Thoughts on running a high powered RF power linear amp on an unregulated supply

[davelectronic]

Ive been thinking, at the moment i am running a 25 watt linear RF amp on a 12 volts regulated supply, i am considering running a large 300 watt linear RF amplifier on an unregulated psu, but well filtered out put stage, although i understand the no load voltage, and the voltage load line drops from such a supply, can anyone see a problem with this, the amp will have an operational range from 11.50 volts to 14.50 volts, so if kept in these perameters can anyone forsee any problems with this.

 

[OLIVE2222]

Hi Dave,

With a nominal 10V secondary you will have (10X1,41)-1=13,1 =mid range of allowed supply voltage.

Whith min 240V (10% tolerance) it's become (9X1,41)-1=11,69V
Whith max 240V (10% tolerance) it's become (11X1,41)-1=14,51V

This assuming a perfect 24 transformer ratio.

Olivier

 

[davelectronic]

Hi OLIVE2222.
Thanks for your reply. I was planning on using a 300 VA toroidal transformer, primary 240 volts, secondary 12 - 0 12 - 0 and parallel these for max current, the transformer is a little over the 12 volts about 12 .80 volts AC, i thought 12 volts x 1.41 = 16.92 minus the rectifier 15.92 volts DC or there abouts, unloaded, add a modest load resistor to bring the voltage down to about 14. 50 volts or just over, i know this will drop when heavily loaded, but was planing on using somewhere in the region of 40000uf of filter capacitance in an attempt to keep the voltage above 11.50 volts.

I dont know if this would work with a load as big as that, but i would not expect the voltage to drop below this.

 

[davenn]

did you pass you amateur licence dave ??

 

[davelectronic]

MMMM i will let you work that out, this is for CB not amateur, and yes i already know your reply.

My maplins mate let me into a little secret, he and others go above there allocated 10 - 50 watts depending on status.

A guy mobile made it in to the margate Kent repeater from sittingbourne Kent , many miles away, more than 10 - 50 watts would cover on uhf.

Any way all us CB ers use power, but mines not 300 watts yet, only a modest 25 watts.
No need for explanations i know already.

 

[davelectronic]

I called the powers to be in the UK, an idiot was wiping me out, i spoke to a top engineer working for the powers above, gave address details down to the last detail, i was told sorry not enough resources, only if it interferes with the emergency services, IE police ambulance fire brigade marine coast guard etc do we get involved, i know what people are using he said to me, but so long as it not a nuisance there is nothing we can do.

That was from a top radio governing body here in the UK .

 

[davenn]

I was just reading through the OFCOM information and rules for CB in the UK

4W is the maximum power with unity gain antennas and I was suprised to see FM only, you guys cant even use sideband

D

 

[davelectronic]

Yep thats right, and you've got the correct governing body, they dont allow it, but as a top engineer said to me, we cant stop it, the guy wiping me out uses a KW of linear power, and they still said sorry mate cant help you, i was asked if he was causing any interference to any services, no i dont think so i said, ok mate sorry but cant help you, i do only use FM i dont have the money for a good multi mode radio for, mmmmm DX ing.

And if i was an amateur, i would not risk losing my status and license.

Ok what i am using even 25 watts is a no no, i get this, but if they did the rounds no one in my county would have there gear anymore it would be confiscated, oh they do return the legal gear once tested, a friend in Deal Kent lost his gear, for disturbing some services, if you live next to a police station dont go robbing banks, joke.

 

[davenn]

I didnt see anythging about 476MHz CB band
I assume there isnt a legal one there ?

 

[davelectronic]

I dont use 476MHz CB band, there is no such thing in CB Radio, i use 80 channels covering frequency's from the list in the link below. What your referring to is 70cm uhf, the amataurs use this on our repeater stations, or go else where that there licence conditions allow.

http://www.g4nsj.co.uk/cb.shtml

 

[davelectronic]

I have a legal scanning receiver covering loads of frequency's, i listen in on that, well two receivers a base and a hand held unit, given to me in a terrible state i repaired them and use one in the living room, connected to a wide band antenna and a pre amp.

If that clears up how do i know what the local amateurs are doing.
One guy in the next town got a ticking off, an amateur for setting up his own repeater station, then a handful of his mates using PMR through it, but he got in trouble with a first warning, i think he heeded it.

 

[davenn]

I dont use 476MHz CB band, there is no such thing in CB Radio, i use 80 channels covering frequency's from the list in the link below. What your referring to is 70cm uhf, the amataurs use this on our repeater stations, or go else where that there licence conditions allow.

http://www.g4nsj.co.uk/cb.shtml

Actually there is --- A lot of the rest of the world has a 476 MHz CB band, it generally covers 40 channels in the 476.000 - 476.999 MHz range
Obviously the UK doesnt

The 70cm amateur band tho its bandwidth varies from country to country generally is within the 430 - 450MHz. Some countries have more or less of that range
We used to have all of that in Oz but in recent years lost the 430 - 431.999 off the bottom end and now have 432 - 450. But it doesnt end there ...
There's hints that we are also going to loose 440 - 450 to commercial services in the near future. Which is a bit annoying as we have a few ATV (Amateur TV repeaters) in that part of the band
and they will have to come off the air permanently as there isnt enough bandwidth for a TV signal in the 432 to 440 which is already allocated to repeaters, beacons, SSB, satellite and digital modes.

