Getting antenna to radiate

[(*steve*)]

Adam - I have SWR meter for my ham radio, but it only measures from 1W up. I am nowhere near 1W. It's matching issue. I said that instead of emitter follower I built common emitter amplifier. With 1.2k in collector, output impedance should be 1.2K. So I tried building Pi matching network from 1.2k to 50 ohms using this site as guide

Try changing the collector resistor to a 100 ohm resistor. Whilst it's not ideal, it will have an impedance much closer to that of your feedline.

Oh, and beware that your resistor may have significant series inductance and parallel capacitance due to the helical nature of the resistance trace in some types of resistors.

 

[CDRIVE]

I'm curious.. What's the bandwidth of the scope you bought?

Chris

 

[GhostLoveScore]

I'm curious.. What's the bandwidth of the scope you bought?

Chris

I haven't bought oscilloscope yet, but I am thinking of buying one with 200MHz bandwidth and experiment with lower frequencies - 14 or 21 MHz band maybe.

 

[GhostLoveScore]

Try changing the collector resistor to a 100 ohm resistor. Whilst it's not ideal, it will have an impedance much closer to that of your feedline.

Oh, and beware that your resistor may have significant series inductance and parallel capacitance due to the helical nature of the resistance trace in some types of resistors.

I'll try that. I'll even try to put 47 ohm resistor and see what I get.

 

[Arouse1973]

That would be horrible circuit if built on that breadboard. I solder everything on a big copper PCB ugly style. Good side is that I can get very short wires.

Adam - I have SWR meter for my ham radio, but it only measures from 1W up. I am nowhere near 1W. It's matching issue. I said that instead of emitter follower I built common emitter amplifier. With 1.2k in collector, output impedance should be 1.2K. So I tried building Pi matching network from 1.2k to 50 ohms using this site as guide http://electronicdesign.com/communications/back-basics-impedance-matching-part-3. But it doesn't work near as good as soldering directly to collector. At least now I get the same signal strength using coax and ground plane antenna as when soldered directly to the collector. Thanks guys for help. When I buy oscilloscope I'll try to improve this circuit and build Davenn's circuit.

Why did you choose a 1.2K as your collector load?
Thanks
Adam

 

[GhostLoveScore]

Why did you choose a 1.2K as your collector load?
Thanks
Adam

1200 ohms is collector load, 12 ohms is emitter resistor. I wanted to try if I would get high gain with this configuration. This should give me 100x gain. It does amplify the signal, but I don't know how much.

By the way, can somebody direct me where to read about amplifiers that only have inductor as collector load?

 

[CDRIVE]

I haven't bought oscilloscope yet, but I am thinking of buying one with 200MHz bandwidth and experiment with lower frequencies - 14 or 21 MHz band maybe.

The economy in Croatia must be booming! 200MHz scopes aren't cheap.

On another note have you considered building a Field Strength Meter?

Chris

 

[GhostLoveScore]

The economy in Croatia must be booming! 200MHz scopes aren't cheap.

On another note have you considered building a Field Strength Meter?

Chris

Sorry, I remembered incorrectly that I was looking at 100 MHz scopes, not at 200 MHz.

I did think about field strength meter, but I ordered RTL SDR and that seems to me pretty much the same. I just keep the both antennas at the same spot and see if the signal is stronger.

 

[CDRIVE]

I did think about field strength meter, but I ordered RTL SDR and that seems to me pretty much the same. I just keep the both antennas at the same spot and see if the signal is stronger.

Damn! I can't keep up with the technology. I had to Google that!

Chris

 

[GhostLoveScore]

It's well known in radio amateur community, probably not so well known outside it.

I have one more question about impedance of the coax. At the point where I'm connecting coax to the board, insulation is peeled off and conductor geometry is not the same anymore. Center conductor is not straight anymore, there is no more insulation between center conductor and braid, and the braid is not around the center conductor anymore. Because of that, at the point where it's soldered, coax impedance is no longer 50 ohms. Is it possible that right there part of the power is reflected back?

 

[Arouse1973]

I'll see if I can work through an example for you and simulate it. Showing you how to match the amplifier to the load. It won't be until tomorrow if that's ok.
Thanks
Adam

 

[davenn]

I'll see if I can work through an example for you and simulate it. Showing you how to match the amplifier to the load

look at the sample circuit I provided on page 1

 

[Arouse1973]

look at the sample circuit I provided on page 1

Yes I have seen that, nice one. I just thought he might like to know how the values are calculated. So yeah we can use your circuit, it would be beneficial to this thread I think if you run through how they came up with those values?
Cheers
Adam

 

[davenn]

Yes I have seen that, nice one. I just thought he might like to know how the values are calculated. So yeah we can use your circuit,

I was trying to get you to simulate that one ... that's what my comment was aiming at

 

[CDRIVE]

Is it possible that right there part of the power is reflected back?

At 146MHz everything is factor. The Amateur Radio Handbooks don't gloss over the importance the mechanics and topography involved with VHF/UHF design.

Chris

 

[GhostLoveScore]

Davenn, I took your advice and flipped through ARRL handbook. I found this circuit


But I couldn't find how to make T1 transformer (collector of 2N2222A). I read that in RF transformers can be without magnetic core. So, should I just wind that transformer on some cylindrical object?

I am not building this, I just want to few things clarified.

Also, I noticed that in most circuits signal is taken with transformer as collector load and not as in FM bugs where it's just taken from collector. I guess this is much easier. Signal that's amplified in primary of T1 will appear at secondary of T1 without much loss.

 

[duke37]

The circuit shows a 50Ω output (forget the LPF) and the transformer multiplies this by 16 to get a load more suitable (800Ω) to the 2N2222 collector.

The transformer needs to be close coupled with low leakage inductance and low capacitance. This is normally achieved by winding on a toroid core but the core material must be suitable for the frequency.

 

[GhostLoveScore]

I actually can try that. I have few FT37-43 toroids here. http://toroids.info/FT37-43.php

 

[davenn]

Davenn, I took your advice and flipped through ARRL handbook. I found this circuit

great circuyit .... I hope you noted it was for 7MHz not 146 MHz

probably a better freq for you to start at anyway till you get some good experience
lots of things change as freq increases

But I couldn't find how to make T1 transformer (collector of 2N2222A). I read that in RF transformers can be without magnetic core. So, should I just wind that transformer on some cylindrical object?

At 7MHz, T1 will be wound on a toroidal core maybe an inch in diameter at the most

flipped through ARRL handbook. I found this circuit

must be a different year to my one which is 2003
in mine chapter 13 is on a different subject
I couldn't find that transmitter you displayed in my copy, Project info and styles change over the years
I was hoping to find more info on T1 for you ....
Have a really good read of the text I would be really surprised if there wasn't some mention of T1

 

[GhostLoveScore]

great circuyit .... I hope you noted it was for 7MHz not 146 MHz

probably a better freq for you to start at anyway till you get some good experience
lots of things change as freq increases

Of course. 146 MHz is too high for cheaper oscilloscopes and oscillators are probably more stable at lower frequencies. And I already have half wave dipole antenna for 7 MHz trimmed correctly that I used on my ham radio Kenwood TS-520. That's one problem less, having correctly trimmed antenna.