate co-ax runs and a switch or relay might be in order instead of the Join-Tenna.
In short, a lot of real-world variables make things way too complex for the logical math to be spot-on.
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