Evaluate: Jumper T20S Gemini Radio Transmitter – Flawed or Unfortunate?


The Jumper T20S Gemini represents an evolutionary step on the planet of radio transmitters, boasting an inner ExpressLRS RF module with twin transmitters. This revolutionary design facilitates Gemini Mode, a function that units it aside within the present radio market. Nevertheless, my exams reveal some potential considerations with its 1W output. Let’s discover these findings in depth.

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New to FPV? You possibly can study extra about FPV drone radio transmitters on this purchaser’s information: https://oscarliang.com/radio-transmitter/

The place to Purchase?

You should buy the Jumper T20S Gemini from these distributors:

Jumper T20s Gemini Radio Transmitter Unbox Accessories Case

Design and Construct High quality

Externally, the T20S Gemini mirrors its predecessor, the T20S, when it comes to design, construct, interface, ergonomics, and RDC90 gimbals. Right here’s a aspect by aspect comparability between these fashions. Notice: My T20S has undergone numerous modifications, particulars of which you will discover right here:

Jumper T20s Gemini Radio Transmitter Compare T20 Original

Under are some detailed photographs of the T20S Gemini version.

Jumper T20s Gemini Radio Transmitter Dissemble Gimbals

Jumper T20s Gemini Radio Transmitter Switches Front

Jumper T20s Gemini Radio Transmitter Switches Back

The important thing distinction is the built-in Gemini Mode transmitter module and its twin folding antennas, designed to be positioned at 90 levels to one another for optimum efficiency.

Jumper T20s Gemini Radio Transmitter Folded Antennas

Jumper T20s Gemini Radio Transmitter Unfolded Antenna

I’m not going into particulars concerning the gimbals, switches, consumer interface and ergonomics on this assessment, since they’re similar to the unique T20S. In case you want to know extra try my earlier assessment: https://oscarliang.com/jumper-t20-radio/. As a substitute, I’ll give attention to what units this mannequin aside.

Gemini Mode Assist

The standout function of the T20S Gemini version is its Gemini Mode functionality.

Jumper T20s Gemini Radio Transmitter

Now, in case you are unfamiliar with Gemini Mode, try on this submit: https://oscarliang.com/expresslrs-gemini/. To place it merely, there are 2 transmitters contained in the radio, which is able to transmitting 2 packets concurrently on separate frequencies, growing redundancy and reliability of the radio hyperlink. It doesn’t essentially enhance vary however considerably improve hyperlink high quality particularly in noisy setting.

Because of the elevated energy consumption and warmth, it has a cooling fan hooked up to the heatsink on the again. The fan could be very quiet and you’ll hardly discover it when operating.

Jumper T20s Gemini Radio Transmitter Back Cooling Fan

With better sophistication comes a better worth level and the necessity for appropriate “true variety” receivers, additional growing the fee. Right here’s an instance of “true variety” receiver: https://oscarliang.com/expresslrs-diversity-receivers/#BetaFPV-SuperD

For individuals who fly briefly to medium distance and open discipline, the unique model is greater than ample for my part. Nevertheless the Gemini model presents higher radio hyperlink reliability in difficult setting.

The T20S Gemini can be appropriate with customary receivers (non variety), merely change to the standard antenna mode within the ExpressLRS LUA script (ANT 1 or ANT 2 modes). To make use of Gemini Mode, merely put Antenna mode to Gemini within the configuration.

Jumper T20s Gemini Radio Transmitter Screen Display Expresslrs Gemini Antenna Mode

Exterior Module Bay

The T20S Gemini may not include a built-in module bay, however it does assist exterior modules. There are screw holes on the cooling fan on the again, particularly designed for putting in a detachable module bay adapter. Nevertheless, understand that this adapter isn’t included within the package deal and can should be bought individually.

Peeling again the rubber cowl beneath the cooling fan reveals the XT30 energy output (7.4V 1.5A) and an information port for connecting to an exterior module.

Jumper T20s Gemini Radio Transmitter Back External Module Output


The T20S Gemini options an inner ExpressLRS module. You possibly can flash its firmware in ExpressLRS Configurator, the firmware goal “Jumper Gemini T-20 2.4GHz TX” is now accurately listed beneath “System”. Updating the firmware was a breeze for me utilizing the passthrough methodology.

