[Elecraft] Antenna Analyzer

[Bob McGraw K4TAX]

Agreed.   I've always said hams cut an antenna a bit long and then trim 
it to get the lowest SWR at the desired frequency. This does not 
necessarily make for a resonant antenna.    My 75M antenna is resonant 
at 3.8 MHz as determined using a Grid Dip Oscillator,  but the impedance 
is about 35 ohms giving a SWR of about 1.4:1 .  At 3.9 MHz the SWR is 
1:1.  The length is actually 123 ft.  But the SWR 1:1 point indicates 
the antenna is 119.8 ft in length.

The resonant electrical length of a 1/2 wave dipole antenna is defined 
as Length being = (492 x K) / Frequency [MHz.]    This does not assure 
it will have a 1:1 SWR at the design frequency, but only that it will be 
resonant at that frequency.   {K = ratio of 1/2 wavelength to conductor 
diameter.  Typically for wire HF antennas a value of 0.95 is used.}

Just remember, the radiation resistance of the antenna will vary 
according to height above ground.  Thus a 1/2 wave dipole at resonance 
will have its center feed impedance vary from some 20 ohms at 1/8 
wavelength above ground to over 90 ohms at 3/8 wavelength above ground.  
And at 1/2 wavelength above ground the impedance is near 70 ohms.  Of 
course ground conductivity will affect the actual results.

73

Bob, K4TAX


On 12/1/2017 1:54 PM, John Harper wrote:
>> Elecraft rigs, including the KX2, have an indicator for SWR that the rig
>> sees. Resonance can be clearly seen by moving up and down the band to
>> find it.
> Not necessarily. Resonance and point of lowest SWR do not always coincide.
>
> John AE5X
> https://ae5x.blogspot.com
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