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Posted: 7/18/2010 8:26:34 AM EDT
Anyone read this?
Anyone build one?

I have a VX7r, and the article shows you a 50w amp homebrew for 30$. The article has pictures, a write up and schematic....
This might be a BADASS addition to my ecoms setup.  
Link Posted: 7/18/2010 8:46:09 AM EDT
Oh - and upon reading it - Can anyone tell me why this could not be made to work for a HT?
Link Posted: 7/18/2010 8:47:14 AM EDT
Be advised that the homebrew amp in QST is designed to work at 30 MHz and below.  Your VX-7 is mostly useful at 144 MHz and above so the QST amp may not help you much.
Link Posted: 7/18/2010 8:49:48 AM EDT
I saw some other similar amps somewhere.
Link Posted: 7/18/2010 9:07:47 AM EDT
The amp there was designed for CW...  I am VERY interested in learning how to DIY - what is different in the basic layout of a CW amp (or an amp in a particular freq) and one for TX or in a different freq?

(you may not see it, but learning is occuring)
Link Posted: 7/18/2010 10:47:12 AM EDT
I am VERY interested in learning how to DIY - what is different in the basic layout of a CW amp (or an amp in a particular freq) and one for TX or in a different freq?  (you may not see it, but learning is occuring)

this is an excellent topic to dig into.  one place to start would be the ARRL Handbook, which unfortunately i would tell you what chapter but my text is sitting on my desk at work.

in general, however, offhand there are five factors you need to consider when specifying or building an amplifier:

1) TX/RX switching
the amplifier itself, or surrounding RF circuitry, must allow any incoming signal (appearing at the *output* of the amplifier) to pass through to the transceiver.  this is generally done with discrete components, an RF relay, or other switching means.  it's important that this low level signal is not attenuated or distorted in any way as this will reduce apparent receiver sensitivity or selectivity.  

2) gain (input power vs output power)
any amplifier will have a gain expressed in dB, a minimum input to realize that gain, and a maximum input before damage occurs.  for example, a 50W VHF amplifier design may have a gain of 10dB, a minimum power input of 1W, and a maximum power input of 10W.  10dB is a 10 fold increase in power, so 1W in = 10W out, and 5W in = 50W out.  an input of 10W still results in an output of 50W.  an input of more than 10W will damage the amplifier.  looked at another way, there is 5W of safety margin on the input.  note also that 0.1 W in will not result in 1W output; the minimum excitation input of 1W has not been met.

3) frequency range
any power amplifier design is going to have a range of frequencies which it operates with relatively uniform gain, and is stable.  this is known as the bandwidth.  it is generally specified as such: "Gain=10dB +/-0.5dB from 140MHz to 170MHz."  depending on the design, cost, etc, outside this range the amplifier may be a paperweight, it may work marginally, or it may oscillate (an amplifier and an oscillator are remarkably similar in design, btw).  very wide frequency (aka "broadband") power amplifiers are difficult to design, and usually end up quite expensive.  

4) modulation compatibility
this is somewhat complicated to explain, and i encourage you first to read through the Wikipedia and other references on Frequency Modulation, Amplitude Modulation, Single Sideband Modulation, and CW.  in short, the characteristics of the signal amplification method employed MUST be compatible with the modulation on the original signal.  if it is not compatible, fidelity of the signal (meaning, relationship of the input to the output) will be compromised at the output of the amplifier.   for example, amplification of AM (and SSB) signals requires what is called a linear amplifier: the output is directly proportional to the input signal, just bigger.   amplification of FM signals is actually somewhat less complicated, and different (more efficient) amplifier topologies can be used.  in addition, RF-sensed TX/RX switching is difficult to do in an AM/SSB amplifier design without relay chatter.  nevertheless, an amplifier vendor will specify what modulation(s) the unit is compatible with.  alternatively, a DIY builder will have to design in an implementation which is compatible.  note that it is in fact possible to make a universal design, but the end user may have to select which mode is in use.  refer to the picture below for an example.

5) heat
the design of any amplifier is hindered by the inefficiency of the amplification process.  due to several factors, RF amplifier efficiency is generally on the order of about 40-70%.  going back to our example 50W VHF amplifier... we know that 50W is going out the antenna port.  how much 12Vdc input power is required?  with 50% efficiency, you would need a total of 100W of 12Vdc input.  where does the other 50W go?  heat!  hence, the designer of the amplifier (either you or someone getting paid to do it) MUST take into account that there will be a "waste" 50W of heat which must be dissipated –– by heat sinks, fans, etc, and generally a combination thereof.  observe the finned construction of the picture below.  note that the cooling approach necessary is highly duty-cycle dependent; a repeater amplifier needs far more heat sinking/cooling capacity than a mobile radio amplifier, since a repeater may be TX'ing for long periods at 100% duty cycle.  



Link Posted: 7/20/2010 8:27:25 AM EDT
I understand the issue.  That makes things much more complex for a broad band amp.
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