A fast tip for these updating: at all times make sure you’re working with the newest LUA script. Seize the latest model, exchange the outdated script in your SD card’s SCRIPT > TOOLS Folder, after which take away the outdated recordsdata, i.e. elrsV3.luac and elrsV3.lua.

If you’re new to ExpressLRS, try my detailed setup information: https://oscarliang.com/setup-expresslrs-2-4ghz/


The Jumper T20S Gemini comes preinstalled with EdgeTX firmware, which is at the moment essentially the most extensively used radio firmware within the FPV group.

A notable function concerning the T20S is its elimination of the SD card slot. And no, it’s not an oversight. In truth, the T20 doesn’t even require an SD card. As a substitute, it boasts an onboard 512MB flash reminiscence. This serves as a storage for what’s sometimes termed as “SD Content material” the place mannequin pictures, audio recordsdata and radio backups are saved. Meaning you gained’t have to buy an SD card individually.

You possibly can entry the storage through the USB-C port: join the cable whereas the radio is energetic, when prompted, choose the USB Storage (SD) possibility.

Whilst you probably gained’t have to mess with firmware when the radio arrives, it’s definitely doable to flash EdgeTX firmware utilizing one of many two strategies described in my tutorial: https://oscarliang.com/flash-edgetx/


The included battery holder accommodates two 21700 batteries, although it’s doable to suit 18650s. This flexibility is a plus for customers with totally different battery preferences.

Jumper T20s Gemini Radio Transmitter Battery Compartment

Batteries usually are not included, and should be bought individually. Get them right here:

21700 Li-ion cells:

18650 Li-ion cells:

For these utilizing 21700, brace for a good match. It takes a agency push to shut the battery bay lid. The holder additionally helps 18650, simply pull the contacts outwards with a pair of pliers, to allow them to maintain the smaller 18650 cells securely. I personally desire 18650 for its light-weight and general weight steadiness, even when it trades off some battery longevity.

Energy Consumption and Battery Life

The Gemini’s energy draw varies with the RF energy setting. Right here’s what I noticed:

Gemini Mode:

  • 0.40A at 25mW
  • 0.59A at 250mW
  • 1.12A at 500mW
  • 0.76A at 1W

Ant1 / Ant2 Modes:

  • 0.28A at 25mW
  • 0.40A at 250mW
  • 0.59A at 500mW
  • 0.60A at 1W

The T20S Gemini appears to make use of a bit extra energy than the unique model probably as a result of cooling fan.

Surprisingly, the 1W setting attracts much less present than anticipated. It’s confirmed by my RF output testing, the radio produce solely round 400mW energy at 1W settings, therefore the low present draw (extra particulars in Output Energy Testing part beneath).

A notable level is the elevated battery consumption in Gemini Mode as a consequence of powering two transmitters. Customers must be ready for a quicker battery drain when utilizing the Gemini Mode.

Right here’s a tough estimation of battery life:

  • In Gemini Mode, utilizing 21700 4500mAh cells, you possibly can anticipate round 12 hours of run time at 10mW, or roughly 4 hours at 500mW. With 18650 3000mAh cells, this drops to roughly 8 hours at 10mW or about 2.5 hours at 500mW.
  • In Ant 1 or Ant 2 modes, utilizing 21700 4500mAh cells, you possibly can anticipate round 14 hours of run time at 10mW, or roughly 7 hours at 500mW. With 18650 3000mAh cells, this drops to roughly 9 hours at 10mW or about 4.5 hours at 500mW.

To optimize battery life, you possibly can make the most of the Dynamic Energy possibility out there within the ExpressLRS LUA script. With Dynamic Energy, the mW output adapts to the mannequin’s vary, sign energy and hyperlink high quality, minimizing energy consumption. This function permits for as little as 10-25mW utilization throughout unobstructed line-of-sight flights.


Other than information connection, the USB-C port positioned on high can be used for battery charging.

Jumper T20 T20s Radio Transmitter Top Switches Slider Buttons Usb C Connector Trainer Port

One of many downsides of the T20S Gemini is its gradual charging. PD chargers don’t appear to work correctly, and it solely assist regular 5V chargers. Consequently, its cost charge is a modest 4W, dropping even decrease because the battery nears full cost.

