Thanks Tom and others for detailed comments on this issue.
I understand the focus on bandwidth, presumably to keep the phasing
constant across 100 kHz or less of 160 meters, and see how that would be
important with fixed phasing. But in my bs/ef, the two forward elements
are fed in phase (equal-length feed lines to a T) and so are the two
rearward ones. The two feedlines from these Ts then run to the shack
where a DX Engineering NCC-1 (W8JI design) is used to adjust the
phasing between the two sets of elements. I'm thinking the variable
phasing should compensate for any changes in phasing across the band due
to narrow bandwidth of the elements, or any changes from variation due
to proximity to trees or in tree foliage change through the seasons. At
worst I would need to re-adjust the phasing control as I QSY up and down
the band. But maybe there's another issue I'm missing.
My first RX bs/ef has only been functioning a few weeks, including the
CW 160m contest, and is still a work in progress. For now the 23-foot
elements are fed directly with the feedline center conductor (no series
coil and resistor yet) so are resonant around 80 meters. They do have
the top hat wires though they are tied off rather haphazardly to
convenient trees. And the equal-length feedlines from the elements to
the Ts are simply laid on the ground, with no choking toroids at the
feedpoint nor a ground rod for the shield out 15 feet from the feedpoint
as I use on Beverage feedlines. About 100 feet of the two hardline runs
going to the NCC-1 are buried closest to the house. There is a ground
rod at the T junction. Elements have twenty 25-foot radials and are tied
to a 2-3 ft. ground rod and the coax shield at each feedpoint. I'm using
75 ohm Pentabond CATV cable for the four equql feedlines (184 feet each)
and 3/4 inch CATV 75 ohm hardline from the Ts to the shack. Because the
elements are not yet matched/tuned for 160, their output is lower than
the Beverages.
Even at this "development" stage, I have found this array to be almost
always better for hearing Europe than any of the three phased Beverage
pairs I have (535 ft and 750 ft stagger phased and 950 ft with 200 ft
broadside spacing). It is really cool to flip the switch to reverse the
phasing and see the difference in f/b, and also to tune through the
phasing range and find other peaks available from the continuous 0-360
degree adjustment, and to optimize the f/b to null out signals and noise
off the rear. I also found this array had some useful f/b on 80 meters
despite the very wide broadside spacing (but the endfire spacing is 1/4
wave on 80). The f/b on 160 meters on the short verticals is
significantly better than from any of the Beverages in nulling out a
persistent, intermittent local noise from the southwest that I've been
unable to track down.
I like W3LPL's suggestion to use the 7-foot posts to keep the wires
above deer level. And also a tip from WW4B to use lightweight fishing
line to tie off the ends, making this the weak link in the system, so
any falling tree branches will snap this line rather than the element
itself or the wire. Replacing the fishing line will be a lot easier than
repairing an element or wire in terms of ongoing, long-term
maintenance. I'll adopt both of these ideas here.
73/Jon AA1K
On 1/30/2014 5:24 AM, Tom W8JI wrote:
But what about an element loaded with a coil at the center or at the
top? Would there be advantages to that approach that would come close
to the short verticals with top-hat wires, or any serious disadvantages?
Jon,
The reason I use the hats and do everything I do in the elements is
bandwidth. Even at my quiet rural location on the quietest hour of the
quietest day, almost any element of reasonable height will have more
than enough signal level. This is why I base load and use a large hat.
While the large hat tends to keep current more uniform throughout the
element independent of coil location, and while more uniform current
increases radiation resistance, that effect is meaningless to me. The
entire goal for me is bandwidth, or a stable SWR vs. frequency.
Bandwidth is also why I load the element with a series resistance for
matching, instead of a network. I want to "swamp out" or dilute the
effects of resonance, minimizing element phase shift vs. frequency
change at the element terminals and preventing drastic changes in
element feedpoint impedance from mutual coupling between elements.
The hat is actually the bulk of the loading, and sets the current
distribution. The coil just cancels reactance. Since it is a series
network with the inductor forming a series tank with the termination
reactance, the lower the reactance used (compared to termination
resistance) the larger bandwidth becomes. You want the loading coil to
be terminated in the lowest capacitive reactance possible, and that is
at the antenna base.
Because voltage and current are out-of-phase above the coil, even with
high current, the impedance increases. This means the tradeoff in a
bottom inductance is increased voltage above the inductor. The antenna
is more "loss critical" above the coil for anything coupled via the
electric field, including a lossy dielectric.
This is a compromise of two things:
1.) Bandwidth
2.) Sensitivity to dielectrics around the element
Getting rid of the hat while the element is close to a tree does
nothing but bad things to both, but no one can say how much. The last
resort for me would be no "hats". Perhaps you can use T elements with
loading wires away from foliage that might change tuning or losses?
73 Tom
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