Stability in yagi designs is directly related to antenna Q, those where the value is high will suffer from wet weather due dielectric loading effects caused by water droplets adhering to the elements surface (molecular adhesion).
In my previous study I looked at average Q-factor between 144 - 146MHz...
The following comparisons take into consideration a much greater frequency range 140 - 150MHz, this clearly shows the cut-off frequency at the HF end. Also revealed is the frequency range at the HF end in which unwanted out of band signals will be attenuated.
As a point of reference competitor designs and the famous DL6WU 10 element standard give a handle on the likely outcome.
Thanks to John Lemay G4ZTR for providing dry and wet analysis of the CQM14DXL long yagi design.
Provided that the cut-off is well above the desired upper bandwidth limit then good to reasonable stability will be the outcome.
When conducting tests odd quarter waves in the feedline should be avoided otherwise impedance transformation can magnify changes! Insertion of a temporary quarter wave coaxial cable section (343mm with Vf of 0.66) into the feedline at the shack end will show if a fault condition/problem is present... If no fault exists then introducing the quarter wave section will make no difference to measured SWR... To be certain do this test when the antenna is dry and then wet!
Derek Hilleard G4CQM
|WA6XX Average Q-factor 66 (140-150MHz)|
|WS8C9 8 element Average Q-factor 35 (140-150MHz)|
|DL6WU 10 element Average Q-factor 27 (140-150MHz)|
|WAXXX10S Average Q-factor 64 (140-150MHz)|
|WA10XX Average Q-factor 39 (140-150MHz)|
|G0KSC 11 element LFA Average Q-factor 114 (140-150MHz)|
|YU7EF EF0213M Average Q-factor 370 (140-150MHz)|
|144NX13S Average Q-factor 360 (140-150MHz)|
|144NX14 Average Q-factor 161 (140-150MHz)|
|CQM14DXL Average Q-factor 623 (140-150MHz)|