23 replies to this topic

[Mr.RC-Cam]

Long ago, back when 2.4GHz was popular and dinosaurs ruled the FPV field, there were some forum discussions about creating an antenna switch for use on a video Rx. But to achieve such a thing at low cost, with minimal RF loss, was a bit beyond our means at the time.

But 2.4GHz is no longer a popular FPV video choice, at least not like bygone days. With the popular wireless video system frequencies using 1.3GHz and below, a low cost (~$50US) RF switch becomes practical. Especially with all the useful RF components that the cellphone industry has made available to us. Long story short, the RF antenna switch idea is something I've been experimenting with and fortunately have had some good success in recent tests.

The latest proto version is shown below. It is R/C controlled (hint: it is the reason for this other post) and allows choosing between two antennas. The current design works well with 900MHz-1.3GHz video receivers, but a version for switching Tx antennas is something I'd like to do too.


BTW, performance is better than expected. In the 900MHz - 1.3GHz bands it has flat frequency response and the insertion loss is about -0.6dB.

It may seem strange to create such a thing when we have diversity controllers that automate antenna switching. But there are situations where pilot control of the antenna choice is useful. For example, since the same receiver is being used with the two antennas, it is a very accurate (and convenient) way to compare antenna performance during a flight. There's other uses too (just use your imagination).

This photo shows how it would mount on a video Rx. I stuck a couple junk antenna's on it to give the photo some scale. It can be used with a variety of antennas, including those with coax cables (such as a patch).


As time permits I hope to iron out the bugs in the version that will work on a video Tx. Ideally it would be the same as the Rx switch design. But making the Tx version (at low cost) has been a challenge. And before you ask, yes I might offer a commercial version of the Rx (or Tx) antenna switch. Because of that, I can't divulge any technical details beyond what you see here. But perhaps my reported success will challenge & stimulate some of you to try your hand at creating a similar DiY project.

[Terry]

Nice job as always
I take it a 5.8Ghz version is not going to happen?
A TX version would be very usefull!


Terry

[Mr.RC-Cam]

Nice job as always

Thanks for the nice words. I had a feeling you'd be interested in something like this.

I take it a 5.8Ghz version is not going to happen?

Using the affordable parts and construction methods, the insertion loss of a 5.8GHz version would probably be about -3dB. So I'm not sure if that would excite you. But a major issue for me is that my RF tools are better suited for 3GHz and lower, so higher frequency designs are more difficult for me to evaluate. Because of these issues I'm concentrating on designs for the 900MHz - 1.3GHz bands.

A TX version would be very usefull!

I agree, a remote controlled FPV Tx antenna switch would be awesome. Up until today, that was something that was giving me some design grief. But, Lady Luck has finally come through and this morning I had success! That is to say, my existing design was successfully modified to act as both a Tx (up to 1 watt) and/or Rx switch. One size fits all.

Time permitting, I'd like to continue improving the design (which means another board spin is needed). One of the things that I want to change is the isolation between antenna channels. Currently I have about 26dB isolation, which is perfectly fine for a Tx antenna switch. But to turn this into a kick-azz Rx antenna switch, I'd like to push the isolation spec to 50dB (without exotic construction it can be tough to do). On the other hand, there might not be much need to do that since the Tx application is probably the one that would get the most attention. What do you think?

There's a lot of possibilities for this antenna switching trickery. For example, electronically aimed tracking antenna (no moving parts), real antenna diversity, and other interesting antenna based gadgets. Perhaps someone out there will recognize the FPV related possibilities and do something about it.

[Terry]

I think with the low price of receivers these days multi RX diversity is here to stay BUT with your switch and multi receivers there is the possibility to reduce the cost and the power consumption
Maybe a 3 RX 6 patch system or 5 patch and a omni.
If the loss could be reduced a bit at 5.8Ghz I would be up for it

But yes TX aerial switching on the plane is where it gets even more interesting.
At 5.8Ghz it would be practical to switch between a patch on the nose to one pointing out the back for some impressive long range. With the use of simple fairing drag could be kept low and the dead spots at the sides are no problem if you have an autopilot.

Or a twin dipole system switched automaticly from the ground RX when the signal is lost via the r/c TX to reduce drops when overflying your self or banking hard.

For me the TX switch is the clear winner because a few dB lost off the TX will not be missed as much as on the RX, if needed a bit more power can always be used.


