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Old Oct 13, 2011, 10:28 AM
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Boulder, CO
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VRx sensitivity improvement

Dear FPV addicts!

A while ago I saw a thread where some folks claimed that a pre-amplifier on the video receiver improved their range. I don't seem to find the thread anywhere. I'm not after a world record here (I'm using 5.8GHz... ), but maybe some of you guys are. So here I go (please forgive me if this is too stupid ):

My receiver has a specified sensitivity of -85dBm (Airwave AWM660RX)
The signal bandwidth is ~6MHz (NTSC).
That gives me a receiver noise floor of -153dBm/Hz.
Thermal noise of a 50 Ohm system is at -174dBm/Hz.

Hmmm....

Being at 5.8GHz should give me negligible noise from nature (the deer around my house don't seem to make much use of this frequency (tested by pointing my high-gain VRx antenna right at them) and the space-aliens also don't seem to contact us any time soon on any of the ISM bands...) Am I giving away some 20 dB's here? Or did I just prove myself stupid - again? Because for a second there I thought I could get one of these:

http://www.minicircuits.com/pdfs/ERA-2+.pdf
(11dB gain@5.8GHz and a Noise Figure of 3.5dB@5.8GHz)

and maybe add one of those in front to knock out the UHF from my RC:
http://www.minicircuits.com/pdfs/HFCN-2275.pdf

A few RF chokes, DC blocking caps and a few drops of solder should give me a gain of 11dB and a NF ~4.5 dB. Total part cost would be ~$10-$20 if bought in lots of ~20 pcs. What gives?

Cheers,
Chyochyo
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Old Aug 13, 2012, 07:23 PM
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Boulder, CO
Joined Nov 2007
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5.8 GHz video-preamp take 2

Hi again,

First of all, don't give up on properly tuned antennas! They are the most important thing in this hobby. You gain much more with good antennas than with the thingy below. But: a pre-amp like this one seems to give another 3-5dB of range. 'Not much!' you say, but 3dB is the same as having a 1.2W transmitter instead of a 600mW transmitter and the thingy below cost just a few bucks to make.

I finally had a moment to put this whole video preamp idea to a test. First I did a few attempts with integrated 5.8GHz WiFi LNAs, but I found that there's very little fun in soldering those tiny things (see picture below).

Today I opted for a much easier-to-solder ERA-2 amplifier from Minicircuits. These are not exactly low noise (~3.5dB NF) but they can be powered directly through the stock AirWave receiver, so you can put it essentially between your antenna and the receiver. Just for thrills I added a 5.8 GHz WiFi bandpass filter to make sure I don't amplify the UHF RC control.

To keep things simple I put the band pass filter in front of the amplifier. This increases the noise figure by another ~1.5 dB. As you can see in the pictures below, the circuit is not a perfect match to the 50 Ohm impedance, but it can't be too bad since the total length of the whole circuit is less than ~10% of the wavelength (the two circuit elements are actually 50 Ohm matched but my solder blobs aren't). I expect to lose another dB or so due to this, but not to worry, since the AW receiver's noise-floor is still well above the one of this amplifier.

It turns out that testing the effect of this pre-amp is harder than to build the little thing. I tried to make the testing quantitative by shielding the terminated VTx (a 600mW ImmersionRC) as good as I could and then to perform all tests on the ground. (I put the terminated VTx in an antistatic bag together with some ferrite toroid and extra caps. I then put this in a tomato can and added a plethora of aluminum foil to make it look like sputnik after an encounter with an asteroid.) I then paced the distance between the receiver and the tomato can until my screen would blue-screen due to the loss of signal.

Sounds easy enough, but I find it is still difficult do be quantitative, since the signal would bounce off from trees or the ground and my shielding clearly didn't provide an isotropic emission pattern Non the less, I found a systematic increase in range with the pre-amp attached. But again: don't forget that using a nicely tuned helix gave much more of an improvement over the (poorly tuned) stock antenna!

The conclusion: While the results varied greatly, I found a consistent increase of range by adding the pre-amp. The increase was never as large as the increase you get from a good directional antenna, but it was the expected 3dB. As a side effect, I also found that the signal would fade more gradually than without the pre-amp. Without the pre-amp the signal was lost completely oftentimes within one step (around 40 yards away from the VTx) but with the pre-amp it would usually get noisy at first but it still seemed flyable for another few paces.

Did any one of you try something similar at 5.8 GHz? I'd be eager to learn from you.

Cheers,
Chyochyo

p.S. You don't need to do this mod with the Sky-RF modules, as these actually provide a better sensitivity out of the box (~5dB better than AW). You can also drop a few hundreds on better receivers, but I personally would probably rather switch to the cheap and proven Sky-RFs instead.
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Old Aug 13, 2012, 09:34 PM
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Hmm, this may be a silly question, but can this be done for 900MHz receivers?
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Old Aug 13, 2012, 10:09 PM
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Boulder, CO
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Quote:
Originally Posted by snelan View Post
Hmm, this may be a silly question, but can this be done for 900MHz receivers?
Depends mainly on two things: First of all your receiver. I have never used 900 MHz but I read that many of them have a rather poor sensitivity. If that's the case, a low noise pre-amp might help you. However, if you fly in an 'RF noisy' environment this might not work. Do you have other 900 MHz sources around your flying area?

