New here and already with doubts (tx)

[Charlie Foxtrot Mike]

Hello everyone! I'm just starting here so after saying hi I'd like to ask about this. I want to build/buy a Transmitter for broadcasting, and looking for it I've found many circuits I could try (guerrilla, spitfire, LULU, etc). Wich one could be right to start? I live in the south of the south of South America, only woods here so I could transmit without stress. The other thing is, I could have one or two cb radios that nobody uses here... Is it possible to modify them for broadcasting? I thank you in advance for any tip or help.  Salud!
CFM

[ff]

Hi CFM!

The way I see it - your building experience should determine your choice about what design to build.  The way I would rank these:

1) Little or no experience building RF projects - Radio Anarchy's Guerilla would be a good starter rig.

http://www.angelfire.com/de/RadioAnarchy/

2) You've worked with small transmitters before but not a lot - the Lulu would do you well:

http://hfunderground.com/blog/?p=72

Or Dave Martin's Corsair:

http://freeradiotx.blogspot.co.uk/2009/06/corsair-transmitter-by-dave-martin.html

3) Dave Martin's Commando is a great rig but you should have some real experience before attempting:

http://freeradiotx.blogspot.co.uk/2009/06/command-transmitter-by-dave-martin.html

There are even better designs out there such as the Dutch Quad, but you really need building experience to get them working.

CB radios are fine to pirate with as long as you don't try to move the frequency too far from 10-11 meters.  The old units that use crystal mixers to synthesize the channels are pretty easy to trick to other areas.  But IMHO if you need to ask the question then you probably won't be successful at this.  It takes some knowledge to convert these.  BTW- Pirating up in that range can be a lot of fun - when the band is right, 5 watts will really kick a$$ - but you lack the large listening audience that exists on 43 and 48 meters.  I hope this helps...

[Charlie Foxtrot Mike]

Hey ff, thanks for the complete answer. I'm completely new to RF electronics, so I guess The Guerrilla would be OK. Only that I guess I have to buy everything from the states or UK, cuz here there's no complete electronic supply store. I'm still thinking about the transformers and wire winding... Well I guess I'll start searching now .
About the cb radio, as it is, it won't broadcast unless it is modified, right? I mean, thinking about transmit for an hour or so with no pause.  Thx again. Now is time to read about electronics. 
CFM

[ff]

About the cb radio, as it is, it won't broadcast unless it is modified, right? I mean, thinking about transmit for an hour or so with no pause.
CFM

You should make sure the final transistor has proper heatsinking.  On older units, often you can get away with homebrewing some extra heatsinking fins that extend up above the transistor, then use a fan.  Of course you'll need to put the chassis in a larger case that will fit everything.  My best advice would be to NOT pirate in the 10 meter ham band or in the CB band.  Lots of angry users will give you the wrong kind of attention.  If you want to use an old CB, make sure you can trick it to operate lower - perhaps in the 11 meter shortwave band or just above.  I have a homebrewed 5 watter that I use in the 25800-26000 area occasionally.  Play it safe, my friend...

[Charlie Foxtrot Mike]

Of course my friend. I want to broadcast cuz I have things to tell and music to show but not to annoy people  . I am a hater myself when people doesn't respect others on Amateur Radio Bands. I was thinking about modify the cb radio, knowing that is possible to use it on 40 meters, maybe is it possible to move a lil further to the 41 meters? Right now I'm searching for the pieces in Aliexpress   there's everything in there. I'd appreciate if you or anyone here could later, when I'm ready to buy, take a look at the list to see if I'm not doing a mistake. Guerrilla Project has begun. Thanks again. Gonna keep you updated (of course the action gonna start in one month or maybe more, depending on how long the shipping takes).
CFM

[Charlie Foxtrot Mike]

Well, had the list almost ready and then found a store here(1500km away, but same country) where good electronic stuff can be found. Searching their catalog I realized there's no IRF510 available, but irf840 and the IRFZ44N. Could I use any of those without having to modify something else? Reading the comments of the site, they say the IRFZ44N works the same. If so, I could buy everything here and not in China. Thanks.
CFM.

Here's the data sheet.

