Welcome guys My name's Callum from DX
Commander
now this video is part of a playlist
because it's all part of building the
nebula class antenna
and should i build it with five eights
or quarter waves
now because when i model and i've
already done all the modeling for you
not today's film today's video is about
the difference between a quarter wave
and a 5 8 and a three quarter wave
actually and where all the resonant
points are okay
because if you take a quarter wave for
seven
megahertz as a 40 where it's a 40 meter
band
it's about quarter waves 10 meters give
or take it's a bit longer actually
it'll resonate again as a three quarter
wave on three times that so three seven
to twenty one
which is the 15 meter band twenty one
point something
okay so we'll take a baseline
of uh something really easy
and what i'm thinking is let's build
this for 30 megahertz which is just
outside
and no more the 10 meter band but
because the speed of light is 300
000 kilometers a second and
we've got 30 million cycles per second
30 million hertz he divided two together
300 divided by 30 is 10 10 meter band
right so
there we are i'm using a piece of
software here
called mmana all right this is it here
um you can follow along you can download
this as well
mmana just do a google search eventually
you'll find it
so what i'm doing to do here is put in
30 megahertz
in fact i'm going to put it at 29.5
for some maths numbers that you don't
need to be worried about but just it's
10 meters all right z2 is
2.5 meters so that's a quarter wave
of the 10 meter band all right and we're
going to feed it with coax at the bottom
wire one there's only one wire at the
base at the bottom
so if we go to the view tab there it is
there hold the control key down and pull
down
and the little red dot at the bottom is
the feed point so we've got the center
of the coax going up to our tree
effectively and then we've got the braid
of the coax with a
chop block or something and we've got a
few radials scattered
on the ground okay quarter wave easy
so we go to calculate and we'll use some
wire
all right bugger all difference between
using that wire or something else but
we'll just do it because there is a tiny
difference
and we'll have a ground setup of let's
have i don't know
16 that says 15 now 16
quarter waves radials just for the maths
okay start so it's a quarter wave now
and if we go to our we can just check
out swr
where it is resident it says it's
resonant
29ish 29.1 or something
we're only going to do this once because
we're going to get onto three quarter
waves in a minute
so it's our best swr we've got an
impedance of about 36
37 ohms now in the antenna books
that's right it is about 36 ohms which
means there's always a bit of loss on a
quarter wave if you
get a perfect match and the more radials
you put down
the more transmission line loss you've
got because
you're pushing it down and down and down
if you put
less radials down four or eight your
impedance will be nearly 50 ohms
but whatever you don't need to know
about that because honestly in swr of
1.3
1.4 to 1 it says it here
you just need not worry about it okay
so if we look at the far field plot now
this is
on the left we've got the plan view so
if we're flying over the top of our
aerial
we're using our special rf glasses and
we're a bumble bee and we can just hover
there
we'll just see this big round
doughnut and the center the very center
of that there'll be a next to no rf
coming off it
because if we go to the right hand side
we've now cut our cake down the middle
into our elevation view
standing on the sidewalk the pavement
looking our antenna
we take a big knife through the middle
and we can literally right through the
middle
and we can see this is what it looks
like now we can measure this as well
interesting stuff and i'm interested in
the second number down where it says 1.1
at the moment
and we're going to drift down we're
going to come to 175
which is 5 degrees above the horizon
so somewhere between three and eight is
where all the dx is
and a minus six sounds a bit crappy
but no manufacturer is going to tell you
what their antenna
actually does at five degrees or horizon
if i was selling this
i'd be saying oh we got peak gain at 1.1
or 1.2 or i'd measure this
next to the c where it's even better i'm
not going to tell them at what angle is
okay this is minus six so the reality is
minus five to minus six is a great base
side for a quarter wave
there's plenty of people out there on
the dx
using a quarter wave on multiple hops
around the planet
achieving minus six minus five minus six
okay so let's not worry it's just a
great baseline
to use all right what happens if we
increase the length of this to
half a wavelength so that would be five
meters
20-ish feet give or take
well what happens is the swr an
impedance goes
cock-eye because it's the wrong length
to to match against
at the moment and but interesting enough
the far
field plot changes this looks great
doesn't it
but until you start measuring at minus
five
minus four point nine so it's only a one
db less than that really
yeah 1db just over 1db better gain on
our dx
and frankly up here where it said 1.1
did it on the quarter wave we're 0.2 now
so
in fact something's gone a bit wrong a
half
wave isn't as good as maybe we we think
it is
so we increase the height again let's go
to where's the height here let's go to
5.75
you'll know why in a minute because i
want to show you what happens to the far
field plot
impedance is coming down back down to
where we're after
but we've got these little lobes up at
the top it's starting to
materialize and it's not until we hit
well a 5 8 is 62 and a half percent so
let's just do
6.25 all right and just see what the far
field plot looks like there
minus five oh that's interesting
so five degrees off horizon it says it
here two point
minus two point seven so we are three db
better
without 62 and a half
however we've got a problem is that
we've got
all sorts of shit going on down here
which means we've got a very high swr
now of interest what happens on a
three-quarter wave
so that would be 7.5 meters long
calculate we then get you'll see
very close to 50 ohms in fact we could