Terry

[Mr.RC-Cam]

To me, the best thing about using the antenna switch (in a multi-receiver installation) is that because only one wireless video Rx is used, RF and baseband video performance is consistent for each antenna.

But like you, I am excited about the remote Tx antenna switching. Not so much for the traditional uses, but more for in-flight (realtime) field testing. For example, it would be ideal to have a standard reference antenna (e.g., our favorite) and pair it up with the antenna we want to test. Both antennas get installed on the FPV model. During flight the antennas are easily switched to make useful real-time performance comparisons. That sort of testing is ideal because it represents real world performance.

For those serious DiY antenna masterminds, such a thing would be a godsend. But of course the regular FPV Joe can use it to switch antenna polarizations, choose between two directional antennas, change from CP to linear, remotely choose low /high gain, and so on. There's plenty of interesting uses for it.

[Terry]

To me, the best thing about using the antenna switch (in a multi-receiver installation) is that because only one wireless video Rx is used, RF and baseband video performance is consistent for each antenna.

By multi-receiver do you mean 3 or more or just 2 ? Not having to adjust the RSSI for each RX would be great but what is the loss for multi switches?
Also you need at least 2 receivers as you dont want to have to switch from the receiver that you are watching the picture from to test a different aerial.
The other draw back is although using the same RSSI means comparisons are better the fact they can not be tested simultaneously means there will be a small error. I guess that if the settling time for the RSSI is short then in practice this will not be a problem but I normally filter out the blips caused by multipath which requires a longer settling time.

But like you, I am excited about the remote Tx antenna switching. Not so much for the traditional uses, but more for in-flight (realtime) field testing. For example, it would be ideal to have a standard reference antenna (e.g., our favorite) and pair it up with the antenna we want to test. Both antennas get installed on the FPV model. During flight the antennas are easily switched to make useful real-time performance comparisons. That sort of testing is ideal because it represents real world performance

Oh yes foe sure it would be great for testing, you know how I love 'field testing' and hate 'lab testing' , well lab testing is good for tuning aerials but the end numbers can be very missleading!

Like you I dont have test gear to make aerials at 5.8Ghz but I would like to compare some that I have now. Im sure that my identical aerials are not identical!

So whats the chance of a TX switch for 5.8Ghz? for testing the losses are not a big problem.


Terry

[Mr.RC-Cam]

By multi-receiver do you mean 3 or more or just 2 ? Not having to adjust the RSSI for each RX would be great but what is the loss for multi switches?

The design will scale up to 1x4 without too much problem. But larger matrix configurations will have more insertion loss.

Also you need at least 2 receivers as you dont want to have to switch from the receiver that you are watching the picture from to test a different aerial.

There's two ways to avoid a second Rx. The simplest method is to switch using a round robin scheme whenever the currently chosen antenna signal becomes too weak (per its RSSI). Of course there would be a momentary loss of signal while it hunts for the next active antenna, but this could be limited to a few hundred milliseconds. But a much more sophisticated way is to use a GPS tracking controller to direct which antenna to select. For example, the pan servo output from EagleTree's EagleEyes could be used to control an intelligent antenna switch (instead of a controlling a pan servo). Think poor-man's electronic beam steering.

So whats the chance of a TX switch for 5.8Ghz? for testing the losses are not a big problem.

When I have a free moment I will measure it to see what the 5.8GHz insertion losses are. But I expect it will be horrible. As mentioned, what I have now is designed for up to 1.3GHz (works at 2.4GHz too, but has ~1.5dB insertion loss).

[Mr.RC-Cam]

When I have a free moment I will measure it to see what the 5.8GHz insertion losses are. But I expect it will be horrible.

I measured the 5.8GHz performance and it was better than expected, but still horrible and certainly not suitable for that frequency. Insertion loss is about -6dB with a switch isolation of a scant 10dB. Keep in mind that the design was for 900-1300MHz, so the 5.8G results are not bad considering the circumstances.

It will require a new design to provide the specs suitable for the 5.8GHz Tx application. I'm not sure if it is wise for me to jump on that right now, but I will put it near the top of my honey-do list. It's too bad you aren't tinkering with the lower frequencies.

[Terry]

It's too bad you aren't tinkering with the lower frequencies.

I wish I could, its not easy living in the UK sometimes!


Terry

[Mr.RC-Cam]

So whats the chance of a TX switch for 5.8Ghz? for testing the losses are not a big problem.

