Re: Dumb Question of the Week: Loop Antennas


Nick Hall-Patch
 

--- In ultralightdx@..., "John H. Bryant" <bjohnorcas@...>
wrote:

Russ,

I was not suggesting connecting the loop to the 2010 through the
outside antenna port but rather replacing the ferrite bar and its
tank coil inside the radio. I no longer have a 2010, but if it has a
single tank coil on the ferrite bar, it "ought" to work like a
charm... since the circuit is expecting a coil of xyz inductance.

That replacement of the internal tank coil was what I was proposing
in my "Dumb Question" note.
Date: Monday, February 23, 2009, 12:02 PM
I've a question which just keeps rattling around in my head:
Back when I started DXing in the early 50s, there were plenty of
pre- and post-War table model radios around with air core loop
antennas built into the rear of the sets. Most often, these loops
were wound spider web-fashion on the hardboard rear partition that
closed off the back of the set. There were some of these antennas
that were wound in solenoid-fashion (like a standard tubular coil)
and glued to the plastic sides of the enclosure, at the rear edge.
In either case, they were moderate-sized air core loop antennas. The
best of these loop antennas had a little trimmer capacitor to peak
the set on a particular frequency.
Each of these sets that I used, was very directional. .. you could
rotate the set (and thus the antenna, of course) and null out
stations quite effectively. My middle school radio buddy Larry did
that with his mom's 5-tube Arvin and heard WBZ Boston from northern
Oklahoma.
***********************
Hi John,

One difference between the All-American 5 and most latter day radios
is that the loop antenna was directly connected to the high impedance
input of the mixer tube. So you had nice sharp tuning, and no
loading down of the loop antenna, which meant solid signal strength.
(Sort of equivalent to the FET amplifiers used with the Sanserino and
NRC loops)

Most solid state radios that use a tuned tank coil on a ferrite bar
have a pickup loop or a tap to a low impedance first stage. I'll
speculate that because AM radio design over the last few decades has
usually been about picking up locals, that the coupling is not
optimized for most effective signal pickup by the tank coil. Generally
the tuning is broader than with the old All American 5 loops. (the
2010's untuned loop is remarkably effective given its design)

I haven't been keeping track I'm afraid, but it would appear the E100
uses a high impedance front end with a parallel tuned tank coil(one
tank coil only, and varying its inductance affects the tuning), so in
theory you should be able to substitute a small air core loop of the
right inductance for its native ferrite loop, but I'm not sure what
would be gained by that, that you're not already getting with the
Slider loop? I suspect you'd be more likely to pick up the E100's
microprocessor noise with a nearby air loop than a ferrite loop does,
but I've been proven wrong before.

Nick

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