I would have to know just exactly how your "Ground R" was constructed to
comment. The two EZNEC variables for ground description are dielectric
constant and conductance. Also there is a lot depending on the T dimensions
and L dimensions in any comparison that would preclude any simple answer
that depended only on ground characteristics.
Also to really see what is going on, you would need superimposed patterns
modeled from a property's real dimensions. The radiation resistance
numbers sound like free space or Mininec ground, both with necessary
interpretation work to navigate.
The answers for an L depend enormously on the ratio of vertical to
horizontal, and also independently on the electrical length of the
horizontal as a top load. The simple comparison does not take account of a
good practice extending the horizontal to approximately 88 feet regardless
of vertical height, to make sure the vertical wire has the most integrated
current, all the way up to the bend, regardless of vertical wire height.
This venture into non-resonance for the sake of performance, reduces the
difference between a T and an L.
In a comparison of a T and an L, set the T using X vertical feet, with say
plus and minus 40 feet for the T wires. For a fair fight, the L must have
the same X vertical wire with an 80 foot horizontal, with the L's NVIS
angles and the enhancement on the side opposite the horizontal taken into
account.
An inverted L with 65 feet vertical and 88 feet horizontal, even over a
commercial grade counterpoise, certainly does not have 8 ohms
radiation resistance. And it is taking advantage of reduced current in a
counterpoise with ham grade radials.
Beyond that, first off the counterpoise should have been evaluated. On 160
antennas requiring a counterpoise, the counterpoise efficiency is the two
ton elephant in the room, with nothing in 2nd place on an awfulness scale.
Regardless of T and L particulars, even an accurate difference is heavily
swamped by any inefficiency in the counterpoise. "A few radials on the
ground" just to get one on the air, can be remarkably lossy.
A peek at the original poster's photos on QRZ, show he was using a tower
to support the bend in his L. Add to any counterpoise loss the losses from
a tower supporting a close vertical wire (either T or L). A tower
supporting the bend of an L or the center of a T should be considered a
secondary winding in a tightly coupled transformer, with the vertical wire
as the primary, and the tower base connected to universal ground through a
big resistor
Those two issues can easily nullify the gain of an amplifier. The total
loss in the two issues smashingly exceeds the difference between T or L
aerial wire choice by an order of magnitude.
After one has taken care of the elephant in the room, back to the question
of T or L, the NVIS-defeating high angle hole in a T or pure vertical is a
real disadvantage for 160 only contests where same continent QSO's can
dominate the score. With the T you can have huge skips if the band goes
longish, losing a lot of Qso's. Worse, you can have your run frequency
taken because the other guy, in a very deep skip zone close to you,
couldn't hear you were there.
The comparison of a given specific T and L is actually a quite complex
question, and there are no simple answers. One has to assess the various
loss issues of the specific implementation and surroundings. No
one-size-fits-all answer. Sometimes one has to model every conductor on the
property, and often because of buried conductors, antenna and otherwise,
has to do it in EZNEC Pro/NEC4.
And often a huge improvement will have nothing to do with a T vs. L aerial
wire.
There are ways to minimize tower current when a tower has to be used to
support the aerial wires, but I've heard none of that discussed here.
73, Guy K2AV
On Tue, Mar 2, 2021 at 2:20 PM Dave Cuthbert <telegrapher9@gmail.com> wrote:
> John, here are EZNEC results for your downward sloping inverted-L and your
> T-vertical.
>
> Inverted-L radiation resistance = 8 ohms
> T-vertical radiation resistance = 12 ohms
>
> How the radiation efficiency compares depends on your ground system. The
> efficiencies for 2.5, 5, and 10 ohm ground systems are compared:
> *Antenna GND R Rad Eff * *T over L*
> L 2.5 76%
> T 2.5 83% 0.4dB
>
> L 5.0 62%
> T 5.0 71% 0.6dB
>
> L 10 44%
> T 10 55% 0.9dB
>
> L 20 29%
> T 20 38% 1.2dB
>
> Dave KH6AQ
>
>
>
> On Mon, Mar 1, 2021 at 10:20 PM John <w5jmw@towerfarm.net> wrote:
>
> > Hello all.I currently have a coaxial inv. L.apex at 50ft and rest going
> > out at abt 45 degrees.The end is abt 20-30 ft high.It has been a very
> good
> > antenna.I used a l because I did not have the option of 2 masts.I am
> > changing a bit.Doing away from vhf/uhf beams.So I have another 50 ft
> > mast.They are abt 200ft apart.My question is…Would a tee type vertical be
> > better that an L.Yes I could elevate 2 or 3 radials.I am using a radial
> > ground on the inv.L.Thoughts on this please.I am not really concerned
> about
> > matching.Just effenciency.Is a tee better? Thanks,john
> >
> > Sent from Mail for Windows 10
> >
> >
> >
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