i'm often asked where do i get a good
antenna
or is this a good antenna unfortunately
you do not see if an antenna is good or
bad before you do some measurements
or you build one yourself that's what we
will do today
be it for satellite reception for laura
weather balloons or even
wi-fi like that we know what we have
and even can save some money great
youtubers
here is the guy with a swiss accent with
a new episode and fresh ideas around
sensors and microcontrollers
remember if you subscribe you will
always sit in the first
row generally antennas are simple to
build
they are just a few conductive parts
soldered to a connector
two things however are essential how
these conductive parts are arranged in
relation to each other
and their dimensions there is an
infinite number of antenna designs
available
as this book full of antennas shows
today we will build a classic ground
plane
and answer all questions about its
capabilities
it is very simple to build with tools
available in every maker lab
and you will see how capable it is
i also will use such a nano vna to prove
that these antennas have a decent
performance and we will simulate
their far-field behavior to calculate
their gain
the first question we have to answer
with antennas
what is the needed impedance our antenna
has
50 ohms because most of the maker
equipment
and the cables we used are standardized
on 50 ohms
i suggest that we start with a connector
because most antennas have one these
four varieties are used in
maker labs sma connectors
they are relatively small and suitable
for high frequencies
up to a few gigahertz we find them on
our lora
as well as on most wi-fi boards please
pay attention
they are sold in standard configuration
as well as with
reverse polarity then they are called
sma-rp both
are not interchangeable i standardize on
standard sma connectors in my lab
and only use rp connectors if a device
forces me to use one
like some access points bnc connectors
they are the standard for lab equipment
like oscilloscopes for antennas
they are not used too often n connectors
they are sturdy and also right up to the
gigahertz range
my spectrum analyzer for example has
such connectors
uhf connectors they are mostly called
pl259
for the male plug and so239 for the
female
socket they were already invented in the
1930s
and they are compatible with banana
plugs
pay attention even if they are called
uhf
they are only good below 100 megahertz
they are the standard connectors for hf
amateur radios
which one should we use for our antenna
most of the antennas used by makers are
for frequencies above
100 megahertz so we will use an sma
connector
we could also use an n-type if we want
to build an antenna with a higher power
but for makers this is not allowed only
if you get an amateur radio license
you are allowed to use high power the
ground plane antenna consists of a
vertical radiating element
which is connected to the inner of the
connector and three
or four radials connected to the outer
ground
of the connector these radials form the
ground plane
which gives the antenna its name as said
before
the different wire length is directly
related
to the frequency of the antenna and if
it is wrong
the antenna does not perform well so how
do we find
the right length we go to a site that
calculates the needed values for us
let's assume we want an antenna for
weather balloons
which in my area operate on 403.5
megahertz
if we leave the velocity factor default
on 95 percent
the radiating element is 17.7
centimeters
and each radial is 19.8 centimeters long
the velocity factor corrects for a
reduced speed
of light in wires by the way and can be
left
on 0.95 for copper wires
shall we cut the wire directly to length
what is a good material if we have such
an instrument
we usually cut the wires longer because
cutting is
easier than stretching a wire unless you
own a harley davidson
as shown in video number two one two
without such a device or if we are in a
hurry
we cut it according to the blueprint we
will later see if this is a good idea
for the construction we can use any
straight wire
bare copper wire for example can be used
for smaller antennas
usually it is not entirely straight
which is mainly an optical problem
recently i bought such 3 millimeter
aluminum
welding rods for my longer antennas they
are very sturdy and
cheap you get them also with smaller
diameters
for short antennas a lot of people also
use copper
or aluminum tubes particularly for
bigger antennas
male or female connectors can be used
depending on your preference
i like this version because it has the
needed holes to connect the radials
and it has a short piece of teflon which
separates the radiating element
from the ground teflon is an excellent
material
mainly because it does not melt even if
you up use it
during soldering to get a good fit
between the connector and the rod
i use such threads they are used in 3d
printing
a small copper or brass tube can do the
same job
i had to grind the rod down to a fitting
diameter
this was no problem because aluminum is
not hard
if you use your soldering iron increase
its temperature to the maximum
because the rods need heat to get to the
right temperature
now it already looks like an antenna
cool
but we need a way to attach the radials
i
use such naked terminals from my
assortment
and solder them straight on the rods
here i used a gas burner
for faster success after attaching
all four radials to the connector our
ground plane is
ready simple but is it also working
as i said before you do not see if an
antenna is good or bad
so i connect it to my vector network
