A couple of points.
1) In an OCF the currents are not equal.
2) The two cables are operating in a (different and largely unknown) mismatched
condition. Using ARRL publications for calculating additional loss due to
mismatch is fraught with danger. As is I might add, using older versions of TLW
software. (If you are an ARRL member and have a T1 line and infinite patience, I
have neither, you can use the absolutely abysmal archive to read about this in
the online June 2014 issue of QST.)
3) OK more than a couple. In a mismatched condition the loss isn't necessarily
doubled for a doubled length.
See http://ac6la.com/swrloss.html, for everything you need to know about this.
Wes N7WS
On 9/18/2017 7:20 PM, Don Wilhelm wrote:
Alan,
The loss is determined by the current in the coax, not that it carries 1/2 the
power. It is the same as for a parallel line - the current is equal and
opposite, so both conductors contribute to the loss.
The total loss should be the same as two runs of coax, but that should be
matched loss. This situation is likely mismatched loss - which according to
the feedline loss with SWR can be relatively small, but present. Consult the
charts in the ARRL handbook and the Antenna handbook for the loss due to SWR -
for reasonable SWR and low loss feedlines it is minimal. The loss due to SWR
is minimized for lines that have a small matched loss. The specification for
LMR-400 is 0.7dB at 30 MHz for 100 feet, so it is small at HF.
In contrast, RG-58 at 10 MHz has 3.6 dB attenuation at 10 MHz per 100 feet.
I may be mistaken (it has happened before), but I am certain that the loss (in
dB) is twice the loss in a single run of coax because the total length of coax
is doubled.
bottom line, with LMR-400 at HF that loss is likely minimal.
For installations that would lead to sharp bends in ladderline or vintage 300
ohm line or close proximity to conducting surfaces, the use of parallel coax
is a good choice for multiband antennas.
Real open wire may be a better choice if it can be spaced with tension on the
wires and run in spaces that are void of nearby conductors, but that is
difficult to achieve in many situations.
Wayne's "run under the house" may negate the advantages of using parallel
conductor lines and make the parallel coax a better choice.
Your situation may vary, but for low loss coax like LMR-400 at HF, that may be
a good choice for multiband antennas. The only "problem" is the weight of 2
runs of low loss feedline to the antenna feedpoint. If the feedpoint is
supported by a support pole, then that obstacle is not a problem.
73,
Don W3FPR
73,
Don W3FPR
On 9/18/2017 9:19 PM, Alan wrote:
Yes, I agree the shields should be connected together at both ends.
I believe the loss is the same as a single run of coax. Each coax has only
half the loss since it carries only half the power, but there are two of them
so the total loss is the same.
That assumes the SWR is the same in both cases. If 100 ohms is a better
match than 50 ohms, then the loss will be lower in the parallel configuration.
Alan N1AL
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