Electrónica Montaje Decametrica Hf

[Capicua]

TRANSOM AND MASTHEAD MOUNTED
VERTICALS ( 10 m wip)
A very common antenna for boats is a vertical,
either a short mobile antenna or a full λ/4, placed on
the transom, as shown in Fig 53. Note that in this ex-
The pattern is again quite directional due to the presence
of the mast and rigging. On 15 meters, where the
antenna is approximately 3/4 λ, higher angle lobes appear.
On 40 and 15 meters, the feed point is near a current
maximum and is in the range of 30 to 50 Ω. On 20 meters,
however, the feed point is a very high impedance because
the antenna is near λ/2 resonance. One way to get around
this problem for multiband use is to make the antenna
longer than λ/4 on the lowest band. If the lowest band is
40 meters then on 20 meters the feed-point impedance
will be much lower. This antenna is non-resonant on any
of the bands but can be conveniently fed with a tuner
because the feed-point impedances are within the range
of commonly available commercial tuners. Tuners specifically
intended for marine applications frequently can




THE BACKSTAY VERTICAL (sloper)
A portion of the backstay can be insulated and used
as a vertical as shown in Fig 57. The length of the insulated
section will be λ/4 on the lowest band of interest.
Typically, due to the loading effect of the rest of the
rigging, the resonant length of the insulated section will
be shorter than the classic 234/f (MHz) relation, although
it can in some case actually be longer. Either
modeling or trial adjustment can be used to determine
the actual length needed. On a typical 35 to 40-foot sailboat,
the lowest band for λ/4 resonance will be 40 meters
due to the limited length of the backstay. Examples of
the radiation patterns on several bands for such an antenna are given in Figs 58 through 60.
The pattern is again quite directional due to the presence
of the mast and rigging. On 15 meters, where the
antenna is approximately 3/4 λ, higher angle lobes appear.
On 40 and 15 meters, the feed point is near a current
maximum and is in the range of 30 to 50 Ω. On 20 meters,
however, the feed point is a very high impedance because
the antenna is near λ/2 resonance. One way to get around
this problem for multiband use is to make the antenna
longer than λ/4 on the lowest band. If the lowest band is
40 meters then on 20 meters the feed-point impedance
will be much lower. This antenna is non-resonant on any
of the bands but can be conveniently fed with a tuner
because the feed-point impedances are within the range
of commonly available commercial tuners.