I bought a copy of that book, thanks for the suggestion Nick. I love reading literature of that nature even if I don't build the end products in their entirety 👍
Ok I'm over in the USA, Muskegon Michigan. I just found your channel, it wasn't easy to find on RU-vid.i was going down a rabbit hole,and BOOM I'm here? So I subscribed to your channel to see how it goes
Have made the walford ham and hale transceiver and am pretty pleased with it. I can get about 3 or 4 watts out using a gel battery. Can increase the voltage up to around 20V and would get more power out. But I am looking to making an amp to fit into the frame I built it in to boost the op with upto about 15V supply both. Have made one of those minipa70 amps from which I can get about 60W with a ameritron 50A supply. They use nice cheap irf530 mosfets, but they were not designed for rf amp use and I concerned about the spurous ops. I have been thinking of using a band pass filter and a lpf with that. I think however a device like the mrf151 might be cleaner. The only problem: have you seen the price of those things and they are getting rare. mrf101 are only £20 from Farnell and they only sell genuine devices of course. I saw the message on here about the mrf101. I think I will compare the data sheets and have a think. That is the problem these days; you see circuits but usually the devices are obsolete, or horendously expensive. Keith G0 ODU By the way, I am surrounded by hills right here!
Hi Keith, thanks for your reply. Those RF MOSFETs can be pricey it’s true. You can chance it and buy some used ones on AliExpress or buy some less powerful ones. You could then use a couple in push pull. Eamon Skelton has a design for a PA which puts out 16W: Building a Transceiver amzn.eu/d/bUayIuy. Hope this helps. 73 Nick
I have learned a lot from this and built it, albeit with a ropey transistor, an IRF 151. Can anyone explain what the 200p capacitor is doing? Does it in any way resonate with the transformer primary - which is broadband, after all. I notice that the capacitor has a good effect on the output waveform, removing a nasty frequency-doubled bump on the wave. I am just enjoying watching voltages on my scope, not building a rig.
Thanks Christian. I presume the 200pF cap is for stability but that's just a guess. I basically just followed the design on the datasheet. Glad you've had some success. 73, Nick
Thank you, Nick. As you see, I am finding that the joy and sorrow of RF is that it's not just clicking functional blocks together like digital. @@M0NTVHomebrewing
Hi there! Thanks very much for watching and commenting. You don't measure the bias before installing the MOSFET - you build it as I did and then proceed like this. First, with no power attached anywhere you make sure the bias trimpot is fully off (i.e. max resistance so no current will flow to the gate). Second, you insert a multimeter set to measure Amps in series with the main power supply to the amp. Third, you apply the main power to the amp - I wouldn't use 50V to start with even if you are going to run it at that. Start lower - like 12V (you can turn it up gradually later). Make a note of the current draw. This is the current the amp is pulling with the bias circuit off. Fourth, turn the power to the bias circuit on (simulating the PTT being keyed). Fifth, CAREFULLY & SLOWLY adjust the bias trimpot until the meter reads a current draw of 250mA + your base current figure. So if it was drawing 50mA before any bias then you'll be aiming for a total current draw of 250 + 50 = 300mA. Once you've got this set and the MOSFET is happy (i.e. not overheating) then you can try connecting the amp with a dummy load and a signal source (e.g. the radio you've built). You can then GRADUALLY try increasing the main amp voltage up to the required level. The transistor I used likes 50V. Sorry this is rather long and technical. The short answer is that the bias voltage is whatever it needs to be to provide the appropriate level of current flow. It makes me realise I need to do a video on this very subject! Thanks again and 73. Nick
Hi there! The transistor is good to 230 MHz according to the data sheet. Check out the data sheet (SD2931-10) and you'll find suggested schematics for 30 MHz and 175 MHz. If you are using it for FM then you should be able to dispense with the bias circuitry and run it in Class C for much greater efficiency. I've never built for FM though so I can't advise you on this I'm afraid. All the best. 73, Nick
No I didn't. I haven't noticed an issue but it is certainly something to watch out for. I think some are more noisy than others. Thanks for the heads up. 73 Nick.
