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ThomasScherrer

40W RC control system

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Hi Thomas!

My very congratulations! I know how hard it is to get a working PA. In an experimental project I tried building one of those to see if it could enhance the rc range: http://hem.passagen.se/communication/1wpa.html. Until now I did not succeed in fintuning the amp because of lacking tools.... (my oscilloscope is broken :c( )

Greets,

Marcus

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well this PA is for 434MHz as I use this frequency to RC all my planes,

I dont plan to use max power, but it is good to have avail if any bad things happens, just a turn on the knob and I have more power !

the lowest step is 100mW input = 100mW out, so that is good for several km as usual.

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Is there any chance of the PA causing damage to receiver when it is close at this power ?

Terry

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yes terry, I will add a filter to the front end of the video RX,

and I will place video RX a few meters away from RC uplink.

I will increase the power slowly, and see what happens,

first the video feed will be lost, way before permanent damage,

and I will only have to use this power if noise or jamming or other bad things

happens to my plane, what if the antenna falls off my plane ?

now I can power up, until I can land safely.

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what if the antenna falls off my plane ?

A little tip for you Thomas, remember to tighten the aerial :)

Terry

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On a more serious note, has anyone ever had damage by getting a 600mW video TX too near a sensitive receiver or similar ?

Terry

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Terry : I have killed an EM406A permanently with too strong RF fields,

about 10m away from a powerfull HF band antenna, feed with 2500 Watts SSB modulation, and it was not even powered while I killed it.

Marcus : try to look at my website it is not that secret :-)

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Thomas, I was thinking of a more practical event that any of us may encounter. Is it possable for us to damage our LNA when the plane is being made ready to fly with a 3W TX or even a 600mW very close ?

Terry

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I think not Terry,

it is un-harmfull to approch slowly and then look for blocking, or jamming.

simply look at a much weaker TV signal 2ch away,

then you approch closer and closer on another channel,

then when you see blocking of the weak signal you know you have overdriven either LNA or RX module, and you should not go any closer than this,

this is the safe way to do this test, it will let you see how far the dangerous distance really is, without the need of 100k$ equipment.

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Perhaps a simplier solution for those of us that are rf design incompetent. I used a Mitsubishi rf power module from rfparts.com, a few caps, a regulator and a box w/ heatsink. http://www.rcgroups.com/forums/showthread.php?t=759494

12v on VCC1 and VCC2 (1st and 2nd stages of amp) and regulated 9v on the bias. I really like Thomas's stepped gain control. I would like to do that too, perhaps a variable voltage reg on the bias? I am getting 10w out into a dummy 50 ohm load.

-dave

Edited by dalbert02

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I think you could make a few changes to your design dalbert.

1. You may want to use 50-ohm co-ax for the two red wires in the IN and OUT.

2. You may want to include a low-pass filter on the output.

Daniel

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Yep I agree with both those. Adding 50ohm co-ax in and out will ensure a good match and reduce the chance of oscilations. A low pass filter is always good practice following an amp, you may even reduce the chances of interference to your own video link.

Terry

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Thank you Daniel and Terry,

I had one guy look at my amp and say the same thing. But then another said that since the case was metal, it was the rf shield, esp considering that the case is the ground for all the circuits. I'll be honest, I don't know enough about rf design to know any better, so please, do keep the suggestions coming. The guy that said that the case is good enough, is that not true? I know opinions are like...well, you know, but could you please educate me a little more on other 'good practices' with rf projects?

Thanks,

-dave

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The case is good for shielding externally, but you may get radiation from the red wire at the output back to the input red wire as both are not shielded and you may end up with parasitic oscillations and burn out your amp.

Daniel

(ex-9V1ZV)

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Thank you.

Here it is again, but this time with coax. Any advice on buying/building a low pass filter?

Thanks again, your expertise is greatly appreciated.

-dave

post-6-1193152353_thumb.jpg

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Better, although the cable could be a tad shorter. Not a problem - I imagine that's RG-58U?

For low pass filters, you may want to try some of the ARRL Handbooks, for low pass filters. I don't have them handy at the moment though. You will need the cutoff around 80MHz I imagine.

Daniel

Edited by Daniel Wee

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No, it is RG-174, is that bad? It is 50 ohm cable used on some of my wireless video stuff and WiFi gear.

Thanks again,

-dave

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I suppose for such short runs it's probably okay. I would personally feel better with RG-58A/U but then that's just me. RG-174 is more flexible but it tends to be more lossy than RG-58 or even RG-213U. I'm a bit rusty with my cables as I've not had to deal with them for a while now so someone may want to correct me, or update me on cable improvements.

Daniel

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For what you are doing the co-ax is ok, I would also connect it so it enters the edge of the board rather than double back for a better match.

You seem to have some test gear so maybe you can build your own filter, a simple pi network tuned to the pass frequency would help. Do you have any way to test for unwanted harmonics ? maybe just a scanner. You may also consider just buying an off the shelf low pass filter.

Terry

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post-6.jpg

My comments: cut all tracks marked with red X away all the way into AMP !

at B you have end stub not a part of transmission line,

so it is creating a capacitor and worse a resonant stub circuit !

at A add a mounting screw, as close to the amp mounting screw,

and make the coax as close to gnd and close to signal as possible,

at C make to open loop MUCH smaller, it is a terrible match there.

also those amps generate HUGE harmonics

at this page:

http://www.webx.dk/rc/uhf-link/uhf-link.htm

you can see how to make a good lowpass filter

use mini spring types from coilcraft free samples and 0805 size capacitors for the filter,

also the decoupling powersupply to amp, use 180pF 0805 size

and two in parallel, on each input to GND, and where the caps are GND connected

add screws to case.

this design will radiate power almost as much on the power line as on the antenna :-) sorry..

electrolyte type capacitors near poweramps are useless in general,

ceramic types MUCH beter, but DONT parallel several different types, like 100pF 1nF and 100nF like I have seen done too many times before,

you will make resonant circuit with tons of hi impedance results,

much better to find ONE value with lowest impedance at the tx frequence,

for 430 to 450Mhz use 120pF to 180pF 0805 or even 1206 for transmitters,

and use two in parallel of same value, same type.

oh darn, I did not know your design is for only 50Mhz, then it is 10 times less critical all the things I just said, however it looks much more like you know what you are doing :-) and if you change things like I said, you will also know how to make a 430Mhz version one day :-) dont take it personal, I just try to be friendly and help.

Edited by ThomasScherrer

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