A coax cable is a transporter. it primairlily needs to do two things for us: 

First we need it to bring the received signal from the antenna towards the receiver, and secondly vica-versa it needs to bring the tansmitted signal towards the antenna.

Obviously we want that job done in such a way, we have limited loss and the signal isnt effected in any other way.

Most commonly used coax types are: RG-58, RG-213, Aircomm, Aircell etc. for most circumstanses these do that job.  

Only when your coax-runs become really long (>40m) or you are using real high power (>2Kw) there might be other coax cables like 3/8 or 7/8 hardline, with less loss and better power handling capabilities 

We are actually lucky that coax cable isnt so difficult to understand. It more or less comes down to: 

 

Buy a line with the lowest cable attenuation
Do not make the cable any longer then necessary

 

The first highly depends on the money you would like to spent on cable.
Overall can be said that: the higher the cost the better the loss.

And secondly, do not make the coax run longer then needed. length is only loss. As always  there could be exceptions:

 

WHAT LENGTH?

 

As mentioned the best length is the shortest, in this way the loss due to the cable is the lowest.
There can be one advantage by extending the length of the cable:

A HALF WAVE coax cable or ANY multiple (1, 1½, 2 wavelength) length of this will show you: “exactly the same input resistance at both ends of cable”,
You can measure the exact SWR from the antenna with these lengths of coax cable.

 

You should always set your SWR at the antenna, so when your SWR is allready 1:1 it doesn’t matter, each SWR measurement anywhere on the line and you will find 1:1.
Only when your SWR isn’t 1:1 but higher your SWR measurements will change according to length, this is when it can come in handy to keep the line a half wave long (or any multiple).

This is the reason you should not set the SWR of the antenna at your tranceiver location.

And there are always SPECIAL CASES where “special” lengths are needed ! 
There are antennas which depend on the coax cable for the to produce “the ground”, some mobile antennas (or plolyester boat antennas) and very small bases verticals could need this Read the instructions before cutting them. Any full size 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. 

Other sitatuations to want specific lengths, would be for impedance matching as with a 2el Cubical quad for example or when you stacking antennas.

 

OPTIMAL LENGTH BETWEEN LINEAR / ANTENNA / TRANCEIVER ??

 

There are stories about the length between an amplifier and the antenna/ transceiver. The above goes for this as well.
Just keep it as short as possible, there is no benefit to make it anything else. 

 

CABEL IMPEDANCE, Other figures

 

The cable has other characteristics: it has an impedance, power handling capability, and loss,  velocity factor etc..
Most commonly used is 50 ohm’s cable.
Sometimes it could be useful to have 75 ohms (for stacking).

Another variety is: 300 ohms, but this isnt coax cable…this is what we call open line.
The open line cable is often used for simple wire antenna’s  multi band antennas.

There is one thing that many do not realise.

 

Open line cable has far less loss then “normal” coax cable. It might be wise to look into it if you are foreced to use exceptional long runs. 

 

Open line can be constructed by yourself if needed, and overall it is rather cheap ! The downside beeing … it cant be in contact with anything … you it needs to be hanging “free”.

But we will stick to the 50 Ohms coax line:  that’s what we needed it is Flexible/ able to lay in corners etc and can be lied down on ground.

The impedance (50 ohms) 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! That will change the impedance of the cable and change the SWR ! Always leave coax in a “natural” flow. Do not bend the cable in straight corners (let the cable take the turn it wants! not sharper, this also counts for storage)
When welded the cable, make sure there isn’t a possibility for water to enter the cable.(vulcanized tape)

 

Speaking about water….DEGRATION springs to mind.

 

Coax cable NEEDS TO BE CHANGED in due time. the “plastic” protection of the cable will suffer from rain and sun, similair like cartires do. Overtime water will enter the coax cable. You could extend the life of your coax by inserting it into a plastic tube so the contact with the elements is limited. Some times coax can last for years and years…but eventually the loss will go up.

This is also the reason why you should store your coax in a dark moist free location.

 

POWER HANDLING CAPABILTIES:

The thickness of the inner cable is responsible for the power handling capacities. The thicker the better.
RG58 witch is one of the thinnest cable can handel up to about 300..400 watts .
Any thicker size cable can handle 1 kw easy
( RG 213 about 1800 watts, Aircell 7 about 2,5 KW at 11 meters and Aircom up to 5 KW.) This depends on other factors like SWR and outside temperatur.

LOSS: with 100 meter length

(old prices hihi)

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)

You can measure your loss simply by measuring the power which goes into the cable and what comes out at the antenna and derive it from that, or by using a VNA they have become cheap these days.

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.)