@@M0NTVHomebrewing definitely worth keeping an eye on. I'd see a raised noise floor (like humps) each side of the carrier, offset by the switching frequency. Often low enough down not to be an issue, but some were seriously poor.
Nick, have you figured out the stages efficiency? Thats a really good measure of how well its working, best i have got was approaching 80% in my 5 watts tx
Hi Ed, no I haven't done this. I basically built it to the Datasheet specifications and hoped for the best! Just getting a clean output with no oscillation was a triumph in itself (you'll understand why this is so significant when you see the final transceiver!). I would like to get into the business of measuring the performance of my RF amps though. I've now got a Siglent Spectrum Analyser so I probably have most of what I need ... apart from the time! Thanks very much. 73 Nick
Great work Nick! Can't wait until you show it working in the next video. Btw did you check out the MRF101 ? - it's around ~23£ and delivers around a 100 watts. Best Regards ,Ash M0LKA
Hi there! It depends upon the MOSFET you use. This particular one was originally used in a linear amp for the 4m band (70 MHz) but I repurposed it for 17m (18 MHz). You'll almost certainly need to adjust the transformer ratios if you go this high though. Best idea is to study the data sheet of your device and see what it suggests for a particular frequency. All I can say is that I would expect the design I used to perform pretty well across the whole HF band - with greater gains at lower frequencies. Hope this helps. 73, Nick M0NTV.
That great. I found with FET's that are high gain low drive. I made a few over the years, using BJT RF transistors like MRF 454/421/455 and 2SC 2290/2879 different bias and tuning and make more heat. I have even used 24V and 50 V BJT as well. 2SC2510 Etc. working with LD Mos now. MRFX 80H /BLF 188XR for 6 meters and 2 meters. watch that thermal past. the stuff sold for computers is reactive to RF and will de tune the amp. I had some repairs come threw that CPU thermal compound was used. That ok for oversupply's like LT 1083 reg etc. to get same power on MRF 421 you need about 3 to 5 watt drive . around 10watt gets appx 100 watt PEP out. drawing 12 amps 14V or so. be careful with 50V and higher FET;s. I had one blow the lid right off and made a dent in my ceiling .was one like you are using but VHF on 6meter. had to much bias and not enough attenuator on input. hear in the USA ,NY state been some great 6meter activity . and some 10 and 12 meter as well. 73's
Thanks very much Robert. That's particularly interesting what you say about the thermal paste. I must admit I thought it was all the same stuff! I've found that the genuine RF MOSFETs are much more tolerant and forgiving than IRF510s. In one rig I killed so many IRF510s I could have replaced them in my sleep! Thanks again for watching and commenting. 73 Nick
@@M0NTVHomebrewing easy way to tell thermal paste is color. wight is good in RF. The gray or copper color is bad. has metal flake in it to improve thermal resistance. CPU's in computer's can make lot of heat. better thermal paste was made for them . in RF world the metal flake silver can conduct in HV transistors like IGBT can arc threw. same with copper type. the newer wight silicone base is great. don't dry out like the old stuff did. when working with LD-Mos you will find soldering to a copper base is best way to get heat out. then either a water block or heatsink to the plate. I still like BJT RF transistors. like MRF 477, 2SC 2312C much better than IRF 510s . and for a KW go with tube like 3-500Z or 8877 Etc. 73's Bob
Hi Phillip, wow - you're fast off the mark! Why would you want to connect your oscillator to the PA? Normally your oscillator would feed into a mixer (or modulator). Sorry if I'm not understanding you right. This amp is intended to be the final part of the transmit chain (but it will have a low pass filter after it too). It has an input and output impedance of 50 Ohms - courtesy of the two transformers. Thanks for watching! 73 Nick M0NTV
Hi Nick, Thank you for your quick response. I was under the impress@@M0NTVHomebrewing Hi Nick, Thank you for your quick response. I was under the impression the chain for a WSPR beacon was software->si5351->rf amplifier ->low pass filter->antenna. Is this too simplistic ?
