1250W CW from a single power FET!!

[eagle1911]

I am just curious why you need that much power to begin with?

Well, as an Amateur the legal limit in the US is 1.5kW. 500W isn't but 1/3 of that, so it's perfectly acceptable to run if it's clean. I'll never use an amp that doesn't have good filtering, and I don't overdrive so the IMD is kept in check.

There are times when QRO is a good thing, and there are other times when it's fun to see how far you can go on 5W or less. I'd like to have access to the full range of the ham radio experience one day, and I'd like it even better if I built some of what I use.

I like building PAs in particular.. something about the power I do find vainly enjoyable...

 

[BOOTY MONSTER]

how often do you think 1500 watts will talk someplace that 500 watts wont ?

 

[eagle1911]

Not very

 

[BOOTY MONSTER]

exactly

 

[Superidgit]

Well, as an Amateur the legal limit in the US is 1.5kW. 500W isn't but 1/3 of that, so it's perfectly acceptable to run if it's clean. I'll never use an amp that doesn't have good filtering, and I don't overdrive so the IMD is kept in check.

There are times when QRO is a good thing, and there are other times when it's fun to see how far you can go on 5W or less. I'd like to have access to the full range of the ham radio experience one day, and I'd like it even better if I built some of what I use.

I like building PAs in particular.. something about the power I do find vainly enjoyable...

Hi eagle,

Forgive give my ignorance I hope I get this right.

So as an Amateur, this means you have a liscense I am assumming and can operate a radio at and up 1500 watts of power. However, me being the local jo riding down the road with 4 watts i am just fine? I cant go pop an amp in my little truck and use 200 watts without a liscence correct? ( yes no liscence here)


What is this 'clean signal' you speak of? First time i have heard the term.

Does this refer to the splattering, the screwing with toaster ovens and what not?

 

[eagle1911]

That's right! the limit is 1.5kW PEP with a license, and 12W PEP SSB/4W AM on CB without one. That's 12W legally of course, obviously lots of CBers run power, and personally I don't care if a CBer wants to run a couple hundred watts from their rig.. I don't even care if they want to run a couple kW as long as it's clean, but with the vast majority of CB amps, particularly the higher powered ones, they are not even close to clean because they are overdriven class C amps being used on SSB or AM when they should not be.. more on classes later.

By clean signal I mean a signal with suppressed amplification byproducts. When transistor devices are used for amplification, they always alter the input waveform in some way. This is distortion. Typically there are two types of distortion, harmonic and intermodulation or IMD. To minimize IMD what is usually done is to run the transistors below their maximum output specifications, thus avoiding overdriving them. To minimize harmonics a filter circuit is needed, but it's a very simple circuit to build generally, so that's minimal effort. These byproducts often cause interference with neighbors electronics, and they interfere in parts of the radio spectrum that are far from the original frequency of operation, on completely different bands.

Another approach to help keep the output waveform as close to the input waveform as possible is to ensure that the transistors are run a class of operation that will allow it. There are several classes of amplifier, with class A providing the least distortion, at the expense of efficiency and gain. Class B is similar to class A, but with greater efficiency and more distortion. Class AB is a point between classes A and B, and usually provides the optimum blend of efficiency and low distortion. Class C has the highest efficiency, but the most distortion.

Upon reading this one might assume that Class A is the "best" and class C the worst, but that's actually far from the case. they each have their purpose.

SSB operation requires a low distortion amplifier to maintain the best sound as well as a clean signal. AM is also best on a low distortion amplifier. CW and FM, however, work great on class C amps because of their characteristics. Many VHF FM amplifiers in ham rigs and commercial/emergency radios are class C.

This is a very simplified explanation of course..

 

[Superidgit]

Thanks Eagle,

So when im on LSB say channel 6 and still hear the the same fool repeating theirself 27,000 times and yet can still hear them on the AM channel 6 this is distortion? or splatter?

And holy crap did i open a can of shit i need to learn.

so operating,building and or looking at an amp need many many many considerations?


and how is one gonna get 50 v outta an alternator without spending tons of money?

 

[eagle1911]

There is a bit more to it than just snatching up a dave-made or whatever, hooking it up and yapping on it, that's for sure.

It's best, when choosing an amp, to use class AB only for HF, including CB. You can read or ask around for info on CB amps known to be class AB. I also recommend using a filtered amp (some Palomars are filtered, not sure about other 11m-only amps). If your amp is not filtered, use an external low-pass filter such as this: RF LIMITED LOW PASS TVI FILTER DF-3000 3KW PEP new | eBay

This filter handles up to 3kW, but there are smaller, cheaper ones out there that handle less power.

To help reduce IMD it's best to find out what kind of transistors an amp uses and look up their datasheet online. You can then use the published power output figure for the transistor as a a guide as to what kind of clean power to expect from the amp.

As an example, let's take an amp that uses SD1446 transistors. Here's the datasheet for the SD1446: http://www.datasheetcatalog.org/datasheet/stmicroelectronics/2805.pdf

You can see that the SD1446 is rated to 70W max. Using this number, you can estimate that 2 SD1446 transistors will give 140W as an absolute maximum without shortening the transistor's expected lifetime. It's good engineering practice (and good operating practice) to only drive the transistors to around 80% of their absolute maximum rating. That means that 2 SD1446s will give about 120W of clean power. At this level we still need the low-pass filter for harmonics, which are always present, but the IMD is reduced.


When you are hearing someone on SSB squawking on AM, you're hearing it because you are on almost the exact same frequency as the SSB signal. This is normal, and not a result of splatter. Two people on the same channel will interfere with each other on some level regardless of mode of operation. When you're on one channel and you can hear someone on an adjacent channel, usually it sounds like garbage not speech, that is splatter, and is the result of huge amounts of IMD being generated by an overdriven amplifier. When I was on Cb a lot I remember hearing guys 2 and 3 channels away from the one they were using. Drove me nuts...


Finally, the 50V device in this thread will be used in a base amplifier in my application, although I'm sure a boost converter could be built to supply 50V from an alternator. It would have to be pretty serious to power a big amplifier though.

 

[Captain Kilowatt]

how often do you think 1500 watts will talk someplace that 500 watts wont ?

It happens a lot if you count televisions and toasters as places.

 

[Kamikaze]

If one carefully reads an SD1446 spec sheet one will not recommend one of these devices for linear amplification.

Pay close attention to the drive power verses output power graph. It is not linear over any range. The graph should have a constant slope at least somewhere.

These devices are obviously rated for 2 way FM work where they do not need to be linear.

No amount of filtering or underdrive will help here.

 

[eagle1911]

No amount of filtering or underdrive will help here.

I'm not sure I agree. The SD1446 might not be the absolute best choice for linear operation, but very often the linear performance of these types of transistors is surprisingly good. I've even seen power MOSFETs intended for switching applications used as RF linear amplifiers with surprisingly good results: http://www.microsemi.com/micnotes/APT9802.pdf

It's not at all uncommon for devices which aren't characterized for SSB to be used for linear applications. The SD1446 is not characterized for SSB, you are correct on that, but when driven to around 100W max (in pairs of course) the IMD is acceptable. W8JI did a nice review of the HLA-150, which uses the SD1446. His only major complaint was the fact that (not surprisingly) RM overstates the output of the amplifier. He then goes on to demonstrate that when the amplifier is overdriven the output is very lousy, which is to be expected. It is noted that when driven correctly the amplifier is about on par with most HF rigs. He also notes that the filtering in that amp works very well.

Review here: RM HLA-150 test