I am frequently asked about the performance of so called 300 ohm transmitting ladder line, particularly on G5RV antenna systems.
My article Feeding a G5RV does not show analysis of a configuration using 300 ohm ladder line. The main reason is lack of published line specifications from the various sellers.
I have however, done analysis of the G5RV based on the theoretical performance of an air spaced line with 0.2mm copper conductors and Zo=300Ω. The results were:
There are two issues with the analysis though:
At radio frequencies, skin effect causes the current to distribute
itself mainly towards the outer surface of the conductor. This means
that a hollow tube of sufficient thickness has similar RF resistance to
a solid cylinder of the same diameter.
Most 300 ohm ladder lines so to the ham market use stranded CCS
conductors, typically 19 strands of 0.2mm diameter CCS. If 30%
Copperweld was used, the cladding thickness would be
about 14Ám. Due to skin effect, this type of conductor will have
performance equivalent to
a solid copper conductor above about 200MHz (which hints its original
Skin depth in copper at 3.5MHz is 35Ám, so unless the cladding thickness is at least 100Ám, line loss will be worse than for copper conductors. The higher resistivity of the steel core and its permeability contribute to much worse performance than copper where the cladding is insufficient thickness. For a 0.2mm conductor (#18) to have copper like performance at 3.5MHz, it needs to be solid copper.
This issue does not affect just 300Ω ladder line, but all ladder lines using CCS conductors, and the effect is greatest for the more expensive, more flexible versions that use a large number of strands. More information is in the article CCS ladder line suitability at lower HF.
When a manufacturer publishes detailed line characteristics for 300Ω ladder line, I will add an analysis based on that data to the G5RV article.
In the meantime, 300Ω ladder line using CCS conductors is a bit of a pig in a poke at the lower end of HF and below.
Essentially, if a conductor is going to have too little copper, you want it all on the outside of the conductor, not buried internally between strands where little RF current flows. In that situation, a single core CCS conductor is better than a 19 strand... yet the market pays extra for 19 strand.
Best of all is a single core hard drawn copper conductor, which is what would be have been used if these lines were designed for HF.
The following analysis focuses on the open wire section of a G5RV
hybrid feed configuration at 3.5MHz. The performance of the open wire
influences losses in the coax section which is in turn very dependent
on the actual length deployed in a particular installation, so to that
extent this only a partial analysis.
The physical parameters of a nominal 300Ω line using 1mm copper conductors and velocity factor 0.85 at 3.5MHz can be found iteratively using RF Two Wire Transmission Line Loss Calculator .
|Length||497.29 ░, 1.381 λ, 100.000 m|
|Line Loss (matched)||0.463 dB|
The table above shows the model. The model assumed zero dielectric
loss on 80m. The calculated matched line loss is
Since the copper cladding is 14Ám and skin depth is 35Ám, a first approximation is that effective RF resistance of the conductor is 1/(1-e^(-14/35)) or 3.0 times that of a solid copper 200Ám conductor. To obtain that effect in the calculator, the conducivity must be divided by 3.0^2, giving 6.4 S/m.
|Length||497.29 ░, 1.381 λ, 100.000 m|
|Line Loss (matched)||1.393 dB|
Rerunning the model with the adjusted conductivity gives the results above. Matched line loss is somewhat higher at 1.39dB/100m. But, matched line loss is not directly relevant to the G5RV scenario.
|Line Loss||4.100 dB|
Recalculated for the load seen in a G5RV and the appropriate length,
the loss in the 300Ω section is 4.1dB, or efficiency of that line
section alone is 39%, less than half the power entering that line
section reaches the dipole feed point.
By this estimate, the loss of the 19 strand 1mm CCS 300Ω line is more than three times that of Varney's original open wire line section (Zo≈500Ω, copper), and about four times that of a home made open wire line using 2mm copper spaced 150mm.
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