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 Post subject: Homemade variable inductor for transmitter projects
PostPosted: Mar Wed 28, 2018 10:50 pm 
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Location: Yukon, OK USA
I have been making inductors like this for my transmitter projects. Easy to make. The ferrite core I have is 3/8" diameter. Then I wrap and tape a manilla folder strip around it. Then wrap about 1" of enamel coated wire over it and then tape it (not shown) with electrical tape. None of this is critical and you can make whatever values you need. Longer core = more inductance. Core centered in coil = max inductance. More coils of wire = more inductance.

The necessary value for the inductor will change depending on the length of the antenna, the value of the tuning cap if used, the frequency of the signal and how the antenna is placed. The antenna placement can be free standing, run up a wall, laying on the floor or anything else, but it always affects how it tunes. So it is really difficult to calculate the necessary value of the fixed inductor. I find it easier to peak the transmitter with the inductor and then go with whatever value is measured. On one transmitter I just left the inductor inside the case and I can readjust for the antenna as needed.

I have found it better to use as little capacitance as possible when tuning the output. So you need to find the closest value inductor available. Using a variable inductor you don't really need the capacitor unless you use a small one for fine tuning. This really comes in handy when using a frequency synthesizer. Changing frequency requires a change in the inductor. You can either switch select multiple inductors or just go with a variable one.


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Last edited by black85vette on Apr Mon 02, 2018 2:24 pm, edited 1 time in total.
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 Post subject: Re: Homemade variable inductor for transmitter projects
PostPosted: Mar Thu 29, 2018 12:31 am 
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Location: Saskatoon
Good idea. If you can make your inductor adjustment fine enough, then you could peak the antenna with it, and eliminate the parallel capacitor completely. That would give you maximum radiated power.


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 Post subject: Re: Homemade variable inductor for transmitter projects
PostPosted: Mar Thu 29, 2018 1:29 pm 
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Location: Milwaukee,WI
Just using approximate numbers. If a output tank circuit required say a 200uh coil and a 150pf cap for resonance about how much larger would the coil need to be to eliminate the cap? I'm guessing that entering in 0pf in a calculator program would spit out an error.


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 Post subject: Re: Homemade variable inductor for transmitter projects
PostPosted: Mar Thu 29, 2018 2:50 pm 
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Joined: Aug Sun 02, 2015 7:34 pm
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Location: Yukon, OK USA
forumuser wrote:
Just using approximate numbers. If a output tank circuit required say a 200uh coil and a 150pf cap for resonance about how much larger would the coil need to be to eliminate the cap? I'm guessing that entering in 0pf in a calculator program would spit out an error.


Depends on the circuit. In my case the inductor is inserted into an antenna system that is way shorter than the wavelength of the frequency so we are inserting enough inductance to match the antenna. The trimmer is just compensating for a fixed inductor that is not exactly right. If you are talking about an LC tank in either parallel or series you can't increase the inductance to eliminate the capacitor because they are forming a resonant element.

Good question and points out that I lacked context for my post. Here is the schematic for the circuit I have been working with using the inductor. L2 is where I use a variable inductor. C6 is the trimmer that can be eliminated.


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 Post subject: Re: Homemade variable inductor for transmitter projects
PostPosted: Mar Thu 29, 2018 10:44 pm 
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Joined: Aug Sun 02, 2015 7:34 pm
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Location: Yukon, OK USA
This may be a good point for some review. Fortunately Bob Weaver has a website that helped me better grasp the antenna system and how all the factors interact. So rather that my feeble attempt at explaining it, here is a good reference;

http://electronbunker.ca/eb/AntennaMatch.html

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