Shack/Antenna evaluation question

tweiss3

Member
This may be an odd question, but I can't find a definite answer. When you are performing a station/antenna evaluation, where on the antenna do you consider as the "point" of radiation for your evaluation. Considering both scenarios below:
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1 - EFHF 80-10M wire antenna, attached to the peak of the roof, is 130' long, other end attached to a tree at 130'. Do you consider the middle of the antenna 65' from the house your point, or is the whole 130' your point of evaluation.

Scenario 2 - Coil loaded vertical (zero ground plane) antenna on roof, 26' long. Is the middle of the antenna the point (13' off the roof), or is the base your point of evaluation?

Just trying to get my ducks in a row if I wanted to turn up power.

nd5y

Member
As far as I know for a single antenna (not a multi-element array) the center of radiation is where the maximum current flows.
For a 1/2 wave dipole that would be the middle (feedpoint).
For an end fed 1/2 wave that would be the middle.
For a 1/4 wave vertical that would be the base (feedpoint).
for a base loaded (shorter than 1/4 wave) vertical that would be the base (feedpoint).

prcguy

Member
The highest current point is where maximum radiation will occur and on the EFHW 80-10 it will vary per band. On 80m it would be in the middle, on 40m there would be a current null in the middle and maximum current would be in two places, now in the center of each half of the antenna. You can look at the maximum current points in this example of an 80-10m OCFD and they will be the same as a center fed or end fed, etc. The Off-Center-Fed Dipole (OCFD)

For the coil loaded vertical maximum current is in the lower portion below the coil, so I would consider the feedpoint where max radiation occurs.