LTE Modem/Antenna modification - Page 1
Forum section: electronics / rf-microwave

author: Tantalum
author_rank: Regular Contributor
timestamp: February 23, 2019, 01:32:48 pm
content: HelloI wanted to have the opinion of persons who know how all that RF stuff work, because honestly I don't have a clue about all that black magic .So I explain briefly my project.I wanted to modify aMikrotik LHG LTEModem/Router/Antenna in the sens that I only would make use of the antenna part (dish and the antenna inside). In other words, I wouldn't use the router and LTE modem (mPCIe USB card).Why?- Because the modem isn't powerful enough (CAT 4, 150Mbit's, no Carrier aggregation possible)- The router part has only a 100Mbit's LAN interface- And the software is outdated and doesn't support more powerful modems (Mikrotik RouterOS v6)Instead of this I wanted to use OpenWRT on a NanoPi Neo2 (4x A53 + Gigabit LAN) and an EM7565 LTE Modem (Snapdragon X16, M.2 USB 3.0), that I would fix on the pole.The original boardA picture of the inside:LAN PoE (passive) port and Atheros router chip on which the LTE mPCIe is connected over an USB 2.0 interface.On the other side you have the antenna part which look similarly (but not the same) to this:The plan- I wanted to let the boardunpowered(because I don't want to use the Atheros chip)- Design a PCB with a M.2 connector to install a EM7565 LTE modem, it would be connected with the same u.FL connector/cable to the original board.- On the same PCB I would add 2 SIM slot, an 2A low noise LDO (5V -> 3.3V) and an USB-C connector (Data + Power)- This PCB would share it's ground with the original board underneath (the antenna wouldn't work without I think).Problems- A lot of peolple are saying to me, that it won't work, or the signal quality would be awful and that the original modem, although less performant based on the spec, would perform better.Are those people right? If so, why?- There is one thing that disturbs me on the original PCB. It's that antenna trace there:Is that white thing a balun?Why does it goes to ground so far away???Are the edges of the PCB also antennas?But they are not angled by 90° as required for MIMO?!?Alternative?I have the following alternative:- I wouldn't add a new PCB. I would only add an u.FL to SMA connector to the device, and use a 1m H155 coax cable to the modem located outside in a waterproof enclosure (the same as for the NanoPi "router" and LAN<->SFP converter)The original board would still remain unpowered! Only the ground would be connected over the shielding of the coax cable.Would it be better???(The 0.50db loss for the coax cable would be ok)Thank you for your help.EDIT:I know, the USB bus is not intended to be used outside on the roof (1m cable), even with ESD protections on the board, but there is no space left inside the Mikrotik LHG to put the NanoPi and it's LAN interface, in it.

author: PaulAm
author_rank: Frequent Contributor
timestamp: February 24, 2019, 09:23:28 pm
content: Instead of trying to reuse an unknown antenna, why not use one of these (or similar)?https://www.amazon.com/Proxicast-Universal-Directional-Antenna-700-2700/dp/B00RJQ8RGCI have one of these connected through 20 ft of LMR400 to a Cradlepoint modem/router. I get around 800K bit transfer rate which, where I'm at, is as good as it gets..The problem with an unknown antenna is that unless you have enough equipment to know what you're dealing with, you're just waving your hands in the air.

author: Tantalum
author_rank: Regular Contributor
timestamp: February 24, 2019, 09:33:14 pm
content: It has not the same gain.https://reho.st/self/491720ced57726f6937ab4d43c12cb28079ef207.pngWell actually, I won't modify anything on the antenna part. I just want to change to modem (the card) and connect it over USB to a routeur (NanoPi), placed ouside of the antenna.The original main board will stay unpowered.

author: radiolistener
author_rank: Super Contributor
timestamp: February 24, 2019, 09:57:09 pm
content: you can try to connect mikrotik antenna to your custom modem. Just make sure that your modem uses the same frequency. There is a high probability that it will works ok. But it means that you just switch off antenna cable from RF connector on mikrotik and switch on to your custom modem.But if there is need to modify PCB and RF circuits, it's better not to do it, because this is very complicated for so high frequency

author: ogden
author_rank: Super Contributor
timestamp: February 24, 2019, 09:57:49 pm
content: That white thing most likely is directional coupler like HHM2510B1 to dump received power into 50 Ohm load resistor at the edge of the PCB. This seems to me like FDD configuration - one polarity for TX and another for RX. I do not see any problem to use just antenna part of that product - reconnect black/grey coax cables to your transceiver and that's it. Thou remember about white thing: that polarization can be used as TX only, other polarization most likely as RX.

author: Tantalum
author_rank: Regular Contributor
timestamp: February 24, 2019, 10:12:33 pm
content: Thank you for yours answers.Yes the new modem will use the same frequencies as the original one (800, 1800, 2100, 2600MHz).https://www.sierrawireless.com/products-and-solutions/embedded-solutions/products/em7565/I just need to change the 2 coax cables because the connectors on the EM7565 aren't the same type.So, to summarize my questions:1. No issue to add another PCB on top (orange on the picture), it won't harm the RF performances?2. No issue if the original main board stays unpowered?3. No issue if the ground is provided from the new PCB on top and that it comes from the USB connector instead of the original RJ45 PoE (passive) connector? In other word, the ground takes a slightly different path on the PCB. I don't know if it harms the performances of the antenna it-self.Thanks!

author: radiolistener
author_rank: Super Contributor
timestamp: February 24, 2019, 10:21:32 pm
content: if RF parts of working PCB will be too close to some conductors, it's performance may be affected.Regarding to the ground, I don't think that cable ground is used as RF ground for antenna, so I think it will not affect performance.

