[Elecraft] Question for Wayne about your OCF Dipole

[Barry]

Fellas,
     I think this subject is being over thought. An OCFD is nothing more 
than a dipole cut for the lowest operating frequency with an off center 
feed. By doing this, it allows reasonable match for multiples of the 
fundamental frequency.

     If a center fed dipole is ~50-75 Ohms at say 80 meters, it will be 
somewhere around 700-1000 Ohms on even multiples. Feeding the antenna 
off the center just puts the feed at a point that reflects a more 
reasonable impedance.  At this point there are two ways to go, the 
Carolina Windom way or the straight OCFD.  I have used both 
configurations and they do really work well if you can get the antenna 
up in the clear. Feeding it with something like LMR-400 keeps the losses 
reasonable even with an SWR that may be 4 or 5:1. In both cases a 4:1 
balun should be used, a current type if you use the simpler OCFD. Every 
Elecraft tuner I have used handles this without breaking a sweat. Happy 
DXing!

73,
Barry
K3NDM

------ Original Message ------
From: "Don Wilhelm" <donwilh at embarqmail.com>
To: elecraft at mailman.qth.net
Sent: 9/18/2017 10:20:22 PM
Subject: Re: [Elecraft] Question for Wayne about your OCF Dipole

>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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