Non typical mosfet question

[9Lives]

Hi everyone, it's been a whole since I posted. I have a couple questions about my base setup aND these mosfets everyone's using now.

I'm using a magnum 257hp threw about 60 feet of very low loss thick coax I bought from a ham site (can't remember) into my sirio gain master mounted almost to the peak of the eve of my one story house. I'd like to get higher but this is what I got for now. It's working pretty well, I've got everything tuned nicely where I'm not running the hell out of the radio. It's doing typically 10-12 bird carrier AM and 60 to 70 ssb. I mounted fans on the rfx unit to keep it cool, and everything is hooked to a scope so I can obsess over my modulation. Lol

I notice with my setup in the lower bands A -C 25_26 mhz my bird is showing more output than the higher h-J bands 28 mhz . I'm trying to figure out how I can smooth this out. The difference isn't huge but I'd say 5 or 7 watts rms.

Is this due to antenna length and things of this nature or is it harmonics the bird is reading on the 30 mhz slug on the lower bands it can't see on the higher? Or is this due to high freq capacitance in the mosfets? I know that in bjt transistors when you get close to the cut off freq the gain can significantly drop due to collector-base and emitter - base capacitance. I'm not sure how this works in mosfets though?

I did read about the miller effect some but a lil advise that's more specific to cb, export radio finals would be helpful. I have a pretty general understanding for electronics. Not pro, but enough to tinker and do some damage. Lol. Thanks

 

[543_Dallas]

Have you took some readings with a dummy load? You're trying to cover a lot of band so what you are seeing may be perfectly normal. Even though the SWR may be fine and roughly the same across the bands the resistance and reactance may change quite a bit...I've never used a gainmaster though.

 

[9Lives]

Ya I'm not even really trying to get that much coverage, just cb plus a lil freeband. I just wanted to me sure I'm not adjusted incorrectly. At first I thought same as mobile cb a plied. Where if the lower channel was more power you adjust the antenna. Then I remembered the effects of those transistors. Just wanted to see if anyone knew.

No, no dummy load . Don't have one. I burnt the last one I made up.. oops. But it's reading virtally no swr, even when I added my amp.

 

[543_Dallas]

That's great that the swr is so low. A dummy load is non inductive/reactive and an antenna is not. The only way to properly test this and know if you radio is adjusted correctly is with a decent dummy load.This applies to radios with fets and BJT's

 

[9Lives]

Ok, I have enough parts to wip up a dummy load w resistors. I may not be able to run it at full power, but I can get close. Once I do am I looking for it to match across the whole board or what ? Can u explain what I should look for?

 

[543_Dallas]

Be sure to use non inductive resistors and keep the lead lengths as short as possible. If it's aligned correctly the power output should be even across the band. If it isn't perfect that's OK.

You said you weren't using all those frequencies. If the ones you do use are pretty close with the antenna I wouldn't worry about building a dummy load it unless you really want to. A few watts difference between 25 and 28 mhz on the same antenna isn't bad. I don't think anything is wrong with your radio.

 

[9Lives]

Ok cool, Its difficult to express things through typing, I was really more or less looking for answer as to why it would do that, if it was the nature of the semiconductor, or antenna phenomenon.

Is that what you're saying is, that you'd have to see what it did on the dummy load?

 

[543_Dallas]

If you were to put an antenna analyzer on your antenna you will find the resistance is different at the top of the band than the bottom. It's not 50 ohms from 25 mhz to 28 mhz You may not see much change in swr but the reactance and resistance may change dramatically. The power delivered to the load will change too.

This happens because of the antenna. It doesn't mean something is wrong with the antenna or the radio. It will happen with mosfets, bipolars and tubes (unless you retune). Maybe someone more qualified will have a better explanation of why this happens.

 

[Onelasttime]

First it is normal for a the tank circuit in export radios to have a pretty steep roll off at both ends. Normaly the radio's are sold with very specific very narrow ham bands as their market niche even if they know they will be pimped out for 11m and so called 'free band".

I had a 1st Gen RCI2950 and it covered 26Mhz-33Mhz or 32,999Mhz. It had it's highest output power numbers in 10m and 11m bands. It did not seem to mind going lower as much as it seemed to mind going higher. I think I was only down 2watts at 26Mhz but at 32 it was a good 8 watts lower than on 10m and 11m. It was pretty flat from 26-29Mhz but once past that it started to roll of rather quickly. I tried a lot of different ways to fix it but the ways that worked best did not flatten it out rather it just moved the point that things roll off at. Since I was not operating up their any ways I just left it alone.

Assuming your radio has a proper alignment and your antenna is remaining resonate and efficient I would not worry about it. Have you checked to see what the swr and is doing. You can specifically check the reflected wattage coming back at those freq's with your meter just rotate the slug since you have a Bird 43. If you had something else you could just switch the jumpers on the back of the unit around and measure the actually wattage in reflect.

The reflected power is climbing as you lose output I would suspect that the coax and antenna are the issue not the tank circuits in the radio.

On the first Gen RCI2950 I have heard that if yo replace the CPU the older units will drop to 24Mhz-29.9Mhz and their existing tank circuits seem to function just fine.

I do not know enough about the Magnum radio. I have used them before but have never owned one. I never liked the feel of the controls or all the dual function controls and such. Just a matter or preference. I do not think Ihave run into any export radio that is flat in power output.

In fact even when people are adding a verrynarrow number of extra channels to CB you see that the power output is not flat and when they do a 10/11m conversion same thing. Even in Ham gear in All Mode HF rigs you see multiple power ratting depending on the band and mode. AM and FM are always about 1/2 of what SSB is for instance and you jump above 30Mhz usually the power drops in half again.

Also keep in mind that few things in life are linear! Man is obsessed with making things linear and expecting them to be linear but normally when you see something linear it is because man has hammered it into submission with cunning and creative programming and circuit design! LOL Usually it is something that is creating the illusion of linearity.

In fact even in biology and medicine complex machines like humans do not have linear function in physiology. For some reason nature like curves over sloping lines. When you do get lines that are straight it is almost always going to be stair steps or zig-zags. The resolution of the test gear and the freq. of the testing plays a role in that. Time is always part of the mix! Even when measuring watt's time is part of the game.

So I would not sweat it unless something is obviously wrong. 3-5 watts seems like nothing to me so long as those are not being reflected back into the radio! That is why I recommended you see what is gong on with reflected rf at those freq.'s as well. A 1/5 watts of reflect all the time could damage the unit so 3-5 watts of extra reflect would be terrible. If you look at your Bird slug you will see two tabs 180° apart from each other that match up with the big arrow. That is what allows you to rotate the slug and have it read in either direction forward or reflected.