Topband: Short receiving verticals question

[donovanf at starpower.net]

Hi Doug, 


A few notes about the short verticals in my 160M passive 
receive array. I use them in my W8JI broadside-endfire 
passive array described in detail on Tom's home page 
and on W5ZN's home page. 

Its important to understand that the loss in the radial system 
of a 160M passive receive array is of no importance, but 
variations in the base impedance of the verticals during 
wet and dry weather could affect the pattern of the array. 
You don't need many radials, but you do need "enough." 

I use eight 65 foot radials under each vertical. Several of my 
verticals are in wetlands that flood during wet weather and the 
variation in ground conditions under the verticals is unusually 
severe. I initially used four radials and found there was nearly 
ten ohms change in the resistive component of the feed point 
impedance between flooded conditions and extreme dry 
ground conditions. Four additional radials solved that problem. 

My radials are simply laid on the surface of the ground. While 
the deer traffic rearranges the location of the radials, that 
doesn't seem the affect the performance of the array. I use 
stranded copper wire, solid wire would easily entrap the legs 
of the deer. 

Dozens of deer inhabit the field where my verticals are located. 
I eliminated deer collisions with the umbrella wires by attaching 
the ends of bottom ends of the wires to the top of seven foot 
fence posts (through a porcelain insulator and short length of 
light rope). I've never had a deer collision since. 

While some users of short verticals install foundations, I've found 
it completely unnecessary with guyed (e.g. top loaded) verticals. 
I simply use a two foot length of one inch diameter rebar. The 
vertical is attached to a 1.25 inch o.d. aluminum tube that simply 
slips over the rebar. Rebar is very inexpensive and easy to install 
an remove and especially convenient for temporary installations 
like mine 

73 
Frank 
W3LPL 



----- Original Message -----

From: "Doug Renwick" <ve5ra at sasktel.net> 
To: "topband" <topband at contesting.com> 
Sent: Thursday, January 30, 2014 3:04:02 PM 
Subject: Re: Topband: Short receiving verticals question 

I have used this same setup for my 4-square 160m receive array for years. 
Since I have to take down and put up this array every spring/fall, I have to 
re-tune each element for the 160m band. I have found that the base loading 
does not have to be exact for the system to 'work'. Last year I decided to 
make inductor substitution box for each element to easily tune each element 
close to 1.830 MHz. The biggest problem with the top hat is deer catching 
the wire/string and bending the element or some rodent eating the string. 
It's amazing how forgiving aluminum tubing is as I can straighten it many 
times without breaking. At the base I use a 2 ft ground rod and 4 short 
radials. I found the use of the ground rod makes a large change in the 
tuning of the element. 
Doug 

-----Original Message----- 

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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----- Original Message -----