[Antennas] Antenna length measurements

[Jack Painter]

> In the good-ole-days, the ARRL handbook had a formula for measuring the
> length of a 1/2 wave dipole that included the diameter of the wire or
> element.  Where we normally use 468/freq in MHz - feet, there was a
formula
> where the diameter of the element was calculated in to get a more precise
> answer.
>
> Does anyone remember what that formula is or where it may be found?  I
> loaned my 04 handbook to my kids so I don't have it.
>
> Thanks
>
> Buck

Hi Buck, just my 2cents here; 468/mhz *does* calculate the 1/2 wave for a
standard antenna within closer tolerances than we would normally ever be
concerned with. In as much as even the lowest impedance feedline changes the
swr at the tuner/transmitter, it is a perfect impedance match that most of
us strive for by trimming or adding tails to the dipole for a perfect swr at
desired frequency. Perfect resonance on the antenna itself is in my opinion
only meaningful if your transmitter was connected directly to it in free
space, a condition that never exists. Resonance has little bearing on
performance from the shack *thru* the antenna when the height above ground,
lossiness of the ground, and characteristics of our particular feedlines
have infinitely more impact than whatever trimmed-length resonance finally
exists on the wire itself.

Using insulated wire for an antenna does lengthen it electrically, but most
of us start with up to 10% excess length and trim to perfect swr at desired
frequency. In one recent case, even 10% was too little margin and I had to
add about 6" to each leg of an 8mhz 1/2 dipole that suspends under a 5 mhz
dipole. Maximum efficiency is in my opinion, the result of a perfect match
in it's installed condition, not whether the antenna is exactly resonant
from a free space calculation.

Best regards,

Jack Painter
Virginia Beach, Virginia