Question for Wayne about your OCF Dipole

[Wes Stewart-2]

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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