testing one two three a very good day everybody my name's callum from uh dx commander now i get a
lot of people say what's the ideal length of coax what coax do i use coax coax coax so first of all
i've got four major tips maybe six if we if we count them all up if you've come from a cp
background you might have the opinion that there is a particular length of coax which is right for
your frequency i promise you that is a myth only because impedance repeats every half wavelength
less velocity factor i'll show you that right at the very end what i mean by that so there's
there's four key things really for me the first one is the frequency okay so what frequency is the
highest frequency right that i'm after because the higher in frequency go the more loss you've got on
the coax i've got two big runs of coax into my development field and they're the most expensive
i could afford because i didn't quite know what i was going to put up there just about do vhf
and uhf at the lengths i'm after then what we can do is we can use some basic coax loss calculators
to work out if we are comfortable with that sort of loss so if you go on the internet
and i just i mean i'm sure there is plenty of tools available for this i just do coax loss
count cue later oh there we are uh i don't know i haven't picked this one before so this
might be contrary interesting uh line type okay well we need to know what all these are
and i don't know but let's just take an average rg 58 uh c whatever whatever that is it's just
showing you what we're on about here now i'm into meters you might be into feet
so i'll just say if you take 30 meters and and multiply it by three in a bit you end up
with 100 feet so ah 30 meters and 100 feet is about the same oh we'll do feet 100 feet about
30 meters and the highest frequency we're after let's say is the two meter band 145 146 megahertz
depending on what part of the world you're in and we'll give it 1.2 swr and we'll put 100 watts into
it 100 watts base station on fm and if we look at the right here we'll see we've got some loss
okay the total loss is six just over 6 db now i've done i've done a video about db
so i know that 6 db is you lose half your power and then half your power again
so let's just have a look there we are power out if it was half and half we'd be 50 to be 25 watts
it says here it's 21 watts so you'd be transmitting 21 watts out of your 100 watt radio
same on receive you'll have a 6 db well 6.7 db loss as well so i don't just there's a lot of
people say oh there's nothing wrong with rg58 well it's not if it's that big right but there's a hell
of a lot wrong with it to use it for a run right and to get a man out or to get your ladders up and
to put some sort of base station antenna on i mean let's have a look rg 58 at let's say 27 megahertz
what happens well we're losing half our power because we're only getting 54 watts out
so it's just under just just under 3 db look so rg58 i promise you give it a miss unless it's
for a short run a little pigtail between a and b all right so and last thing is let's just have
a look at the minimum the minimum i would use i don't like rg213 but the minimum i'd use is this
at 27 megahertz that's the 10 meter band 11 meter band you see how it's gone up to
78 so that's not too bad the loss is only um just over a db okay interesting that if you
come down to let's say that seven megahertz 7.2 give it 7.2 2 megahertz which is the 40
meter band you'll notice that there's a hell of a lot less loss because well there is
so what we have to do is when people say what's the best coax for this or whatever
it's what frequency is the first thing and the second thing is how much loss am i prepared to
accept all right once you've got the loss you're prepared to accept i mean i spoke to a big vhf
uhf guy who is comfortable accepting 3db because he's got a particularly long line he's got to go
up a tower as well he's got about three db loss he knows that so well if he wants 100 watts out he
just puts three uh 200 in all right because that's three db it's double and he's got a received
preamp to soak up the difference all right so you can get away with it if you know what you're doing
all right we still haven't got which coax have we so once i've decided that the minimum
i want is rg213 then i start looking at other things so let's have a look now
at for instance messi are not selling anything here pay alone mess it in polonie uh correct
this is how to read somebody's coax charts okay let me accept i haven't been here so let's have
a look okay so this is how you'd read just a data sheet from some manufacturer wavelength 85 meter
band three and a half megahertz right feet and meters so let's take we were doing 50 meters
there's 50 meters which is 164 feet it turns out and then we would look down for the band so
2 meters 144 looking down here i've got a useful output of 57 what doesn't tell us it's it's funny
this isn't it this is treating us like imbeciles okay because i come got the db loss but as you can
see it's about two and a half it's nearly three db at that frequency if i come down frequency wise to
wavelength 28 just sort of cb ish 50 78. so we end up with 78 of our which i mean if i was getting 78
to 80 frankly i can cope with that all right so you got a trade-off between the frequency
um the length and we kind of done that and i've shown you how to read a couple of tables the
next thing is i'm particularly interested in the connectors so this happens to be
so this is ssb electronics air cell 7 i think yeah sl7 and you can see the construction
is a pl259 but it looks like an n-type construction there's a rubber gland in there
rubber grommet it all screws together it's waterproof once it's on the thread isn't
waterproof as i have proved i'm making a video at the moment where my two meter
station is completely trashed because i forgot to put some vaseline on the threads my whole fault
um so now these aren't 10 a penny all right the old days of rg21 crappy rg213 with those pl259s
you know 10 for a dollar or whatever it is with all that soldering and braid and everything else
i've done a couple of videos on how to fit these very modern pl259 but i'm particularly
interested in all the quality of the pl259 i can buy for my coax so the the four coaxes i
personally recommend as a minimum would be the seven mil verse seven millimeter
version which is just over uh six millimeters quarter inch just over a quarter of an inch
messi and paloni do a seven and a ten mil and ssb electronics to um air cell seven and uh flex sl sl
