Weather Effects on Antenna Performance
Posted on September 30, 2012
This summer I built a ground mounted part 15 AM system as described here and have begun to accumulate data on its operation as reported here. It is a bit early to draw firm conclusions from the measurements but some things of interest have been observed.
The amount of “freight carried” is shown by the field strength produced by the system and to measure this I decided to use my base coil loaded antenna in my basement as the receive antenna and the Yeasu FRG-100 receiver and a DVM for signal strength measurement. The signal derived from the field strength and which drives the S meter circuits was brought out from the receiver for display on a DVM. The DVM reading was correlated to the RF voltage at the receiver antenna input by use of a RF source where the voltage can be set to known levels. From these data a calibration curve of DVM voltage vs RF voltage input was drawn and an equation was derived from this curve which allows for calculation of the RF signal voltage input from the DVM voltage reading. With the receiver connected to the indoor antenna it is possible to measure relative field strength produced by the transmission system without weather effects previously associated with the outdoor receive antenna. Concurrent measurement of the antenna system parameters is provided by use of a current transformer and oscilloscope as described in this post.
Field Strength Change With Rain
The antenna field strength decreased by as much as 3.7 dB after two days of heavy rainfall. It is not yet possible to explain this decrease lacking more data points but it was observed that the decrease in FS was slow after the beginning of the rain. During and after 1 inch of rain on the first day there was no decrease in FS and the decrease seen began on the second day about one half of the way through the day which produced another 1.08 inches of rain. The largest decrease in FS occurred on the following day with no rainfall with the maximum decrease of 3.7 dB. The FS began slowly increasing on the second dry day and is still increasing on this third dry day and presently is -1.3 dB relative to the FS before the rain.
Ground Resistance Change with Rain
The antenna system feed point R was measured at the end of an unusually dry month of July to be 57 ohms. Using a model that this resistance is the sum of the ground resistance and the loading coil effective resistance, the ground resistance is calculated as Rg = Rfp – Rcoil = 38 ohms. The measured (on the second dry day) Rg after 2.08 inches of rain in two days was 17 ohms. Paradoxically, this halving of Rg was accompanied by a decrease in the measured FS. Measurements of the antenna voltage, current, and input power show little change from those taken before and after the rain except for the v to i phase angle. This change in angle only explains 9% of the reduction which amounts to -0.8 dB. It was noted that there was an increase in the feedpoint power of 0.25 dB after the rain which is probably due to the transmitter’s response to the lowered feedpoint resistance, but the second dry day data reported above were taken after the phase angle was set to 0 and the antenna power restored to the before rain value of 82 mW in an effort to factor out the changes due to antenna tuning and changes due to antenna system input power. This technique of separating effects by normalizing the tuning and power is intended to show the changes due to other effects. These “other effects” dominate the decrease observed and are not yet explainable pending gathering more data.
There is a saying used when I was involved with medical research that states “With the data you do not have we can establish what you do not know.” Well, gathering data thus far has established some things which I know but has also illustrated that there are things which I do not know. For research inclined people such as I this is exciting because there is more to be discovered. For those who just want the answers this is frustrating but such is the nature of the beast. I hope as this progresses to be able to provide some reliable answers with confidence and will keep you updated.