Figure 1:
Figure 1 above shows an optimal horizontal antenna set-up, with its cabling path between antenna (left) and transceiver (right).
The blocks depicted in the figures play an important role in removal of annoying unwanted common-mode currents.
Horizontal antenna (Beam, rotary dipole, wire dipole etc.) set-up.
The green block (BAL) represents the BALUN.
This network ensures smooth transition between the symmetric antenna and the a-symmetric coaxial cable. It ensures that, during transmission no current returns to ground over the outer surface of the coaxial cable, and during reception, no common-mode noise current originated in the house or environment penetrates into the wanted low signal path. A good quality (Common-Mode Choke type) current BALUN is recommended here.
The yellow blocks (LI1 – LI3) represent the LINE ISOLATORS.
These networks remove any residual common-mode currents that may exist on the coaxial transmission line.
LI 1 is typically installed at the tower base or immediately underneath the wire antenna, close to ground, where it can be connected with the shortest and largest section possible wire to a good RF ground. Basically this Line Isolator removes most of the residual common-mode currents induced in the coaxial cable, routed in the “near-field” of the antennas.
LI 2 is typically installed in long coaxial cable runs, and serves the same function as LI 1 and LI 3. It’s deployment is optional, and depending on the particular situation.
LI 3 is typically installed where the cables enter the house or shack. It is the most interesting place to install a Line Isolator, since it separates the noise polluted safety ground connected to the TRX, from the “clean” RF ground connected on the antenna side. (The line Isolator is to oriented so that the “clean” RF ground side is at the antenna side).
Remark:
LI 1, LI 2 and LI 3 are often given different names, because they are deployed at different positions of the transmission line, but electronically they are exactly the same devices: coaxial common-mode chokes with high impedance at the frequency of interest.
The blue block (TRX) represents the RADIO. Typically the transceiver is connected via its power supply to the safety ground of the house electrical installation. This ground is effective for protection of the equipment against electrical hazards, but has an RF impedance which is far too high to be useful for draining RF common-mode currents or noise. It can be improved by installing a local low impedance RF ground close to the radio equipment or if impossible due to the location of the radio’s, one can rely on the RF grounding of the Line Isolator(s) further down the transmission line path.
Figure 2:
Figure 2 above shows an optimal vertical antenna set-up, with its cabling path between antenna (left) and transceiver (right).
The blocks depicted in the figures play an important role in removal of annoying unwanted common-mode currents.
Vertical antenna (Shortened (Loaded) or fill size monopole, inverted L, etc.) set-up.
The green block (ZM) represents the Impedance MATCHING circuit.
Verticals installed above a good ground plane are excellent low angle radiators and as such, very suitable for working DX. This is especially true at the low HF bands where horizontals often become impractical. Often however, loading techniques need to be applied, in order to achieve practical heights. Loading verticals, lowers their feed point impedance to values that can easily get below 25 Ohms. In such cases, some sort of matching needs to be applied in order to achieve a reasonable VSWR value. This matching circuit can be a narrowband L / C network or a wideband UNUN impedance matching transformer.
Similar like in the horizontal antenna set-up, the yellow blocks (LI1 – LI3) represent the LINE ISOLATORS.
These networks remove any residual common-mode currents that may exist on the coaxial transmission line.
LI 1 is typically installed immediately after the impedance matching circuit, near the vertical, and close to ground, where it can be connected with the shortest and largest section possible wire to a good RF ground. Basically this Line Isolator ensures that the vertical doesn’t “see” the coaxial outer braid as a radial and thus blocks the majority of common-mode current potentially flowing back to the radio and causing interference during transmission. Similarly, it blocks Common-Mode noise and disturbances, potentially coupling in the wanted signal path during reception.
LI 2 is typically installed in long coaxial cable runs, and serves the same function as LI 1 and LI 3. It’s deployment is optional, and depending on the particular situation.
LI 3 is typically installed where the cables enter the house or shack. It is the most interesting place to install a Line Isolator, since it separates the noise polluted safety ground connected to the TRX, from the “clean” RF ground connected on the antenna side. (The line Isolator is to oriented so that the “clean” RF ground side is at the antenna side).
Remark:
LI 1, LI 2 and LI 3 are often given different names, because they are deployed at different positions along the transmission line, but electronically they are exactly the same devices: coaxial common-mode chokes with high impedance at the frequency of interest.
The blue block (TRX) represents the RADIO. Typically the transceiver is connected via its power supply to the safety ground of the house electrical installation. This ground is effective for protection of the equipment against electrical hazards, but has an RF impedance which is far too high to be useful for draining RF common-mode current or noise. It can be improved by installing a local low impedance RF ground close to the radio equipment or if impractical due to the location of the radio’s, one can rely on the RF grounding of the Line Isolator(s) further down the transmission line path.
General Remarks:
An RF GROUND (RF GND) is not the same as a safety ground A safety ground must have a low resistance at the mains frequency (50 Hz or 60 Hz) in order to protect electronic equipment from shock hazards, due to leakage possible currents in fault conditions. An RF ground however must also have a low impedance at the RF frequencies of interest. This can only be accomplished by interconnecting multiple ground rods, spaced apart and connected with a short large section cable to the antenna and /or transmission line set-up.
Do not forget that Common-Mode RF feedback and noise can also flow over antenna rotor cable and control cables. These cables (located in the antenna near field on one side and the radio room on the other side) should also be provided with Wideband Common-Mode Chokes to obtain an optimum result.
