On my Cushcraft MA160V, the resonant SWR point has lowered from almost 5:1 to 1:1 without the Amidon unUn in line. All 26 of the buried radials are attached. The main difference is that the normally dry ground is saturated from 6 inches of rain in the past two weeks. Can the water saturated soil really make this much of a difference? The last time this antenna showed this good of an SWR, was when I had no radials attached, in dry dirt, as expected- Of course attaching radials only began to show the true impedance mismatch, thus the need for the Amidon UnUn to match the coax to the antenna- Bottom line, the antenna is acting like I disconnected the radials, which are very much intact- My guess is that as the soil drys out, my SWR at sresonance will once again rise, and i will need the UnUn again for matching reasons- does this sound right>? > > 73 Paul N0AH Paul,
What you experienced could be a misleading indication as ground conductivity with moisture depends a lot on the actual type of soil. with a vertical that is less than a quarter wave the direct feed impedance should be very low. What could be happening is that in order to get to a 1:1 you probably have a significant loss resistance which brings in the 1:1 reading. This is similar to what placing a non inductive resistor across your feedline. IMHO it is better to measure with a bridge the actual base resistance with a ground system of known efficiency on 160. If you have 26 radials all 1/4 wave long on 160 the amount of moisture (fresh water moisture) in the ground you should not get that kind of variation and not for an antenna that has a low feed point impedance. On a dry salt lake or high salinity soil the results with mixing water would be more noticeable but I don't think this is what you have there. My point here is that one of the reasons the FCC requires AM stations to have a ground system as part of their application is so there is a constant radiation with a certain power level that will maintain the radiation pattern and microvolts per meter at a given distance. A good adequate radial system is supposed to prevent the very phenomenon you described. Herb Schoenbohm, KV4FZ > > > _______________________________________________ > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
