Laszlo,
As I understand, you are mounting this on the roof of your building, so
the ground system has to stay within the 20x26m roof area.
From my model in EZNEC, it looks like it would be better to use the
single horizontal 22m top wire. This produces the highest radiation
resistance (about 15 ohms) and better efficiency and bandwidth. This
does have about 0.7 dB F/B, with the higher signal in the direction
opposite the open end of the top wire. To reach resonance with the
relatively small ground system (described below), you will need about 10
uH of inductance in series with the feedpoint. You can match 50 ohms by
tapping up on the loading coil with the bottom of the loading coil
connected to the ground system. Loss in the loading coil should be
under 0.2 dB with a coil Q of 200 or more. The bandwidth is pretty
narrow - about 35 kHz for under 2:1. This is with #14 wire throughout.
Using the four sloping top wires, I found they each only needed to be
about 14m long to obtain resonance (no loading coil or capacitor).
Since the wires slope down, they tend to cancel part of the field from
the current in the vertical section and this lowers the radiation
resistance to about 7 ohms. The pattern is quite omni, however, it is
down about 0.5 dB from the inverted L configuration at a 20 degree
elevation angle. Due to symmetry, it does have a null at high angles
(while the inverted L has a lot of signal at high angles). The 2:1 SWR
bandwidth is about 25 kHz.
I didn't look at the slightly sloping 20m top wire, but I would expect
it to be slightly worse than the longer horizontal top wire.
I assumed a ground counterpoise with a perimeter wire around the roof
(20x26m), with 8 wires from the feedpoint. Four wires go to the corners
of the rectangular perimeter wire and four go the the midpoints of the
sides of the rectangle. It looks like a rectangular spider web. Since
your antenna is quite elevated, actual earth losses should be low, but
coupling to the building roof, wiring, and plumbing will likely change
the resonance from what my model shows (which is assumed to be floating
at 30m above ground). Adding more radial wires to the counterpoise will
reduce the inductance needed to resonate the inverted L. If you don't
use a current balun (choke) at the feedpoint, the shield of the coax
will become a radiating part of the ground system and change the tuning
of the counterpoise - which can be good or bad depending on length and
routing.
If you need more details, let me know.
73, Terry N6RY
On 2009-09-04 1:14 PM, Laszlo wrote:
> I like to make a inverted L for 160m.I have limited space.
> I live in a block of flat in the inner city, the house is 30m high, houseroof
> is only 20x26m.
> The vertical part of inverted L will be 18m.
> My question:
> 1.Which is better efficiency?Using only 1 top wire(22m) in horizontal or 1
> top wire sloping down a little(about 20m) or using 4 top wire(each is 17m)
> sloping down at 45 degree?
> 2.If I use 4 top wire ,antenna will radiate 4 direction? Normal inverted L
> radiates only one direction? Lot of difference or not?
> 3.I have limited space for radial,no space for 1/4 wave radial ,I have only
> 15m(45 foot) for each direction?
> Anybody tried to use trap radial? To use a coil or capacitor in each
> radial? Or not?
>
>
>
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