DIY Minimal FPV backpack

Concept

The idea is to make our own VTX using the bare TX module and some basic electronic parts. This allows for a much smaller footprint. And while you're at it make sure to add a proper circularly polarized antenna.

The VTX module

You can find the bare modules at goodluckbuy (as a set), ebay, sdshobby, rangevideo, foxtechfpv, etc. Now be careful when matching these transmitters with receivers since there are different versions available. The choice of module is important. The most important differences are the frenquency band, output power and power supply requirements.

Some require a steady 3.3V and nothing more. If you feed these with more than the specified 3.3V you'll end up burning them. There are however versions available that have a built in voltage regulator. These you can run from 5V. On rangevideo.com the difference is clearly listed. On other websites it's not always clear what should be used, in that case using 3.3V is the only safe option. You can buy either one as long as you feed the correct voltage on usage (see power supply later on).

Regarding output power the most common version is the 200mw version. There also is a 10mw version available. That last version is really only for micro and indoor use. 200mw combined with a proper circularly polarized antenna gives a range of over 1km.

Now on to the frequency band. These TX58xx modules all work on 5.8GHz but they don't share the same bands. There are 3 bands available as you can see in the picture below:

:

Hobbyking/foxtech/... sells band E products from the picture above. Rangevideo/goodluckbuy/... however sells bare modules using band A. These 2 can't be mixed! More information on this post @ RCGroups: http://www.rcgroups.com/forums/showpost.php?p=20064589&postcount=436

The pictures in this tutorial show a TX5823 module that I bought from goodluckbuy.com. It comes in a set with the receiver module. It's listed as a 3.3V module and the specifications show it's a band A module. So it shouldn't work with the RC5305 and RC5808 from foxtech/hobbyking.

Unless you're working with diversity, antenna tracking or some other exotic features it really doesn't pay off to make your own receiver. These RC5308 receivers can be bought for less than 25 USD! So if you want to go that way make sure to pick a proper band E transmitter.

Adding an antenna

Now that we have the module the first thing to do, BEFORE ADDING POWER!!, is to work on the antenna. If you power it up without antenna you'll burn the module.

I won't explain how to create a circularly polarized antenna here. There are enough tutorials available on the web to get you going. If you really don't know where to start go to this topic on RCGroups.com by IBCrazy: http://www.rcgroups.com/forums/showthread.php?t=1388264

I prefer the skew planar wheel version on both transmitter and receiver. Why? Just because I find them easier to build and this combo gives me enough range. Make sure to look for silver plated copper wire since this makes the soldering job much easier!

0.6mm Silver plated copper wire ebay source: http://www.ebay.com/itm/0-6mm-Silver-Plated-Copper-Beading-Craft-Wire-7-5-yards-/270752569922?pt=LH_DefaultDomain_0&hash=item3f0a1c5a42

Coax cable ebay source: http://www.ebay.com/itm/12in-RP-SMA-male-to-RP-SMA-plug-RF-Pigtail-Cable-RG316-/150590703162?pt=US_Radio_Comm_Coaxial_Cables_Connectors&hash=item230fe7c63a

If you don't want to get into this you could also use a small piece of coax cable matching 1 wavelength. This is the bare minimum but it works.

To solder the antenna on you need to connect the inner part of the coax cable to the ANT marking and the outer part (shield) to the GND marking. Best results will be achieved if you connect both ground points so that you properly shield the coax cable all the way.

This picture shows how you can run the antenna wire on the side of the module for support. Your antenna will be catching a lot of wind so make sure to glue the antenna and the contact points so you don't lose connection in flight.

Power Supply

Next point is the power supply. Now that we have an antenna mounted it's save to apply 3.3V to the VCC (+/red) and GND (-/black) points on the other end of the board.

On the previous picture you can see the servo lead I used for that. There are many options to get this voltage from. You'll need to keep one thing in mind though. If you use the same power supply as your RC equipment you'll end up with noise generated by the ESC and servo's. This noise will translate into white horizontal bars in you video signal. If you don't know what I mean just try it.

The easiest solution for this is to use a separate battery. Sure it also adds the weight of an extra battery making it less interesting. From that battery you still have to run a 3.3V regulator anyway. But it is a noise free power supply.

Another solution is to work from the main battery but add some filtering before. For this google LC filter and you'll find a lot of tutorials (for example: http://www.fpvuk.org/how-tos/lc-filters/). The basic filtering exists of a capacitor (400uF or more) and a ferrite ring. The capacitor needs to have the leg marked with a white stroke on the negative (black) wire! The positive (red) wire has to be wrapped around the ferrite ring. If you reverse the capacitor it will blow up.

picture from: http://www.rcgroups.com/forums/showthread.php?t=1339007

That's for filtering the noise out. You still need to get to the 3.3V level. For this you can use a voltage regulator like pictured below. These things have 3 legs (check datasheet for correct pinout, mine was GND, VOUT, VIN). If you use one of these you'll have to work with the 5V from your BEC since dropping a full lipo voltage would cause the regulator to overheat and shut down.

So you still need that bec in between. Alternative option is to go for a 3.3V BEC right away. For instance this one from hobbyking: http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=17159&aff=268919. I haven't tested that myself yet so I can't say how noisy it is (or isn't).

An even cheaper power supply was found by user Whakahere on RCGroups. For more details see this post: http://www.rcgroups.com/forums/showpost.php?p=21097510&postcount=772. Link with most information of the module itself is http://www.ledsales.com.au/catalog/index.php?main_page=product_info&products_id=810. You'll need to add some parts yourselves for setting it to the right output and some capacitors for cleaning the signal. You'll find a lot of sources on ebay if you search for KIS-3R33S: http://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l2736&_nkw=KIS-3R33S

Getting a video signal

Almost there. We have a working video transmitter. Now we just need to feed the analog video signal into the module. For this I used the white wire on the servo wire.

Camera selection and wiring is another point I won't covere here. It's good to know that there are micro camera's available that work on the same 3.3V we already have for our video transmitter.

The complete Assembly

And to complete this article a picture of my MS Composit Swift II with the module (and 9v camera :-)) mounted.