The AKK FX2 vtx is a $20 30.5mm stackable VTX made by AKK for 5.8GHz FPV. Key specs and my take on these are:
  • Smart Audio (choose channel/band/output power in Betaflight OSD)
  • User selectable  25mW/ 200mW/ 500mW/ 800mW (suitable for low race power up to ludicrous 800mW output)
  • 7-24 input (2-5s, possible 6s with a capacitor)
  • Microphone (Your DVR will have sound from the quad)
  • Digit LED and push button (You can adjust power and band/channel easily without betaflight OSD)
The AKK FX2 is an excellent 30.5mm stackable VTX that can be had for less than $20 with free shipping
I've had the AKK FX2 in my main race quad now for 3 months because of less than ideal experiences with longevity from other VTXs in the past. I can say that after numerous race meets, fun fly sessions and solo practice it has held up perfectly with no issue performing exactly how I want - no fuss. Furthermore when flying in a group I've had no problems of channel bleeding or power-up blasting which are the features that TBS and Immersion RC promote as key features in their premium VTXs that cost nearly twice as much.

More importantly than this though, the FX2 gives an excellent signal. I don't find myself wanting more than 25mW except when flying around dense trees when I use 200mW for extra penetration. I have had no need to run 500mW or 800mW other than just to test that they work. For disclosue I use an emax RHCP pagoda antenna and a RealAcc 8dbi patch (review here) on my Aomway Commanders (review here) and a UXII MMCX stubby on my quad.

None of the above are 'killer features' but again, just do the basics that you'd hope for in a good vtx. What is unique about this model though is that it sits tidily and firmly in the flight stack of my Floss 2. Alongside the Sunrise Cicada all in one ESC/FC (review here) I use it makes for a super solid stack that has not cable ties, double sided tape or hot glue. Additionally given the way the MMCX connector exits cleanly out the back I can either use the included pigtail but my new preference is to use the UXII stubby MMCX CP antenna. This is super light and super compact on my floss 2 - no protruding antennas or mounts to be broken. Lastly the base of the board is dead flat. I didn't see the advantage of this until I saw FC board like the Kakute with the elevated gyro - the FX2 definitely clears this with standard standoffs. Alternatively you can fit a skinny receiver like a FRSKY XMplus between the FC and vtx like I've done below:
Picture           My AKK FX2 that has been in use for 3
solid months 

Such a minor detail but the flat base of this vtx means you can make a super low stack


Proof my quad gets use! Plus my XMplus receiver as it sits wedged under the VTX

                                                                                     My super low stack all back together.
In summary, all of these 'little things' done right make for a really good vtx ESPECIALLY since it can be had with free shipping under $20. These are sold exclusively from AKK with free (and fast) shipping or from Amazon.

If 30.5mm mounting is not your thing, the FX3 is based on a 20mm stack and the X2 is a board that will require the aforementioned cable ties or double sided tape while the nano3 will mount to the back of your runcam micro similar to how the FullspeedRC TX200 (review here) does it. The Nano only accepts 5v but does do smart audio.

The FX2 is also available in the 'ultimate' variant which will output up to 1200mW but it's hard to recommend this for quadcopters - even for long range the pros like FPVProvo recommend only 200mW and rather focussing on your aerials and receivers.
The options from the article above:
30.5mm model from this review:

AKK FX2 5.8Ghz 37CH 25mW/ 200mW/ 500mW/ 800mW Switchable FPV Transmitter support OSD          (configuring via betaflight flight control board) $19 at time of writing
20mm model with a practically identical feature set:

AKK FX3 5.8Ghz 37CH 25mW/ 200mW/ 400mW/ 600mW Switchable FPV VTX with MMCX Integrated OSD FC (Configuring via Betaflight Flight Control Board) 
$16 at time of writing
Non-stacking vtx with same features as FX2:

AKK X2 5.8Ghz 37CH 25mW/ 200mW/ 500mW/ 800mW Switchable FPV Transmitter Work with Betaflight OSD FC 
$16 at time of writing
Nano version to stack on the back of your micro cam:

AKK Nano 3 5.8GHz VTX Support OSD Configuring with mounting hole, a stackable vtx designed for Runcam micro and Foxeer micro. 
$14 at time of writing
FX2 version with up to 1200mW transmitting power.

AKK FX2-ultimate 5.8GHz VTX Support OSD Configuring Upgraded Long Range Version 
$26 at time of writing
This is the 1.6g circular polarised antenna that plugs directly into the vtx if your frame will fit. Also available in every other conveivable connector.

Realacc UXII Stubby RHCP MMCX-J 5.8GHz 1.6dBi Super Mini Short Antenna $16 at time of writing
If you are reading this between the 6th and 12th of June, AKK currently have a sale on as below, click on the picture or any link above and it will take you to their sale pricing: