Connecting two LTE/GSM RF antenna connectors in parallel

Thread Starter

tomerbr

Joined Oct 16, 2017
32
Hello,

In one of my projects, I use a GSM/LTE modem (SIM7000G by Simcom)1640861365720.png1640861386578.png
In the current design, we use a U.FL connector for the antenna.
We use a flexible printed antenna that is connected to the U.FL connector and glued to the interior of the enclosure.

Now, the customer wants the option to connect an external antenna but he doesn't want the end user to open the enclosure.
I added an SMA antenna connector in parallel to the U.FL connector.

My questions are as follows:
1. Is this design common practice? Considering the the U.FL antenna is always connected, will another, un-terminated SMA connector can interfere with the antenna reception/transmission?
2. Will connecting the second, external antenna, can interfere with the device reception/transmission? - In that case we have two working antennas connected to the same line.

See schematics of the relevant part attached.

Thanks,
Tomer

1640861365720.png
1640861386578.png
 

Yaakov

Joined Jan 27, 2019
5,488
You will need to have some means to disable the internal antenna. Two antennas in parallel will not provide a proper match.

One way you could do this is with an external jumper across a jack that exposes the internal antenna's output and the modem's input. That would allow the user to disconnect it and connect an external antenna.

More elegantly, you can consider this article: https://www.digikey.com/en/articles/rf-switches-simplify-multi-antenna-systems
 

drjohsmith

Joined Dec 13, 2021
377
Two antenna in parallel will totally kill the matched impedance, and you will not pass Rf approval,

We use these sort of devices

https://www.murata.com/en-eu/products/connector/switchconnector

so when the cable for the external antenna is plugged in, the internal antenna is dis connected,
but you say the user does not want to access inside the case.

There are other manufacturers around,

If you use an RF switch,
then the RF layout is "interesting" else you will not pass RF approvals,
 

Thread Starter

tomerbr

Joined Oct 16, 2017
32
Thank you both for your answers.
Can you suggest a way to switch between them?
I guess using MOSFETS is not an option as the RF is not DC and 2 MOSFETS on the RF line in parallel to switch between the antennas will not work.

Is there another way to do it?
I can control it with a software.
Automated way is preferable (sensing when the seconed antenna is connected) but even switching when the signal is via the MCU is an option.
 

Yaakov

Joined Jan 27, 2019
5,488
Thank you both for your answers.
Can you suggest a way to switch between them?
I guess using MOSFETS is not an option as the RF is not DC and 2 MOSFETS on the RF line in parallel to switch between the antennas will not work.

Is there another way to do it?
I can control it with a software.
Automated way is preferable (sensing when the seconed antenna is connected) but even switching when the signal is via the MCU is an option.
The link in my reply addresses that.
 

drjohsmith

Joined Dec 13, 2021
377
Thank you both for your answers.
Can you suggest a way to switch between them?
I guess using MOSFETS is not an option as the RF is not DC and 2 MOSFETS on the RF line in parallel to switch between the antennas will not work.

Is there another way to do it?
I can control it with a software.
Automated way is preferable (sensing when the seconed antenna is connected) but even switching when the signal is via the MCU is an option.
Are you aware of what VSWR you need to achieve to meet the desired characteristics from the antenna to meet regulatory approval ?
 

Thread Starter

tomerbr

Joined Oct 16, 2017
32
I have found this RF switch BGS12SN6E6327XTSA1, from its spec the frequency range looks fine to me but it states that it is made for Bluetooth and WLAN. Will it work for GSM/LTE as well?

I would have preferred an SMA connector with an internal switch, are you familiar with such a part?

Thanks again,
TOmer
 

Yaakov

Joined Jan 27, 2019
5,488
I have found this RF switch BGS12SN6E6327XTSA1, from its spec the frequency range looks fine to me but it states that it is made for Bluetooth and WLAN. Will it work for GSM/LTE as well?

I would have preferred an SMA connector with an internal switch, are you familiar with such a part?

Thanks again,
TOmer
I don’t know that device. It might work. WiFi would need bandwidth out to 5+ GHz. Sorry i
can’t help more.
 

drjohsmith

Joined Dec 13, 2021
377
1:1.8 I think, but the modem already have an FCC approval so I just need to match the antennas to it.
Check the FCC approval,
In my experience, The FCC approval for the module
is only valid if you plug into the module with the approved antenna

You still nee to have your product approved to sell it,

In practice, if your just coming out with the specified track, to a compatible connector / antenna then passing is "easy"

But if you now want to put active parts in the Tx/Rf path
its real easy to put a via a few thou out and fail approvals,
 
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