 Aircell 7  Aircom plus

 

 

CONNECTORS:

 

PL 259 is most commonly used type. although this type is not 50 ohms, I know my mind is a bit extra-careful but I often use N-connectors these are 50 ohms so any loss which just might exist with PL 259 connectors is reduced.
Use only the amount of connectors you need and that is 2 !
Once’s tested the S.W.R this will probably be the same next time. How nice it is to see those needles going up, but again this is loss …extra connectors, S.W.R meter etc.

 

COAX AND VELOCITY FACTOR

 

The shielding and the core of a coax form a capacitor. Through the use of insulation material, the value of the capacitor is bigger then it would be in air, and this has the effect of slowing down the signal. This can be of great importance in some applications, although for many purposes it does not needed this information 😉

 

Velocity factor
The speed at which the signal travels is normally given the abbreviation VF and this is the fraction of the speed at which the signal travels when compared to a signal traveling in free space. So VF for a signal traveling at the speed of light would be 1.0, and for one traveling at half the speed of light it would be 0.5.

The velocity factor of the coax cable is found by VF = 1 / SQRT (dielectric constant)

One important factor of a coax cable in some applications is the wavelength of the signals traveling in it. In the same way that the wavelength of a signal is the speed of light divided by the frequency for free space, the same is also true in any other medium. As the speed of the wave has been reduced, so too is the wavelength reduced by the same factor. So if the velocity factor of the coax cable is 0,66, then the wavelength is 0,66 times the wavelength in free space.

In some instances lengths of coax cable are cut to a specific length to act as an impedance transformed or a resonant circuit, then this needs to be taken into consideration when determining the required length of coax cable.
The advantage of using a coax cable with a low velocity factor is that the length of coax cable required for the resonant length is shorter than if it had a figure approaching one. For example, 1/4 lambda 75ohm coax (VF * 1/4 lambda length) for transform the impedance from 125ohm to 50ohm.

If you are going to need to make electrical lengths of coax cable for any reason, measure the VF. I have seen the VF to be variable and NOT to be as the manufacturer has provided.

 

Dielectric materials

There are different types of materials that can be successfully used as dielectrics in coax cables. Each has its own dielectric constant, and as a result, coax cables that use different dielectric materials will show different velocity factors.

Coax Insulation Material	Dielectric Constant (Er)	Velocity Factor
Polyethylene	2,3	0,659
Foam polyethylene	1,3 – 1,6	0,88
Solid PTFE	2,07	0,695

Dielectric constants and velocity factors of some dielectric materials used in coax cables.

 

On a side not Wire for antennas and velocity factor:

When using insulated wire for a wire beam you have to change the lenght. VF of the wire is between 0,98 and 0,95.

 

Choosing the right coax with the same name ? 

 

If we take RG58 or RG213 there are many manufacturers around the world producing it.

If you need to select any of them,  you need to inspect the outshield of the coax.

 

IF THE DENSITY OF THE OTHER SHIELD IS LAGER (more wires) this MEANS IT HAS LESS LOSS. THIS IS ONE OF THE PRIMAIRY ASPECTS TO SELECT COAX 

 

ANTENNA TUNERS

Do not use it.
If your antenna is not resonant at the design frequency 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/capacitor)
Most people use the antenna tuner directly after the transceiver, it is only logical that the SWR does not become better It is NOT an ANTENNA tuner but only a CABLE tuner!
Remember a dummy load had an SWR of 1:1,1 but was a terrible antenna the same counts for an ATU.
The ATU is the joker in our system.

DONT USE IT!

ONE EXEPTION:
There is only one way that the ATU may be used for what it is ment for.
That is with non- “conventional” types of cables and antennas. (that 300 ohm line for example).
The case here is that these impedances are way of 50 ohms.
But neither will be used by 99,9 % of us 11 meter DXers.

PS Taking a look at most antennas, you’ll often find coils in them. You could look at it as an actual “fixed antenna-tuner”, but this is of course not what we are speaking about.

 

RECEIVING PRE-AMPLIFIERS

The device, which can improve a received signal.
Works on 12 volts has an input and output connector (commonly PL259)

 

There is only one reason why you should use a pre-amp
To make up for the loss produced by connectors and cables.
But often it is put where it should not be ….at the transceiver his location, this could help in the old-days due to bad receivers but these days the transceivers all are perfect and really do not need one.
The right location is at the bottom of the antenna, only then it might bring stations a bit clearer.
Yes almost forgot: The signal which your antenna does not pick up, can not be amplified by a pre-amp..

The most important factor of a pre-amp is its noise-figure. This is the noise generated by the device itself. Say you have 2dB loss and a 3dB noise figure you reception will become worse !

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. (You can notice a big difference on 144 mhz and 934 mhz)

Professional pre-amplifiers like those from MIRAGE or ICE have a noise figure of < 1dB and a gain variable up to 20 dB.

The amount of power they can handle is not that high, Of course with remote switches and a sequencer this is a problem you can overcome.