SSB Carrier Supression

[Mikebits]

Like I said, in a pinch.
Sounds whacky but check out page 31 of this pdf.
https://www.electro-tech-online.com/custompdfs/2009/01/211man.pdf
I worked at a place where we would burn in 100W HF radios with light bulb dummy loads.

 

[Mikebits]

That has to be the worlds worst peice of a crap circuit I've ever seen.....

Gee, Thats kinda harsh, even if you forgot the i before e rule...

Recall that glass house saying before you throw a stone.

 

[flat5]

In 1960 I had a 5 watt transmitter (11Q4096 was the call sign)
and used a 6v dial light bulb for the dummy load
Not the easiest part to buy these days.
6.3v_0.25a_bulb.jpg]

 

[JimB]

In a pinch, a 100W light bulb will work as a dummy load.

A light bulb in not recommended! The resistance is not constant, varying considerably with applied voltage.
In the good old days of valve (tube) transmitters with pi-output networks, a light bulb was fine, but for a transistor PA which is expecting a constant 50Ω load (usually) the lamp is a bad idea.

JimB

 

[Space Varmint]

In 1960 I had a 5 watt transmitter (11W4096 was the call sign)
and used a 6v dial light bulb for the dummy load
Not the easiest part to buy these days.
6.3v_0.25a_bulb.jpg]

house bulb

You can use any incandescent. It's about 72 ohms which is close enough. I got a dummy load and been using it. The rf isn't as bad with it but it's still there. No, I've made up my mind. I'm gonna put this in a cabinet and just shield everything from the amp.

 

[Analog]

Can we make this thread even longer?

 

[Nigel Goodwin]

house bulb

You can use any incandescent. It's about 72 ohms which is close enough. I got a dummy load and been using it.

Incandescent bulbs don't have a constant resistance, and 72 ohms is no where near enough to be 50 ohms - the idea of a dummy load is that it's accurate so you can use it for transmitter alignment.

An incandescent bulb is particularly bad for SSB, as the power varies from zero to full power as you speak - so the resistance of the load will vary with it.

The rf isn't as bad with it but it's still there. No, I've made up my mind. I'm gonna put this in a cabinet and just shield everything from the amp.

It's normal to build radio's in shielded cabinets, you should have been doing this all the time.

 

[audioguru]

house bulb

You can use any incandescent. It's about 72 ohms which is close enough.

A 120V/200W incandescent bulb is 72 ohms when it is hot and is about 5 ohms when it is cold. I have never seen a 200W house bulb.

I just finished measuring the resistance of two 120V/100W bulbs and they are both 10.0 ohms. In parallel they are 200W and are 5.0 ohms.
120V squared/5 ohms= 2880W!
So your transmitter is driving a short circuit until the bulb warms up when you are screaming continuously into the mic.
Which will burn out first: a light bulb or a transistor?

 

[duffy]

Which will burn out first: a light bulb or a transistor?

This is a function of the cost of each component - the more expensive one burns out first.

 

[audioguru]

This is a function of the cost of each component - the more expensive one burns out first.

An incandescent light bulb costs $1.00. A little transistor costs $.11.
The little transistor driving the big incandescent light bulb is shorted but the light bult is still shining.

 

[duffy]

Next you're going to tell me the toast doesn't always fall butter-side down.

 

[Space Varmint]

You have a good >80 watt dummy load?
Pump the 80 watts into that and see if the "feedback" is gone or if you can sniff where it's coming from. I'm thinking a good ground or shielding won't matter so much in this scenario (or Sinatra, as my spell checker thinks). The power amp should be shielded in any (metal) case.

wha-da-ya thinks?

No, it's an el cheapo dummy load. An MFJ. It's small, supposed to be 300 watts but it's no Cantenna. So it leaks big time. I could probably transmit a couple of miles off of it...lol.

I know for sure it's the rf getting back in it now. If I hold a note like whistling in the mike where I'm putting out constant power, I can hear the rf break in on my signal. At 8 watts max output, when I'm speaking normally, it bouncing around about 3 watts so I'm safe. I found my old Diamond SWR/Power meter too so I've measured on both.

 

[flat5]

Well I guess shielding never hurts. You might as well use feedthroughs too for the dc power.

 

[flat5]

"Can we make this thread even longer?"
I, for one, really enjoy SV's building threads.
I'm glad he is sharing this adventure with us.

 

[Space Varmint]

An incandescent light bulb costs $1.00. A little transistor costs $.11.
The little transistor driving the big incandescent light bulb is shorted but the light bult is still shining.

lol. You don't use a transistor. You just take your RG-58 cable and screw a PL-259 right into the light bulb socket and key down.

Now if your talking about the final output transistor, try more like 25 dollars or more.

 

[kchriste]

Even if you are using dual shielded coax, high quality N connectors, dummy loads, shielded enclosures, etc, you'll still pick up the transmitter on a decent receiver if they are tuned to the same frequency. Unless you are running full duplex, what you need to do is mute the receiver with the PTT signal from the transmitter.

 

[Space Varmint]

"Can we make this thread even longer?"
I, for one, really enjoy SV's building threads.
I'm glad he is sharing this adventure with us.

You know flat5. Home brewing is just the funnest thing in the world! I started out building little direct conversion receivers using headphones on CW. Then I built some CW transmitters. If anybody wants some good hands on experience then I would pursue my ham radio ticket. It gives you license to experiment on the air and will really hone your analog and digital skills if you have the patience to add some good bells and whistles.

I was kinda lucky, that right when I started getting into some home brewing, I met this guy on 2 meters who did it all the time. We became good friends and he taught me allot. This guy would spend a weekend and stay up all night and build a complete QRP transceiver and take it camping. He said he contacted Russia with 1 watt camping out on top of some mountain in the state of Georgia. He was cool. We would go to lunch and he would draw block diagrams on napkins the whole time we were there. I would have like 12 napkins to bring home each time...lol.

 

[Space Varmint]

Even if you are using dual shielded coax, high quality N connectors, dummy loads, shielded enclosures, etc, you'll still pick up the transmitter on a decent receiver if they are tuned to the same frequency. Unless you are running full duplex, what you need to do is mute the receiver with the PTT signal from the transmitter.

Yeah, that's not the problem. The problem is rf getting back to some lower power stages in the transmitter. It could be my PLL oscillator which will cause my frequency to shift as the power goes up & down. It might me getting into the mosulator which is a complete disaster. I kinda don't trust the inter-linking cables between sections either. I'm using RG-174U which is real thin coax. It may not have enough shielding. I know I have some unshielded connectors I need to re-do. But basically, I get rf from the 80 watter getting into somewhere it shouldn't be and causing my signal to sound distorted.

 

[kchriste]

Like flat5 mentioned, "shield the heck out of everything!"
The RF could still be getting in through the MIC or somewhere else. If it was getting into the PLL you'd notice a lot of FM happening as well I would think. Do you have a frequency counter? RG-174U is good for between PCBs and will handle 80 Watts at 30Mhz with no problem. I wouldn't use it as a 100ft antenna feed line though.

 

[flat5]

You could also decouple stages more generously
Throw a few chokes and .1 caps around the low level circuits.
Can you see with a scope RF on the power supply?
I don't know if that is meaningful when you have an eighty watt
amp next to the scope, what do you think?