Tinkering with a mobile/cell phone repeater booster, very high-tech RF

 Tinkering with a mobile/cell phone repeater booster, very high-tech RF.

I have been tinkering with improving mobile phone reception in remote areas (or buildings that have poor phone reception). There are two main ways to do it.

First, an external antenna is mounted on the vehicle with a cable to an internal antenna in a phone cradle. The external antenna is much better than the phone antenna and away from the car shielding. I have this system installed in our camper and it worked very well on our trip to Winton in outback Qld. My mobile is one bar better than my friend's phone. Cost~$400 plus fitting.

Second, as above but with a booster. The booster is a bidirectional amplifier with additional filtering. The boosters are carrier specific, so always use a Telstra one for remote areas. The boosters allow all phones to use them. There is a teardown of one of these, see below and they are very sophisticated. I have been playing with one with my test gear just to see how they work. Amazingly well. Cost ~$1000.

Both systems only work within a specific distance from a cell tower. Mobile phone towers are purposely limited in distance, about 20 km, to limit interference between cells. Phones have a GPS so the system knows where they are. It is possible that towers in remote areas might go further, but not sure.

Another alternative is to use Skylink satellite internet with phones. Works extremely well but is costly and bulky. $200 for hardware, then $150/month.

The device from an eBay advert for the booster with its external and internal antennae, currently $300 off (where I bought mine).

The next is a spectrum analyser with the booster signal.

The third is just the two antennas connected; the signal is lost in the noise.

The fourth is the Telstra coverage around Winton and Longreach, outback Qld.

https://www.ebay.com.au/itm/165975550917?hash=item26a4e9ebc5:g:s5kAAOSwlXBc26dO:sc:AU_Regular!4211!AU!-1

The device from an eBay advert for the booster with its external and internal antennae, currently $300 off (where I bought mine).

Cel-Fi Wave app

The next three images show the Cel-Fi Wave app. It gives more clues on how the device works. The booster connects to a phone via Bluetooth.

The first is the dashboard showing the (low) received signal quality and strength. The antenna separation is to stop the device from going into positive feedback from TX to RX.

The carrier is shown as unknown but should show Telstra. I was having problems talking to the cloud servers during set-up. The booster is specific to carriers as they use filters for the specific bands that the carrier uses.

It also shows the band format, it covers 4G and 3G systems. 5G is mainly being introduced to urban areas in Australia by Telstra. It is likely that the more remote areas will continue using 4G, although 3G is being phased out in 2024. There is a 5G/4G booster coming out but has not hit the market yet. The antenna I am using is 3G/4G/5G.

This image shows the specific band that the booster is using, band 28. The booster has physical SAW filters or duplexers/quadplexers to allow it to only boost one band. The details include channel parameters, signal strengths and the like. From this table, I can tune my spectrum analyser to the right signal to see the amount of boost.

This image shows which bands it can use. There are 11 channels in 3 bands (B3, B5 and B28) corresponding to those that Telstra uses. The channels are 5, 10 and 20 MHz wide depending on band and format. It is pretty remarkable that a device so small and low-cost, ~AU$800, can work across such diverse bands

Internal workings

https://www.waveform.com/pages/cell-phone-repeaters

There is a very good blog post and video on a teardown of one of these repeaters. I won't repeat what is in them, rather just add a bit on its operation.

http://www.kerrywong.com/2021/07/10/cel-fi-go-x-cell-signal-booster-extreme-teardown/

https://www.youtube.com/watch?v=DCKqpoCvSDw

The booster/repeater is a multi-band bidirectional full-duplex amplifier. It scans the available channels and bands from the external antenna and selects the best channel for signal quality and strength. It then boosts just that channel to the slave antenna and then to the phones using it. Boosting more than one channel with less filtering could interfere with other services.

The first task on the external antenna end, to the left, is to filter the right band via a duplexer or triplexer/quadplexer. Then it must go through a duplexer to split the RX and TX channels, usually tens of MHz apart. From there it goes through physically different amplifier modules, for each band.

The second image shows the reverse side with multiple separated amplifiers on each end. The middle is for power supplies and control.

The image is for a USA booster and only has two bands. The Australian Telstra version would have three. The circuit board is populated as required for available bands.

This image shows the type of duplexer/quadplexer that is used in the booster. They are specific to particular bands. Sawnics is a Korean company specialising in ceramic filters. There is a bit of déjà vu for me as I had worked with newly available ceramic filters when I was in high school in the early 1970s. We were trying to use them as an alternative to crystal filters for SSB.