this post was submitted on 08 Aug 2024

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IWTL I Want To Learn

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IWTL how to Build an RF Amplifier (self.iwtl)

submitted 3 months ago by Isoprenoid to c/[email protected]

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I want to build / design an RF amplifier that can boost the signal from an AMT-MW207 kit.

Design goals:

Boost the signal a couple of watts
AM signal
525 - 1605 kHz baseband range

I've been searching for RF amplifier designs but many of them are too big (10's of Watts), or are hard to implement. It's been difficult trying to find something that can instruct me clearly. I'll have to take into account things like impedances and the like.

I have an electronics background, so if you can only point me towards a book or other resource, even that would be helpful.

I'm going to be checking out 'Experimental Methods in RF Design', hopefully it can point me in the right direction.

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[–] [email protected] 6 points 3 months ago* (last edited 3 months ago) (1 children)

Well, this sounds super illegal. Mediumwave is a pretty busy block of spectrum. There's the 160 meter amateur band in my jurisdiction, which technically is medium wave, but still above commercial AM stations. And you need a licence, which I'm pretty sure from the way this post is you don't have.

Using somebody else's band, besides being a crime, is just kind of a dick move.

I'm not quite sure how much help I should give until we clear that all up. What's your use case, exactly? A couple watts isn't that much higher than the <1 that's actually allowed sometimes.

[–] Isoprenoid 1 points 3 months ago* (last edited 3 months ago) (2 children)

Using somebody else’s band, besides being a crime, is just kind of a dick move.

This depends on the jurisdiction, and the amount of interference. I'm not from the US.

The amount of power I'll be using won't even interfere with suburban neighbours.

https://en.wikipedia.org/wiki/Short-range_device

What’s your use case, exactly?

Transmitting music to AM receiver radios in a house.

[–] [email protected] 1 points 3 months ago* (last edited 3 months ago) (1 children)

This came across as someone who might not know what they're doing. If that's not you, my apologies. For example, you didn't list the input power or how many dB you're looking for, which is pretty central to the operation of any linear amp.

What this device's output, exactly? The first amp that I'm using for the transmit chain on my build would give around a Watt out, which is close. I can't remember how low the frequency range goes, but I wouldn't be surprised if the same vendor produces one for lower-frequency use as well as SW.

It looks like this is a near-field device, by the lack of a building-sized radio mast. What's your plan to manage the resulting SWR?

[–] Isoprenoid 1 points 3 months ago (1 children)

This came across as someone who might not know what they’re doing.

Yup, that's why I'm here. I thought that was the point of "IWTL"?

What’s your plan to manage the resulting SWR?

I have a NanoVNA so could build a small dipole. High efficiency isn't really the point.

I'm not going for anything fancy, I just want to know the basics of how to design and build an RF amplifier.

What this device’s output, exactly?

It's only in the mW region. The effective transmission is only within a metre (3 feet) of the device.

how many dB

Gain can be small, even if it's 10 or 20 dB. I want to learn the principles of design and then I can work from there.

[–] [email protected] 2 points 3 months ago* (last edited 3 months ago) (1 children)

Yup, that’s why I’m here. I thought that was the point of “IWTL”?

It is. In that case I'll recommend you go with a different project. Same as if it was IWTL how to make a small amount of Florine gas - sure, there's neat things you could do with that, but maybe stick to things not likely to cause a civil disturbance. You say it wouldn't effect neighbors but I'm kind of skeptical. A typical suburban noise floor at 1 MHz might be around -100 dBm. Apply the inverse-square law to divine how far your signal - if actually transmitted - can go.

Radio is really cool, I'm not trying to discourage you here, or be patronising. I'll say that I'd guess many commercial BJTs will work to amplify 1MHz in this power range, albeit with distortion. How to design a circuit that will be linear is something I haven't personally looked into much. I'd guess that the know-how to do that would amount to a good crash course in many of the other analog concepts you might need, like what near-field means.

Actually, that specific one might not be involved, so I'll throw it out there. An antenna needs to be 1/4 wavelength long, or it's not really interacting with waves, but just local fields. Something also within a fraction of the wavelength may feel the field (it's in the near field), but not much in the way of a wave will be produced off into the distance, for the same reason you can't flip or snap a rope with just your fingers. MW is defined as being over 100m wavelength, so any antenna below 25m is electrically short in those frequencies.

I have a NanoVNA so could build a small dipole. High efficiency isn’t really the point.

Efficiency might not be the point, but not frying your amp is good. Maybe your low-power amplifier can handle Watts of standing wave, but I wouldn't assume it. Like I covered, no small antenna will make a difference here.

[–] Isoprenoid 1 points 3 months ago* (last edited 3 months ago) (1 children)

Efficiency might not be the point, but not frying your amp is good.

I don't want to break the amp, but I'm expecting my first attempt to be non-ideal.

I’d guess that the know-how to do that would amount to a good crash course in many of the other analog concepts you might need, like what near-field means.

Yes, that is exactly why I am asking here. I was hoping someone could point me in a direction.

MW is defined as being over 100m wavelength, so any antenna below 25m is electrically short in those frequencies.

Yes, I would likely use a Hertzian dipole. Which is inefficient. So not likely to cause much distant interference.

[–] [email protected] 1 points 2 months ago

Hertzian dipole

Ah, I see what you mean now. I was thinking you meant a proper half-wave dipole, which is the wrong answer.

[–] [email protected] 1 points 3 months ago

I have no clue what it does..but more power to you as long as it doesn't affect others or hurt anyone I am for you. Doing a deep dive now.