Inductance in parallel

I'm jst assuming that you have a computer with Internat access right in front of you, so...

Googling "parallel inductance" delivers "about 1, 030,000" hits, at least some of which are good.

Correct

Correct.

Correct

It sounds like you're trying to make a long wave coil cover medium wave.

A simpler way to get the lower inductance might be to split the long wave coil in half i.e. tap it half way down; but you could add parallel inductance instead. The only problem with this is that inductors are lossy and it will lower the Q. You need to use a good quality parallel coil, and screen it, or, if your other coil is a ferrite rod antenna, mount it in such a way that it reinforces the signal in the main coil.

The short answer is yes, paralleling two equal inductors halves the inductance.

Just be careful that any magnetic fields from the two inductors dont interact, else the resultant inductance may be considerably different, and dependent on the physical arrangement.

This program can help your do the calculations:

This is corrent.

If your work without cores (i.e. air coils) it is more difficult:

Generally, yes, as others have posted, inductances in parallel combine just as resistances: L(net) = 1/(1/L1 + 1/L2 + ... + 1/Ln) As you noted, the inductances must not share magnetic field. If they do, things get more complicated, but you can calculate the effects, or just measure them. In fact, it's under the condition that the turns completely share the magnetic field that the inductance goes as the square of the number of turns. That is far from the case with typical solenoid RF coils. For example, a 1" diameter coil of 25 turns, 1" long, will have an inductance of about 11.2uH. A 2" long coil of 50 turns (two of the original end-to-end) will have an inductance of about

One thing that others have not pointed out, that I've seen so far anyway, is that large coils can have self-resonances not far above their operating range. You will need to be careful that those resonances don't mess with operation of your circuit.

If there is any way you could switch the original inductance out and switch a good single one in its place, instead of paralleling one, I think you stand a better chance of being happy with the performance. There are also other issues like LC ratio (operating impedance level) that probably won't be optimum either, and limiting the "detractors" as much as you can would be a good idea.

Remember that the resonance is 2*pi*f = 1/sqrt(LC). If the capacitance range stays the same, you need to make the inductance 1/4 as large as it is now to double the frequency. To triple the frequency, you'll need to make the inductance 1/9 as large is it is now. That would mean paralleling the original with 1/3rd its inductance, or 1/8th its inductance, respectively.

Cheers, Tom

Tom Bruhns wrote: in part ..............

. Thanks to all for all the adviceand the caveats also. As somebody asked; yes I am thinking of gettiing a low frequency recevier to 'move up' into the broadcast band without tearing the guts out of it. Possibly by just tagging some extra inductance in parallel with three existing and themselves fully shielded range coils (it's a TRF). At least I could remember some of the basic formulae without even opening a text book! The practicality of your advice is much appreciated. The possibilty of reducing inductance to one quarter of a single inductor and then along with various stray capacitances wondering about interstage coupling via the existing arrangment and maybe ending with very low Qs and loss of gain? sounded somewhat finicky. Thankyou. Terry.

Receivers use variable capacitors to tune LC circuits though some like the WW II ones used tuning inductors which had powdered iron or an iron core in the coil to vary the inductance. The tuning sections not only make things work better but as you can easily tell seperate signals on different wavelengths.

From the formula you wrote above you can see that according to your frequency needs

C= 1/(39.5 * F^2*L)

L= 1/(39.5 * F^2*C)

Depending on what you want to change. Parallel-series inductor has been thoroughly explained by the other guys. You want to know whether to add in series or parallel capacitors ? This page should explain things fast.

Increasing bandwidth is also going to increase your noise as the 2 are directly related. However you can can increase selectivity by cascading LC networks and/or do a whole lot of other things on the different sections of your receiver.

lemonjuice