1.) This property involves the "Q" or figure of merit of the antenna. For example using solid wire tightly wound on a ferrite rod has low "Q". Adjacent channels will also be received because the bandwidth of signal reception is reletively large for the purpose of receiving one channel. The bandwidth is large because there is turn-to-turn coupling of the signal, producing a capacitor effect which negates the inductance effect. When the turns are spaced equal to the wire diameter, the effect is reduced. When winding loops with a large diameter and short length, the wire must be spaced 2, 3, or even 4 times the wire diameter to absolutely minimize the effect. Side note: the diameter of a coil also has an effect, and adding in the wire coupling effect makes for low "Q".
2.) Litz wire vs. solid wire is an expensive small improvement. The improvement is that the Litz wire has a very low rf resistance at larger wire diameters. For the AM-BCB we are talking #18 wire size or better, else the cost outweighs the results. Typical is 660/46 Litz, with a very low rf resistance versus #18 solid Copper with a large rf-resistance- due to the skin effect of the large perimeter of the wire. Litz has many, many small wires with near-ideal perimeter. Each strand is inductively coupled by the weaving and bundling of the numerous wires. The trade off is in the DC resistance. Litz has a high DC ohm, and solid wire a low DC ohm. However "adding" the AC and DC resistances together we find that the overall lower general or total resistance belongs to Litz wire: the reduction of rf resistance is larger than the increase in DC resistance. Solid copper gets very rf resistive due having the signal presented to a large area, AND without significant penetration of the wire. Litz wire has each strand nearly matched to twice the rf signal penetration: thus for signal, it behaves very well for the intended frequency range.
I will have to leave Q #3 to someone else.
Paul S. in CT
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--- In ultralightdx@..., "Pollock,Raphael E" <rpollock@...> wrote:
What about the following issues:
1. Tight center wound coil versus loose winding around a larger length of the ferrite bar?
2. Litz versus standard single or multistrand wire for coil winding?
3. If creating a secondary coil winding for direct coupling purposes where is the best place to position it: directly around the primary tuned coil winding or off to the side, and if the latter, how far away from the primary winding?
Thanks for sharing your expertise!!
Raph Pollock
________________________________
From: ultralightdx@... <ultralightdx@...>
To: ultralightdx@... <ultralightdx@...>
Sent: Sun Apr 11 13:26:54 2010
Subject: [ultralightdx] Re: Why are some radios so directional?- FARMERIK
I'm not an engineer, nor even an Amateur, but I can start the answer in a simple fashion.
Essentially, the ratio of the ferrite rod length compared to its diameter determines the nulling quality. The larger the ratio, the better the nulling. A second minor factor is the length of the coil winding compared to the length of the ferrite rod. The effect is small, and generally a winding length of 1/3 to 1/2 of the ferrite rod length improves nulling a small bit by keeping the coil away from the ferrite rod cylinder-ends.
Sometimes one encounters a non-cylinder shape that is wider than thick. One can presume the circumference of the shape as an equivilent circle. Extreme cases exist, like so-called "Antenna Strips": some have widths 5 or 6 times the thickness. The reasoning is to present a large surface area to the signal received (a so-called apparent diameter). An easy test is to hold the radio antenna vertically, and tune in the station. The cylinder will not receive well, if at all. The ferrite antenna strip can be tuned due to the wide width facing perpendicular to the station.
When coupled with good rf circuitry, the front-end of a radio is greatly improved, preserving sensitivity, with a low noise-floor (that enhances the nulling effect).
Paul S. in CT
--- In ultralightdx@...<mailto:ultralightdx%40yahoogroups.com>, "farmerik" <farmerik@> wrote:
Can any of the engineers here explain why some radios are VERY directional using the internal ferrite loops?
With my PL-600 I can easily separate stations in different directions by only a fairly small number of degrees, perhaps 15 or 20, The 310 seems good at nulling stations too, and the 911 series are supposed to be very good at that also, as well as a few others. Many other radios cannot do this. I can't completely null out a station that close in direction, but I can make a target much louder than a pest, and switch back and forth between them. What ever causes this would be very good to incorporate in any external antenna designs, if possible. - FARMERIK