Hi Phillip, Ah you didn't mention it was for a beacon - I understand! I think if you've got one of those all-in-one black boxes that Hans Summers sells then you probably can feed it directly into an external RF amp but two questions arise: (1) I thought WSPR was a low power mode so you probably wouldn't want to connect it to something as beefy as I built. (2) I'm not sure what the power output of your beacon is - probably not much. The amp I built needs at least 250 - 300mW drive so you might need some kind of driver first. Hope this helps! Keep going - it's all good fun :)
I have some problems!!! How rf power amplifier works I have a walkie talkie that have a rf power amplifier module inside and powered by 7.5v and have 5watts of power into the antenna How this is happening? We know our antenna have a impedance of 50 ohm so if: I = v÷r P= v × I I= 7.5v ÷ 50ohm=150mA P=7.5v×150mA=1.12 watts So why ican see 5watts in my power meter
Thanks for your question. The answer is ... it depends! The output of your final amplifier will depend on several parameters: the gain of the transistor; the bias current running through it; the voltage that is feeding it; the power of the signal coming in from the driver amp; the class of amplifier etc. Without a schematic it would be impossible to say for sure. The class is important though - a class A will draw lots of current but waste half it's power on heat that you have to deal with whereas a class E will be highly efficient and you'll get more power out and less wasted. Some of the small HF QRP rigs are now using class E finals. I'm afraid the calculation is not as simple as you think. Check out Charlie Morris' RU-vid channel where he walks you through the calculations of how a power amplifier works. 73 Nick
@@M0NTVHomebrewing thanks for answer. I have some modules from motorolla shw5066 they can produce 7watts of power with 7.5 volt battery and when I opened it ididnt see any transistor!!! I can send you some pictures of inside of it
@@ltlt6117The simple answer is impedance. The output amplifier draws a bigger current at that supply voltage to generate the power. Therefore the amplifiers actual output impedance is significantly less than 50 ohms. They then transform this impedance to 50 ohms. It's the reason why an amplifier on a 12V supply can deliver hundreds of watts
Brilliant video mate!!! I subscribed on your channel !! Could you please write to me details about the book? I didn't found that. Greetings from New Zealand
Hi Rico, thanks very much for watching, subscribing and commenting! The book I mentioned is 'Building a Transceiver' by Eamon Skelton (EI9GQ) & Elaine Richards (G4LFM) published by the RSGB (Radio Society of Great Britain) in 2014. You should be able to get a copy on Amazon etc. I'm hoping when Cycle 25 gets going I might even be able to make some contacts with some ZL stations - not managed it yet though! Thanks again. 73 Nick
The transformers on input and output are necessary to match the very low impedance of the RF power transistor to 50Ω. On the input that will insure that maximum power is transferred from the driver stage to the power amp. On output you are matching a nominal impedance at the collector/drain (based on a calculation of how much power you want out) to the 50Ω of the following low pass filter and antenna. These are wideband transformers and the output one is built to handle significant RF current. Hope this helps. 73, Nick
Hi there, I don't have any downloadable schematic or layout for the PA at present but you could always watch the video and take a screen grab at the appropriate points. At 4:45 I show the SCHEMATIC and at 9:26 I show my LAYOUT. Hope this helps. 73 Nick
Some confusion with that diagram it has ground symbols and power supplies are + & - positive and negative so if you build a power amp with mains supply (ground, positive, neutral) 240v it has ground which you would ground the casing to stop electric shocks in case there is a fault and hot lead comes off so would you connect ground to the heat sink and negative?
Thanks for commenting. There shouldn't really be any confusion with the circuit diagram. There is ground, +50V DC for the main power source and +12V DC for the transistor bias circuit. I wouldn't go anywhere near a 240V AC mains supply! My 50V comes from a powerful DC boost convertor. The actual schematic comes straight from the official data sheet of the MRF 151. 73, Nick
@@M0NTVHomebrewing If you get a meter on the 50V or the 12V line and reverse the probes it should say -12V or -50V not Ground which is 0V Example +12V 0V -12V so 0V would really be the reference point where an AC waveform crosses between + and -
