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RF Amplifier

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bd13

New Member
Does anyone know how to make a amplifier/preamplifier for an rf device that runs at 49mhz? i have this old r/c car that i took apart and used the circuit boards to make something else, and it only has maybe a 100 ft range. i want to improve that range as best i can, but i don't have that kind of skill to do it. i just got these old radio tubes, or vacuum tubes, i think, from a 1960's electrics kit, and i know those can be used to improve range. i'm not saying i want to use them in an amplifier but it would be good to know how to use them, like in a ham radio. i just need this to amplify the transmitter signal a lot so i can get about 1000ft or more out of it, but it also needs to be portable, and not huge. can anyone help? thanks

the transmitter runs at 49 mhz with a 9v battery
 

stevez

Active Member
Be careful - there are regulations that govern amplifiers in many countries.

Vaccum tubes are viable technology - if you have access to all of the components. You will find that a solid state (transistor, integrated circuit) technology is probably more appropriate and less expensive.

Amateur radio operators are permitted to construct amps for 50 mHz and other bands. You'll find lots of information in publications like the Radio Amateur's Handbook. A 10 yr old copy will have plenty of relevant information.

Good luck.
 

stevez

Active Member
If the amp described is class C then it will be compatible with FM or CW (unmodulated carrier) only - class C isn't linear if memory serves me right. Applying AM or similar input might generate a more powerful signal but will also generate lots of unwanted stuff as a result of the non-linearity, I think.
 

hamfiles

New Member
I'm not an expert on this by any stretch of the imagination, but I believe the inductor/capacitance reaction in a class C amp makes the output appear linear. The collector current does flow in pulses, but the output voltage is a linear sine wave because of the tank action of the inductors and capacitors, and the amp will reject all frequencies except the resonant one. On this one, C2 is adjusted to get a clean sine wave on the O-scope.
 

stevez

Active Member
Hamfiles - maybe we can both learn a bit, especially if someone with more knowledge jumps in. That's the great thing about the forum.

Like you, my knowledge is limited but it's my understanding that class C amps, by their nature, are not suited to anything but FM or CW. They are more efficient so you can get more power out of a transistor. I think it has to do with linearity - the relationship between input and output - if you looked at a graph of a class C amp it would not be a straight line. Other classes of amps are more linear and a graph of the input/output would appear to be a straight line within the operating range of the amp. The significance of this is addressed in the next paragraph.

An AM signal appears at the input as many frequencies around the carrier frequency - with departures from the carrier exceeding the audio or modulating frequencie. The strength or amplitude is also varying (hence Amplitude Modulation). An FM or CW signal is different in that the amplitude (for the purpose of this discussion) is constant. In FM the frequencies are varying but the amplitude is constant. With AM the input signal level is all over the place and is being processed by a system that is non-linear. At the very least what comes out won't be looking like what went in - in addition it's my understanding that the departures from linearity allow the generation of all sorts of extra junk. A square wave generator is an extreme example where there is an abrupt change - that abrupt change generates lots of harmonics.

The LC circuit, while being resonant, is probably very broadbanded so it might clean up some of the junk it's probably not going to attenuate the unwanted stuff very much.

Many class C circuits can be adjusted to operate linearly but then by definition they become another class (A or AB) - at a lower power output.

So, that's more or less how what I've learned plays back in this situation. More insight or explanation would be helpful.
 

Roff

Well-Known Member
Well, I'm not a class C amplifier expert either, but I'm pretty sure you can get AM by replacing the RF choke with the secondary of a transformer and applying the modulating signal to the primary. This effectively modulates the supply voltage, which in turn modulates the carrier amplitude. You could also do it without a transformer, but it's a little more complicated.
 

bd13

New Member
thanks, but....

thanks for the info, i will try that class c amplifier as soon as i get all the parts. but in the meantime i found this topic on 'radio amp'. one of the links had plans for a simple pre-amp, which i also could use, but i would still prefer a amplifier for the transmitter. the link was http://www.hearsat.org/preamp.gif, so i decided even though its not what i really need to give it a shot anyway. but before i put it together i would like to know, will this work for my 49mhz receiver? thanks
 

hamfiles

New Member
You will probobly need to beef up your remote transmitter, you can do this 2 ways.

1. modify existing circuit

2. add rf amplifier


Simplest way to modify transmitter is to change output transistor to more powerful one, modify the bias network for higher voltage, and apply said voltage. This is a little hard because its easy to mess up a board this way.

For adding RF amp, you may want to find out if your device is AM or FM. I think a small transmitter at 49 MHz would probobly be FM. There are many 49 Mhz FM systems. Alot of walkie talkies are 49 Mhz FM. That being said, you want to add your amp at the output of your antenna. I think Motorolla makes some specialty chips that act as single-chip RF amp stages.

One thing you might be able to do is try raising the supply voltage a little on the transmitter. If you use 9 volts, maybe try 12 volts. Sometimes a couple of volts can give you a little more power. But keep in mind that the circuit is made for a specific voltage, so raising it will lower the active region on the output transistor, raising distortion. You may also fry your transmitter.
 
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