RG6/U with CCS centre conductor – shielded twin study discussed a synthesised synthesised shielded twin instead of ordinary two wire line for an example HF multiband antenna.
The original scenario then is the very popular 132′ multi band dipole:
- the famous 40m (132′) centre fed dipole;
- 20m of feed line being parallel RG6/U CCS quad shield with shields bonded at both ends;
- 7MHz where we will assume dipole feed point impedance is ~2000+j0Ω (a lowish estimate, it could be double that depending on height).
We will consider the system balanced and only deal with differential currents, and matched line loss is based on measurement of a specific sample of line (RG6/U with CCS centre conductor at HF).
This article will calculate the same scenario with three feed line variants:
- 150Ω twin line with the same CCS conductors as the RG6;
- 600Ω twin line withthe same CCS conductors as the RG6 (ie the spacing is increased to increase Zo); and
- 600Ω twin line using 2mm HDC.
The loss under mismatch depends not only on the transmission line characteristics and length, but also on the load and the current and voltage distribution.
Above the 150Ω twin line with same CCS conductors as the RG6 has loss almost identical to the synthesised twin shielded in the original article. Almost all of the resistance in the coax is in the CCS centre conductor, so I assume that the loss in the twin CCS is approximately equal to that of the synthesised twin. Dielectric loss is less than 1% and can be ignored.
Above the 600Ω twin line with same CCS conductors as the RG6 (ie the spacing is increased to increase Zo). Almost all of the resistance in the coax is in the CCS centre conductor, so I simply assume that the 600Ω twin line with same CCS conductors has 150/600 times the matched line loss. The loss is considerably lower at 0.354dB in this scenario, due to the higher Zo.
Above the 600Ω twin line with 2mm HDC. The loss is considerably lower again at 0.061dB in this scenario, due to the higher Zo.
Note that in all these cases, the load impedance and length of the line form an important part of the evaluation scenario.
So, we can identify that two factors result in the quite poor performance of the synthesised shielded twin:
- high resistance per unit length of the thin CCS conductor; and
- low Zo.
Improving both of these factors in the third scenario reduces loss under this mismatch scenario by a factor of 50.