I would be suprised if an amateur in the UK will get away with doing anything illegal for any time. If he's caught it will not go well for him.
The UK has always had very strict licencing regulations compared to the rest of the world.
Amateurs in other countries will loose they licences if caught TX'ing out of band and you should see the huge fines they have to pay, some of them top $10,000

I ensure I do as Im supposed to, there's too much to loose!!

 

[davenn]

But back to the original topic

300W amp .. holy crap haha
A regulated supply would be better but failing that, you could just use a really decent deep cycle car battery... something in the 100Ahr area. The battery will provide a lot of good smoothing and help overcome the voltage drop problems
Im assuming you are going to run this off 12 - 14V ?

Assuming 40 - 50% efficiency of the amplifier, not uncommon. 300W of RF is going to need a PSU capable of 600 - 700W . so lets say 650W for middle of road calculations
650W / 13V = 50Amps

I wouldnt even consider using a transformer based ( linear) PSU
for those sort of current requirements I would use a switchmode PSU
A transformer for a linear supply capable of 50 Amps is going to be big and very heavy.

D

 

[davelectronic]

Yes 70cm UHF over here is in the 430 - 450mhz band under 450 MHz, i thought TV used 600 - 700 plus MHz, channels analogue where over here and higher i think, at the moment the 300 watt linear is an idea, not for continuous use, its powered in stages from 50 watts up to 300 watts, yes the site over here sell smp's to power them 60 amps is the highest i think, the twit that keys over me has the psu and the linear, and a plug in the wall linear, tube i think.
No i am undecided if to use a unit that big, biggest ive used some years ago now was 100 watts, i thought that did well, where i live 300 watts, other than late at night would wipe out loads of peoples TV's especially on an 18' glass fiber vertical, no i dont know to be honest, part of me dont care, if you where going to die would you give a f...k, well you might, i dont know.

http://www.kcb.co.uk/shop2/index1.html

My favorite shop.

 

[davenn]

i thought TV used 600 - 700 plus MHz, channels

ATV = Amateur TV as in TV done by amateur radio operators

This is real TV as in 25 frams / sec. Not to be confused with SSTV (Slow Scan TV)
which isnt TV its just sending individual pics as a data transmission from your computer.

There are 2 forms of analog ATV...
1) the standard VSB Visidual SideBand. This is the commercial standard for TV transmission mode for analog TV. The video is used to AM modulate a carrier, then one of the sidebands is partially removed, usually by filtering.
The audio is applied as a subcarrier FM signal and is usually offset from the main carrier, commonly 5.5 or 6.5 MHz. Depending on where you are in the world.

2) the other form that is mainly used by amateurs, mainly cuz its a heck of a lot easier to set up, is straight FM TV. This time both the video and audio use FM. Again the Audio subcarrier is offset from the Video by 5.5 or 6.5 MHz

For either system the total bandwidth is ~ 7.5MHz per channel

A lot of amateurs have, in recent years, also been playing with digital TV and in general use the same modulation modes used in terresterial DVB systems. This means we can use DVB TV settop boxes (which are readily available and cheap) to demodulate our signals.

Some guys do use the mode s that satellite TV uses and they make use of the satellite STB's for decoding.

I have built up ATV gear on 1.2GHz, 2.4GHz, 5.7GHz 10GHz and 24GHz using FMATV

on 24GHz we are using a 80mW Gunn Oscillator transmitter into a 40dBi gain dish and getting an easy 76km.

D

 

[davelectronic]

Well ive learnt some thing new tonight, i thought you was referring to normal TV type broadcasts, those last list of frequencies are very high, at 40db over i should think distances like that are easily attainable, not my cup of tea, but i might google it find out more about it, where stuffy over here, ive never heard of it until tonight.

A few amateurs put the pc's to good use and incorporate them into there activity's, shipping is one of them, every thing they talk about in the English channel from fishing to commercial i know about, in years gone by it was part of my job, including diving in my own time for dock side work / boats / surveys, thats about the only interesting thing amateurs here do.

 

[davenn]

Well ive learnt some thing new tonight, i thought you was referring to normal TV type broadcasts, those last list of frequencies are very high, at 40db over i should think distances like that are easily attainable, not my cup of tea, but i might google it find out more about it, where stuffy over here, ive never heard of it until tonight.

its a lot harder doing ATV cuz of the wide bandwidth. For example, that 80mW with 40dB gain does help. But spread over some 7.5MHz insrtead of the usual 15kHz for a narrow band FM NBFM signal you loose 30 db straight away. ~ 120km is about the limit with that power level and antenna gain. A couple of my mates managed to do that a couple of years back.

Also adding to the problems is the very hi path loss due to water vapor/humidity, it can add an additional ~ 0.5 dB loss / km.

And last but not least, the higher the gain the dish antenna, the more difficult it is to point to the guy at the other end of the link. There is only ~ 3 - 4 degrees of beamwidth, thats 2 degrees either side of the centreline, go further than that and the signal disappears. Its really a pencil thin beam, you have to be quite accurate in pointing the dish, else contact is never going to happen.

That actually is part of the fun of it. all the challenges

cheers
D

 

[davelectronic]

As i say ive never heard of it, i will google it and find out more. Its not a lot of power, but a very high frequencies, yes i know NFM and WFM plus the other modes.