From function generator to WA2EBY amp

[ikrase]

I've been working with some RF using a JDS function generator and a homemade WA2EBY class AB amplifier (~20-50 W output, aimed at the ISM application in the ISM band).

Unfortunately, the function generator simply does not provide nearly enough power to drive the class AB amp (which wants something like a watt input).


And I am *not* very good at RF, or analog electronics in general -- I was barely able to successfully breadboard the absurdly simple amp.

Does anybody have any suggestions as to a reasonably low cost, low effort way to pre-amplify this? Most of the cheapie ebay modules are either power amps or input LNAs, and Mini-Circuits is simply orders of magnitude out of budget. However I found this: https://www.ebay.com/itm/1MHz-1000MHZ-3W-35DB-HF-VHF-UHF-FM-transmitter-RF-Power-Amplifier-For-Ham-Radio/322712581412 -- does anybody know anything about this amp or its practicality?

I am not running a communication application, so I only care about distortion and noise as far as is needed to keep from polluting the spectrum, and can accept heavy filtering as a method to keep things clean.

[scdreger]

With 20-50 watts output at HF, you certainly should care about noise and distortion. Your neighbors on the spectrum sure will.

[HB9EVI]

Before building any intermediate amp, you should urgently construct a 50 ohm dummy load; you cannot go 'on air' with any kind of amp under construction; and if you are no radio amateur, you should keep away from sending over antennas completly.

[PA0PBZ]

They call it an FM amplifier and the first amplifier element has no bias so it could be a class C amp, I guess that's not what you want.
And for the other responses: op did not say it was for going 'on air', so I hope he is responsible enough.

[ikrase]

Yes, dummy loads are what this is about thus far. I made an appropriately high power one. Unless the slight reactance of my resistors is already allowing too much radiation.


Edited OP to indicate what I actually mean about signal integrity. The point is that I'm not expecting modulation to be preserved even slightly and that "just filter everything" is a viable option in a way it wouldn't be for many applications. I will be checking the spectrum. I know that for the type of power amp I made, it's typical to follow it up with a massive low pass filter when doing communication.


They do indeed mention "FM" but that item looks like they did the typical Ebay thing of vomiting up every possibly related keyword on the title or description. I don't see what makes it specifically for FM -- (non)linearity of amp classes?



Edit: I'm re-evaluating this project now. I am going to need to do some more research, as non-amateur RF law seems to be a bit of a rabbit hole.

[bd139]

You’re fine if you don’t radiate anything. But you probably are. Even a 20mW oscillator on a breadboard acts as a nice CW transmitter. The only thing that is saving you is the PA makes a crap antenna.

I would get a license first. It’s not difficult and covers your ass.

As for the original problem, AFAIK you’re limited to 1W at the antenna port and 4W ERP on the ISM bands so this is already pushing way past the line.

[Lord of nothing]

It’s not difficult

it is and pricey.

[HB9EVI]

what do you consider 'pricey'? for any electronic nut it should be childs play to acquire a license; and the legal stuff cannot be such a huge problem either

[Lord of nothing]

Leistungsstufe D ........................................................................................ 6,54 Euro

Monthly
mean 78,48€ per Year.

+ once Exame + Callsign 

[bd139]

Leistungsstufe D ........................................................................................ 6,54 Euro

Monthly
mean 78,48€ per Year.

+ once Exame + Callsign 

You have to pay monthly? One off cost here.

[Lord of nothing]

You have to pay monthly?

You pay it once per Year. 
My Cellphone Contract cost me 5€ per Month. 

[bd139]

Ok that’s sucks. I’d probably just be a pirate there

[Wolfgang]

Hi,

what you need is a driver amp. If interested, look at what I have built for the HF bands:

https://electronicprojectsforfun.wordpress.com/rf-module-gallery/the-amplifier-module-gallery/medium-power-hf-amplifiers/

These are "lab amps" that are tolerant to misuse.

BTW, the WA2EBY amp is a counterexample of a good amp design, for the following reasons:
- Its marginal. As Mr. Kossor writes himself, he is running the transistors far out of specs, and he does not expect them to last forever
- Insuffient cooling, cannot be reasonably fixed due to the high Rth of the transistors used.
- Only for AM, no key down CW or FM, or the final transistors will desolder themselves from the PCB under full load
- Misuse intolerant. Overload, missing antenna, wrong output filter, glitches, ... prepare for a new round of transistors
- No protection (High SWR, overcurrent, ...)

As a newbie, I would look for amps that never die, and I would run them only at half of their rated power and cool them well.

Much success !

[ikrase]

Cool, thank you! (I suspected that "driver amp" would be the term but the word "driver" gets used for so many different purposes....)

This will make some good studying.

[dietert1]

For the sake of completeness:
The original poster was asking about a RF amplifier board at ebay. I have one of those and can offer a gain measurement. The amplifier was measured with a 30 dB attenuator (small signal). The gain curve starts at about 42 dB at low frequencies and drops below 30 dB above 1 GHz.
This is not the broadband amplifier advertised on ebay. It requires some work to correct the extra 10 dB at low frequencies. The board has unused solder pads between the two chips that can be used to place a filter.
Bias current is way off - much to high for the given cooler. At 12V i measured about 570 mA. Without additional cooling, current starts running off as soon as it gets hot and it will certainly burn after some minutes. They did not put heat conductor between board and cooler. I never tried 15 V.
The board was very cheap and you get what you paid for (crap). It came from a seller "thanksbuyin". Better buy something serious from Mini-Circuits.

Regards, Dieter