The good news is that making a good LNA at 900 MHz would likely be a bit easier, as most components can be larger. The noise figure of easy to use, integrated amplifiers is usually a bit better at 900 MHz compared to multi-GHz.

Best,
Chyochyo
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Old Aug 13, 2012, 10:13 PM
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Hmm, sounds a bit promising. I will look into this a bit more.
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Old Aug 13, 2012, 10:28 PM
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Boulder, CO
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Originally Posted by snelan View Post
Hmm, sounds a bit promising. I will look into this a bit more.
Best of luck. Please let me know how it goes.
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Old Aug 13, 2012, 10:47 PM
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Originally Posted by Chyochyo View Post
Best of luck. Please let me know how it goes.
Will do! Might be a bit until I find time to look into it, but I'll PM you or post here.
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Old Aug 13, 2012, 10:56 PM
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Can I buy one, actually two, from you? And how did you power the amp? how many volts?
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Old Aug 14, 2012, 01:50 AM
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Originally Posted by Chyochyo View Post
Hi again,

....... but they can be powered directly through the stock AirWave receiver, so you can put it essentially between your antenna and the receiver........
thanks for the great info... in fact, I've just placed my order for the ERA-2 and the HPF and is expecting them to arrived in a couple of days...

I am not using the same type of receiver as yours, but can you please let us know how do you power the LNA via the AirWave receiver so that we can have a reference?

cmPang
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Old Aug 14, 2012, 02:38 AM
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Great additions! Sure to purchase some soon!
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Old Aug 14, 2012, 09:12 AM
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Boulder, CO
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Originally Posted by KillerCut View Post
Can I buy one, actually two, from you? And how did you power the amp? how many volts?
Hi KillerCut,

Unfortunately I don't have time to sell these. I can certainly try to help you here if you have any questions.

Cheers,
Chyochyo
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Old Aug 14, 2012, 09:21 AM
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Boulder, CO
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Originally Posted by cmpang View Post
thanks for the great info... in fact, I've just placed my order for the ERA-2 and the HPF and is expecting them to arrived in a couple of days...

I am not using the same type of receiver as yours, but can you please let us know how do you power the LNA via the AirWave receiver so that we can have a reference?

cmPang
Hi,

Great! Post your results here. Be sure to make the circuit as small as you possibly can to get a decent performance.

I powered it through the AirWave receiver. These have a DC power supply attached to the antenna port (see here: http://www.rcgroups.com/forums/showp...&postcount=786). The ERA-2 need a DC current at their input. The recommend current is 40mA. The airwave deliver a bit less (~25mA if you attach an ERA2), but that should be OK. The amplifiers actually have a bit less noise at the lower current and can still deliver enough power for this application.

If you have another receiver, you might need to power it with an RF-tee. You can typically omit the inductor at the cost of reduced gain and increased noise figure. I'm not sure how low-noise your receiver is to begin with so the results might vary.

Please let us know your results here!
Best,
Chyochyo
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Old Aug 14, 2012, 09:22 AM
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So how do you power it? How many volts? I'd like to build it if you wont sell it. Also which rf chokes and dc blocking caps did you use? If you can do a step by step diy guide that would be great.
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Old Aug 14, 2012, 09:24 AM
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Originally Posted by FirstPersonView View Post
Great additions! Sure to purchase some soon!
Hey, FPV!

If you have less shaky hands than me you should try some integrated LNAs with better performance than the ERA-2 I just can't seem to solder them.

Cheers,
Chyochyo
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Old Aug 14, 2012, 09:33 AM
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Originally Posted by KillerCut View Post
So how do you power it? How many volts? I'd like to build it if you wont sell it. Also which rf chokes and dc blocking caps did you use? If you can do a step by step diy guide that would be great.
Hi KillerCut,

Please see post #12 above. The ERA-2 needs ~20-40 mA flowing into it's output. Please note that if you are planning to connect a clover leave or an SPW you will need a blocking capacitor or an appropriate band-pass filter at the input as well. You don't need anything at the input if you go for a helix.

You can find a basic circuit diagram on page 3 in the ERA-2 datasheet: www.minicircuits.com/pdfs/ERA-2+.pdf. You can omit the inductor (I recommend to omit it since it is very hard to solder a 6GHz compatible inductor!) as long as you have a high enough DC voltage. For instance, if you have 12 V DC you can use a 213 Ohm resistor and loose only about 1dB. With the wrong inductor you might lose much more than that.

-Chyochyo

Please note again that I used the ERA-2 simply because it's easy to use. There are much better amplifiers for this job but they likely would require a nice circuit board.
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