Topband: Short receiving verticals question

[Jon Zaimes AA1K]

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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