What's the worse case insertion loss (dB) that you would allow with your 5.8GHz Tx? Just for reference, I'm aiming for <2.5dB at 5.8GHz, but it could be a bit more. So if you require the antenna switch to have less loss then that would be good to know.

BTW, I've convinced myself that an electronically steered (servo-less) antenna tracker for 900MHz - 1.3GHz FPV would be a worthy project experiment. My design goals are to have five patch antennas in a pentagon shape to cover 360^o (pan only emulation, no tilt). A single A/V receiver would be used and the antenna selection would be based on the servo control signal from one of the telemetry based FPV tracking controllers (EagleEyes or equiv). With a diversity controller the directly overhead situation could be handled by a sixth patch antenna (with second Rx) aimed at the sky, but this extra trouble would probably be overkill.

Even with the cost of five patch antennas (or other suitable aerials) it seems like a practical tracking antenna solution. The GPS based antenna switch would avoid the RSSI method limitations. As a bonus, it would provide unrestrained 360^o operation without troublesome slip rings or the nuisance of servo unwinding.

[Terry]

What's the worse case insertion loss (dB) that you would allow with your 5.8GHz Tx?

3dB , any more than that and other methods win out.

5 patches to cover 360 is good , I cover it with 4 but have to use low gain wide beam patches or even dipoles with reflectors on my 5.8Ghz system.
I gave up on tilt long ago as for me I found it more trouble than its worth so I agree with you.


Terry

[Mr.RC-Cam]

3dB , any more than that and other methods win out.

Although 5.8GHz RF design is very unforgiving, I should be able to stay under the -3dB insertion loss.

I'm currently working on a new 1x2 antenna switch PCB (will use higher performance parts to support your 5.8GHz wishes); I'll come back in a few weeks and report the project's status.

In the meantime I will continue to evaluate the 900MHz-1.3GHz design. I've completed the firmware (added some user-programmable options), so it is now a fully working prototype. I hope to start the tracker's 1x5 switch design soon, but first I need to move some other projects off my plate.

[Terry]

I'm currently working on a new 1x2 antenna switch PCB (will use higher performance parts to support your 5.8GHz wishes); I'll come back in a few weeks and report the project's status

Im not sure what I would use them for first as like you I have a quew of projects waiting for my time but Im 100% sure they will be used


Terry

[Mr.RC-Cam]

I've been working on the PC boards for the revised 1x2 and new 1x5 RF switches. It's slow going, but I've made progress.

The PCB layout tools I use have some really nice features (perhaps too many to learn in my lifetime). The coolest trick I like is the 3D modeling, which can accurately show how the finished board will look, including how it will fit inside the intended enclosure. To use this feature every component must be created with special 3D information, which can be a bit of work. But the end result is really cool and can save time in the long run since it helps find layout mistakes.

I've uploaded an example 3D video of the 1x5 RF switch prototype PCB (with it positioned in the metal case). What I see directly on the CAD screen is very high res, but of course the quality looks horrible on YouTube. Here's the link to the video (best if watched with Youtube's 720p resolution setting):



In the video you can see there are 6 SMA connectors. One for the Rx, five for the antennas. The ten green LED's point the compass direction of the FPV model and will indicate which antenna is active. At least that is what I have in mind, all of which is subject to change as I continue to work on the design.

[Terry]

Very nice!

Terry

[Mr.RC-Cam]

I'm expecting that FedEx will be delivering the redesigned PCB's this week. So it won't be long before I know how the new design works on 5.8GHz. Fingers are crossed!

[Terry]

Mine too,ready and waiting!

Terry

[Mr.RC-Cam]

Sad to say, but the revised board's performance at 5.8GHz is about the same as the old revision. Not the result I was looking for, but I knew the risks of a 5.8GHz switch design were going to make this a tough adventure.

Here's a picture:

[Terry]

ugg thats a shame I was sure there would be some improvement.

Has the switch found uses on lower frequencies yet?


Terry

[Mr.RC-Cam]

It's just experimental stuff, so no compelling uses. But just about everything we take for granted in this hobby today was at one time a solution looking for a problem.

The electronic antenna tracking PCB (5-way switch) is stuffed, but I need to find time to write the firmware. There's some more important projects in the way at the moment, so it will have to wait for a few weeks. Here's a photo of the little fellow: