VHF power transmitter (lab experimnt) - Page 1
Forum section: electronics / rf-microwave

author: Dajgoro
author_rank: Frequent Contributor
timestamp: January 22, 2024, 01:25:04 pm
content: HelloDoing a lab experiment with low plasma ionization, and it needs 300W of RF power(30-70MHz), but instead of buying expensive lab equipment for a second lab I figured a VHF radio amateur transmitter could do the same job.I just needs to be a carrier, no modulation at all. I tried looking around the web but best I could find was 100W, or FM radio transmitter, but that's too high frequency. Where could I look for such a transmitter?

author: xrunner
author_rank: Super Contributor
timestamp: January 22, 2024, 01:35:01 pm
content: Look for the ham transceivers labelled "HF/50 MHz". Almost all will output 100W at 50 MHz. Most ham transceivers for the HF bands will handle the 6 meters (50 MHz) ham band.If you need 300W you'll need an amplifier also.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: January 22, 2024, 01:39:06 pm
content: Yes, it has to be 300W otherwise it wont work at all. I'm googling more but still nothing, its like I'm missing keywords.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 22, 2024, 01:44:48 pm
content: This is not a product recommendation, but should give you an idea on what to look for:https://mfjenterprises.com/collections/ameritron/products/als-606xGiven that your interest is only between 30-70MHz, a multi-band linear may be overkill, and you might be able to get away with a cheaper single band 6 meter/50MHz FM Class-C amplifier.Another possibility would be to modify a 27MHz "CB" amplifier (which theoretically are illegal, but seem readily available).https://www.ebay.com.au/itm/185467688681?hash=item2b2ebc56e9:g:sqcAAOSwXBdirCPu(I can't vouch for the build quality or suitability for your purpose)

author: Dajgoro
author_rank: Frequent Contributor
timestamp: January 22, 2024, 02:06:01 pm
content: I sent an email to MFJ, as for modding things, I'd prefer not to if I can avoid it. If I go down that path I'll end up soldering infinity vacuum tubes and the place will look like doc browns lab.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 22, 2024, 04:57:31 pm
content: Any used Ham Linear that has 10 meter in it would work just fine a little over 30 MHz.I suppose 50 MHz would be better?Almost ALL Ham Linears that work up to (and slightly over) 30 MHz. 50 MHz is not that popular,In US, there is a minimal change you need to do to make any linear work on ten Meters. It's a kinda stupid law, You give them your Ham Call Sign and they tell you which wire to cut. Everyone knows that anyway,I would look at Ham clubs (sometimes they have classifieds on their website) or go to a Hamfest. I do not think things are much different in the US as in Croatia.I have seen lots of the CB linears. You never know which harmonic you are on with some of them.I would get a Ham Linear and then sell it when I was done with the project, you might even break even.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 22, 2024, 05:18:43 pm
content: Maybe you can go for a 2m band kit and just fiddle a little bit with the tank circuits (more windings, larger caps). MRF300AN/BN push pull could easily do this.NXP also has app notes and sample PCBs for this.

author: jwet
author_rank: Frequent Contributor
timestamp: January 23, 2024, 03:24:08 am
content: I believe there is a 4M band in the UK if I remember correctly. US only has 6M and 2M (50 and 144) that aren't so close. Try a British Dealer like Waters and Stanton, etc.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 23, 2024, 02:14:44 pm
content: Quote from: Dajgoro on January 22, 2024, 01:25:04 pmI doubt many amateur transmitters would be designed for 100% duty cycle, 24/7 operation. You may need to derate the amplifier for your application. I wonder also how robust they are or the other end, maybe they have enough protection, they won't allow you to drive your particular load. I assume you are not running this experiment open loop and are planning some method to close it.If you are planning to talk with some of the amplifier companies, I suggest you make sure to describe in detail what your requirements are. Not just what you have posted.

author: CaptDon
author_rank: Super Contributor
timestamp: January 23, 2024, 03:01:44 pm
content: In as much as the very term 'V.H.F.' usually indicates 100MHz to 300MHz no wonder you are not having any luck. It is doubtful you will find any 'ham band' amplifier to suit your needs. As some have mentioned a '6 meter no-tune' solid state amplifier may be wide enough to cover 30 to 70 but it is a crap shoot. You will probaably end up with an R.F. synthesized signal generator for frequency generation and a small amplifier driving a bigger amplifier. All in all more costly than you expect. Most of the Amplifier Research units are designed as broadband to start with and many can be driven directly from signal generators but now your 'used equipment' total landed cost will approach $5000.00 U.S.D. and if you buy used make sure they have a return policy because half of the junk on Ebay is broken or mis-represented!! (I know, I just bought a Wavetek 3000 that is A.F.U.'d and 'returns not accepted') B.T.W., most 'tube type' amplifiers will require retuning of the grid, plate and loading controls with each frequency change.

author: p.larner
author_rank: Frequent Contributor
timestamp: January 23, 2024, 05:30:25 pm
content: why not build an amp using a 4cx-250b valve?

author: TimFox
author_rank: Super Contributor
timestamp: January 23, 2024, 06:56:49 pm
content: Quote from: CaptDon on January 23, 2024, 03:01:44 pmStrictly speaking, the “f” bands were defined many decades ago to correspond to decades in wavelength.mf = 0.3 to 3 MHz, or 1000 to 100 mhf = 3 to 30 MHZ, or 100 to 10 mvhf = 30 to 300 MHz, or 10 to 1 muhf = 300 to 3000 MHz, or 100 to 10 cmetc.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 23, 2024, 08:19:57 pm
content: Quote from: joeqsmith on January 23, 2024, 02:14:44 pmThe trick is to go for a 500W part and run it with 300W. That should work even 24/7.

author: baldurn
author_rank: Regular Contributor
timestamp: January 23, 2024, 09:22:50 pm
content: Quote from: joeqsmith on January 23, 2024, 02:14:44 pmPeople do run them at 50% duty cycle for extended periods with FT-8. Often the needed derating for such use will be specified.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 23, 2024, 10:09:31 pm
content: What about this one:https://www.dxworld-e.com/product-page/50-70mhz-linear-amplifier-500w-6-4m-band-board-mrf300-included

author: TimFox
author_rank: Super Contributor
timestamp: January 23, 2024, 10:23:30 pm
content: Vacuum tubes suitable for RF power purposes were often rated two ways:"ICAS" = intermittent commercial and amateur service"CCS" = continuous commercial serviceE.g., the popular 6146 beam power tube was rated at 27 W CCS and 35 W ICAS plate dissipation, below 60 MHz.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: January 24, 2024, 03:43:09 pm
content: Still no reply from MFJ email, I tried calling them on phone now (waited till it was 9:30) but nobody picks up."why not build an amp using a 4cx-250b valve?"I had such ideas but I don't have a reliable schematic, and that would be sort of adding a project on top of a project."I doubt many amateur transmitters would be designed for 100% duty cycle, 24/7 operation."It does not have to work 24/7, its just for an experiment that they run for a bit and then they fiddle with it."https://www.dxworld-e.com/product-page/50-70mhz-linear-amplifier-500w-6-4m-band-board-mrf300-included"That looks promising, so I just give it 50-70MHz in from a regular waveform gen and that's it?

author: CaptDon
author_rank: Super Contributor
timestamp: January 24, 2024, 07:24:10 pm
content: One problem using the solid state amplifiers is the horrible impedance mis-match of your load circuit. Those power amplifiers want a 50 ohm load and those old 600 watt units were bench queens spending more time on the repair bench than in actual operation. The newer model is better but with your load characteristics you'll probably still wreck it in short order. These plasma experiments are being done at many universities. What power amps are they using and do they buy them ten at a time as they get blown up daily? Reminds me of the dipshits putting new klystrons and CFA tubes in Navy radars. They require a very careful installation procedure and careful tune up. The dweebs would just bolt a new one in, attach the connectors close the door and press 'GO'. IF it worked, well good enough, if it instantly ruined the new $50,000 U.S.D. tube, well who cares, the tax payers will just buy another one.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 24, 2024, 11:32:19 pm
content: Quote from: Dajgoro on January 24, 2024, 03:43:09 pmWell, a beefy PSU plus an output filter and some thermal management also need some attention ...

author: Dajgoro
author_rank: Frequent Contributor
timestamp: January 25, 2024, 12:03:07 pm
content: Quote from: CaptDon on January 24, 2024, 07:24:10 pmExactly why I'd try avoid making my own. Best case is to have somebody who makes and sells such things have it setup all and send over.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 25, 2024, 02:11:22 pm
content: I have a linear amp that would work. It has two 4cx350 output tubes and a very wide ranging matching network, It is a US Military GRC-106. It is rated at 100% for FSK teletype communications at over 300 Watts/ Weighs 30 kilos. Needs BIG PS. Maybe there are some surplus military rigs available where you are.This GRC-106 amplifier will put out full Watts with only 100mW Drive. So you could use a signal generator.If you get a rather traditional HAM Linear with two cx 500 tubes rated at over 1000 watts, It should work at 300 W continuously.Then you have the Drive Problem. You need a Transmitter that will also put out about 40 Watts continuously.The Tube amps have a good matching network and are not as sensitive to mismatch.The single 500 tube amps might be a little light for your purposes. There are a lot of dual 500 tube amps available used here in US. Many Brands. Used to be very popular. Heathkit, Drake etc....They will only go a little above 30 MHz but some have been modified for 50 MHzI do not know what you would be using for a Radiator (antenna) Might be helpful. And what is the impedance of the radiator?Tube amps typically have good Matching (Tuning) networks but they cannot match everything.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: January 25, 2024, 02:22:10 pm
content: There is no military surplus in this area, and if there is its unknown condition from somebody's attic.So I think I found everything, here it is:Signal gen:https://www.ebay.com/itm/384375810124?hash=item597e95684c:g:GsUAAOSwMc9hNZOCPreamp:https://www.ebay.com/itm/111775220411?hash=item1a065282bb:g:0QgAAOSwnyBZwkRaAmp:https://www.dxworld-e.com/product-page/50-70mhz-linear-amplifier-500w-6-4m-band-board-mrf300-includedSWR:https://www.ebay.com/itm/354581681877?hash=item528eb716d5:g:ATEAAOSwiQJj6Y6JPSU:https://www.tme.eu/hr/en/details/hrp-450-36/built-in-power-supplies/mean-well/If anyone knows of better replacements for what I linked post.

author: LaserSteve
author_rank: Super Contributor
timestamp: January 25, 2024, 11:53:56 pm
content: See if Communications Concepts Inc is still in business. Helge Granbergs (Spelling?) Motorola app notes form the basis of their designs. Also consider ISM band amplifiers...Would 40.6 Mhz work for you? Also surplus FAA VHF/UHF rigs show up from time to time and run often 500 watts.Steve

author: pienari
author_rank: Regular Contributor
timestamp: January 26, 2024, 07:16:08 am
content: Hi.I think if you build amp you can integrate pwr/swr meter there.https://www.dxworld-e.com/pwr-swr-meterI got one in my 1.6kw hf amp and it works fine.Dont buy too small psu, 20A 36-48v at least.I have used old 48v server psu.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: January 26, 2024, 09:09:24 am
content: It says:"Need bridge-tandem match (directional coupler) to work with.Minimum voltage from directional coupler (pwr swr bridge) +2,55V"Where do I get that?

author: pienari
author_rank: Regular Contributor
timestamp: January 26, 2024, 10:41:14 am
content: It generate voltage from antenna line.Display module needs 12v dc.There is Microchip cpu so 0-5v inputs.

author: radiolistener
author_rank: Super Contributor
timestamp: January 26, 2024, 11:16:56 am
content: note that amateur VHF power amplifiers are intended to work with proper 50 Ω load.Since you're planning to use it for plasma, if your load is not 50 Ω it can damage such power amplifier or it just will not work due to triggering high VSWR protection.Also note that experimenting with plasma at VHF frequency can produce a lot of unwanted emission at air band and military band, so you can be punished by government for for interfering communications for aircrafts and military.300 Watt is high power for VHF and frequency is very close to important bands like aircraft, so if you will use it for experiments, very soon you can expect guests, some kind of SWAT team with helicopters, etc...At that frequencies plasma ball can work as a good enough antenna, so if you're still want to experiment with it, in order to avoid issues with government and military services, I suggest you to do it for at least within good Faraday cage and within underground basement to reduce emission. But note, that it still can lead to issues with government and military services due to unwanted interferences.

author: CaptDon
author_rank: Super Contributor
timestamp: January 26, 2024, 02:24:19 pm
content: Only a second harmonic of his intended frequency band would be in the VHF military / aircraft band. Aircraft frequencies are 108MHz to 137.99MHz so primary frequencies from 54 to 69MHz could be a problem. These experiments are often done with what we would consider really crude techniques. A single power tube like an 813 or 810 in tuned self oscillation mode generating a huge R.F. potential coupled into the plasma chamber. Frequency is crudely controlled by tank resonance and power delivered is based solely on plasma apparent resistance. The bottom line is that the R.F. generator produces a huge no load voltage to start the plasma. Once the plasma develops it loads down the R.F. generator. Plasma resistance / impedance is all over the place. A lot of plasma stuff also uses magnetrons very similar to those in microwave ovens where the voltage to start the plasma is a kilowatt or more at an open space impedance of 377 ohms.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: January 29, 2024, 09:22:51 am
content: The antenna is 50 ohm, an it is run in a sealed metal vacuum chamber, and the experiment has been going on for a while now without any issues (its ran by researches, not me lol).