Hello
to everybody,
My name is Chavdar Levkov
LZ1AQ and I am new in this forum as a writer.
As a reader I have followed it for a long time.
I have some comments about the discussed topics. It is a long post
so excuse me for that.
1.
Wideband
small loops are working almost in short circuit mode – they are loaded with
amplifier with very low input resistance e.g. 6 ohm.
In order to increase the
short circuit current we must reduce the loop impedance preserving the loop
area. Fat conductor or paralleling loops is the obvious solution. Crossed
parallel connection of coplanar loops
is the most effective way to do that. To reach the same low inductance you
must use very fat conductor which is not practical. This method does not have
any advantages when normal impedance matching is used or in the case of tuned
loops.
2.
The crossed small
loops have the same radiation pattern as a single small loop with one very
important exception – the limitation of 0.1 wavelength perimeter is no longer
valid for crossed loop.
This can be easily checked with NEC model.
So we can use the small loop properties in much wider frequency range.
3.
The effective height
of the wideband
loop (working in short circuit mode) is determined
by the Area/L
ratio.
You will not find any difference in reception between crossed and single loops
if this ratio is the same.
4.
I do not recommend to use multi-turn loops – single turn loop has the highest
efficiency.
I do mean
loops winded as inductor. I do not assume crossed coplanar
loop connection as a multi-turn loop and I do not use this term to
avoid misinterpretations.
5.
The main problem in
active wideband loops is that there is no impedance matching in its classical
form. The induced current is very small and often the system is noise limited
by the internal noise of the amplifier.
Using crossed coplanar loops is a way to avoid this problem.
My experience is that two crossed circular loops with diameter larger than 1 m
with alum. tubing of 14 mm diam.
have sufficient current
and the noise floor
is
limited by the external noise (using common base amplifier as described in my
article). At least up to 10 MHz.
On higher frequencies, in quiet rural locations,
the external noise might become sometimes very low and in this case the system
becomes noise limited by the internal noise.
6.
The closely spaced
phased array
I build with small active loops or dipoles
is actually 2 element array.
3 antennas are used
in order to have 4 directions. Its RDF is as any other 2 element
array with active dipoles(verticals)
and slightly higher RDF for loops.
7.
There is a
difference in polarization sensitivity between
arrays with vertical loops and vertical dipoles. The loop array is highly
directional for vertically polarized (VP)
waves (better RDF than dipole array and is
almost as 4-square for 2el array). This array has zero sensitivity in main
direction for horizontally polarized (HP) waves but
has two HP
side lobes at
+- 45 deg to the main direction.
The dipole array has zero sensitivity for HP waves at all directions. It is a
pure VP antenna. The take-off angle of the dipole array is lower compared
the
loop array.
Practical consequences are that the loop array directivity is different
compared to dipole array.
Very often
with one array I have
high F/B ratio while with the other array it might be low
and vice versa – it depends very much from the propagation. When we have nearly
HP waves
the directivity is minimal for both arrays.
8.
The phased array with active loops has one
advantage compared to
active vertical dipole (or GP) array . In a small yard usually there are too
many parasitic non-resonance conducting objects. A dipole array (with high
input impedance amplifiers)
often loses its directivity probably
due to capacitive interactions with these close
objects . The loops are much more stable.
Both arrays are influenced from nearby resonant antennas
and
measures to de-tune TX antennas must be taken
if the space between
antennas is small.
9.
For those who have an experience with small single loops it must be mentioned
that the loop array is much more quiet compared to a single loop apart from
directivity benefits.
The reason is that a single vertically placed
loop has in its pattern a lobe toward zenith which is eliminated in phased
connection.
10.
About
CAT5,6
cable -You do not need to measure it
- the specifications are widely available. As a balanced line it has certain
advantages which will benefit the noise immunity of the receiving system.
The only questionable parameter is the delay (which is important when building
phased arrays). I have measured the difference in the signal delay between two
30 m
cables
from the same manufacturer. Using the same colored pairs, the difference is
below
1 -2 ns. The accuracy was limited by my measuring equipment.
Usually these cables are marked in factory at each meter and
for the experiment were cut precisely at the corresponding
markings.
For more details you can read my page at www.lz1aq.signacor.com
, also in http://active-antenna.eu/application-notes/
There you can find also
links to other pages with similar topics .
Happy New year to everybody,
73 Chavdar LZ1AQ
lz1aq@abv.bg
P.S.
This message was delayed almost 3 days
since I was
not aware that I must subscribe to this forum in order to write messages.
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