QRP
900 GRAMS HYDURAULIC TUNED SMALL MAGNETIC LOOP
My
name is Alexandre Grimberg, my call sign is PY1AHD. I
have born in Rio de Janeiro in 1949.
I recieve the PY1XLS call sign on the
year 1978, and in 1987 I became “Classe A”
with the call sign PY1AHD.
I
am a lawyer working on the security electronics business
since 1974 on my own company called Radiocom (www.radiocom.com.br).
I remember when I was young boy that while
all my friends was playing football I was at home developing
all kind of scientific experiments.
Today
after 50 years I am still developing my own toys.
For
the last four years I am designing and field testing small
magnetic loop antennas for QRP HF back pack operation
and the loops became my FT817 best friends.
The
first step to build a magnetic loop is to find out a variable
capacitor. If you have a 50 Pico Farads maximum and a
10 Pico Farads or lower minimum capacitance split or butterfly
capacitor in your hand, you just have the passport to
the amazing small magnetic loop world.
The
small magnetic loop described on the text can tune by
the home made 5 Pico Farads to 50 Pico Farads variable
capacitor any frequency between 10 meters to 20 meters,
and with the parallel association of a 180 Pico Farads
capacitor it covers the 40 meters band.
Working
QRP (less then 10 watts) allows me to design some very
interesting variable capacitors from metal tubes (piston
type) to sliding capacitors made on printed circuit boards.
If
you want to run 100 watts of power you can look for a
big plate spacing split, butterfly or the high isolation
vacuum type capacitors. High isolation capacitors are
expensive, big, heavie and hard to find.
If
you design a 100 watts loop you will need to know the
risks that the RF exposure generated by the high concentration
of RF energy on the loop. So is recommended keep at least
15 meters distance from the loop.
What
is a magnetic loop antenna?
Threre
is a quantity of written information about the small magnetic
loop antenna. I will try to explain to the newcomer in
simple words how it works.
1)
Think on the loop as an RF transformer. The primary, the
excited loop is 1/5 of the main loop. It is constructed
with any kind of self supported wire (can be thick copper
wire, coaxial cable shorting the braid with the inner
wire, etc...). The secondary, the main loop is the conductor
circle closed by the variable tuning capacitor that irradiates
the radio frequency energy. Remember that there is no
electrical contact between the excited loop and the outer
loop.
2)
The magnetic loop antenna is a high “Q” device.
The band width is narrow and the user is constantly requested
to retune the antenna for the lowest SWR transmission
any time there is a frequency changing.
3)
The high “Q” of the magnetic loop works like
a front end tuned filter circuit. It
acts as a pre selector greatly improving the adjacent
channels rejection.
4)
The small magnetic loop can be used on the vertical plane
or on horizontal plane. When you turn the loop on the
vertical plane you can kill sources of electrical noise
and other kind of interferences.
5)
The small magnetic loop works perfectly at low heights,
normally a 1 meter diameter loop works perfectly only
at 1 meter above the soil.
6)
And the last and most important advantages of the small
magnetic loop:
NO COUNTERPOISES.
This
antenna was presented to CQ Magazine readers by Dave Ingram
K4TWJ on his World of Ideas column on the October 2006
issue and translated to Spanish to the December 2006 issue
of Radio CQ Magazine.
The
antenna including the control serynger, the cable and
the connector have a total weight of 900 grams, and when
I have the adjusting serynger on the acrylic support on
my belt results on a weight 200 grams lower.
In
practice I have only 700 grams of antenna on my back pack.
..
..
..
Antenna
weight |
Excited
loop |
The
excited loop is 1/5 of the main loop size. It is constructed
with any kind of self supported wire, can be thick copper
wire, coaxial cable shorting the braid with the inner
wire, etc...
Water
tuned sliding variable capacitor
To
make the sliding variable capacitor I cut two printed
circuit board rectangles. The fixed one measure 180 milimetrs
by 58 millimeters and the sliding one measure 58 millimeters
by 70 millimeters. Do not forget a 10 milimetrs by 10
millimeters extension on the lower right side to of the
sliding printed circuit board that will accommodate a
slot to solder the coaxial loop braid.