Estimated charging instances are as follows:

  • 21700 4500mah batteries: Roughly 9 hours
  • 18650 3000mah batteries: Slightly below 6 hours

Enjoying Simulators

The Jumper T20S Gemini is plug and play with standard FPV simulators comparable to Tryp and Liftoff. When linked, it’s routinely acknowledged within the simulator and exhibits up as “Radiomaster TX16S”. Whereas it’s the unsuitable mannequin, it doesn’t have an effect on utilization, and the setup stays the identical as all different mainstream remotes.

Tear Down

Disassembling the Jumper T20S Gemini may be considerably cumbersome as there are lots of screws entails. To start out, gently pry out the rubber inserts on the again. Then, unscrew the ten screws which maintain the back and front housing collectively. Don’t overlook to loosen the 2 high switches throughout this course of.

Jumper T20s Gemini Radio Transmitter Dissemble Remove Screws

Be cautious of the three cables connecting the back and front housing, which must be unplugged first earlier than pulling the radio aside.

Jumper T20s Gemini Radio Transmitter Dissemble Pcb Cable Connection

All of the switches are linked to the mainboard utilizing cables. This design alternative simplifies any restore or replacements you may have to do down the street.

Jumper T20s Gemini Radio Transmitter Dissemble Main Pcb

The construct high quality and inner designs are largely just like the unique T20, aside type the PCB shade (was once inexperienced however now black), and the RF module boards.

Jumper T20s Gemini Radio Transmitter Dissemble Rf Board

Output Energy Testing

Listed here are the facility measurements of the Jumper T20S Gemini’s inner module at totally different output energy ranges. These measurements have been taken utilizing the ImmersionRC Energy meter V2.

Gemini Mode

Energy Ranges 25mW 50mW 100mW 250mW 500mW 1W
T20S Inner Module 21mW 47mW 86mW 211mW 462mW 404mW

Antenna 1

Energy Ranges 25mW 50mW 100mW 250mW 500mW 1W
T20S Inner Module 21mW 45mW 85mW 208mW 450mW 490mW

Antenna 2

Energy Ranges 25mW 50mW 100mW 250mW 500mW 1W
T20S Inner Module 25mW 55mW 101mW 233mW 482mW 425mW

Curiously, at 1W, the output is round 400-500mW. I repeated the check 3 instances, tried totally different ExpressLRS configurations, and in addition tried increased enter voltage, related outcomes. Might this point out a flawed inner RF module, or am I simply unfortunate with a foul unit? Anyway, 500mW appears to be extra promising, so I examined it additional for consistency.

Jumper T20s Gemini Radio Transmitter Testing 500mw Output Power

After a 20-minute check at 500mW in Gemini mode, the output stabilized at round 450mW. The cooling fan appears to be efficient in retaining the module cool. I’d most likely keep away from utilizing 1W setting with my T20S Gemini radio, and stick with 500mW.

How you can Setup

I’ve a tutorial on find out how to setup the T20S from scratch: https://oscarliang.com/setup-jumper-t20s-radio/


Jumper T20s Gemini Radio Transmitter Side Bottom

Gemini Mode is especially well-suited for pilots who function in RF-noisy environments or require a extremely dependable hyperlink for his or her flights. Its attraction extends to each hobbyists and professionals who may expertise interference points with customary transmitters.

You should buy the Jumper T20S Gemini from these distributors:

Jumper is the primary to launch a radio with built-in Gemini appropriate module. Whereas their pioneering effort is commendable, there appears to be points with the 1W output – whether or not a singular defect or a broader high quality management drawback – is a priority. Thankfully, the 500mW efficiency appears to be stable in my testing, providing a dependable various for many customers.

In case you want to run Gemini at 1W output energy, at the moment your finest guess is the BetaFPV SuperG module which I’ve reviewed: https://oscarliang.com/betafpv-superg-nano-transmitter-module/

In case you want to run 1W settings with out Gemini Mode, get the unique T20S, which can be cheaper: https://oscarliang.com/jumper-t20-radio/


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