[TowerTalk] OCFD: Should I be able to do better?

[Joe Subich, W4TV]

On 2021-05-27 4:50 PM, David Gilbert wrote:
> Joe,  you are of course correct, but I think maybe he said 576 mohms,
> as in 576 milliohms, although he didn't type it very clearly.

Except that 50.1-j0.576 Ohms at 6.68 MHz and 33+j7.11 Ohms at 7.15 MHz
do not make sense.  Generally, the real part of impedance *rises* above
"resonance" (along with the inductive reactance).

On the other hand, the lowest band on an OCFD generally shows minimum
SWR lower than would be desired.  EZNEC shows "resonant" points (to the
nearest 25 KHz) for a 69' wire 35' above ground fed 63% from one end:

     6.9   MHz  94.1 +2.43 Ohms  (2.13:1)
    14.1   MHz 201.9 +3.56 Ohms  (1.02:1)
    21.175 MHz 519.2 +11.4 Ohms  (2.57:1)
    28.3   MHz 113.9 +2.7 Ohms   (1.76:1)

The typical solution would be to place a 220 pF capacitor in series with
the long leg to 'tune out" the reactance on 40 meters.  The effect of
the capacitor is progressively less on the higher bands but would raise
the real impedance and resonant frequency slightly.

If I were trying to build a 40/20/15/10 OCFD, I'd start with 70 - 72'
with 80/20 split and a 150 pF cap in the long leg (plus a 4:1 Guanella
balun *followed* by *two* K9YC chokes of 11 turns RG-400 on FT240-31
cores).

73,

    ... Joe, W4TV


On 2021-05-27 4:50 PM, David Gilbert wrote:
> 
> Joe,  you are of course correct, but I think maybe he said 576 mohms, as 
> in 576 milliohms, although he didn't type it very clearly.  He couldn't 
> ever have had a VSWR of 1.012 with j576 ohms.
> 
> 73,
> Dave   AB7E
> 
> 
> On 5/27/2021 12:01 PM, Joe Subich, W4TV wrote:
>>
>>> The closest resonant frequency is 6.68MHz, with an VSWR of 1.012 and
>>> an impedance of 50.1-j576m ohms.
>>
>> That is *NOT* a resonant frequency - not with -j576 Ohms.  Your measured
>> value at 7.15 MHz (+j7.11 Ohms) is much closer to resonance.  I would
>> estimate the actual resonant frequency ( j0.0 Ohms) to be about 7.13
>> MHz.  Note: *Resonance* is defined as j0 ohms regardless of the real
>> part of the impedance.
> 
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