[Elecraft] Antenna Analyzer

[Don Wilhelm]

Bob and all,

The "classic" formula for a wire half wave dipole (known to most hams) 
is 468/F (length in feet and frequency in MHz), and your "492 x K" 
factor is correct.  Sorry for those who use metric, you will have to 
convert)

However, that may not lead to a resonant antenna due to a number of 
factors - height above ground, proximity to other objects, diameter to 
length ratio of the wire among the most obvious factors.

In his years on this planet, L.B. Cebik long preached that "cutting 
formulas" such as 468/F were only an approximation.

Armed with an antenna analyzer and a calculator plus measuring tape, you 
can cut an antenna to be 'resonate' at the shack end of the antenna 
feedline in two tries.  The following technique appeared in QST 
Technical Correspondence for May 2017 (yes, it was my contribution).

The steps are as follows:
- Cut the antenna according to the 468/F formula and add 10%.  Write 
down the length.
- Put it into its final position and with the antenna analyzer, measure 
its resonance point (take your pick of whether that is point of lowest 
SWR or the point where the reactance goes to zero).
- Multiply the length of the antenna by the frequency you picked to call 
the resonant point.  That will yield a number to replace the "468" in 
the cutting formula.
- Use the resulting number and divide by the frequency where you want 
resonance to be.  That is the new length of the antenna.
- Cut the antenna to the new length and put it up and operate.

This works well for HF.  VHF may have other factors involved, so you may 
have to check the results again after the initial adjustment, but the 
principle remains the same.
This method takes into account the antenna surroundings and shortens the 
"cut and try" process considerably.

73
Don W3FPR


On 12/1/2017 3:22 PM, Bob McGraw K4TAX wrote:
> 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.}
>