Re: Why are some radios so directional?- FARMERIK
--- In ultralightdx@..., "Pollock,Raphael E" wrote:
> What about the following issues:
> 1. Tight center wound coil versus loose winding around a larger length of the ferrite bar?
Paul S. covered this one.
> 2. Litz versus standard single or multistrand wire for coil winding?
Litz will improve sharpness of tuning, which can make an apparent improvement in nulling, as adjacent-channel stations are slightly more attenuated.
> 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?
There are issues here. John Bryant published an article in our Files section, in which he demonstrates a coupling winding on one end of the ferrite rod. You don't get as much coupling that way; the way to get the most coupling is to wind over the center of the existing coil, and to wind in the same direction. In addition, the coupling winding would ideally match the impedance of whatever's connected to it. If you know the inductance of the internal coil and the characteristics of the external loop (it's inductance) you can get a close match across the band.
But -- many radios will overload with that much coupling! If you're DXing weak signals and there aren't any flamethrowers on adjacent channels, you might pull it off, but as you can't easily relocate the coupling winding, it may be better to offset it. My Zenith 705 went nuts when I tried this, and an older Panasonic portable doesn't even like getting close to the Terk. These radios were designed for the antennas they came with.
It takes a LOT of signal to overload the DSP Tecsuns. In my experiments coupling an impedance-matched loop to a resonant coil, I've been winding the coupling coil right over the center of the resonant coil. I wrap one layer of thin, narrow electrical tape (not the good 3M stuff) over the resonant coil first, mainly to protect it as I make modifications. I got a package of different-colored tape at Lowe's so I can color code the transformers for LW and MW. If you're coupling to a random-length wire, wrap the coil around the center of the one on the rod. That requires yanking out the rod, which ruins the Litz wire, and there goes your "Barefoot" classification forever, as you'll have to rewind it.
The PL-380 is very flexible and really benefits from a better antenna. If you're not afraid to do some soldering inside the radio, get some FT114-61 toroids from http://kitsandparts.com/, wind 55 turns (240 uH) of #24 magnet wire (spaced over about 2/3 of the core), and then try coupling whatever antennas suit your fancy, varying the number of turns on the coupling loop for best results. You can put the toroid just above the top edge of the radio, leaving the stock ferrite rod in place, and not have to drill any holes in the radio.