digging through an old radio manuals from the 20's and 30's...
Can a champ circuit be used as a simple low power modulator?
What precautions should a DIY use when stretching in to RF?
RF on the bench
Moderators: pompeiisneaks, Colossal
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Andy Le Blanc
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- Location: central Maine
RF on the bench
lazymaryamps
Re: RF on the bench
What are you tryin' to do? Flee power AM?
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Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
Re: RF on the bench
Trying not to exceed flea power AM, outside the work shop.
Looks like what I can scab together, even crudely, can be an issue.
Looks like what I can scab together, even crudely, can be an issue.
lazymaryamps
Re: RF on the bench
Still not graspin' what you're tryin' to do ...... 
If things haven't changed in the last years, I think the FCC limit for free transmission is 100 mW of RF power. If you're tryin' to mimic a classical AM transmitter by having a separate modulation amp transformer modulating the DC power feed of the carrier amplifier, you need about 50 mW of audio power to stay inside legal limits......a single 12AU7 should do.......
if you'd care to be somewhat more specific, maybe I could give you some hints.... as it happens, I've been fiddlin' with RF for close to 40 years... roughly from DC to microwaves...
( and yes - I have been involved in RF transmissions of 1-2 kHz carriers at multi-kW levels.................)
If things haven't changed in the last years, I think the FCC limit for free transmission is 100 mW of RF power. If you're tryin' to mimic a classical AM transmitter by having a separate modulation amp transformer modulating the DC power feed of the carrier amplifier, you need about 50 mW of audio power to stay inside legal limits......a single 12AU7 should do.......
if you'd care to be somewhat more specific, maybe I could give you some hints.... as it happens, I've been fiddlin' with RF for close to 40 years... roughly from DC to microwaves...
( and yes - I have been involved in RF transmissions of 1-2 kHz carriers at multi-kW levels.................)
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Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
Re: RF on the bench
I've been looking at old pre war xmitter builds.
but even with simple two and three tube modulated sets, the anticipated
power output far exceeds imposed limitations.
looking for ways to play with it (build and test real circuits) on the bench without running afoul of legal issues.
I don't want to step on anybody, or pirate either, but I've got references from
the earliest transition to tubes, and very early radio I've been dying to explore.
Whats the etiquette
but even with simple two and three tube modulated sets, the anticipated
power output far exceeds imposed limitations.
looking for ways to play with it (build and test real circuits) on the bench without running afoul of legal issues.
I don't want to step on anybody, or pirate either, but I've got references from
the earliest transition to tubes, and very early radio I've been dying to explore.
Whats the etiquette
lazymaryamps
Re: RF on the bench
That's about what I thought.....
well - as find in the books, you'll need a stable oscillator circuit at the carrier freq, - then a good follower to isolate the oscillator.
Then if you transformer modulate a low gain stage - a 12AT7 at a fairly low B+ should do, you should be roughly within legal limits.
You will of course need some filters along the signal path to get rid of harmonics...
If there seems to be to much power, you can always dissipate it in a screened dummy load. It's not the final power level that counts, - it's how much you radiate.
An early 60s copy of ARRLs "Radio Amateur Handbook" will also give you lots of practiacl tips and ready formulaes for a lot of the stuff you need, like the oscillators, filters etc.
Actually, the old RAH's are quite packed with good, practical info for tube builders.
well - as find in the books, you'll need a stable oscillator circuit at the carrier freq, - then a good follower to isolate the oscillator.
Then if you transformer modulate a low gain stage - a 12AT7 at a fairly low B+ should do, you should be roughly within legal limits.
You will of course need some filters along the signal path to get rid of harmonics...
If there seems to be to much power, you can always dissipate it in a screened dummy load. It's not the final power level that counts, - it's how much you radiate.
An early 60s copy of ARRLs "Radio Amateur Handbook" will also give you lots of practiacl tips and ready formulaes for a lot of the stuff you need, like the oscillators, filters etc.
Actually, the old RAH's are quite packed with good, practical info for tube builders.
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Cliff Schecht
- Posts: 2629
- Joined: Wed Dec 30, 2009 7:32 am
- Location: Austin
- Contact:
Re: RF on the bench
I like the Armstrong regenerative receiver. In a single circuit he produces a high gain, high selectivity and high sensitivity device with a single shitty triode. To boot he proves the existence of controlled positive feedback as well the idea of non-linear gain-bandwidth product. Pretty neat stuff!
Cliff Schecht - Circuit P.I.
Re: RF on the bench
The problem with most regen RX designs, is that they usually have very high self emission......
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Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
Re: RF on the bench
Appreciate the replies...
Just come down off the roof, cleaning the chimney and stove pipe.
had some sun to melt off the access to get up there to do the chore.
digging around today I found a few gauges of wire to wind some coils,
found an old ganged tuner rotor and stator to play with.
Looks like an oscillator and modulator aren't too bad to build.
both can be simple and practical
It's finding a suitable RF tube to build an amp around and figuring the modulation method that works for whats in the bone pile...
A modulation tranny might be tuff, but it looks like I can try a couple methods
theres "Heising" plate modulation, and "series cathode modulation",
Going even simpler, a bare bones hartley around a 12au7 looks very practical.
No RF amp there, modulate with simple wire loop on the grid circuit.
one air coil and about 5 parts for a BOM.
I have been very impressed with just how simple it can be.
so a very high self emission... I should scab together a field meter too.
The grid was a new thing once, the Armstrong "repeater", RF is repeated
back into the grid, and as soon as you "amplify" you have a practical range, and sharper reception.
compared to the massive scale that "American Marconi Co." spark gap needed, it was huge leap.
Just come down off the roof, cleaning the chimney and stove pipe.
had some sun to melt off the access to get up there to do the chore.
digging around today I found a few gauges of wire to wind some coils,
found an old ganged tuner rotor and stator to play with.
Looks like an oscillator and modulator aren't too bad to build.
both can be simple and practical
It's finding a suitable RF tube to build an amp around and figuring the modulation method that works for whats in the bone pile...
A modulation tranny might be tuff, but it looks like I can try a couple methods
theres "Heising" plate modulation, and "series cathode modulation",
Going even simpler, a bare bones hartley around a 12au7 looks very practical.
No RF amp there, modulate with simple wire loop on the grid circuit.
one air coil and about 5 parts for a BOM.
I have been very impressed with just how simple it can be.
so a very high self emission... I should scab together a field meter too.
The grid was a new thing once, the Armstrong "repeater", RF is repeated
back into the grid, and as soon as you "amplify" you have a practical range, and sharper reception.
compared to the massive scale that "American Marconi Co." spark gap needed, it was huge leap.
lazymaryamps