Use a preamplifier to transmit a weak signal

[Mr. Scram]

This may be a naive question as I'm not well informed when if comes to RF, and that's probably an understatement. However, some Googling on the subject doesn't tell me a lot. I've seen Dave and others use usually cheap preamplifiers to amplify a received signal before reviewing it with some other equipment. Would it be possible to also use such a preamplifier to improve the signal of a weak transmitter? Various modules for sale on the well known international marketplaces come to mind. Their transmit power is often woeful and I wondered whether this could be a cheap albeit hacky fix.

The simple logic is bigger wiggle out equals stronger signal, but something tells me it may not be that simple. I'm not seeing it done either, which should probably tell me something.

[CJay]

Yes, absolutely it is possible, an amplifier is an amplifier.

'We used to 'broadcast' UHF TV test signals via a Labgear distribution amp at a repair shop I worked at, strong enough to be received int he workshop with a very basic loop antenna on the back of a telly and a half decent test of the tuner module.

[cdev]

If you mean a receive preamplifier, then by all means do it, assuming your intention is to amplify a weak signal you are trying to receive. And its output is the front end of your receiver

Any amplifier can be subject to instability and become an unintentional radiator if the input and output are not isolated from one another and it is operated into a impedance mismatch. Water leaking into a GPS antenna was the cause of this in a story I read on Time Nuts.

A coastal fishing community found itself with a problem causing lack of reception of GPS signals in port. The FCC was called in and the problem turned out to be caused by a boat which had a GPS whose antenna (it had a built in LNA) had suffered water incursion. The effect of this was to cause it to self oscillate in the GPS band. Needless to say the antenna was replaced.

If you want an outdoor signal to be receivable indoors - there is no need for any kind of active system, simply connect two antennas to one another by means of a coax cable- this will allow the signal to get indoors from the outside.

[Mr. Scram]

If you mean a receive preamplifier, then by all means do it, assuming your intention is to amplify a weak signal you are trying to receive. And its output is the front end of your receiver

Any amplifier can be subject to instability and become an unintentional radiator if the input and output are not isolated from one another and it is operated into a impedance mismatch. Water leaking into a GPS antenna was the cause of this in a story I read on Time Nuts.

A coastal fishing community found itself with a problem causing lack of reception of GPS signals in port. The FCC was called in and the problem turned out to be caused by a boat which had a GPS whose antenna (it had a built in LNA) had suffered water incursion. The effect of this was to cause it to self oscillate in the GPS band. Needless to say the antenna was replaced.

If you want an outdoor signal to be receivable indoors - there is no need for any kind of active system, simply connect two antennas to one another by means of a coax cable- this will allow the signal to get indoors from the outside.

I'm specifically talking about using a preamplifier I see people using to boost received signals to boost a weak transmit signal, as per my original post. Transmit, not receive. Think of a 433 MHz module with a disappointing output and range, and attempting to increase the range by amplifying the signal with a preamplifier before it hits the sending antenna. I should add that I'm not trying to blast a massive signal into the ether as I'm well aware that's both obnoxious and potentially illegal, just improve a substandard signal to more reasonable standards as a stopgap fix and troubleshooting measure.

[pigrew]

Yes, you can use a "preamplifier" (likely a LNA?), as a PA, albeit with low output power and low efficiency. (LNAs and pre-amplifiers are generally biased for linear operation and also have wider bandwidths).

I'd highly suggest using a spectrum analyzer and/or power meter in order to measure the output power and if there are any/many out-of-band signals output from the existing transmitter, so that you can choose an amplifier with the proper gain & output power.

If your amplifier is too non-linear (and has wide-enough bandwidth), it'll easily transmit at multiples of the carrier frequency. These would need to be filtered out before the antenna (or a lower bandwidth amplifier chosen).

 If the power is too high to be the PA's input, you'll need to add an attenuator on its input to get the signal at the proper level for the LNA. If the PA's output power is too high, you'll need to add an attenuator between it and the antenna.

Your preamp may (or might not) be more prone to reliability problems or oscillations with mismatched sources/loads.

