IB Coils

[Rick S]

Carl,
The problem I have been running into is the coils I reproduce are either a exact match with Heneries, but low with resistance or exact match with resistance and low with Heneries. The Rx coil has no components with it, just the coil. Tx has coil, parallel resistance and capacitance. How would you go about adjusting the reproduced coils to match? I have study the L network which is close to what is needed. The Chebyshev networks. This one is way to much for what is needed, but has nice tables that you plug in numbers for the resonance and frequency wanted. Main question in the design of the coil(s) do you make them below the measured coil, then add components to match or do you look at it from another direction? I have tried using the scope and matching the frequency and resonance, but there is alot of trial and error in this process. I would like to get closer with math than guessing with components. I have read that capacitance is the way to go for resonance,frequency and impedance matching. Any help would be appreciated. Rick S

[Carl]

Re: IB Coils

The coils are basically the "L" component in an "LCR" tank circuit. On the TX side, the LCR tank is part of a Colpitts oscillator (usually). On the RX side the LCR tank feeds some sort of preamp stage.

The center frequency is set by the "LC" product, so it's important to get the inductance correct for the amount of capacitance you have, or to adjust the capacitance if the L is not right. The only way to adjust the capacitance (assuming you don't want to open up the control box) is to add some across the coil, which means you can only adjust for a lower-than-needed inductance. So try to keep the inductance less than or equal to what you need.

The resistance of the coil helps determine the "Q", or quality factor, of the tank. Basically, this is equal to resonant frequency divided by bandwidth, so a higher Q (less resistance) equates to less bandwidth. This is good in that it make the TX frequency "tighter" and gives lower RX noise. It's bad in that it makes the TX and RX more sensitive to frequency alignment.

So, what I would try is to get the inductance as close as possible, tweak the coils with a cap if necessary, and don't worry about the resistance.

- Carl

[Rick S]

Re: IB Coils

Carl,
Thanks for the info. Does impedance come into play here? What is the easiest way to get the impedance match? Is it the inductance and capacitance matching of the tank circuit that makes the impedance work. Also, is the frequency divided by Q equals the band wideth? Is there another way that is simple to determine the band wideth? Many thanks, Rick S

[Carl]

Re: IB Coils

Impedance is the combination of the real resistance and the reactive inductance & capacitance. In the case of IB detector coils, we're mostly interested in getting the reactive part right because this is what sets the resonance point, and the resistance has secondary effects of setting the bandwidth and maybe affecting coil current. Yes, resonance frequency divided by Q equals bandwidth.

I still think the easiest way to build a coil is to first roughly measure the resonant point of an original coil, which can be done with a frequency generator and either an oscope or a DMM if the DMM has enough bandwidth (probably not). You can also measure BW with this setup. You should measure the coil capacitance (they often have capacitors in the coil), and knowing the capacitance and resonant frequency you can then calculate the inductance as being L=1/[(2*PI*f)^2 * C] where PI=3.1416. Wind a new coil to achieve the same rough inductance (trial & error), maybe on the low side, then add capacitance to make it resonate at the right frequency.

- Carl