Topband: T Vertical feed

[Paul Christensen]

> My question is regarding the feeding of the antenna, should it be
> straight with 50 Ohm cable? Should I make some impedance transformer?
> (I have FT240-61) that I could use.

Based on your description, I just ran your "T" in 4Nec2 with the NEC/4.2
engine.  Your T should have a base Z near 21+j110 at 1825 kHz and will be 
resonant
at 1620 kHz which will help to push the max current point up the vertical
radiator and further away from the ground surface.  Keep in mind that the 
base Z of a T will vary quite a bit depending on ground condition and number 
of radials deployed.  The last 160m T I modeled for N4NN showed very close 
accuracy when compared to an N2PK VNA measurement after assembly.

You could use an ATU back at the shack end.  VSWR on the line near the base 
will be
roughly 15:1.  Use low-loss coax like LMR400 and total loss over 100
feet will be about 0.5 dB + tuner loss.  VSWR at the tuner end, owing to
line loss, will be about 12:1 in this example.  *However*, depending on the
exact line length, the Z at the tuner will be as high as 700 ohms, and as 
low as 4 ohms, so a high
quality shack ATU will be important especially when line length results in a 
line input Z in the single digits.
Not many commercial tuners do well on 160m with super low-Z terminations.

Or, build a low-pass L network at the antenna base after measuring actual 
base Z.  If it's near the 4Nec2 model, the L will consist of 14 uH series L 
and 1253 pF of shunt C across the base.  Make them both variable if possible 
and use high Q parts to
minimize loss.   A variable L with a typical Q of 200 would result in 5.5 
amps of coil
current (ref. 1500 watts) and a total network loss of 0.09 dB + matched line
loss.  I would keep the input coaxial feedline on the ground or in the
alternative, under the radials.  Use a high-quality 1:1 ratio current-choke
at the input to the L network.

Paul, W9AC



----- Original Message ----- 
From: <ct1ilt at sapo.pt>
To: <topband at contesting.com>
Sent: Wednesday, January 25, 2012 2:27 PM
Subject: Topband: T Vertical feed


> Hi Everyone,
>
> just finishing puting up a T Vertical (exactly in T shape) using 2
> supports at the ends.
>
> The vertical portion is 18m and the T is 17m to each side (34m total).
>
> I am laying radials from 20m - 40m (have about 20 right now).
>

> Thanks for help, getting ready here for CQ WW cw 160M.
>
> 73's Filipe CT1ILT aka CR6K
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK