Posted by
k6dgw on
Aug 26, 2020; 12:01am
URL: http://elecraft.85.s1.nabble.com/Inverted-L-for-160-meters-tp7664325p7664329.html
**interspersed ...
On 8/25/2020 4:23 PM, kevinr wrote:
> From what I can find, and what I can calculate, five wires, each ~130
> feet long, could make a nice inverted L for 160 meters. One leg for
> the vertical and horizontal portion of the antenna. Four legs for the
> counterpoise (ground plane). There are many broken wire antennas
> stored in my shed which are fodder for the radials. Do all of the legs
> for the counterpoise need to be 1/4 wavelength or can I substitute
> some shorter lengths?
**Radial elements on the ground can be of any length, longer [up to a
point] is better. Their purpose is to provide a lower loss path than
the earth for the return circuit. Elevated radials should be tuned,
usually 1/4 wave, to be effective. You can think of them as the "other
half" of the vertical radiator, and if you had room and could fold them
down, you'd have a center-fed half wave antenna fed at 180 deg.
> The feedpoint should somewhere above 50 ohms impedance. As I add
> more radials that number will reach 50 ohms asymptotically.
**Radiation resistance will be somewhere in the 50 ohm ballpark.
Generally, the radiators are not resonant due to physical constraints
and there will be some reactance in the feedpoint impedance. That is
generally cancelled with a series capacitor [center conductor to
radiator] or base inductive loading.
>
> I can get the vertical part up to 70 or 80 feet above ground with the
> rest of it horizontal. Most of the radiation should take place from
> the vertical part since it is closer to the feed point.
**True. For the most part, the horizontal part acts as a capacity hat.
Shipboard antennas for 600 m were often one or more horizontal wires
between two masts with a downlead to the transmitter. Most [nearly all]
of the radiation came from the downlead.
> But there should be some effect from the direction of the horizontal
> portion. How strongly does the direction of the horizontal portion
> effect the radiation pattern of the antenna system?
**It will have a small effect. So will the arrangement of the radials
if they are asymmetric in spacing around the radiator or in length. In
every case I've seen, the "Fly Poop Axiom"++ applies: Unless you're a
broadcast station seeking to maximize field strength at the limits of
your effective coverage, the effect is way below the other vagaries of
Top Band.
> My property allows me to point it from 300 degrees around to 200
> degrees so I have plenty of options. Between 200 and 300 degrees
> there is a road used by loggers, and the folks maintaining the towers
> at the top of this mountain. They can break any antenna lower than 80
> feet above ground. Spar poles and cranes clear out any dead limbs
> across the road.
The effect of having one of them take down your antenna will be far, far
greater than that produced by precisely aiming the horizontal wire over
their territory. Keep it away from the loggers ... with today's
equipment, they rarely look up anyway. It's the Fly Poop Axiom vs the
Elephant Poop Axiom. [:-)
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
++Effects and changes so small they are like trying to pick fly poop out
of the pepper." You can use your imagination for the elephants [:=)
>
>
> Inquiring minds...
>
> Kevin. KD5ONS
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