Sharing one MIMO LTE antenna between two modems

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WahroongaFarm
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Sharing one MIMO LTE antenna between two modems

Post by WahroongaFarm » Sun May 27, 2018 9:16 pm

An antenna topic that comes up from time to time is "Can I use a single antenna to provide service to two modems?"

As long as the service is generally from the same direction or tower; then the answer is probably ... yes.

However to provide isolation between the two modems, a splitter combiner is required for each antenna feed.

Image

e.g. https://www.ebay.com/itm/141852500475

Notes:
  • A good cavity combiner/ splitter will add a 3dB loss from the antenna to each modem in the receive direction, however the transmit direction is more or less lossless. It also provides ~27dB of isolation between the two modems such that when one modem is transmitting, the spurious signal received by the other modem is dropped by 27dB, hopefully enough for both modems to operate successfully in tandem.
  • Always use high quality low loss coaxial cable and make the connection between the antenna and the modem as short as practically possible.
  • A MIMO antenna would require two splitter combiners.

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JimHelms
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Re: Sharing one MIMO LTE antenna between two modems

Post by JimHelms » Sun May 27, 2018 10:32 pm

I agree with most of what you say, except the splitter that one should use. I would argue that the best one is https://ltefix.com/shop/accessories/com ... r-divider/

Of course, I could be biased toward product selection but, then again, it is how I stay in business. This is a great topic and one that many people can benefit from. It is also one that I have been quizzed on one a regular basis.

Looking forward to exploring the possibilities further.

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Re: Sharing one MIMO LTE antenna between two modems

Post by WahroongaFarm » Mon May 28, 2018 4:18 am

JimHelms wrote:
Sun May 27, 2018 10:32 pm
I agree with most of what you say, except the splitter that one should use. I would argue that the best one is https://ltefix.com/shop/accessories/com ... r-divider/
Woops I 100% agree. :)

Perhaps an opportunity to explain a bit more about how a splitter/ combiner works.

The basic terminology http://www.radio-electronics.com/info/r ... torial.php

RF combiner: An RF combiner is used to combine RF from a number of different sources. This is achieved while maintaining the characteristic impedance of the system. Dependent upon the type of combiner it may introduce additional loss by using resistors, or it may be use transformers in which case it could in theory be lossless.

RF combiners can be used in a number of different applications. They are used for sending several signals along a single feeder, and they may also be used for circuits where several RF signals need to be brought together.

RF splitter: An RF splitter is the reverse of a combiner - in fact splitters and combiners utilise exactly the same circuits - the inputs for one form the outputs for the other. As the signal is split a number of ways, there is an associated reduction in signal level between the input and the output dependent upon the number of outputs for which the signal is shared.

Telecom Hall with pictures http://www.telecomhall.com/what-is-spli ... biner.aspx

Image

Further technical explanation https://www.minicircuits.com/app/AN10-006.pdf (pdf)

The above article explains why simply 'joining the coaxial cable' in a 'T' configuration is a bad idea; due to:

a). the lack of isolation between the two ports or (in our case) modems
b). impedance mismatch

In summary the ltefix splitter combiner is well suited for 3G/ LTE applications, as it achieves:
  • 3.1dB insertion loss in the splitter direction
  • around 0.1dB or so in the combiner direction
  • 27dB isolation between ports.
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Re: Sharing one MIMO LTE antenna between two modems

Post by JimHelms » Sat Jun 16, 2018 3:59 pm

Several customers have inquired about installing two routers inside a single narrow band 2400-2700MHz MIMO antenna/enclosure.

Their objectives include sharing the single MIMO antenna with both routers.

I have experimented in the past with several methods of combining the signals to include using a combiner which is too large for this type of application.

The ideal combiner should be small enough to fit inside the antenna enclosure and still allow adequate room for the two routers.

Below is one combiner that may work -- although it needs to be tested. It is still a little confusing on what factors distinguish a splitter from a combiner from a splitter/combiner combo. The builder insists this module is a combo unit.

SMA Female to 2 x SMA Female Splitter 1 to 3GHZ2.jpg
SMA Female to 2 x SMA Female Splitter 1 to 3GHZ2.jpg (60.19 KiB) Viewed 2237 times
SMA Female to 2 x SMA Female Splitter 1 to 3GHZ1.jpg
SMA Female to 2 x SMA Female Splitter 1 to 3GHZ1.jpg (67.75 KiB) Viewed 2237 times


The specs on this module are:

Connectors: SMA Female to 2 x SMA Female
Rated power: 2 W
Frequency range: 1GHZ ~ 3GHZ
s21> -3.8dB (3db branch removal power, the actual insertion loss is less than 1db)
S11 <-15dB
S22 <-15dB


The antenna's internal connectors are shown in the image below:

WiFix-dm25x19-internal.png
WiFix-dm25x19-internal.png (319.59 KiB) Viewed 2237 times


If testing of this module is successful, I will have another module built with the following changes:

1. Replace the SMA Female (input) connector with a SMA Male Right Angle connector to attach directly to the SMA Female antenna connector.

2. Replace the two remaining two SMA Female connectors with 10 inch RG178 pigtails with U.FL connectors for the modem.


Thoughts?

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