they do a 10 mil all right and then they do ecoflex they do eco flex 10 and then they
do an eco flex 15. now that's quite expensive i bought 100 meter drum which is 330 something feet
but that was something like six pounds a meter but they're the two main runs to my antenna field one
was for hf transmit and one's for uh was for vhf but i've just had donated to the channel a
hundred and i can't tell you who a hundred meters of uh lmr lbf450 something like i think it's 7 8
massive run i don't know how we're going to get it to the field that's the only problem
but anyway so we've got our frequency we've got our length we've got to make sure that the coax
we buy we can get some super nice easy to install connectors and then you've got to fix your budget
because you might say oh yes that at that time eco flex 15 callum was talking about absolutely
perfect so let's just have a quick look at that actually eco flex 15 dx shop does it here we are
it's who i bought it off myself i didn't get a deal but no mind eco flex 15 plus
is that what i bought i don't know but let's view it 10 pounds a meter that's about 12 13
now is there a lot here we are so this is how to read a loss chart so i'm over at the right
hand side of this screen and immediately i'm going to do say on average 10 megahertz
my loss for 100 meters 300 and something feet at 10 megahertz is less than 1db all right if we go
right up to 2 meter band at 100 meters it's whoa what am i doing it's 300 and it's uh 3.23 uh db
just over 3 db 400 meters and i'm into probably about 75 so i will be losing half my power there
it doesn't have a particular spec for 28 megahertz but if you look at 50 megahertz 100 meters is 1.87
10 meters or 30 ish feet is going to be 0.18 so that's why i chose eco flex 15 other than the fact
you've then got to fix your budget do you go i've spent 500 pounds on this radio and you want me to
spend 600 pounds on the coax but unfortunately the antenna and the coax is more important than
your rig most radios can do anything i promise you if you if you're just going to lose all your
all your power in the coax you'll receive and you transmit that's not good for man or beast is it
if you're in europe ssb electronics and messi and polonie and i know in the us you've got all
the traditional stuff lmr 400 and there's some times andrews there's a few companies you know do
if you're actually looking at seven eight and one inch hard line and that sort of stuff you're not
even watching this video you know what you're after okay but just beware of rg58 and rg213
and by the way not all rg213s are the same all right there's some real rg213 and you'll pay good
money for that and then there's some really cheap nasty stuff it's the size of our g213 but it isn't
i was going to talk about velocity factor briefly just before i go so let's get one of my books out
i might have to refocus the camera that's above me i'll have a look in a minute
velocity factor quarter wave so so if you've got some antenna oh you see me there you've got some
antenna here right that's the ground there's just some vertical right and we got some coax
what will happen is the next take because a lot of these companies quote velocity factor so i'm
just downloading this data sheet as well and i'm going to have a look at verlos t here we are it
says it here velocity i'll just zoom up for you guys velocity factor it says 0.86 so eco flex
15 has got a velocity factor of 0.86 what does that mean
so sometimes when we're making a phasing harness or whatever else we specifically want to cut the
coax the exact size now i'm just going to tell you what i use on holiday and why i use it
so in the main i like to take with me a half wavelength of coax or the 80 meter
band so roughly 80 meters happens to be 3.75 megahertz right because i know if i take a half
wavelength of coax it's going to repeat every half wavelength at that frequency every wavelength of
the 40 meter band every two wavelengths at the 20 meter band and i know that every half wavelength
if you if i've got a a 50 ohm whoops you said degrees there if i've got a 50 ohm exactly a
50 ohm antenna here it i can put any length of coax on the 50 ohm coax on there and i will read
50 ohms at the other end okay if i'm actually got let's just use different color
blue if though i've got a 45 ohm antenna there after half a wavelength
what happens and i actually don't know how this happens so you can't quote me on it but something
happens to the impedance it goes up and down like a sine wave and i think this is where we need um
smith charts but ignoring that i happen to know it exactly half a wavelength
we get a repeat here of our 45 ohms because if you measure it at various points it may be below
or above that and in fact it's really cool stuff you can do on matching sections with coax stubs
but and that's not today so what is this velocity factor so let's just take 40 meter band
so um we'll we'll take we'll take 7.38 because i happen to know that's exactly 80 meters
so half a wavelength of 80 meters is going to be 40 meters however i don't need 40 meters of coax
for my exact half wavelength i would divide that well actually i'd multiply it but divide that by
the velocity factor so 40 meters times 0.86 which is our velocity factor we put it here
and we'll do that on my calculator wherever my calculator is 40 meters
exactly 40 meters actually the 40 meter band is actually 42 meters on average 42 times 0.86
36 that's why i always take 36 meters of coax with me but if you are from the cb world and you are
convinced that you need a very particular length of coax you are in a myth world i promise you
unless someone has specifically designed an antenna that needs a particular length of coax
but bear in mind it would have to be a particular velocity factor as well okay i think you find
doesn't matter all right so to recap we've got the frequency we need to pick um the loss you're
prepared to accept the budget you have right because it's pointless you're going oh yeah
i'll do all that but i can't afford it and in my opinion what connectors you want so you can buy
cheap but you'll be buying it again i promise you some of my coax is years old because i bought the
best i could afford at the time and it was a lot i bought some of these connectors i've used several
times taken them off this this patch leads probably got smaller and smaller and smaller
and i'll probably go scrap now but i'll remove the content of the connectors
and reuse them you can't do that with 10 of penny ones all alright so stick
around for more stuff we'll do on this and i'll see you on the next one bye for now
you