At 08:49 AM 2005-10-16, Bob K0RC wrote:
>I slapped together an 80m dipole using the L=468/F formula and find
>that it resonates about 200 KHz lower than expected.
>My target frequency was 3.775 MHz. That's 124 feet on my calculator.
>
>I built it using some left-over solid copper 12 gauge plastic
>jacketed house wire. I center feed it with 100' of RG-59/a coax. One
>end is up 60 feet, the other end about 40 feet. Using my AEA HF
>Analyst, it dips at 3.585 MHz with a VSWR of 1.6:1. This is a 5.3%
>difference in length from the formula.
Just for fun, I put your antenna and feeder into EZNEC+ over average
ground. Here's a look:
At Antenna
==========
Bare #12 conductor F=3.809 R=72.8 SWR(50 ohm)=1.46 or L=472.3/F
#12 w/0.05" PVC insul F=3.681 R=67.1 SWR(50 ohm)=1.34 or L=456.4/F
Looking into 100' of 75 ohm, VF=0.66 (lossless)
===============================================
Bare #12 conductor F=3.799 R=69.7 SWR(50 ohm)=1.39
#12 w/0.05" PVC insul F=3.663 R=63.0 SWR(50 ohm)=1.26
It's worth remembering that 468/f is just a working approximation
which gets you in the ballpark. It assumes that the antenna will
resonate about 5% lower than a free space half wavelength (due to end
effects, etc.). I don't think "468" is etched on a stone tablet.
Of course, your immediate environment will also impact resonance and
feedpoint impedance. Further, coax impedance and velocity factor are
subject to normal manufacturing variations.
And test equipment never lies. 8-)
73 Terry N6RY
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