Coaxial Cables

Question 1

What is the purpose of a coaxial feeder?

Answer 1

To provide a low loss link between the Transmitter or Receiver and the antenna.

Question 2

What are the three types of cabling used within the CIT trade?

Answer 2

Flexible Coaxial (URM)
Semi Rigid Coaxial (LDF)
Rigid Feed (Line)

Question 3

Define the term “Impedance”?

Answer 3

Electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied.

Question 4

What are the two common impedance values

Answer 4

• 50 ohms (Ω)

* 75 ohms (Ω)

Question 5

State the characteristics of Rigid Feed?

Answer 5

• Rigid feed is a special type of coaxial cable designed to have minimum loss.
• In Rigid Feed no electric or magnetic fields extend outside of the outer conductor. The fields are confined to the space between the two conductors, resulting in a perfectly shielded coaxial line.
• The space between inner and outer conductor is mostly empty. Some cables within this series are sealed and filled with an inert gas.
• The inner conductor is held away from the wall of the outer conductor by, bead, washers or a spiral wound filament.
• Rigid feed is generally found with transmitter halls due to largely the expense and poor weatherproofing properties.
• High frequency (HF) Transmission generally operates on a higher power rating. Therefore, a Co-axial feeder that is capable of taking this kind of power is required.

Question 6

State the characteristics of Low Density Foam Cabling?

Answer 6

• The inner conductor of LDF is constructed of a solid copper core.
• As the LDF series of cables get bigger in diameter, the solid inner conductor changes to a hollow core. Connector pins are screwed in rather than soldered.
• Semi rigid coaxial cable uses a foam diametric, this will have about 15% less attenuation that solid dielectrics but is susceptible to moisture absorption
• The outer shield is solid corrugated copper which affords a certain degree of flexibility but not to URM standards.
• Jacket is normally made from PVC and should be resistant to both ultraviolet and water ingress.
• For LDF cabling purposes the code 4-50 relates firstly to the diameter of the cable and the second part confirms the impedance of the cable.
• LDF is also 50Ω impedance.

Question 7

State the characteristics of Uni Metric radio Cabling?

Answer 7

• The inner conductor of URM cabling is stranded copper wire, this increases the flexibility of the cable.
• The inner conductor and outer conductor are separated by a solid polyethylene (PE) dielectric.
• The outer conductor is braided copper wire which provides the shield and further increases the overall flexibility of the cable.
• This can have an adverse effect as when the cable is bent, as it creates gaps in the shield layer.
• Some flexible coaxial cabling can have double braiding or thin foil to counteract this but does sacrifice flexibility in the process.
• Jacket is normally made from PVC and should be resistant to both ultraviolet and water ingress.
• URM 67 is 50Ω.

Question 8

State the precautions that must be observed when installing coaxial cabling?

Answer 8

• Bend Radius
• Damage to sheath
• Crushing Cable

Question 9

If using cable jacks, how should the cable drum be orientated?

Answer 9

If using cable jacks or rollers, the cable drum should be orientated that the cable is reeled from the top of the drum.

Question 10

When burying cable, what should the minimum distance be for installations below footpaths?

Answer 10

0.5m

Question 11

If burying cable, what form of coaxial cabling should be used?

Answer 11

“Bury Direct” Coax Cable

Question 12

If the above cable is not available, how should the cable be protected?

Answer 12

Use buried conduit manufactured for carrying buried cables.

Question 13

How are connectors classified?

Answer 13

• Type of Fitting

* Impedance

Question 14

Name three different variation of LDF connectors fittings available?

Answer 14

• In-lines
• Elbow
• Flange

Question 15

Prior to and on completion of installation of coaxial cabling, what electrical checks are required to be conducted?

Answer 15

• Continuity = Less than 1 Ω

- Insulation = Greater than 20 MΩ

Question 16

What are the test parameters for these two tests?

Answer 16

• Continuity = Less than 1 Ω

- Insulation = Greater than 20 MΩ

Question 17

When installing coaxial cabling, what is required to be incorporated?

Answer 17

Swans Neck

Question 18

What are the reasons of why this is required to be incorporated?

Answer 18

To remove any strain from the terminating connector.

To provide spare capacity to facilitate any repairs if required.

Question 19

State three different methods for channelling coaxial cabling?

Answer 19

• Cable trays
• Ty-wraps
• Cablocks
• Standoff supports
• Catenary

Question 20

On completion of installing coaxial cabling, what is the process of weatherproofing the connections?

Answer 20

self-amalgamating and densopol tape to weatherproof terminated coaxial cabling.

• Firstly, we apply the self-amalgamating tape around the connector joint ensuring that the rear of the connectors is also covered.
• A layer of densopol tape is then applied on top of the self-amalgamating tape and smoothed to prevent water ingress.
• Both tapes should be applied to same direction as the thread of the male connector. Densopol should be applied from bottom to top to prevent water ingress.

Question 21

State six locations of where a coaxial feeder label should be labelled?

Answer 21

Coaxial Cables should be labelled at the following locations;

i. At the antenna
ii. Before and after any joints
iii. At the base of the mast/tower
iv. Before entering any buildings or bulkheads
v. When leaving any buildings or bulkheads
vi. At the termination bracket

Correctly labelling feeder systems afford quicker and simpler fault rectification.