author: Tantalum
author_rank: Regular Contributor
timestamp: February 24, 2019, 10:33:31 pm
content: Quote from: radiolistener on February 24, 2019, 10:21:32 pmThe new modem will be located at the same place as the original one.And I will use a LDO instead of a switched voltage regulator, to avoid noise.Quote from: ogden on February 24, 2019, 09:57:49 pmOk, thank you. I will have to check, which of the 2 connectors on the new modem acts as TX.I have another question.Are standard ESD protections for USB 2/3.0 enough to protect the modem from incoming EMI events through the USB cable?USB bus/cable (1m long) aren't meant to be used on a roof(Unfortunately there are no high speeds USB isolators available)

author: ogden
author_rank: Super Contributor
timestamp: February 24, 2019, 11:28:45 pm
content: Quote from: radiolistener on February 24, 2019, 10:21:32 pmCable ground will not affect performance, thou you can't say that it is not used in antenna. At LTE frequencies cable ground (shield) definitely is connected to antenna element. Think of dipole.Quote from: Tantalum on February 24, 2019, 10:33:31 pmNo it's not. Do not confuse ESD protection with lightning (induced) surge protection. Energies differ by many orders of magnitude. If you ground your USB-interconnected boards properly, USB cable will not be an issue, Ethernet cable going to indoor equipment - will.

author: Tantalum
author_rank: Regular Contributor
timestamp: March 05, 2019, 08:43:05 pm
content: Quote from: ogden on February 24, 2019, 11:28:45 pmThank you for all this explanations.Well actually I was planning to use an Ethernet to (SFP) fiber adapter, so no surge issue with the network equipment.The board will be powered with a 48Vdc power line, there will be a isolated (2kV) DC-DC 12V converter on the board inside the metal enclosure fixed on the pole.But I have another problem related to that issue.The pole on the roof, is currently not earth grounded. What should I do?- Should I include an earth ground wire with the DC power cable, and connect the metal enclosure to it?- If I use GDTs and MOVs at the power input, should there be only one between Vin-GND, or also 2 others between Vin-EarthGND and GND-EarthGND?- Should the metal enclosure fixed on the pole, be electrical isolated from the metal pole?Thank you!

author: radiolistener
author_rank: Super Contributor
timestamp: March 05, 2019, 09:50:53 pm
content: Quote from: Tantalum on March 05, 2019, 08:43:05 pmFor what reason you're thinking about ground? Does your metal box will be powered directly from mains? If so, then connect metal box to the ground will be good idea. It will protect maintenance personnel from high voltage which may be randomly connected to the metal enclosure.

author: Tantalum
author_rank: Regular Contributor
timestamp: March 05, 2019, 10:03:58 pm
content: Quote from: radiolistener on March 05, 2019, 09:50:53 pmTo protect the equipment.Quote from: radiolistener on March 05, 2019, 09:50:53 pmNo.I will use a DIN rail 48Vdc PSU probably combined with a DIN rail 60V surge arrester type 2, located inside the house (10m cable).

author: ogden
author_rank: Super Contributor
timestamp: March 05, 2019, 10:10:36 pm
content: Quote from: Tantalum on March 05, 2019, 08:43:05 pmYou shall do proper lightning protection/earthing of the mast *and* enclosure. If DC cable that goes inside building will be only current path to the earth, in unlikely event of direct lightning strike lives or property may be at risk. You shall consider to consult or hire local certified electrician specializing in outdoor communication equipment installation. Also there are loads of installation information from point-point data transmission equipment manufacturers: Ubiquiti, Cisco, Mikrotik.

author: Tantalum
author_rank: Regular Contributor
timestamp: March 08, 2019, 04:51:20 pm
content: Ok thanks!Have anther question.I'm designing the adapter board with the M.2 connector. On the datasheet of an M.2 LTE modem I saw that schema.At those frequencies listed, is it not more efficient to use ferrite beads instead of capacitors in the pF range?

author: Tantalum
author_rank: Regular Contributor
timestamp: March 19, 2019, 07:17:51 pm
content: Yet another question :-/Does it make a difference if I make the smallest PCB as possible, like on the first picture, or a pcb which uses the full surface (2nd picture)I mean, the antenna is behind the existing PCB, BUT, technically it gets the LTE signal from the reflection from the dish. So in other words, does a PCB which will cover the whole surface of the existing PCB will make any difference or should I keep it as small as possible???Well, the 700-900MHz signal won't be reflected from the dish, but they won't have any issue to pass through the PCBs. I'm wrong?

author: ogden
author_rank: Super Contributor
timestamp: March 19, 2019, 09:04:51 pm
content: Quote from: Tantalum on March 19, 2019, 07:17:51 pmWhatever you change in the original design - it will change RF performance for some degree. Question is how much and which band will be affected most. Nobody can tell for sure unless it is measured. Difference between two PCB options you show could be insignificant. Thou smaller PCB is better, smaller center-aligned PCB is even better

author: Lord of nothing
author_rank: Super Contributor
timestamp: March 27, 2019, 10:51:08 am
content: since he is from NL he should keep in Mind that the must fit into the regulation.good look to get a certificate for that mode.

author: Tantalum
author_rank: Regular Contributor
timestamp: March 27, 2019, 01:41:10 pm
content: Not from NLActually, I don't care about the certificate, this mod is not for sell

author: Lord of nothing
author_rank: Super Contributor
timestamp: March 27, 2019, 01:55:58 pm
content: You are not permitt to power it up.Its like with screw arround your Car. If you change anything you need to check it by an Technician who hand you out a Certificate.

author: Tantalum
author_rank: Regular Contributor
timestamp: March 27, 2019, 01:57:35 pm
content: ok