My FPV Ground Station

Well... Yes it does need a little help! But... I'm finally working on it and determined to get it finished. I'm done with the "Spend" phase and just getting started on the "Cut and Drill" phase. Then all I have left is the "Wire & Solder" phase and I'm ready to go. I've been collecting this stuff for almost a year. I think it's time I get serious about putting it all together. I figured if I started a build thread, I'd feel committed to finish it. Once this is done, I'll drop my OSD, camera, and transmitter into the Phantom and I'll be set.
...Tiger

 

That looks like its gonna be sweet. Look forward to seeing your progress.

 

Interesting parts assortment. What is the silver box in the front left side with the two large coaxial connectors?

 

Looks like a preamp for the RX. Appretly boosts the incoming single like like so:



This look pretty heavy duty though with that giant patch antenna.... guess this must be a pretty long range setup. Looks like some of those 10+ kilometer rigs.

 

Good idea! Thanks for the nice video.

 

Just spent about an hour searching for info on that NGHobbies pre-amp. Except for this video, all the feedback I found on forums were from people that said installing the pre-amp didn't help or made things worse. Interesting how this video test came out so much different. I'm thinking this video looked good because of errors in the test procedure. Hard to say exactly what's happening without more detail.

Here is a thread of interest: http://www.basement-rc.com/forums/index.php?topic=460.0

One thing is clear from everything I read, this pre-amp is very wideband and has no front end at all. They are just plugging it in between the antenna and receiver. Because it's wideband it's going to amplify everything it sees. They show gain specs from 900MHz to 5.8GHz and you can be certain that it has even more gain below that. This is going to cause nothing but overload and intermod in the receiver. What they are doing here is a really really really horrible idea! I'll post more about what I'm doing and why shortly.

...Tiger

 

While Tiger is correct, I have seen things not always as the theory says, so mileage may vary with tests. I once hooked up a TV antenna and Radio Shack TV preamp up to a scanner and heard Roseburg city police in Wimer! I am also building a ground station, my order is being shipped at the moment.

 

I think I have to disagree with Tyson (PP) on this one. When you find that theory does not follow practice, you can be pretty certain that you left something out to the equation when you looked at the theory. This is not unusual, because in real world applications, it’s sometimes difficult or even impossible to grasp all the variables and plug them into the analysis. What often happens, especially in the hobby arena, is that people lack not only the technical expertise, but also the proper tools to attack the problem. Given that, what generally happens is that they apply what theory they do have and then simply “try something”. There is nothing wrong with that and it’s the logical thing to do, but you just want to keep that kind of success in perspective. In tricky technologies like radio, apparent success may not be as sweet as it looks at first. What might work for one person on a given day in a given location, may not necessarily work for another person. It might not even work for the same person if he moved a mile away. That might be what I was seeing in the reports I was reading about the pre-amp tested in the video posted earlier. One person’s sensational success seemed to be followed by many reports of failure by others. I don’t pretend to know exactly what the facts are in this case, but I would guess it has a lot to do with the RF environment in which the tests were made and the other equipment involved in the test. Here is Southern Oregon we are blessed to have an unusually clean RF environment due to the low population density and the associated lack of industrial and commercial activity. You can take something that works fantastic here and move it somewhere else and it might not work at all. That said, our great location doesn’t mean that we can get away with ignoring theory and good design practice. It just means that we have a much better chance of getting away with a less than optimum design here. Remember that something seemingly working today is not necessarily guaranteed to work tomorrow. Also, in the case of something like Tyson's scanner, if it doesn’t work tomorrow, it’s not a big deal. In the case of your expensive FPV aircraft, it’s a totally different story. You need to design your system for the best possible performance under all circumstances. A few empirical test results are no substitute for sound theory and good engineering practice. You want to build the best system you possibly can within your budget and technical ability. Success is never guaranteed no matter what you do, but violating good design practice is always the road to failure.

