Hi Steve,
Poor performing ground systems affect directive receiving antenna performance in three fundamental ways: - degrading the feed point VSWR - by degrading feed efficiency which can upset the performance of phasing networks in a phased array. - allowing intrusion of feed line external shield currents into active or passive feed point matching networks The directive performance of Beverage antennas is relatively insensitive to ground quality. At the termination end of a single wire Beverage the total termination resistance (resistor plus ground resistance) drifts as ground conditions change, but Beverage side lobe and rear lobe performance isn't very sensitive to sub-optimum termination resistance A poor preforming ground system a t the feed point of a single wire Beverage has relatively little affect on the 160 meter efficiency of a single wire Beverage because there's normally much more signal at the feed point than needed. The biggest affect of a poor preforming ground system at the feed point of a Beverage is that undesired signals conduct from the outside of the shield of the feed line into the feed point matching network . We can suppress these unwanted signals either by using a low impedance ground connection (multiple ground rods and short radials) or by using a high impedance feed line choke (the commonly used 9:1 transformer with isolated windings provides inadequate feed line external shield isolation). A sparse ground system at the Beverage feed point allows unwanted signals to easily couple into the feedpoint matching network degrading the sidelobe and rear lobe performance of the Beverage. A sparse ground may be further degraded by frozen ground. A poor preforming ground system also degrades phased arrays (arrays of short verticals or Beverages) in a similar manner. - by degrading the VSWR at each feed point of the phased array as ground moisture varies or when the ground freezes. potentially upsetting the signal level balance and phase relationships at the phasing network. - by degrading the side lobe and rear lobe suppression of the array if feed line external shield currents intrude into the active or passive matching network at the feed point of each phased array element. 73 Frank W3LPL ----- Original Message ----- From: "Lee STRAHAN" <[email protected]> To: "VE6WZ Steve" <[email protected]>, [email protected] Sent: Wednesday, December 7, 2016 10:23:40 PM Subject: Re: Topband: RX ant-Frozen ground When we at Hi-Z see deterioration it is often the output cable running to the receiver where the typical aluminum shield no longer makes great contact. Also if it's one of the Hi-Z Antennas amplifiers make sure you are seeing 4 to 5 Volts DC at the element or amplifier input. Foliage touching an element has often been the culprit as well. Once even a spider crawled across the amp terminals and the 5 volts killed it and left a partial short. You just never know. Lee K7TJR -----Original Message----- From: Topband [mailto:[email protected]] On Behalf Of VE6WZ Steve Sent: Wednesday, December 07, 2016 1:36 PM To: [email protected] Subject: Topband: RX ant-Frozen ground I’m wondering what experiences are out there regarding small vertical array, or beverage performance in the winter over frozen ground. The reason for this question is that over the last 2 weeks or so, my HI-z (24’ vertical) SDR skimmer antenna at my remote QTH has shown somewhat diminished performance and a dramatic increase in AM inter-mod byproducts on 160. During the somewhat hasty install of this antenna (the SDR “RBN skimmer” antenna was not considered an important part of the station) I had some trouble driving the ground rod for the HI-z amplifier and only ended up with about 3’ of copper pipe driven into the ground. During this past summer and fall the performance was good. Winter has been here for a while. Last night it was -28 deg C. Today, that rod is now probably sitting in a block of solid ice. (ok, not solid ice, but the conductive dissociated ions from the native salts distributed within the soil are now locked in ice) In truth I am not really concerned about this particular antenna, but I do plan to install an 8 circle array as well as some beverages this summer and this problem got me thinking about making sure I have a “seasonally stable” ground system up here in the frozen north. Some quick research points to potential frost depths ranging from 6’-8’ depending on snow cover and soil type. Perhaps driving deeper rods could help, but it seems to me that there will still be a seasonal change of consequence. Literature associated with various short vertical array products indicate that 3’-4’ rods should be adequate, but is this true in a northern climate? For the most part, I could care less whether the array works in the summer…..it’s during our winter DX season that I want performance. Perhaps a very carefully designed and symmetrical short ground radial system could stabilize things, but I know with phased rx vertical arrays this can be a big problem and cause phase imbalances. Perhaps on my beverages a small radial system at the termination would be helpful. I am not new to rx antennas, having built and used numerous single wire beverages, reversible beverages and end-fire phased beverages as well as passive and active (Hi-z) phased vertical arrays at my home QTH, but have never “noticed” a problem. BTW, it is possible that there is something else going on with my SDR antenna other than a ground problem, and I will be checking that out next time I’m at the station. However, I think I’ll still want to consider the frozen ground impact even if there is something else going on. Any experiences, measurements or ideas from those living in places where it snows a lot, you have something called a “block heater” for your car, and when you work outside you last about 5 minutes before your hands and toes are numb?? de steve ve6wz. Steve Babcock Lead Geologist, Mature TAQA North Ltd. D +1.403.724.5147 M +1.403.870.0082 2100, 308 – 4 Avenue SW, Calgary Alberta Canada T2P 0H7 www.taqaglobal.com TAQA is the brand name of Abu Dhabi National Energy Company PJSC If you no longer wish to receive email from TAQA, please click to Unsubscribe This message contains confidential information and is intended only for the individual named. 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