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Antenna combining options - best practice

 
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Olecranon
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Joined: 26 Mar 2015
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PostPosted: Thu Mar 26, 2015 4:38 am    Post subject: Antenna combining options - best practice Reply with quote

Hello everyone,

This forum looks very interesting. Should be some good reading to keep me busy for awhile.

I have a few questions regarding the practical application of a cellular antenna setup that I'm hoping to get help with. I'll try to be concise.

background : I'm setting up an external antenna(s) to use for cellular broadband. My carrier is sprint which uses multiple frequency bands, and I have the ability to select between them. The cell tower is 5 miles away, and I have line of site. This is a 50 ohm system.

◾Band 25: 1850-1915 MHz (transmit)
◾Band 25: 1930-1995 MHz (receive)

◾Band 26: 814-849 MHz (transmit)
◾Band 26: 859-894 MHz (receive)

1) Yagi vs parabolic grid.

I needed a band 26 antenna (814-894Mhz) and had to choose between using a 13dBi Yagi or a 15dBi parabolic grid. I ended up going with the parabolic grid antenna for the higher gain. Is there any compelling reason to use a Yagi instead? The only advantage I see with a Yagi is that it has a larger horizontal and vertical beam width so it doesn't need to be pointed with as much accuracy. Does this sound correct?

2) Combining multiple antennas (different frequencies)

As I mentioned, I have two bands I can switch between. Would it be possible to use a combiner and connect two different antennas to the same feed line that are tuned for different frequencies.

For example : 804-894Mhz grid antenna and a 1710-2170Mhz grid antenna connected to a combiner and feeding the cellular device.

The reason I would want to do this is to switch bands on the device without having to physically swap antennas. It seems like receiving would work ok, but transmitting would have issues since half the signal would be going to an antenna with the wrong frequency? Would a combination of high pass/low pass filter alleviate this issue?

3) Combining multiple antennas (same frequency)

Any issue with stacking two identical antennas with identical cables into a combiner? Antennas would be same make/model connected to the same make/model patch cable. The antennas would be mounted in the same vertical plane about 3ft apart.

Thanks for any assistance you can lend. I just want to do this right the first time.
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admin
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Joined: 03 Jan 2007
Posts: 204

PostPosted: Thu Mar 26, 2015 8:18 am    Post subject: Reply with quote

For your point 1, there is a direct relationship between directivity (how directive your antenna is, this is what gives you gain) and the beam width (how wide or in other words, how non-directive your antenna is). So higher gain is typically lower beam width.

Point 2, you are talking about a splitter. If your low band antenna has a large impedance at the highland, and the highland antenna has a large impedance at the lowband, you may get some natural diplexing. Otherwise you could lose up to 3 dB in energy if the antenna has low efficiency but an impedance close to 50 Ohms for it's non-made for use case. Really, it probably won't matter and you can just use a splitter.

For point 3, you can do that, and the net effect is a higher gain system (more directive as well). 3ft is a fine separation for that frequency, they will have uncorrelated fading (wavelength is only 1/6-1/3 meter).
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Olecranon
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Joined: 26 Mar 2015
Posts: 2

PostPosted: Thu Mar 26, 2015 3:33 pm    Post subject: Reply with quote

Excellent! Thank you very much.

In regards to point #2:

Both the high band and low band antennas have a 50 ohm impedance rating. The cabling, and splitter would also be 50 ohm.

Wilson does make a diplexer made for this very purpose, but the frequency range isn't an exact match for my application. I might go ahead and try a splitter since I can use i for option #3 if it doesn't work well.

One final question. Is it possible to estimate the bandwidth of an antenna? I've heard of people using 2.4Ghz wifi grid antennas for receiving cellular signals in the 1850-1995Mhz cellular band. Supposedly there is only a 0.5dBi drop due to the mismatch, but that seems optimistic.

I'm trying to decide if I should get a 2.4Ghz 24dBi grid antenna to use for the high band, or get a 19dBi parabolic grid that specifically tuned to 1710-2170Mhz?

Thanks again for the assistance.
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