[TowerTalk] [Bulk] A Capacitively Fed 102 Degree Vertical for160 Meters

[tedc at nucleus.com]

Grant,

If your feedpoint Z is 25 ohms then your antenna is a
90 degree antenna.  I cannot use a wire antenna due
to lack of tall trees in the area.  I could probably
switch in combinations of fixed mica capacitors but
VA6POP who lives here in Calgary advises that he
could make the variables and I am planning to use the
auto tune route.

Ted


------- Original Message -------
>From    : Grant Saviers[mailto:grants2 at pacbell.net]
Sent    : 3/22/2015 6:45:39 PM
To      : tedc at nucleus.com; towertalk at contesting.com
Cc      : 
Subject : RE: Re: [Bulk] [TowerTalk] A Capacitively
Fed 102 Degree Vertical for160 Meters

 Mine 160m tunable T works almost exactly as you have
modeled.  It is 
12ga wire supported by a fir tree.

My tuning is 3 x 2000pf caps (Russian screw mount
transmitting micas 2 x 
1000pf 1kv paralleled for more amps) in SERIES
shorted by 8a 250vac pcb 
relays.  It works out for my vertical 85' to top x
two 33' top load 
wires to make a T, seven radials 125' elevated 10'
that each series 
capacitance (short), 2000pf, 1000pf, 666pf moves
resonance 50KHz.  1.5:1 
swr bandwidth is 50KHz.  Interestingly the open
circuit cap/relay 
voltage stress is always the same for each
capacitance value and no 
problem at QRO, about 370v IIRC, but nothing is ever
hot switched.  The 
pcb relays are rated 5kv contacts to coil which is
more important, IMO.  
I made a rotary switch frequency selector with diode
steering to select 
the 4 relay combinations.

Since the antenna is 25 ohms at 1810 resonance I use
a 50:25 ohm TLT and 
the caps/relays are on the 25 ohm side which is why I
paralleled the 
micas to easily handle more amps.  I also use a
ground shunt, a bunch of 
16ga wire on two FT240-31 for about 7000 ohms at
1.8MHz, only about 5 
watts in it at QRO.

So far all is fine after a 3 years and a few 160m QRO
contests. However, 
I do want to move the top of the T about 6' out from
the tree, it is now 
1' out and from some research I think the tree
coupling is costing me a 
few db's.

Grant KZ1W

On 3/22/2015 16:56 PM, tedc at nucleus.com wrote:
> I have been doing some design work for a vertical
> that will give me less than 1.3:1 VSWR across the
> entire 160 Meter band with either very little manual
> adjustment or with the right circuitry the antenna
> will self adjust.
>
> My design is based on the fact that at approximately
> 102 degrees of length a vertical should exhibit a
> feed point impedance of 50 +j100. The feedpoint must
> be isolated from ground so a very large inductive
> shunt must be used to ground the tower for static
> discharge, something in the order of XL of 2000 ohms
> or greater so as to have little effect on the
> feedpoint impedance. Based on using a triangular
> tower with a 12inch (30.48 cm.) face a tower of 137
> ft. (41.76 M.)would be required.
>
> By feeding the tower via a variable capacitor with a
> range of 820 pf to 2600 pf a match of less than 1.3:1
> is obtainable over the entire band. If the operator
> had favorite phone and cw operating frequencies fixed
> values of capacitance can be switched in remotely or
> a remote means of varying the capacitor must be
> employed. A monimatch type device could also be used
> to control a stepper motor and the antenna could
> adjust its self.
> e
> As with all verticals an effectve ground system is a
> must, be it radials on or in the ground or an
> elevated system.
>
> This same antenna could be used in a Four Square
> configuration if one has enough space for an even
> more extensive ground system.
>
> My current situation prevents me from actually
> building this antenna and it has been primarily a
> design exercise incorperating some ideas that I had
> used in a 40 Meter design some years ago. The 40
> Meter design worked very well and I could obtain a
> VSWR of less than 1.2:1 anywhere in the band. Any
> thoughts or idea's as to the relevance of this design
> is apreciated.
>
> Ted Clapahm VE6AMR
>
>
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