1 watt 433mhz - 2.4ghz rf amp... from SCRATCH! - Page 2 - RC Groups
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Jul 18, 2013, 01:44 PM
RC beginner
yes, we agree on the sad state of affairs, specially in the fpv area. i cant count the number of times i go out with my 10mw micro vtx expecting to at least skirt the field and get wiped out by images from some guys in the next county. its the main reason that, even though im a great video plane fan, i avoid the fpv forum here. you will get trampled by even hinting at legalities. not to mention the plain lack of consideration for fellow hobbyists.
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Jul 19, 2013, 04:32 AM
Stuart
srnet's Avatar
Quote:
Originally Posted by dave1993
even though im a great video plane fan, i avoid the fpv forum here. you will get trampled by even hinting at legalities. not to mention the plain lack of consideration for fellow hobbyists.
Indeed so.

What I do find amusing is the justification for using excess power in the TX, by upgrading to say RFM23BP, its 'needed' to overcome 'interference' or an excessively powerful on-board video TX. Where do they think the 'interference' is coming from ?

A few years ago GWS introduced a tiny 2.4ghz RC controller, great for park flying, slope soaring and the like. The power output was only 10mW, but it had a particularly sensitive receiver (on purpose) to overcome the lack of TX power.

The techie guy said it had been made low power on purpose, as this was a far more friendly method of sharing a scarce RF spectrum. Why use 100mW or 1W over the park when its just not needed ?

Great TX for traveling, small light and the batteries used to last forever.....
Jul 19, 2013, 04:35 AM
Stuart
srnet's Avatar
Quote:
Originally Posted by dave1993
even though im a great video plane fan, i avoid the fpv forum here. you will get trampled by even hinting at legalities. not to mention the plain lack of consideration for fellow hobbyists.
Indeed so.

What I do find amusing is the justification for using excess power in the TX, by upgrading to say RFM23BP, its 'needed' to overcome 'interference' or an excessively powerful on-board video TX.

A few years ago GWS introduced a tiny 2.4ghz RC controller, great for park flying, slope soaring and the like. The power output was only 10mW, but it had a particularly sensitive receiver (on purpose) to overcome the lack of TX power.

The techie guy said it had been made low power on purpose, as this was a far more friendly method of sharing a scarce RF spectrum. Why use 100mW or 1W over the park when its just not needed ?

Great TX for traveling, small light and the batteries used to last forever, which you would expect as it was only using a 10th or 100th of the battery power of some other systems.
Jul 19, 2013, 06:12 PM
RC beginner
all true. i do have some gws stuff but never heard of that unit. do you have a link or what it was called?
Jul 19, 2013, 06:13 PM
RC beginner
all true. i do have some gws stuff but never heard of that unit. do you have a link or what it was called?
Jul 19, 2013, 06:14 PM
RC beginner
lol!
Jul 20, 2013, 02:49 AM
Stuart
srnet's Avatar
http://www.gwsus.com/english/product...ransmitter.htm

The Gamestick one, and it was only 4dBm not 10dBm.

The receivers were real small light and cheap too.

I have it for my Gambler DLG, which is great for over the park and light wind slope soaring, never had a range problem.
Jul 26, 2013, 02:46 AM
Registered User
Quote:
Originally Posted by srnet
I have as you might have guessed I have tried that for the 'other' project.

An LNA in front of a RFM22B gave me a 12dB performance gain, so about 4 times the range. The RFM22B may well be popular for RC these days but they really are quite deaf.



Most RC most modelers would want to present a public image of being considerate and law abiding. However If they want more range, and a lot do, what is legal or not is of little concern.
The numerical gain of a LNA does not get added to the receiver sensitivity (that's a common misconception). Maybe you could have a closer look at some theory of NF in cascade amplifiers. Basic microwave engineering. The NF of the first amplifier predominates. Assumes the first stage has lower NF than the following stages.

http://217.34.103.131/applications/mcl_nf_calc.html
Jul 26, 2013, 07:28 AM
Registered User
I may have missed something, but it does appear that gain adds directly in a cascaded system, it is the noise factor that does not add. The noise factor of the first dominates over the system, which would allow for a high gain amp (but noisy) to be placed after a lower gain, but very low noise amp and come up with an over all better system. Some trades would have to be looked at as the gain of the first stage also has an effect on the overall noise factor of the system.
Jul 26, 2013, 10:13 AM
Stuart
srnet's Avatar
Quote:
Originally Posted by costi_n
The numerical gain of a LNA does not get added to the receiver sensitivity (that's a common misconception). Maybe you could have a closer look at some theory of NF in cascade amplifiers.
No need thanks.

When I said 'An LNA in front of a RFM22B gave me a 12dB performance gain"

I meant just that, it was a real life measured performance gain, I could receive packets that were transmitted with 12dB less power than without the LNA in place. .
Jul 26, 2013, 10:25 AM
Stuart
srnet's Avatar
Quote:
Originally Posted by Aeroengineer1
I may have missed something, but it does appear that gain adds directly in a cascaded system, it is the noise factor that does not add. The noise factor of the first dominates over the system, which would allow for a high gain amp (but noisy) to be placed after a lower gain, but very low noise amp and come up with an over all better system. Some trades would have to be looked at as the gain of the first stage also has an effect on the overall noise factor of the system.
An amplifer must add noise, and the case of the LNA I was using it was quoted as 0.9dB maximum.

Not sure it helps to discuss the theory around these things, as the RFM22 receiver is a fairly broadband device, and clearly by adding gain you are amplifying out of band signals too, and the effect that has on the receiver is not so easy to theorise about.

The LNA was actually a 20dB gain device, but only produced a measured 12dB performance gain in the RX sensitivity.
Jul 26, 2013, 01:55 PM
Registered User
Quote:
Originally Posted by srnet

The LNA was actually a 20dB gain device, but only produced a measured 12dB performance gain in the RX sensitivity.
12dB "performance" gain is HUGE. it means that rfm22 has a realy bad LNA inside.
Can you share us what type of LNA you are using? and how you measure the real gain added by LNA
Jul 26, 2013, 02:30 PM
Stuart
srnet's Avatar
Quote:
Can you share us what type of LNA you are using?
The LNA used was mentioned earlier in the thread.

Quote:
and how you measure the real gain added by LNA
Not sure I should need to expand on what I said earlier;

"I could receive packets that were transmitted with 12dB less power than without the LNA in place"
Jul 26, 2013, 02:59 PM
Registered User
Probably how are you sure you accurate reduced the power by 12dB
At short distance the Tx leaks can be comparable with antenna power.
Do you mean you effectively expanded range four times, that is 12dB, right? but at the boundaries of signal losing.
Even Jim Drew or Sanders will be proud of such LNA...
Jul 26, 2013, 03:12 PM
RC beginner
hey, renato, long time no see.

i get very accurate gain indication by using rssi readings from my transceiver modules. verified and calibrated by the highly technical "long walk" method. fortunately i only have to do that once for each type of module.


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