[ff]

Searching their catalog I realized there's no IRF510 available, but irf840 and the IRFZ44N. Could I use any of those without having to modify something else? Reading the comments of the site, they say the IRFZ44N works the same. If so, I could buy everything here and not in China.

Can't say for sure.  The thing is, these garden variety MOSFETs are designed for passing DC current - or - low frequency AC.  The 510s and their ilk were designed back in the 70s for automotive service.  They were used for such "AC" applications as flashing turn signals and the like.  We are taking what is basically a low frequency device and using it far outside its design parameters.  That's why I can't tell you for sure if one will work and another one won't.  I know from experience that 510s, 511s, 520s, and 530s will usually work in these designs.  But even 510s will vary in performance in these circuits due to manufacturing irregularities and HF performance is not a concern with the quality control people.

Looking at the IRFZ44 sheet I immediately see that the Miller charge is MUCH higher, which will hamper its RF performance because they require more drive at RF.  You really should try to get a few 510s (or 511s) if you plan to play with these circuits.  You'll have enough debugging to do without making yourself yet more work by using a compromised device.  If you have a Paypal account you can usually find what you need on EBAY and Paypal automatically does the currency conversion.  Good hunting!

[Pigmeat]

Fred, would it be possible for him to salvage a mod tranny from one of those old CB's and use it in the Guerrilla?

[ff]

Fred, would it be possible for him to salvage a mod tranny from one of those old CB's and use it in the Guerrilla?

Absolutely, Pigmeat!  The impedance match would be close enough for the 5-10 watt range.  If built flawlessly, his Guerilla will probably put out about 5 watts of carrier, or maybe 6.

[Charlie Foxtrot Mike]

Great to know! The irf510 that I need, must be the IR original? Cuz the Vishay are available almost everywhere. In fact, the whole list in Aliexpress costs about 30 usd , but then I'll have lots of each component... What's not bad (as long they aren't too bad quality). I'm so motivated that I already started learning basic electronics .  I'll keep you informed guys. Thanks and salud!
CFM

[ff]

The irf510 that I need, must be the IR original? Cuz the Vishay are available almost everywhere. In fact, the whole list in Aliexpress costs about 30 usd , but then I'll have lots of each component...

Go with the Vishay.  They get the genuine dies from IR to build their devices.  Vishay should be fine if you are buying from a reputable vendor.  I would recommend you stay away from generics however.  There are LOTS of counterfeit parts out there.  Even Radio Shack back in the 90s was selling counterfeit IRF510s in their bubble packs that were being made in Haiti.  International Rectifier sued them, forcing RS to re-label their counterfeits as "IFR510".  Study hard my friend.  Electronic theory is a worthy pursuit and I salute you for your studies.  Also, if you don't know basic electronics you'll never be able to debug your project.  73!

[Zoidberg]

I could have one or two cb radios that nobody uses here... Is it possible to modify them for broadcasting?

Yup, it's possible.  Several years ago I heard some fellows, apparently from Brazil, using Zamin CB transverters near the 40m band.  My 2008 logs have some of the info I was able to find: http://www.hfunderground.com/board/index.php?topic=642.0

These CB transverters have been used in the 6800-7000 kHz pirate bands too, by at least one US op, which he confirmed privately to me.  I think he was just testing some borrowed CB transverters, but I logged those shows several times a few years ago.

BTW, my mediafire links to audio from those 2008 logs have expired, and I don't recall whether I uploaded those to my archive.org account.  If not, I'll try to remember to do so.  I recently copied all of my old off-air radio recordings from CD/DVD to a new external hard drive, which is in storage at the moment.  Next time I rotate it back home I'll check.

[Zazzle]

Hi there,

about the question regarding "which MOSFET to use". This is fairly technical now but may offer some insight into the technical background.

Basicly, almost every common (Power-) MOSFET can be used in the HF Bands up to 50Mhz. _When_ the features are suitable and it's driven properly, they all will perform about the same.  The issue here is "driving properly".

