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Coax Cable

Coax Cable

Postby qrz11 » 13 Feb 2011, 13:16

* TNX to 19 SD 348 *

Coax Cable:

Image

The way to take the received signal from the antenna towards the
receiver and visa versa. the way to bring the transmitted signal from the
transmitter towards the antenna is through a coax cable.

This is the only purposes of the cable. Most commenly used are RG-58,
RG-213, Aircomm, Aircell. There are bigger types like 3/8 or 7/8 hardline
but this will cost and perhaps it is of no use to use these types simply
because the lenght between your antenna and tranceiver is short enough
to there is hardly any loss! You might start thinking about those when
you need more then 50 meters.

(At 11 meters I could not see the difference between 3/8 and Aircomm
Plus over 30 meters only in my wallet!)
Very lucky we should be, that there are no big misunderstandings about
cable. It is as simple as:

Buy a line with the lowest cabel attenuation
Do not make the cable any longer then nesessary
The first highly depends on the money you would like to spent on cable,
It is the case that the more expensive the cable the better it will be.

The second is misunderstood by some, the lenght. As said, you only have
to keep one thing in mind, keep it as short as possible.
There are numerous tales making the round about the lenght. Only one
is true.
Without going to deep a HALF WAVE coax cable or ANY multiple
(1, 1½, 2) of this will show you: exactly the same input resistance
at both ends of cable.

In normal words:


Already told you that SWR should be measured at the antenna her
location. You might see other values when you measured anywhere else.
But with a cable exactly an ELECTRICAL half wave long you will see the
same values again!.

So it has no use as some do to make the line between the amplifier and
the radio a quater-wavelenght long or anything else! NO USE ! The
currects and voltages are only exactly the same at a half wavelenght.
To calculate the electrical halfwave lenght of a cable you need its velocity
factor.

Other figures

The cable has other cahateristics: it has an impedance, power handling
capability.
Most cables are 50 ohm, sometimes however you need other. For
example when you are going ahead with stacking you could need a 75
ohm cable.
Another variaty is: 300 ohms, this is what we call open line.This was used
very commen in the old days. I still use it for wire-antennas. You could
make this line your self, but before starting playing around with this you
do need to know how imedances work how to calculate with them etc. For
example this line can not touch anything else!, so its a bit harder to
handle.
We will stick to the 50ohms, thats what we needed Flexible/ able to lay in
corners etc!

The impedance of the cable is produced by it thickness, the distance
between the two conductors and the material between those conductors.
With the knowledge of this you can now make up your own mind, Is it
wise to bend coax cable in a tight curve?
No you just might changes the distance between the two conductors!
Do not bend the cable in straight curves (let the cable take the turn it
wants! no sharper, this also counts for storage)
When welded the cable, make sure there isn't a possibility for water to
enter the cable.(vulcanised tape)

POWER HANDLING CAPABILTIES:

The thickness of the inner cable is responsible for the powerhandling
capabitlies. The thicker the better.
For example in my younger days, often my RG-58 would get burned
aswell as the connectors at both ends.
But then again what is a 12 years old boy thinking when he is putting a
half KW in RG58.
Any thicker size cable can handle 1 kw easy for example the quite thin
Aircell 7 is capable of handling about 2,5 KW at 11 meters.
Aircom can handle even more.


LOSS: at 27 MHZ with 100 meter lenghtname

RG 58-------------6,6 dB
RG 213------------3,4 dB
AIRCOM plus-----1,5 dB
AIRCELL 7--------4,6 dB


You can calculate easy with these numbers for example with only 20
meter cable using RG-58 we come to
6,6 / 5 = 1,32 dB loss (100 meters RG 58 was 6,6 dB loss we want the
loss at 20 meters: 100/20=5 ..... 6,6/5= 1,32dB)

So when we stay below 50 meters Aircom Plus is good enough we only
reach a loss of 0,75 dB then. (not included the connectors etc.)

SPECIAL CASES:

There are antennas wich depend on the coax cable for the to produce
"the ground", some mobile antennas and very small bases verticals could
need this. Read the instructions before cutting them. Any fullsize antenna
does not have this problem! And just make sure you do not use types
mentioned before unless there is no way to overcome this.
When you want to combine antennas (stacking) other features come in
read the chapter Stacking.

CONNECTORS:

PL 259 is most commenly used type. although this type is not 50 ohms,
I know my mind is a bit extra-carefull but I only use N-connectors these
are 50 ohms so any loss wich just might exsist with PL 259 connectors is
reduced. Honestly I have to say I never noticed any difference on 11
meters between those connectors.
Use only the amout of connectors you need and that is 2 !
Onces tested the S.W.R this will probarbly be the same next time, it is
nice to see those needles going up, but again this is loss ...extra
connectors, S.W.R meter etc. As said before I am a bit extra-carefull but
I am confident that all those pre-cautions will resolve in less loss!

Image

ANTENNA TUNERS

If your antenna is not resonant at the design frequencie and you need an
ATU to make sure the SWR stays below the wanted numbers. You have
done something wrong!

There is only loss in an ATU due to extra connectors, and the inside of the
ATU (coil/capacitator)
The antenna tuner is miss guiding you, The SWR does not become better
It is NOT an ANTENNA tuner but only a CABLE tuner!
Remeber a dummyload had an SWR of 1:1,1 but was a terrible antenna
the same counts for an ATU. The ATU is the joker in ouwer system.

DONT USE IT!

There is only one way the ATU may be used, with other types of
antennas (Zeppline) or cables. (that 300 ohm line for example) But
neither will be used by 99,9 % of us 11 meter DXers.

RECEIVING PRE-AMPLIFIERS

The device, wich can improve a received signal.
Works on 12 volts has an input and output connector (commenly PL259)
This for me is an un-know region at the 11 meterband. In my new setup I
will include a proffesional one and will put down the results.

Ofcourse I bought in my life once such a device availible from the
CB-market, only to find out the noise level was improving with great
numbers perhaps when I used bad cable it could have been an
improvement.

The mentioned noise level is the quality figure of the pre-amp. At higher
amateur frequencies these devices do come in handy because they do
make up for all the loss in cable connectors etc. (I can notice a big
difference on 144 mhz and 934 mhz)

Profesional pre-amplifiers like those from MIRAGE or I.C.E. have a noise
figure of < 1dB and a gain variable upto 20 dB.

Image


The amout of power they can handle is not that high, ofcourse with
remote swiches this is a problem you can overcome.

I can see only one reason why you should use a pre-amp: It is to make
up for the loss produced by connectors and cables.
So often it is put where it should not be ....at the transceiver his location.
The right location is at the bottum of the antenna, only then it might bring
stations a bit clearer.

Yes almost forgot:

The signal wich your antenna does not pick up, can not be amplified by
a pre-amp.
Once a local station had one of these CB- pre-amps in his car, ofcourse
this is not an improvement.....but hey..it looks nice.

email me: 19SD348@planet.nl




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qrz11
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