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" <k7tjr@msn.com>
To: "VE6WZ Steve" <ve6wz@shaw.ca>, topband@contesting.com
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:topband-bounces@contesting.com] On Behalf Of VE6WZ Steve
Sent: Wednesday, December 07, 2016 1:36 PM
To: topband@contesting.com
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
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