[Lf] [Fwd: [Lowfer] NDB Coils]

[Andre' Kesteloot]

Lyle Koehler wrote:

> The coils you describe are probably efficient (I don't know about the
> losses in the fiberglass cores), but don't have much inductance by
> LowFER standards. A single layer solenoid of #10 wire, close wound on a
> 3 inch diameter by 12 inch long coil, would have about 9 turns per inch
> for a total of 108 turns. The inductance would be about 208 uH. Spacing
> the turns by an amount equal to the wire diameter, which is a good rule
> of thumb for obtaining maximum Q with solid wire, would reduce the
> inductance to about 50 uH. Since it takes 2000 uH or more to resonate a
> typical LowFER antenna, you'd need to connect quite a few of these coils
> in series to make a conventional loading coil.
>
> For those who feel that the wire in the loading coil is part of the 15
> meter limit, I suppose you could build a conventional pi-network output
> circuit into the "transmitter" using one of these coils, and dispense
> with the external loading coil completely. K6STI's coil calculation
> software predicts a series resistance of about 1.6 ohms for a 108-turn
> coil of copper, very slightly less for silver. I'm too lazy and rusty at
> the math to do the calculations, but a quick P-Spice simulation yields
> the following results: With an antenna equivalent circuit consisting of
> a 250 pF capacitor in series with a 20 ohm resistor, and a coil
> inductance of 208 uH, the input capacitor on the pi network would need
> be about 0.22 uF and the output capacitor about 3600 pF to achieve
> resonance in the 180 kHz range, and to provide a 50 ohm load to the
> transmitter output circuit.
>
> For comparison, suppose the inductance is increased to 2900 uH, the
> input capacitor is reduced to 0.01 uF and the output capacitor is
> reduced to 15 pF. With a coil Q of 200 (about 16 ohms series
> resistance), the voltage on the antenna would be about 10 dB higher than
> it would using the 208 uH coil. This second circuit is very similar to a
> typical LowFER installation where a small variable capacitor is used
> across the top of the coil for fine tuning, and a larger capacitor is
> used between the bottom of the coil and ground to provide a closer match
> to 50 ohms (for those who want their transmitters to see 50 ohms). It is
> still actually a pi network, and if you make the transmitter box big
> enough, it will fit inside.
>
> This is a very long way of saying the coils are too small...
>
> les at highnoonfilm.com wrote:
> >
> > I've been given the chance to purchase a couple of surplus Nautel antenna
> > tuning
> > coils salvaged from decommissioned NDB's. They appear to be made of
> > silver wire (#10 gauge) wound over fiberglass cores. The coils are 10-14
> > inches
> > long and 3 inches in diameter. One of them also has a small ferrite variometer
> > in the center that is adjusted using a knob on the side of the cabinet.
> >
>
> --
> Lyle, K0LR
>
> http://www.computerpro.com/~lyle
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