On a sunny day (Sat, 13 Jun 2015 18:25:31 +1000) it happened Kevin Foster wrote in :
There are many ways,
1) use a varicap (that is a diode whos capacitane depends on reverse voltage, those exists up to several hundred pF and work in the 0-30 V range, varicaps can be put in parallel too.
2) If the coils have an ferro core move that in or out, that is called permeability tuning, so purely mechanical, old car radios did it.
3) Sometimes, depending on the Q factor needed, you can use a permanet magnet to partly sature an iron core and tune over a big range that way.
4) Moving tap on a coil (purely mechanical).
5) Screw core in and out, rod with knob, purely mechanical.
Not in parallel but try this; in parallel with your coil have two parallel capacitors and place the variable resistor in series with one of those capacitors. That will give a variation in resonant frequency.
Can you tell us the frequencies involved, power levels, how much variation you seek etc?
No.There are gapped ferrite cores with a hole drilled down the centre leg where you can trim the inductance of the assembled core by screwing a ferrite plug into the centre leg and bridging the gap to a controllable extent.
EPCOS may still make them, and Ferroxcube. Here's a data sheet for a Ferroxcube RM5 core
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which you can buy from Farnell. They may also sell the adjusters.
If you're working with cores that can be saturated, you can put an extra winding on the core and run enough DC through it to get the core close to saturation. That'll decrease the inductance and, if you start with a frequency somewhat lower than what you want, the change in inductance will allow you to tune it to the frequency you want.
And you don't have resistors in there reducing your antenna efficiency and possibly going up in smoke when you transmit.
There' variable-geometry schemes that work beyond 500MHz, and while they're not going to be efficient with brass tubing from the hobby shop, they'll probably be better than pots.
Wanting to use a pot is one thing, and having that actually work is quite another. There's a reason that varactor diodes and trim caps exist, after all.
You haven't given us any details as to what you're trying to do, nor why. That makes it difficult for us to help you. If you can crispen up your question some, you'll get more helpful answers.
** Got one of those inside my valve AM tuner as part of a high Q notch filter at 9kHz to eliminate inter carrier whistle.
The pot core adjustor has a very small range, about +/-1% of frequency.
The OP is holding his cards close to his chest but I figure he needs a much bigger range and when he says "variable caps" he means tuning gangs - cos trimmer caps cost less than pots.
All I want to do is demonstrate that I can tune the resonant frequency of a coil with some (any) combination of fixed capacitance and variable resistance.
Meaning a pot and fixed cap, as in an RC tuned circuit. No physical adjustment to the coil itself.
It is something of an experiment, hence the lack of detail in my question.
The reason for using a pot is because a calibrated dial face is needed and the alternative of a large tuning cap is impractical.
A coil has only a self-resonance with its own stray capacitance, you're probably meaning a LRC circuit.
Adjusting the resistive portion of a LRC circuit has a second-order effect on the resonance frequency. The adjustment has a much larger effect on the circuit Q factor (resonance quality.
Forget it.
This has a smell of the crystal-radio set builders. You'll get nothing out of the receiver if you add resistance to the tuned circuit.
There's three solutions, two of which (saturable inductor core and a DC winding, voltage-variable-capacitance diode) have already been covered. The third, is to have a two-section coil, one inside the other, where the inner coil can be rotated to couple or anti-couple to the outer coil. Wire them in series, and you have a knob-tuned inductor,
The capacitors can be any value you want. I'll try to explain in approximate terms how it works, with both capacitors equal value.
When the series resistor is very high then the one capacitor in series with the resistor is effectively out of circuit and you get the resonant frequency (f1) of LC. And good Q.
When the resistor is very low then both capacitors are in circuit and the lower resonant frequency (f2) is L2C or (f1)/sqrt(2). And good Q.
As the resistance is varied the resonant frequency varies between those limits, with Q getting worse. Not nice but might be enough.
Look up "reactance modulator".
Try breadboarding or simulating the circuit to get a feel for it's behavior.
I used to have a radio that was tuned in a similar way. Two quite large circular fan shaped coils that could be moved together by a knob on a shaft attached to one of them.
As Piglet explained, yes, but varying the reactance of either L or C nega tively affects Q. If you can tolerate that (what's are your bandwidth limit s?), you're done, your parts count just goes up a bit.
And you need a knob-and-dial tuning indicator.
And you'll be doing noise testing. What level?
What other constraints? Required tuning range as a function of center fre quency? Will you be working at a fixed center frequency or will you want/ne ed to retune them together later? Bandwidth? Must all of the coils be ident ical (you're forbidden to squish turns, the simplest method as mentioned)? Must all other bits (tank C, interconnect hardware, environmental cover, et c.) be identical too? What's already in place and still in design? Fixed-ph ase or steerable array? Budget? Customer/boss/teacher/somebody's requiremen ts/vision/whims? XMT or RCV or both? Power level? Fiddling time budget now and later per element?
All of these (and more) narrow your options.
Is this to win a bet, or is this just the task that you drew out of a cla ss assignment/job situation?
If it were me, and I absolutely had to produce a knob-and-dial I'd look a t adding a varicap (or gyrator if you wanted to impress) to each antenna ta nk, with its control voltage delivered via a pot. Varicaps would also help a lot if you later want or need to steer via computer...
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