tune this
swr resonance where is it absolutely
resonant
29.7 okay
two five or something i i didn't read it
but you'll you'll see
that a three-quarter wave gets a better
match
than a quarter wave so we are literally
at 50 ohms so our swr
is next to nothing one to one as they
call it
1.08 to one and if we go to the far
field plot on a three-quarter wave and a
lot of people will tell you that a
three-quarter wave is a bit of a cloud
warmer
a cloud warmer being we're warming the
clouds up because all our rf is going
straight up
but if we go back down to our five
degrees off our horizon business which
is
175 it's minus 3.2 so in fact
a three-quarter wave isn't as bad as
what a lot of books
and all the people on e hammer qrz would
make you believe
so minus 3.2 we are uh was minus six
something wasn't it yeah so we're three
db
better so that's doubling your power
receive and transmit
pretty good isn't it however
we would love to get the this pattern
uh 6.4 i think is a nice
shape let's just have a look at 6.4
minus 5 2.2 so it's another db it
wouldn't be great to get the height
of 60 somewhere between 60 and 65
percent
i've picked 64 in this case minus 2.2
is 62 better than that 6.2
we had minus 2.2 far field
2.5 so in fact 64
6.4 is great because if we go
too high so eight meters right
go eight meters now we're losing this
gain
so it just starts to look more and more
crappy
right nine meters so that's 90 percent
of a wavelength
of interest i might as well show you
this
so we're losing so minus 7.6 so 5 db
worse off than we were at 66.4
64 which is a great number to have
we won't get a very good match on 64 but
what we'll get
is this great pattern so somehow
minus 2.2 with our peak gain
about 15 degrees off horizon at nearly 2
db
nice antenna okay however we've got this
reactance and not very good impedance um
so the trick is what we do
is we actually electrically build it for
three quarter wave
but load it to get the physical height
down to 64 percent because 7.5
if you remember gave us a fantastic
match 53 ohms
1.08 do you remember so what i do with
the dx commander 40 meter element
is it it's just an accident right the
the
4d 10 meter dx commander pole i think
the factory tell me it's
9.5 or something but it's not fully
extended it's 9.25 maybe 9.3
and as a test few years ago i went up
and came back down
a little bit which just loaded this
court away for 40
quite nicely however it ended up being
electrically a three-quarter wave for
the 21
megahertz 15 meter band however
the height the physical length
is about 64 65 percent which is why when
the propagation's in for 15 meters
it's a cracking antenna vx commander so
that goes up 9.2 meters i think and
comes back down 1.9 meters
so why am i telling you all this because
when we build
the nebula class antenna which is a
quarter wave
for 80 meters so it's it's
lengthening all the elements really bar
40
is i'm going to build it with
three-quarter wave
electrical lengths but with fold overs
to make them
5 8 or 64 percent physical lengths
all right so we'll get a different
pattern we'll get the better pattern on
the nebula class than we get on a
regular dx commander
there's two advantages to that one
that's a little bit
hey you hardly notice it a couple of dbs
not a lot
but there's much better matches
to 50 ohms much better so you'll get
this
literally down down to 1.1
now there's some manufacturers that have
a hell of a problem getting 40
and 15 to work on the same antenna
that's because
40 all the time will want to resonate on
21.
so which is why this fold-over little
boomerang match i call it
works so well because it retains the
very wide bandwidth for 40 meters
but also gives you a great match for 21.
so for the nebula class for
antenna for the elements on the nebula
class
the elements for 20 and up so 20 17
15 12 10 6 and so on so forth
will all be three quarter waves 5 8
actually
interesting just before i go i want to
show you what happens
when you keep increasing the lengths do
you
does it do you get a trick so if it's
resonating at
two and a half seven and a half the next
time it's going to resonate is 12 and a
half
which is one and a quarter wavelength
and you'll see
uh sure enough it's you get not bad
impedance swr 1.46 what happens to the
pattern though
so the pattern is
not bad actually minus five you've got
all this wasted gain at the top
but that's just interesting so the next
time it's going to resonate
we've got one and a quarter it won't
resonate one and a half but it will at
one and three
quarters now this is only recently
discovered this
this is a rather nice looking antenna
because
you see these lobes down here look at
this
at five six seven degrees off the
horizon one point three it's just come
down a couple
point two so we're 6 db better
if you can get one and three quarter
waves trouble is a terrible null just
above 20 degrees here if you've got
anything high
high angle coming in which is one of the
reasons
the dx commander is also fantastic on 70
megahertz which is the four meter band
so there we are in a nutshell quarter
wave versus 5
8 and how to make a three quarter wave
into a 5 8. either with a coil at the
bottom like the manufacturers do
but for a ground mountain you might as
well go up and come back down
so be it now this is all part of a
playlist the antenna field
which is where we streamed the grass
we've put the cables down we've got the
cables now into the shack
next thing i've got to do is tidy up the
cables put so239s on them and everything
we've got to get a radial field down and
then finally the last bit of this
is putting the nebula glass antenna up
but i will be doing a video of how i've
modeled that to make sure
all the harmonics aren't going to
interfere with each other
using three quarter waves as 5 8. all
right so my name is callum from ds
commander have a jolly good day
thanks for joining me and listen if you
like my stuff please consider
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gives me another extra confidence that
i'm doing the right thing okay so having
you along for the ride
would uh would warm my heart all the
best now and i'll see you next time
bye for now
