[Elecraft] Question for Wayne about your OCF Dipole

[Don Wilhelm]

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