Ferrite cores for 80-110mhz VHF transformers? - diyAudio
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Old 22nd October 2012, 09:03 AM   #1
bigwill is offline bigwill  United Kingdom
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Default Ferrite cores for 80-110mhz VHF transformers?

I am a complete novice with VHF circuitry, it's all a bit mysterious, and sadly beyond the limits of my oscilloscope!

I've been experimenting with little FM transmitter bugs, which is fun - and I've been wanting to increase the power of my circuits. So far I've built what everyone else has probably built at some point - the one transistor VHF oscillator with a 2n3904 or similar, consisting of a tank circuit in the collector and a small capacitor between the collector and emitter for positive feedback for oscillation. Audio is fed to the base which modulates the operating point / transistor capacitance to produce FM.

The output of this oscillator is obviously high impedance and the base of a bigger RF transistor at 100Mhz is probably fairly low - transformer coupling would work well here. I'm also interested in transformer coupling the output stage to the antenna.

I'm probably going about this completely the wrong way, but where on earth do you get ferrite cores or rods suitable for this band?? I know they exist but I can't seem to find anything from the typical outlets, most of the ferrite products are listed for RFI supression, and I have no idea how that correlates with their usefulness as a transformer core. I'm guessing most of these materials will result in a uselessly low Q.

I'd happily wind an air-core transformer but I can't seem to find anything about this either!
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Old 22nd October 2012, 11:23 AM   #2
DUG is offline DUG  Canada
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Palomar Engineers | Amateur Radio

good place to start
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"You can't stop the signal, Mal. Everything goes somewhere..."
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Old 22nd October 2012, 11:26 AM   #3
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Cable TV 75ohm to 300ohm balnenced adapter have suitable cores in them as does many older set that have both 300ohm and 75ohm inputs usualy have such a transformer mounted either in a little PC board or in a little metal box.
are some good sources.

Type 43 is what you want to look for as that is what the above are typically made of as well as a few others.

A lot of times an air core transformer is used in such circuits as well.

Here are a few links that may help you,

Broadband Baluns

Inductor Calculators

ferrite cores for vhf - Google Search

Cheers !!

jer
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Old 22nd October 2012, 12:22 PM   #4
dmills is offline dmills  United Kingdom
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Type 43 or 61 for those frequencies, probably wound on a FT37 or FT50 toroid.
KitsandParts in the US do a wide range of RF magnetic components.

Personally, I would probably go for a source follower after the oscilator to provide some buffering before your amplifier stage.

You might wish to look into an 'L Match' as a relatively narrow band way to match the output of your amplifier to the aerial system, a small air cored coil and two trimmer capacitors is the usual approach.

Regards, Dan.
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Old 22nd October 2012, 12:36 PM   #5
bigwill is offline bigwill  United Kingdom
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Thanks everyone

Will type 43 really work? I can honestly say magnetics aren't my strong point - I keep seeing references to it only being useful in the low VHF range - but perhaps in a different application? RS stocks toroids in this material though, and I'll happily use it if it works.

Or, as dmills says, use a source follower. I'm guessing some kind of JFET like an MPF102 will work? How anything works at all at 100mhz is still a mystery to me!
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Old 22nd October 2012, 12:37 PM   #6
DF96 is offline DF96  England
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Keep the power down otherwise one of your neighbours might complain to OfCom. You could find all your electronic items confiscated, not just the oscillator.
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Old 22nd October 2012, 01:35 PM   #7
bigwill is offline bigwill  United Kingdom
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Join Date: Dec 2004
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Quote:
Originally Posted by DF96 View Post
Keep the power down otherwise one of your neighbours might complain to OfCom. You could find all your electronic items confiscated, not just the oscillator.
Luckily I don't really have any neighbours I really don't need much power anyway, but it's interesting to experiment.
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