VHF/UHF preamp ckt doesn't work

[SteveNielsen]

I built this circuit in the hopes of boosting an antenna signal for digital TV reception and it does is the exact opposite, it literally cuts the signal in half. I've tried smaller in/out caps and eliminated the diodes at the input. I've checked the specs on the transistor and it should function at UHF frequencies. I am real rusty on transistor circuit design and was never all the good at it to start with. The input/output Z is supposed to be 50 ohms and I realize that my balun and digital box Z are 75 ohms but I'd expect it to at least boost the signal level enough to see an increase regardless of the mismatch.

[budm]

Re: VHF/UHF preamp ckt doesn't work

How are the components are being put together? Shortest legs as possible? Ground plane? a short length of trace or wire can have very high impedance at UHF.
http://www.electroschematics.com/447...nna-amplifier/
http://www.sentex.ca/~mec1995/circ/activant2.html
Do a lot of search how to build UHF amplifiers, RF is like a black art.

[SteveNielsen]

Re: VHF/UHF preamp ckt doesn't work

Wow. Speedy response. Thanks Bud Thanks for the links.
I'll upload some pics later, my camera isn't here right now. I kept the leads a short as I could building it on a little RS project board and it's in an Altoids tin. The in/out leads are the longest, about 1" each but inside the tin. I worked in RF stuff for several years (long ago and forgotten most of it) and you're absolutely right, all kinds of mysterious things go on in RF. It gets pretty weird.

[SteveNielsen]

Re: VHF/UHF preamp ckt doesn't work

Here are a couple of pics of my second attempt at building it. I know the soldering isn't that beautiful.

[Elysarian]

Re: VHF/UHF preamp ckt doesn't work

I recall UHF being very picky about stray inductance and capacitance back when I was doing this sort of thing on a regular basis (most of my stuff was VHF/FM for audio only though I did build some UHF circuits for video/TV applications).

Are the two wires down the right-hand side of the board by design?

Also, losing the diodes was a good start, they'd have demodulated an AM signal (I've built AM receivers that consisted of just an L/C tuned circuit and a diode in the past)

Also, the cut-off frequency of the 2n2369 may be a little low? - I'm seeing it listed at 500MHz, US UHF band only starts at 470MHz (and goes up to 890)

The BSX20 appears to have a similar bandwidth.

This means the amp should be fine for VHF but not for UHF as you'll get unity gain or less.

[Agent24]

Re: VHF/UHF preamp ckt doesn't work

I don't know much about RF but protoboard looks like a bad way.

Most RF designs I have seen do it 'manhattan style' or with custom PCBs.

[SteveNielsen]

Re: VHF/UHF preamp ckt doesn't work

Elysarian:
The wire on extreme right is B+ and next is ground. I tired to match the layout to the schematic as closely as I could.
As I recall a diode in series would demod an RF signal, not sure about ganged like this, but the reason I removed them was due to their internal capacitance.
I think you're right about the transistor bandwidth and that's probably the biggest problem.
My previous exp was with 2-way VHF-FM gear and a little UHF thrown in too.

Agent24:
I'm not familiar with what "manhattan" style is. Is that like building things atop one another?

[T30M31]

Re: VHF/UHF preamp ckt doesn't work

Have you considered using a monolithic amplifier such as the Mini-Circuits MAR-6SM+ which covers DC to 2 GHz?

[SteveNielsen]

Re: VHF/UHF preamp ckt doesn't work

Yes I have and I thank you for the very good suggestion. I may do just that. I've ahd a little trouble finding a source for the MAR-6 though. I was curious as to why this design didn't work, at this point I am satisfied with the explanation that the bandwidth limit of the transistor is the big reason for it not working.

This is why I love this place, someone like me can bounce ideas off other minds and get good feedback. It's awesome. Thanks.

[T30M31]

Re: VHF/UHF preamp ckt doesn't work

You can find the MAR-6 and other Mini-Circuits parts at RFParts.

http://www.rfparts.com/minicircuits.html

[SteveNielsen]

Re: VHF/UHF preamp ckt doesn't work

Thank you very much indeed. Site bookmarked. I have a schem for using one and I'll see about getting the parts together for the project.

[Agent24]

Re: VHF/UHF preamp ckt doesn't work

Agent24:
I'm not familiar with what "manhattan" style is. Is that like building things atop one another?

Looks like this: http://makezine.com/2011/09/01/manha...dding-chassis/
I always like how it looks when done well, but I've never used it myself. I just make standard PCBs with KiCad.

I see many people use it for radio receivers, so I thought it might be suitable for your project - but I am not an expert in RF stuff so I could well be totally wrong!

[SteveNielsen]

Re: VHF/UHF preamp ckt doesn't work

That is really interesting. Thanks.

[redwire]

Re: VHF/UHF preamp ckt doesn't work

Steve, the output impedance of a common-emitter amplifier is roughly equal to the value of the collector resistor. In your case: 1.5kohms, which is way above 75 ohms. I believe this is why you are having so much loss. Also, check the DC collector voltage to see if the transistor is biased happy, say 1/2Vcc.
With RF amplifiers, usually you will see an output transformer or an added emitter-follower to provide the low output impedance.

[SteveNielsen]

Re: VHF/UHF preamp ckt doesn't work

That's good info redwire, thank you. I will check the collector DC and try a smaller resistor on the collector.

[SteveNielsen]

Re: VHF/UHF preamp ckt doesn't work

The collector voltage is about .3 volts lower than supply voltage. Sub'ing an 85 ohm in place of the 1.5K made no appreciable difference to the collector voltage. Tests were done with and without a 50 ohm resistor load on the output.

[redwire]

Re: VHF/UHF preamp ckt doesn't work

The collector voltage is too high- the transistor is almost cutoff at 0.0002mA (=0.3V/1500) quiescent current instead of 6mA. It might be shot.
I can't see your resistor colour codes, but check with an ohmmeter if they are wrong.
Lowering the collector resistor will make it eat batteries or overheat the transistor, that's why an output transformer is used to match 75 ohm loads.
It's an art to do a good RF amp and I think this circuit is too basic a design to begin with, so I wouldn't ask too much of it...

[budm]

Re: VHF/UHF preamp ckt doesn't work

You cannot just change the Collector resistor without changing the bias resistors to get the VC to be at 1/2 VCC. AS redwire indicated, the circuit will draw more current also.
I.E: PS is 12V, Rl is 75 Ohms, so @ 1/2 VCC, you will have 6V across 75 Ohms which means the current through the resistor will be .08A (6V/75 Ohms) or 80mA. You will then calculate the Base current required to have Collector current of 80mA.

[SteveNielsen]

Re: VHF/UHF preamp ckt doesn't work

Thank you both for the info. This is where I am really rusty, get lost in the math, and forgot most of what little I knew and I never grasped it that well to begin with.

I know the amp design is cheap and dirty and the author said it is not perfect but said it does work, so I'm not expecting a lot out of it. However it is a learning experience for me and maybe it will sink in to my old brain, that's really the reason I keep playing with it.

I am looking at some better designs. I like the MAR6 amp and will probably try building an RF amp with that device. I know I could just buy an antenna booster but it's more fun to try making things and at least I can learn from it.

Edit: I did measure the resistors before using them and the actual values are correct.

[Radio Fox]

Re: VHF/UHF preamp ckt doesn't work

As it says in the original article, the frequency range for that circuit is 14 to 550 MHz. So if you are trying to amplify UHF TV signals, then you need a transistor with a higher fT.

http://users.belgacom.net/hamradio/s...eband_MAR6.htm