I can't say too much more without design specifics. What module is your transmitter? What amplifier are you planning to use? What sort of antenna? Is everything 50-ohm matched?

And, as always, please keep transmit powers to legal levels.

[9aplus]

Please look here:
HackRF TX amplifier modification
https://lna4all.blogspot.com/2013/04/lna-for-all-low-noise-amplifier-for.html

[CJay]

I'm specifically talking about using a preamplifier I see people using to boost received signals to boost a weak transmit signal, as per my original post. Transmit, not receive. Think of a 433 MHz module with a disappointing output and range, and attempting to increase the range by amplifying the signal with a preamplifier before it hits the sending antenna. I should add that I'm not trying to blast a massive signal into the ether as I'm well aware that's both obnoxious and potentially illegal, just improve a substandard signal to more reasonable standards as a stopgap fix and troubleshooting measure.

You're likely to be breaking the law by attaching an amplifier of any description.

It's quite easy to make a perfectly OK RF module work very poorly if the manufacturer recommendaitons for layout and antenna coupling/design aren't followed properly.

Are you sure you couldn't make improvements that way?

[Dennis Frie]

Any specific reason you prefer to use a LNA (normally intended to be used in a receive chain) rather than a PA (normally intended to be used in the TX chain)?

For a LNA to be useful, the input signal must be really low power (well, depending on the LNA, but in general).

[cdev]

Why don't you look up the regulations and post them here?
 

I'm specifically talking about using a preamplifier I see people using to boost received signals to boost a weak transmit signal, as per my original post. Transmit, not receive. Think of a 433 MHz module with a disappointing output and range, and attempting to increase the range by amplifying the signal with a preamplifier before it hits the sending antenna. I should add that I'm not trying to blast a massive signal into the ether as I'm well aware that's both obnoxious and potentially illegal, just improve a substandard signal to more reasonable standards as a stopgap fix and troubleshooting measure.

Why don't you improve its antenna? The size of a monopole for 433 MHz is around the distance between the tip of my thumb and index finger when both are extended.  If the law specifies the legal limit of a 433 Mhz device by power, then as long as you stay under that power level you're okay. Courtesy to hams who share the band would also require the output be minimal to accomplish whatever it is you want to do. The antennas Ive seen on 433 MHz devices seem always to be designed for compact size, not range. A quarter wave wire working against the PCB's ground is likely to give you more range than a coil Assuming its legal. It may not be. I don't know. Look it up.

Just from monitoring, the signals from most 433 Mhz devices are quite short and emitted at intervals of a minute or more, so they don't seem likely to pose problems to other uses of the frequencies. Even though when I listen at 433 MHz - and I pick up a lot of stuff when I do (it seems to be all weather sensors, a bunch of them) none of them would seem to be diminishing the usefulness of the band for others. 

That would seem to be important to me.  Look up the legal situation. If you cant address the situation at 433 MHz within the limitations of your equipment without breaking some law, you should get another kind of module that gives you longer range capability legally. And because an antenna might focus the signal in one direction, its kinder to other users of the frequencies too. If you need still more range, I bet there are devices that can utilize some kind of license.

[Mr. Scram]

I realized I never posted the solution I opted to go for in this thread, which is rather rude after the helpful replies.

Due to the comments here I did some more homework and figured out the transmitting chip/board in question could be configured in multiple ways by using different passives on the same layout, as specified by the datasheet. I found that I could achieve what I wanted by simply ordering the correct passives, and replacing them on the board I had at hand. They specified not only the correct values, but even the brands, so you could be pretty sure everything should be working as intended.

That's a much better solution that the one I was looking at, and the helpful replies steered me in the correct direction.

[TimFox]

In general, a unit sold as a "preamplifier" is designed to accept weak signal power and output somewhat higher power, and is optimized for low noise added to the signal.  However, the output power is to be roughly the same as the normal input power for the receiver, etc. that it drives.
You must read the spec for how much power it can deliver (not very high) and the corresponding input power required.
The usual term for an amplifier that accepts a medium power from the source and delivers higher power to the load is "power amplifier" or "post amplifier".
It will be specified accordingly.