There are several reason why the IRF510 is used so often:

• It's widely available (from misc manufacturers).
• It's cheap (we all managed to blow up a dozen FETs on a single day full of tinkering, eh? ;p).
• With a Drain-Souce-Voltage of about 100V it's suitable for most PAs up to 30W, which is enough for most people.
• It has a relative low RDS(on) resistance which qualifies it for pretty low PA output impedances (and therefore high power).
• And most important: the Gate-Source Capacitance is relative low. In fact. that low that not much attention needs to be paid when designing a driver stage.
• (Another, but not too important reson is the relative low Gate-Source Threshold Voltage.)

Why are those values important? Well, the perfect MOSFET for a HF PA should have:

• A low Drain-Source(on) Resistance Value (RDS(on)). The lower the better. It defines how how much power will be converted into heat (during the transition phase but mostly the full-on state). This value isn't really critical. It affects the PA output impedance  but does not much harm. It limits the maximal obtainable output power. But a bigger heatsink solves the problem in case the chosen FET is able to deliver the desired output impedance and power.
• A high Drain-Source Breakdown Voltage (UDS). It should always be twice the expected voltage on the output. More is better because a bad SWR will reflect power back into the PA which caused voltage peaks. Voltage peaks above the breakdown voltage will, in fact, just cause the inner structure of the FET break down. BAM!
• A low Gate-Source Capcitance (Cds). This is most critical. A MOSFET behaves almost like a perfect capaciter when driven with AC (especially HF). Which means that the driver circuit basically sees a capaciter on the output and must be able to deal with reactive loads. On lower frequencies (say... below 20Mhz) that isn't much of a problem when the GS-Capacitance is relative low. But the bigger this value becomes (or the frequeny) the more attention needs to be paid. Wrong coupling can cause oscillation (harmonics, which cause excessive heat in the FET and Filter) or a not fully driven FET (which will likely cause the FET to become terribly hot). So, when using High Power MOSFETS, one needs to pay attention to the driver stage. It must be able to deliver enough power and (depending on the driver) an inductive compensation must be added to take care of the capactive behaviour of the FET. But this may cause more problems.

Like most times, there is no such think like a "perfect MOSFET for RF use". Here goes the rule: pick two out of three.

• A low RDS(on) means: a low DS breakdown voltage.
• A Low RDS(on) and high DS breakdown voltage means: a big GS Capacitance.
• ... and so on.

And yeah, that also not the full truth. There's wayyy more to add.

Greetings,
~Zazzle

[Charlie Foxtrot Mike]

Sorry for the delay... This bad habit of working, I cannot leave it.  Thanks guys for your great replys, there's so much info to absorb... But it's exciting to start re-discovering this world. About my study, we'll, I have a couple of books... One of them really didactic and the other one it's hard stuff (for me). I'm moving step by step.
OK this is how it looks so far : in 3 days I go to the cap city, so I think I should find almost everything out there. In the worst case I'll have to get the irf510 from eBay or some other seller and then wait. I was reading the diagram from station QRP and discovered I have some of the tube they use in the 8 watts TX. So I went to the guy of the TV and radio service and asked about that project (he has no IRF510s btw). Then he told me: "I used to build and repair the transmitter for the first guy (a reverend) who was here with radios (in the 70s), I know about TX and Rx a lot." and I was like  . He picked up a lot of designs for FM, am and SW... And told me to choose any...  Many of them were too low power(am and sw) or too high (fm) but much easier to build.  It's gonna be my last card...   So I'll be back on the 28th, hope with everything ready to start soldering.
Pd: one thing I have realized here, is that it seems there's no pirate stations in these latitudes. At least in a language I could get. It makes it even more interesting for me to accomplish this task.
That's for now people. Thanks again ff, Lex, Zazzle, for your time and passion. Have a nice weekend.
Charlie F. Mike

[Charlie Foxtrot Mike]

Hi guys, how u doing?  I'm still travelling across the country and no much time near Internet connection. So I'm still gonna be in debt with you all. Soon I'll write again (next week).
Yesterday I found an amplifier circuit using the TDA2003. There are 2 of them (ready to use), both 6 watts, one is mono and the other one is stereo. They are cheap, so if it works, I could get one. What do u think? Or should I build it?   

Talk to you soon, have a nice time. Salud!
CFM