On 10/20/2015 9:17 PM, amdx wrote:
On 10/20/2015 3:03 AM, rickman wrote:
On 10/19/2015 7:55 PM, amdx wrote:
On 10/19/2015 2:14 PM, rickman wrote:
To be a bit simplistic, the amount of signal captured is proportional
to the loop area; the number of turns has little to no effect on that.
I'm pretty sure that is not correct. The signal strength is
proportional to the number of turns *and* the loop area. I will
have to
dig out my notes on this, but some factors (like Q) even out with
various changes in antenna parameters such as number of turns, loop
size, etc. But signal strength is proportional to the area of the loop
and the number of turns.
From
http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm
E = 2pi w S µR e / λ
λ is the wavelength in meters
w - the number of ML turns;
S – is the area of the windings in m2;
μR is the effective magnetic permeability of the ferrite rod SML.
μR is
always less than the permeability of the material used and depends from
the size, geometry and the way the windings are constructed. μR = 1 for
aerial loops.
The product:
А = w μR S (3)
is called effective area of the SML.
Correct me if I'm wrong,
A 1 meter square loop with 5 turns would equal 5 square meters.
A = 5 sq. meters.
A 2.23 meter x 2.23 meter 1 turn loop would equal 5 square meters.
A = 5 sq. meters.
A 5 meter x 5 meter 1 turn loop with a series inductor would equal 25
sq. meters.
A = 25 Sq. meters.
A 5 times increase in A (S) means about a 7db increase in signal
strength. (minus losses caused by series inductor)
Does that all seem right?
I forgot to include the following definitions.
Е – is the voltage between antenna terminals in uV;
е – is the intensity of electromagnetic wave in uV/m.
Not sure where you are going with this. For the purpose of calculating
the received signal strength of an antenna without factoring in
resonance, the area is just the area of one loop (S = pi r^2), not the
loop area times the number of turns. The number of turns (w) is
multiplied by the loop area in the formula along with the relative
permeability of the core material to get the effective area. Is that
what you mean? The post that Jim made explicitly stated, "the number of
turns has little to no effect on that", with "that" meaning "the amount
of signal captured", or E in the above formula. That is the point I was
correcting.
So why do you feel the need to include a series inductor with the 25 m^2
1 turn loop?
I don't know what the inductance of a 1 turn 25 m^2 loop is, but I think
it would need a very large variable capacitor to tune it.
(Gut feeling) Just want to keep it under 1200pf. Because I have that
size variable inductor.
That's not the question. I'm asking why you think this antenna needs an
inductor and the other two don't. I'm guessing this is the only
configuration you are considering. I'm not sure how practical a 5 meter
tall loop will be if you are really serious about building it. If you
make it from copper pipe it will be not only large, but heavy and
require a lot of support to be used outside in winds.
The capacitance needed will depend on the frequency you wish to tune. A
round 5 meter single loop will be 29.5 uH. At 1 MHz it will require
somewhat less than 1 nF if I've done the math right. I've got this in a
spread sheet, but I've never verified it is set up correctly.
If you want to work at lower frequencies you can use a smaller antenna
radius and more turns which will increase the inductance letting you use
a smaller cap to tune it. L ∝ r * N² Cut the radius by X, increase the
number of turns by X and the inductance increases by X. Signal strength
will only go down by a small amount related to the ln().
--
Rick