Cris,
I think you will have better control over a dedicated vertical section. I
recently assisted in the design of a Marconi "T" for N4NN. We used 600-ohm
open feeder line for the vertical section in order to create a "fat
radiator." While it looks like a transmission line going up to the T, both
conductors radiate with equal phase and are connected as one wide conductor.
For your 66 ft. high, 132 ft top wire, it will resonate at approximately 1.5
MHz and that's good since in resonating lower, the maximum current point
will rise above the base and base Z will also increase which has a secondary
benefit of minimizing loss in a low pass L network. I assumed 30 surface
radials, average ground, radial length equal to radiator height. At 1825
kHz, 4Nec2 shows your antennna's base Z to be 23+j175 with #14 wire. Z will
change with effective radiator thickness. Taking that value and entering
into TLW, a base L network will require a series L of 12 uH and parallel C
of 430 pF. I would make both pieces variable unless you make an actual base
Z measurement after construction with an RF bridge or VNA. A high-pass L
would be preferred to minimize network loss but unfortunately, a high-pass L
will not work with that base Z. Assuming a rough Q of 200 for L, loss is
about 2% with most of it occurring in the series L with 1.4 amps of current,
referenced to 100-watts.
For you or anyone interested, here's the .NEC input file of the antenna Cris
is proposing:
CM GM4FAM T Top
CE
SY hgt=66 'Height of T Top Above Ground
SY len=66 'Length of each T side
GW 1 10 -len 0 hgt 0 0 hgt #14
GW 2 10 0 0 0 0 0 hgt #14
GW 3 10 0 0 hgt len 0 hgt #14
GS 0 0 0.3048
GE 1
GN 0 30 0 0 4 0.003 24 #14
EK
EX 6 2 1 0 1 0 0
FR 0 0 0 0 1.825 0
EN
Paul, W9AC
----- Original Message -----
From: <cris@gm4fam.plus.com>
To: <topband@contesting.com>
Sent: Wednesday, January 11, 2012 5:41 PM
Subject: Topband: Marconi Tee
> Greetings Topbanders
>
> I wish to put up a Tee antenna for TB using a mobile tower as a support.
>
> 66ft high with approx 66ft on each of the two top wires.
>
> I can insulate the trailer supporting the tower from the ground using wood
> blocks and rubber car mats.
>
> Am I
> a) better off using the mobile tower as the vertical section of the tee or
> b) putting a supporting cross arm at the top of the tower with a vertical
> wire dropping down to the feedpoint (ie using the tower as simply an
> aerial support)?
>
> If b) what should be the minimum distance of that vertical wire from the
> tower?
>
> Any thoughts?
>
> 73 Cris
> GM4FAM
>
>
>
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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