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@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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|