Re: Why are some radios so directional?- FARMERIK


Jim - The 705 is a nice little Zenith, but I think you would really like a 790 Navigator MUCH better. Look here-

You might want to try the LW and old MB too. They are more directional than most Zenith portables, [no surprise] and more sensitive than any other model I have, including the InterOceanics with more IF stages. The defeat-able AVC, and manual RF gain is nice for DXing too, and the audio is great on an efficient Hi-fidelity speaker. - FARMERIK

--- In ultralightdx@..., "jim_kr1s" <jkearman@...> wrote:

--- In ultralightdx@..., "Pollock,Raphael E" <rpollock@>

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

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

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
<> , 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.


Jim, KR1S <>

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