As shown in the photos below, the radio8z transmitter and antenna are located in the back yard near bushes. The loading coil is tuned by moving a small series coil closer or farther from the main coil. The best performance at resonance historically results in a total power into the antenna of 85 mW.
With generous rain this spring and summer the bushes have grown enough to surround the antenna and touch the antenna at some points. After trimming the bushes a little the measured power to the antenna system was 41 mW with the phase angle 45 degrees inductive. More trimming of the bushes and tuning the coil brought the power up to 65 mW at 10 degrees inductive. The expected 85 mW could not be achieved due to the coil tuning being at the mechanical limit before reaching resonance.
It is thought that more radical bush trimming will be needed to restore the expected power but doing so will ruin the aesthetics so the small gain in signal strength expected is deemed not worth the effort.
It is apparent that the bushes being in such close proximity to the antenna greatly affects the system performance. Come November when the leaves drop it will be interesting to check if the system performance improves and by how much.
Neil
Would that be from moisture? I would have thought that wood would be RF transparent at these frequencies.
Artisan, that would be my best guess. The bushes are close enough to the radiator to increase the capacitance to ground, hence the inductive phase difference between antenna voltage and current.
I'll try to repeat this measurement when the leaves are gone.
Since I first posted I had a chance to do a test/listen drive and the range is about what it usually was previously, maybe a bit more noise. By theory, a drop from 85 to 65 mW is a 13% drop in far field field strength...not negligible but also not a lot.
Neil
Well, the leaves are gone and I did some maintenance on the outdoor transmitter. All is well.
Our Small Radio Arena
Radio8Z's experience matches general observations made over several years of tending both AM and FM part 15 operations.
Changes to antennas, transmitter outputs and the surrounding terrain, are typically observed as very slight using whatever method of evaluating signals.
We all go through periods of looking for the big break-through when we hope that some new trick will exhibit a huge jump in range while still satisfying the rules, but in reality the differences are always very slim and barely noticeable.
I recently tried to boost the reading on the S-meter of the Grundig SATELLIT 750 radio by using a different antenna on the Wholehouse 2.0 Transmitter, and I "think" the meter went up by one hair.