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Old 22-05-2018, 13:20   #1
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Antenna Location Issues

I'm looking for some advice from someone knowledgeable about antennas. I have heard that placing two VHF antennas in close proximity can cause a noticeable degradation in signal quality. I don't understand it, but that seems to be the case. I am wondering if two very different types of antennas, in this case wifi and cellular, will also have the same issue. I am looking to install one each of these antennas on my stern arch, and for ease of wiring and aesthetics it would be best to have them located fairly close to each other. Will this hurt their performance? Also, will having the wifi antenna in close proximity (about 8") to an aluminum pole that the wind generator is mounted to be a big issue?


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Old 23-05-2018, 06:49   #2
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Re: Antenna Location Issues

The mutual interaction of two radio antennas is not an issue. It is a natural behavior. All antennas would work best if they were located to be free from surrounding conductors.

The effect of nearby conductors on an antenna is increased if the nearby conductors happen to be of a length or configuration that causes them to resonate at the same frequency as the first antenna is operating at. In order to reduce interaction between two antennas with a mutual resonant frequency, the physical separation should be at least one or two wavelengths.

If two antennas are not mutually resonant and are resonant on widely separate frequencies, the effect of one on the other is reduced, and closer physical spacing can usually be tolerated.

Antennas for cellular telephones can be resonant over a very wide band of frequencies. Usually they are in the radio spectrum above 500-MHz.

Antennas for WiFi are usually designed for much higher frequencies, some as high as 5,000-MHz.

Before assessment of how much interaction might occur between an antenna for "cellular" and an antenna for "WiFi", it would be necessary to know the frequencies involved.

Note that as frequency increases, wavelength decreases. An antenna designed for 5-GHz means one wavelength is only a little more than two-inches. If you have a one-foot separation of that antenna from another, it has about five-wavelengths separation.

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Old 23-05-2018, 06:56   #3
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Re: Antenna Location Issues

That is great information. Thank you.
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Old 23-05-2018, 07:08   #4
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Re: Antenna Location Issues

What CW said.
One major issue is two of a similar frequency in close proximity. For instance, VHF radio and AIS. If these antennas are too close, like within 10 feet, when the AIS transmits, it will likely desensitize your VHF reception, knocking out all but strong signals. In an extreme case, when AIS transmits, it could override your squelch and you would be hearing each burst.

You may have seen crowded installations of multiple antennas. Typically, to keep them from interfering with each other, special filters must be added. Part of what I do for a living is tuning and troubleshooting these types if filters.
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