Coax Cable only needs to do two things:
First we want it, to take the received signal from the antenna towards the receiver. (in the best way it can without loss)
To bring the transmitted signal from the tranceiver towards the antenna. (again in the best way it can, without any loss).
These are the only purposes of the cable.
Coax has several aspects to keep in mind:
1- The impedance: (for 27MHz all tranceivers and antennas use 50 Ohms) So we want (in almost all cases) 50 Ohms cable.
2-The loss of the cable: Overall you could say the bigger the diameter of the cable the less loss it will have. But equally important is the density of the outer coax braid (it needs to be thick and fully cover the surface. This will decrease any loss, hence a better cable.
3-Flexibility: Where it is no problem to bend a piece of RG-58 cable this will become more difficult when you use coax cable with larger diameter.
Straight down a mast it isnt a issue but around the rotor problems are building-up. It is not uncommen to use different types of coax between the antenna and the tranceiver.
Most commonly used coaxcables are types like: RG-58, RG-213, Aircomm, Aircell. These are still rather flexible and easy to use by the operator. Roughly you could say the bigger the diameter the less loss the coax has
There are bigger types like 3/8 or 7/8 hard line but this will cost and perhaps it is of no use to use these types simply because the length between your antenna and transceiver 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 you will not see the difference between 3/8 and Aircomm Plus over 30 meters only in your wallet!)
As mentioned, the best length is the shortest. This is the only way to keep the loss of the cable as low as you can.
There is maybe ONLY ONE exception :
By extending the length of the cable:.
A HALF WAVE coax cable or ANY multiple (1, 1½, 2 wavelength) length will show you: “exactly the same input resistance at both ends of cable”. In normal words:
You can measure the exact SWR from the antenna with these lengths of coax cable.
If your antenna is truly 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 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).
So always measure the SWR at the antenne input and never and the end of the coax line (unless you use that 1/2 wave lengths rule).
And there are always SPECIAL CASES:
There are antennas which 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 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.
When you want to combine antennas together(stacking) other features come in, read the chapter Stacking.
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.
Most commonly used is 50 ohm’s cable.
Sometimes it could be useful to have 75 ohms (for stacking). Another variety is: 300 ohms, this is what we call open line.
This was/is still in use for simple wire antenna’s often multi band antennas.
We will stick to the 50ohms, that’s what we needed Flexible/ able to lay in corners etc!
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!
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.(vulcanized tape)
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.)
LOSS: with 100 meter length
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.)
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 only use N-connectors these are 50 ohms so any loss which just might exist 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 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 😉
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)
Coax cable electrical length
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.
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|
|Foam polyethylene||1,3 – 1,6||0,88|
Dielectric constants and velocity factors of some dielectric materials used in coax cables.
When using insulated wire for a wire beam you have to change the length. VF of the wire is between 0,98 and 0,95.
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!
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 impedance’s 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.
The device, which can improve a received signal.
Works on 12 volts has an input and output connector (commonly PL259)
This for me is an un-know region at the 11 meter band.
In the new setup I will include one of the below marked pre-amps and will put down the results.
Of course,i also had once such a device available 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)
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 this is a problem you can overcome.
There is only one reason why you should use a pre-amp
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, 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..