Output Tuning

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Output Tuning

Postby K9ACT » Mon Oct 11, 2010 9:58 am

My prime reason for opting for Class D was a shortage of big variables around here. What I forgot about was the fact that my high level antenna tuner does not cover 160. This came as a dull thud when I was ready to move off the dummy load.

I have an L-net type tuner I made to use my 80 m dipole on 160 so this is the first thing I tried. I got a nice SWR null with it on my 160 dipole but the waveform was not a clean sine but the guy I contacted with it said it sounded great. Ignoring that, what was interesting is that tuner uses a loading type variable (close spacing) and produced no arcing at all.

I then substituted my low power MFJ tuner which nulled out and produced a clean waveform at 100 and 200 W. I did not modulate it as it is rated at 300W.

I opened up the MFJ and measured the C's and L and replicated this with larger caps but they arced with 100W and no modulation. The injput C required is about 126pf but the largest cap (3/8" spacing) I own was only 100pF so I put back the MFJ after setting the input cap to 100 and measured the L and other C when this nulled.

Using this cap in the tuner, it arced over when I modulated at 100W.

Somehow, I am building some sort of high voltage transformer instead of an antenna tuner and it's time to start over.

The obvious question at this point is, what is the difference between a tuner with Class D and the output tuning of Class E? All I want to do is load a 160m dipole with this transmitter.

I notice Steve's 400W Class E has 1:1 transformer, L input and two caps for tuning and loading.

If I change my output transformer to 1:1 and replicate this network, will I accomplish what I need?

There are lots of neat calculators on line to help design these sorts of networks but they all want parameters that I have no idea how to determine.

For example, they want input resistance and reactance to calculate input Z and likewise for the output. I presume I can use the numbers from my antenna analyzer to get the output but the input is a mystery. All I know is the DC resistance but don't know what the transformer does to this nor where to get the reactance.

I guess I am rambling but perhaps there is s nugget in here somewhere.

js
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Re: Output Tuning

Postby kf1z » Mon Oct 11, 2010 11:08 am

Are you saying that you want to convert your rig to Class-E ?

That's sort of what it sounds like.

After all....
With Class D, you must provide a near 50 ohm load (or whatever design is...) but in thisa case it is 50 ohms.
So you either have to switch antennas, or use a tuner.

With class-E, you'll want a less than 3:1 swr at 50 ohms, but then tune the amplifier (tune/load controls).

Either way, you're 'tuning' something, unles you can just switch antennas...


To be honest, I'm not sure how far you can stray above 3:1 swr... but I THINK, as long as you can tune the Class-E output for the correct voltage/current, and have a good waveform, you are good to go.

I use mine into 2:1 ( 75 meter dipole, cut for 3900kc, used it on 3700 the other night to talk to you, just retuning the transmitter)


For your 200 watt rig....
Use at least 4 cores for each transformer (you need 2 seperate transformers).
Then use for 75 meters, a 4 to 5 uH coil, 500pf tuning cap and the loading cap of at least 3000pf.

Then add in another 12? uH for 160, ( this will require experimentation to get the correct amount)


But also what you HAVE to do, is 2 seperate shunt capacitors on each drain buss.
Then extra shunt C switched in for 160 meters.


So I'm not sure which you want to do... rebuild a tuner that will handle the Class-D with no arcing, or rebuild the rf deck into Class-E?
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Re: Output Tuning

Postby K9ACT » Mon Oct 11, 2010 11:48 am

kf1z wrote:Are you saying that you want to convert your rig to Class-E ?

So I'm not sure which you want to do... rebuild a tuner that will handle the Class-D with no arcing, or rebuild the rf deck into Class-E?


All I want to do is get it running on 160 into a 160 dipole as simply as possible.

On 80, my Heathkit tuner works just fine into the 80 meter dipole with its KW rated caps.

I do not understand why I need half inch spacing on caps for 160 with a resonant antenna.

The current status is big spacing on the Ant side and the best I have is 1/8" for the trans side but it arcs with mod on 100W. I don't think the caps in the Heath tuner are any wider than 1/8" and there is no arcing on 80.

It's not obvious that using 10kv doorknobs for most of the capacitance will work any better.

js

js
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Re: Output Tuning

Postby kf1z » Mon Oct 11, 2010 11:58 am

Sorry, I somehow overlooked that it was a 160 meter antenna.


When you switch to 160 meters.. are you adding capacitance across the transformer?
(drain to drain, or from both drains to ground?)

If I mis-tune my E-rig, I arc across the tuning components.

So possibly you need to adjust the L in the tuner?

not sure.... I'm not thinking clearly about this at the moment...... :D
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Re: Output Tuning

Postby K9ACT » Mon Oct 11, 2010 7:21 pm

As my Firefox likes to say, "well, this is embarrassing"....

I hooked my antenna analyzer to my 160 antenna and it showed, R=5, X=20, SWR =9.

I went out to have a look and saw nothing unusual from a casual look. The North end looked just fine but a view through the binos showed that it was broken about 15ft from the far end but somehow, both ends got wrapped around branches and acted (mechanically) for all the world, like a whole piece of wire.

It was a lovely day for antenna work and it is now resonant at 1885 and no sparks.

Sorry for the panic,

Jack
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Re: Output Tuning

Postby kf1z » Mon Oct 11, 2010 7:41 pm

Good :!:
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Re: Output Tuning

Postby K9ACT » Tue Oct 12, 2010 7:46 pm

And now for the rest of the story.....

Even after fixing my antenna, my big caps in the tuner still arc under modulation and we discussed this to the boredom of the rest of the net today until K9WEK said: "why do you need a tuner"?

I couldn't think of any answer other than that there is no other way of tuning this Class D design.

He say: "if your antenna is really 50 ohms at the frequency of interest and the transmitter has a 50 ohm output, you should not need a tuner".

I say: DUH!

Turns out that with no tuner, the SWR is 1.3 at 1885 and about 1.5 10kc either side.

He also made up a spice model for a T tuner so I could juggle C's and L's all day, (blowing no FETs) to understand why mine was arcing.... too much L and not enough C.

If anyone is interested, I would be happy to post the Ltspice model.

js
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Re: Output Tuning

Postby Gilly » Wed Oct 13, 2010 12:16 am

Hi Jack


What stage are you up to now....

Have you still got hum or is it all fixed up....


Wayne
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