...Tiger

 

GPS – Yes the little silver box in the lower left is a pre-amplifier. In this case it’s a very low noise PHEMT (GaAsFET) amplifier manufactured by Angle Linear (anglelinear.com). This is a small company located in Sourthern California that specializes in low noise pre-amps for space and commercial applications. As most would probably not suspect, the purpose of this amplifier in my application is not to amplify the signal. The real purpose is to lower the noise figure of the receiver. There is no published noise figure for the receiver, but looking at the design, I’m guessing that it is pretty high. I am hoping to lower the noise figure by about 6db using the pre-amp. This is the equivalent of increasing the transmitter power four times without any of the potential issues that can result from having a higher power transmitter on the aircraft. Another way to accomplish this would be to increase antenna gain (noise free gain), but that would result in a narrower receive beamwidth, which is not at all desirable in this case. Many people think that a pre-amp is just to increase signal strength. Reality is that most receivers have adequate sensitivity (gain). What they usually don’t have is good noise performance. You really don’t want to increase signal strength, but rather you want to increase the signal to noise ratio. Increasing signal strength can actually cause all kinds of problems that can degrade performance. In a pre-amp, you generally want the lowest gain you can get that will allow the desired improvement in noise performance (there is a formula for that). In the case of the amplifier I selected, the gain is only about 15db and the noise figure is about 0.6db. I think this will be a really good match for my receiver. I will be testing all of this on lab equipment when my ground station is complete. We’ll see how I did.

The other thing that goes hand and hand with this is the gray box in the lower right that has all the screws on it. This is a six cavity band pass filter. This is to eliminate any out of band signals from reaching the input of the pre-amp and receiver. Like all broadband pre-amps, mine will have hundreds of times more gain at lower frequencies than the frequency that I’m actually trying to receive. This can be problematic so you really need the filter. That’s the main thing that’s wrong with the way they used the pre-amp in that video that was posted. The published specs for that amplifier show that it only has a gain of about 10db at 5.8GHz, but has huge gain down at 433MHz where the control transmitter is. It has even more gain yet when you get down to VHF where you get hit by FM, TV, and two-way radios. With no filter on the pre-amp input, the results can’t be pretty! The filter is of critical importance. Depending on your RF environment, it may even be more important than the pre-amp itself.

…Tiger

PS: For anyone that might be thinking about a pre-amp, Angle Linear is owned by Chip Angle. I met him over 40 years ago when he was working for TRW Space Systems. We were both doing RF engineering for a company in Los Angeles. He’s a super nice guy and a long time ham radio operator. If you mention that the amplifier if for ham radio, you can generally get a sizeable discount (Note that FPV requires a ham license!). Also, if you tell Chip the exact frequency you intend to use, he’ll optimize the amplifier performance for your exact frequency (no charge).

 

Thank you Tiger for the great explanation. I am looking forward to your ground station build and am sure it will be amazing.

 

I think I’m about finished with my ground station layout and am about ready to start fabrication. I have done the design in a CAD program called “DraftSight”. This is a professional grade 2D CAD drafting program available FREE from Dassault Systems, the makers of SolidWorks ($12,000.00). The program is totally compatible with AutoCad, which is one of the programs I normally use. DraftSight reads and writes AutoCad “DWG” files as well as other popular formats like “DXF”. Anything designed in this program is easily transferred to any major CAD or CAM program for CNC machining or whatever. If anyone out there is following my ground station design and is interested in being able to read and/or edit my design files, I highly recommend that you download and install DraftSight. The FREE download is available here: http://www.3ds.com/products-services/draftsight/overview/ This program is basically a perfect clone of AutoCad LT ($1400.00). It has all the features of AutoCad plus a few more. It even shares the same command set. If you learn this program, you have also learned to use AutoCad, which is considered the industry standard for Computer Aided Drafting. This is a very powerful program!