analyzer
if you want to know more about this
device and how to use it
feel free to watch video number 359
if you are not interested in measuring
an antenna you can go
straight to the next chapter where you
get the results
next question what is a good antenna
it has to behave like a 50 ohms resistor
to the cable or device attached to it
then it radiates the full power into the
air
otherwise a part of the power is
reflected
and lost here we see the antenna's
impedance
in a smith chart and the reflected power
in an swr curve these curves are also
explained in video number 359
let's start with the radials at 90
degrees
instead of the 45 degrees shown in the
blueprint
here are the measurements the swr curve
has its minimum at around 370 megahertz
but the lowest swr is 2.2
a lot of power is reflected and the
antenna is not optimal
why is that the smith chart shows us
that the antenna is resonant at this
frequency
the point is directly on this line
but unfortunately its resistance is not
50 ohms a 50 ohms resistor sits here
our antenna has a resistance of only
21.9 ohms
this is why a part of the power is
reflected
let's now comply with our drawing and
bend the radials to a 45 degrees angle
the minimum did move 8 megahertz but
the swr became 1.1
much better than before if we look at
the smith chart
we see that the resistance is now 45.5
ohms much closer to the 50 ohms
maybe we could tweak a little more but
because we have to accept
that the surrounding also has some
influence
i stop here you see we created a nearly
perfect antenna
for cheap unfortunately on the wrong
frequency
we wanted 403.5 megahertz
the swr on 403.5
is 2.7 still usable but
as usual on this channel we want more
how can we shift the minimum to 403.5
megahertz
we reduce the length of the radiating
element
after the first cut we are back to 365
megahertz
the length of the radials does not have
a significant
effect so i cut them to the exact length
as requested by the blueprint and
continue with the resonator a few cuts
later
we are at 403.5 megahertz
the swr is 1.07
and the resistance is 51.5 ohms
success in addition the swr
stays way below 2 for the whole weather
balloon band from
401 to 406 megahertz
very good indeed the radiator
is now 17 centimeters long the
calculator said
it should be a little longer but not a
lot
this is the proof that the calculator
provides usable results
and you can build such an antenna even
without a
nano vna you can solder the terminals
directly to the connector
and remove the screws or if you want to
use the antenna outdoors you can add
conformal coating
or color just make sure you cover the
sma connector with tape
before doing so i will not do it now
because of the freezing temperatures
outside
but still i can replace my not so nice
looking antenna
of my weather balloon tracker on the
roof by the way
this simple antenna could receive
signals from weather balloons
more than 200 kilometers away we talked
now about
return loss and resonance what else is
vital for an antenna
its far field pattern and its gain of
course
to find this out we can use simulation
software
you can get costly professional packages
but fortunately
we also get a cheaper solution called
m-m-a-n-a-g-a-l
i modeled our antenna and calculated its
behavior
mounted two meters above the ground
here we see that it evenly radiates in
all directions
this is what we expected because it has
a vertical
radiator but how does it radiate in the
vertical direction
here we see that it does not radiate a
lot
directly upwards the power not radiated
vertically
is radiated in other directions and
creates a gain of 5.3 dbi
at an angle of 5 degrees for example
very good if the other station is at a
similar height
5 dbi is a good gain for a
non-directional antenna
it multiplies the output power of your
device by a factor of 3.
such an antenna even can be used for
satellite traffic
because other than most people think
satellites
hardly ever pass directly overhead
most passes are in this range and here
our antenna does not lose a lot
let's check the dimensions of a ground
plane antenna for other applications
for wi-fi its measurements are 2.8 and
3.1
centimeters still feasible for do it
yourself
and what about an antenna for weather
satellites at 130
megahertz 50 centimeters for the
vertical element
possible with the one meter long
aluminum rods
here i would use an end connector
because it is more stable
and the mounting of the three millimeter
rod is easier
conclusions it is straightforward to
build a decent ground plane antenna for
our desired frequency
they get bigger and bigger with lower
frequencies when i was young i had such
an antenna for 27 megahertz
the single element was nearly 3 meters
you have to choose the material
according to the usage
indoors or outdoors and mechanical
stability
using a vna we can trim our antennas to
a good swr
without such a device we get a good
performance if we stick to the results
of the calculator
ground plane antennas have an excellent
far-field behavior
and are perfect for many applications
one last thing in the intro i showed you
a chapel antenna
which looks completely different this
one is for 868 megahertz
lora this antenna is more complex to
build
and i would not try it without a vna on
hand
but it works and delivers a slightly
different radiating pattern than the
ground plane
maybe even better suited for satellite
reception as always
you find all the relevant links in the
description
i hope this video was useful or at least
interesting for you
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thank you bye