Re: How To Calculate Ferrite Loopstick Inductance Values for Ultralites? Part 2

ferrite61 <dxrx@...>

The other part of your question is quite difficult. One inductor generally does not do both AM-Band and the Beacon-Band *well*. There are compromises. This is due to the frequency range of both bands. AM is 520-1710kHz and Beacons are 200-500kHz. Even lower in frequency is the International Long-wave band at 150-270kHz.

Because these other frequencies are lower, a larger-valued inductor is needed. As noted Gary's Long-Wave inductor is 1700uH, about 5 times the value of an AM-band inductor. This requires great skill in producing one. Many turns are needed using generally thin wire. Thus the ferrite length is also large. About the only compromise is to use a stronger ferrite with a larger diameter. The problem is that these stronger ferrites are also conductive, and one must insulate the coil from the ferrite "just in case", and as a matter of good practice.

These qualities have led to the use of "ferrite sleeves" or tubes with a large diameter, but rather thin ferrite thickness. Again, the sleeve *must* be insulated from the wire.

What is good? Well, for Beacons, 1800-2500 uH is ok. For International Long-wave 3000-4000 uH. As a compromise involving physical size and usually $100 or so, a multi-ganged variable capacitor (four sections of 360-400pF) and be wired up. Now the tuning range is extended by capacitance: not by inductance. Doing this also involves using a *reducer* so that the 1/2 rotation of the variable is increased to 3 turns. I would recomment 2 of these for 18 turns: the slightest movement will pass a station (or beacon) or two!

Paul S. in CT

--- In ultralightdx@..., "ferrite61" <dxrx@...> wrote:

Since you already have a reasonable estimate of the inductance, the tuning capacitance is based upon that. For about 680 uH (0.681 mH) tuning to the bottom of the AM-band is 138 pF, with the top end having a lower capacitance of about 12.5 pF. That range is too low (meaning the inductance is too high). Reducing the inductance is accomplished by removing turns of wire from the ferrite bar/rod. Without a general formula, its a trial-and-error process. But a general rule of thumb can be used.

Since you have 80 turns and the measured inductance is 680uH, we can scale-down the inductance by ratio of the square of the turns. For example a new inductance can be estimated by 60x60 / 80x80. Thats 3600 / 6400 times the 680uH giving an estimate of 382.5uH. This example is quite suitable in UL-Radios, especially the new DSP sets.

On the other hand if you need more inductance adding turns is needed and an example of 40 turns with 150 uH would solve as 60x60 / 40x40 times the 150uH. 3600 / 1600 x 150 is 337.5uH. Again this answer is suitable.

In general, most newer sets use "the reverse of the original specification" being 250uH and 360pF. Today, for purposes of better low-end reception, most manufacturers like 360uH with 250pF. Some are even 470uH and 200pF.

So now that's done, the quality of the inductor becomes important. Centering the turns on the bar/rod is important, and also spacing the turns of litz a little will help. Try to keep the length of all turns less than 2/3 of the ferrite length... optimal is 2/5 of the length. Fine-points include insulating the wire from the ferrite by using a layer or two of heat-shrink tubing. The object is to get the sharp tuning in the tuner, because the filtering (IF) stages will reduce the sharpness... you may hear an adjacent strong station on most UL-Radios.

Paul S. in CT

--- In ultralightdx@..., "jerry_popiel" <jerry_popiel@> wrote:

This question is likely for the Experts. How do you know what Inductance values are acceptable for MW and LW Listening? That is, I have an older 5.5 inch Ferrite Bar Antenna with approx 80 turns of Litz wire on it, that I am using to couple to a PL 380. It measures 0.671 mH on my DM 4070 LCR Meter. Is that Inductance Value acceptable, or what is / or how do I know what is the required value?

Or does this depend on the length and diameter of the Ferrite Bar - such as the 122 turns and 1700 uH of 100/44 Litz wire that Gary DeBock has said is required for his 7.5 Ferrite Rod Antenna for the Pl 380? Any replies would be very much appreciated


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