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I tuned an RF amp of 140Mhz to FM 105.8, have some questions

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mbarazeen

Member
Hi,

If any RF experts here please help me answering few things.

I found an old satellite phone, for 140MHz and i could drive its amplifier part with a crystal oscillated FM modulator by tuning its coils.
I have RF power meter and a kind of spectrum analyzer to see the frequency and outputs.

With about 500mW input the amplifier delivers 27W, using an output transistor C1946. Two of the coils always get very hot and i can feel the heat coming out from it. I think it was doing the same with original design for 140MHz too, but not to full level.

What is the exact reason for this heating? RF induced eddy current in the coil copper? what could be the best way to reduce it? i have not changed its orientation from original design. one of the coil is having a bobbin without ferite rod, other one is air core.

Any feedback would be helpful, i can post later if a diagram would help.

Thanks
 

k7elp60

Active Member
Most likely the coils are getting warm and/or hot because of current thru them. The amplifier is either being driven to hard or the design for the physical diameter of the coil wire is in error.
 

cowboybob

Well-Known Member
Most Helpful Member
... The amplifier is either being driven to hard or the design for the physical diameter of the coil wire is in error.
That, and:

What is the frequency of your xtal controlled signal being used to drive the amplifier? With no (zero) modulation, are you able to tune the tank coil(s) to a peak? Having done that (if possible) what does the spectrum analyzer show?

I'm thinking harmonics.
 

mbarazeen

Member
the xtal is 52.9 Mhz and doubled to 105.8. I used a 50 ohms dummy load, it was matched for higher power delivery by tuning the coils, infact SWR will be closed to 1:1 since the power meter itself having built in dummy load.

Spectrum shows 104.8 peak and couldnt see any higher level harmonics. output transistor is showing modarate power discipation. I didnt check the current drawn, can be checked.

any ways that i can try 2minimize the heat?
 

cowboybob

Well-Known Member
Most Helpful Member
Well, is the heat a problem? Is it altering the freq, or something? If there is no discernable damage being caused by the heat, why be conerned?
mbarazeen said:
... it was matched for higher power delivery by tuning the coils, ...
This leads me to believe that you've simply exceeded the design limits of the circuit.

Either back off on the driver level or detune the coil(s) to arrive at a heat level that suits you (and the circuit).
 

mbarazeen

Member
That, and:

I'm thinking harmonics.

you are right about harmonics too, i couldnt see it earlier since i mistakenly didnt select the right range, now i found there are 3 components, nearly 63, 71 & 78 MHz, i dont know where it comes from, i can only see 71 MHz component in the modulator. if i impliment a good filter will it resolve the problem? should it be between amp and modulator, or just before antenna?

Thanks to all for your valuable suggestions
 

cowboybob

Well-Known Member
Most Helpful Member
... i can post later if a diagram would help. ...

That would be helpful.

What you're seeing are what would be called "spurious" harmonics: those not related to the center frequency of the xtal controlled driver. Their source(s) are difficult to pinpoint but are generally related to circuit design and the purity of the driver sig.

As you may know (or at least suspect), any amplifier is designed to operate best within a specific frequency bandwidth, the tighter the better. This is both for the purity of the final signal as it is for the efficiency (power produced) of the amplifier.

Your device was designed for 140MHz. You are asking this same circuit to operate at 105.8MHz. It would be an exceptional (very, very rare) circuit that could do that cleanly, let alone efficiently.

Since the harmonic(s) are most probably being generated within the final amplifier's design and component arrangement, the only option for elimination is a "post process" filtering scenario. This solution would not, of course, eliminate the basic problem of the over heating in the final amplifier stage.

So, in essence, You're asking the device to perform way outside it's design envelope. The only realistic solution is to replace the current tank components (and other stuff as well) with appropriately selected components for the frequency you want them to work with.

Finally, if you can live with the heating issue, you could ( and should, for legal reasons) filter out the spurious elements of the signal with filters placed after the final amp. This will, of course, reduce the final signal strength, not be an easy task and may introduce signal purity issues themselves.
 

mbarazeen

Member
cowboybob, Thanks for your good explanation, i will soon post you the diagram so that you can help me out to suggest component values for new frequency. Is there any tools available online, that will help me designing RF?
 

cowboybob

Well-Known Member
Most Helpful Member
... i will soon post you the diagram so that you can help me out to suggest component values for new frequency. Is there any tools available online, that will help me designing RF?

Good, that will help.

And, yes, there are tools for designing RF amplifier circuits. We won't know what's appropriate until we can see the type (A, B, etc.) of the current amplifier. I will caution you that the alteration you envision may not be achievable in the context of the device you have.

I am curious, though, why you've selected 105.8MHz. This frequency is allocated within the range assigned for commercial FM analog transmissions (US, UK and other nations) and is a common frequency used with MRI devices.
 
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