I have no idea why Dassault developed this and offers it for FREE unless it’s their way of getting under the skin of their biggest competitor. Note that if you load this program, you will be asked once a year to renew its activation and every month or two you may receive an email promoting their other products. They will also send you notices for free upgrades and support offerings periodically. Note also that on the right side of the page linked above, there are a bunch of support links including an excellent video series (all FREE). If you install the program, you will want to view the getting started videos to help you configure the screen options to your liking. As I recall, the default program has some really ugly screen settings (like white on black). It’s worth your time to fine tune the program options to make it nicer to use. I gave a talk on Draftsight at the local Robot Club meeting a few months ago. I’m going to see if I can find my notes and list of recommended changes in the setup.

Anyway… I expect to upload my design here in the next couple days. I just have a few more tiny changes to make before I start cutting and drilling! I just wanted to get this up first for anyone that might want to load my files or might be looking for a professional grade CAD program that won't bust the bank.

...Tiger

PS: I don't know that this might happen, but lets not get any CAD discussions going here in this thread. It would be a lot more beneficial and less distracting to start a new CAD thread in the "General Discussion" section. I'd like to keep this thread focused on the ground station if we can. Tnx!

 

Well... I'm doing a terrible job of making progress on the project. Just about the time I thought I was really going to get rolling, I got slammed with work. I'm going to at least post my basic layout here tonight so you can see what I'm doing. There are a couple screw hole sizes that need to change on this, but basically it's what I'm making. I'm going to post this here both in the original DraftSight (DWG) format and also a PDF for those that are not interested in the CAD stuff. For those doing DraftSight for the first time, I just posted some screen setup changes you'll want to make to the program over in General Discussions / "CAD Software". http://southernoregonrc.com/index.php?threads/cad-software.4817/#post-23107

I hope to post a picture of this soon which will help this drawing make more sense. Basically what this is is a 8.5" square plate (same size as the antenna) that mounts on the back of the patch antenna. The plate attaches 2" behind the antenna using two 3/8" x 2" x 8" aluminum bars. All the equipment is sandwiched between the plate and the antenna. A more elaborate drawing would show all this much better, but I'm lazy and only want to draw enough to allow me to locate and drill all the holes in the plate. I did add a "Comment" layer to the drawing tonight to label most of the mystery parts. You may want to explore the "Layer" command in DraftSight to turn ON and OFF various features (layers) in the drawing. Also, you might want to explore the "Block" command. All the major assemblies in this unit are defined as "Blocks" so you can drag them around the layout to reposition them and all the various parts of the assembly stay together. Everything is drawn to perfect scale so you can use my "Blocks" to put together your own layout.

Note: The CAD file is at the bottom in zip format. I couldn't figure out how to attach it any other way.

...Tiger

 

Agreed. That will be handy being all one unit on the antenna. Is there any concern about have all the electronic right up against back of the antenna like that as far some sort of interference?

 

Mike - I'll eventually post drawings for the two 3/8" aluminum plates (what you are calling a "block"). The lower plate has a standard 1/4-20 tapped hole so the whole thing mounts on top of a tripod nicely. The top plate has tapped holes for an offset bracket that allows the unit to mount below and in front of the tripod head. This allows for the mounting of a LCD monitor above the tripod head. Which option you choose depends on personal preference. I'll probably start off using the lower mount since I will be using goggles. Then I'll eventually switch to the upper mount so I can have an LCD for other people to view while I'm flying with the goggles. Lots of output options with the video distribution amp so I have isolated signals for everything (goggles, LCD monitor, video logger, etc.).

Jonas - There is absolutely no concern about the close proximity of the equipment to the antenna. That was taken into account early on.

...Tiger

 

Your layout looks great John.

 

Mike - Thanks for the offer. I finally looked and I think I'm ok on the 3/8 x x2". The piece I have is just long enough to get the two sides I need assuming I don't screw something up. Wish we had a place in GP to buy aluminum. I use to get it at Frost Boats, but that dried up. Now it's all the way over to White City Metals when I need something. Ugh!

Thanks! ...Tiger