..
The
copper over the fixed printed circuit board is over half
of the board.
It
is important to cut a small rectangle measuring 2, 9 centimeters
by 2 centimeters on the right top of the fixed plate to
keep enough room for the right side of the loop conductor
(coaxial cable).
The
size of the coaxial braid from the end of the loop to
the sliding plate slot is 250 millimeters.
The coaxial loop cable
extremes are fixed in place by three plastic ties on each
side on the main base.
.
On
each side of the fixed capacitor plate I have carefully
glued with cyanocrilate (Super Bond) two pieces on plastic”
U” channel guides.
This guide is made for the hobby and modeling market in
California by Plastruct by the reference “90583”.
The
fixed rectangle capacitor plate is glued to the acrylic
base by double faced Scotch tape.
The
acrylic base measures 26 centimeters on each side, 9 centimeters
on the top and 5 centimeters on the smaller side.
The
antenna “boom”
The
antenna “boom” is made on a one inch diameter
water plastic pipe. The tube is 1 meter long and the serynger
fits with little pressure inside of the tube inner wall.
..
I
developed a hydraulic system that can control the moving
plate position resulting on a variation on the resonating
frequency of the antenna between 14 megahertz to 30 megahertz.
..
The
seryringer is a 20 millimeters type and the excursion
is 7 centimeters.
I have made a “U” shaped acrylic support transpased
on my belt trousers that keeps in place the frequency
calibrated 20 milliliters serynger that permits tune the
antenna atached on my backpack.
..
40
meters adaptor
A
180 Pico Farads fixed capacitor is connected to an alligator
clip and lowers the resonating frequency of the antenna
to cover the 40 meters band.
..
..
Keep
in mind that working on 40 meters with this antenna we
can expect only
4% efficiency compared to a 40 meters dipole, but despite
of this supposed low efficiency be prepared for great
surprises
Making
the loop
The
loop is made on RF 52 ohms coaxial cable. The coaxial
cable reference that I use on this type of loops is the
RGC-213. This cable have a 0,25 centimeters diameter inner
copper wire that will keep the magnetic loop self supported
on a perfect circle shape.
The
total extension of the loop circle is 254 centimeters,
resulting on a circle of approximately 80 centimeters
in diameter.
The
coaxial loop cable is short circuited soldering the braid
to the inner copper wire.
On
the left coaxial loop extreme leave 1 centimeter of the
inner copper wire involved by the braid and make a good
solder using a clean 100 watts soldering iron.
On
the right coaxial loop extreme leave 1 centimeter of the
inner copper wire soldered to the braid.
On this side you must leave 25 centimeters of braid on
a semi circle shape to be soldered on the slot of the
moving printed circuit board sliding plate.
These 25 centimeters of braid will allow the free movement
of the capacitor sliding upper plate.
Water
tuned loop schematics
..
What
is the efficiency of this QRP magnetic loop?
According
on G4FGQ Reg Edwards software:
Band |
Efficiency |
Loss
in db (ideal antenna) |
S-Units
|
| |
90.7%
84.5%
73.43%
60.50%
37.80%
4.6%
|
0.4
db
0.7 db
1.3 db
2.2 db
4.2 db
13.3 db |
0.1
0.1
0.2
0.4
0.7
2.2 |
I
hope that with this project I can estimulate the imagination
of the radio amateurs all over the world.
I
would like to say that I have more then 500 Dxs over the
last four years with a FT817 and one of my loops and many
of them with other hfpack stations.
One
of the most interesting was a pedestrian mobile to pedestrian
mobile QSO with G0SBW Tom in England on 17 meters band
registered on the Hall
of Fame of the HFpack group as a world distance record
on this category.
Another
one that impressed me takes place when I was walking on
the sand of the paradisiacal beach of Coroa Vermelha in
Bahia with the 817 and my Traveling
Loop when I reach VR2XMT Charlie in Hong Kong
returning to my call with a 5 by 9 signal report.
e-mail:
py1ahd@ig.com.br
