Progetto per una verticale di WA7RAI

Ho trovato questo facilissimo progetto per una verticale per i 20 metri che ha anche un discreto guadagno sul radiotore isotropico circa 3,1 db
Le misure sono inglesi ed anche il testo, ma si può sintetizzare così:

tratto radiante 1/4 lambda
supporto metallico pari a 1/4 lambda
Lo stub (adattatore di impedenza) in parallelo all'alimentazione è semplicemente 1/8 lambda x 0.66 in RG-213 cortocircuitato al termine
4 radiali (contrappesi) sotto al punto di alimentazione di lunghezza pari al tratto radiante, collegati al polo freddo
un buon piano di terra alla base del supporto (sostituibile con radiali accordati).
I disegni sono visibili cliccando sul link nel testo in inglese.



A LOW ANGLE VERTICAL ANTENNA for 20m

Published and Copyrighted 11/20/2007 by S. C. Smith, WA7RAI • All rights reserved




For those who would enjoy working some serious DX on 20 meters but cannot install a beam antenna, this slightly modified ¼ w ground plane vertical may provide a solution.

While directional gain is certainly an asset, one cannot overlook the important role that a narrow elevation beamwidth and a low angle of radiation plays in successful DXing.

This design is based on the premise that a specific set of dimensions and conditions plus the addition of a shorted stub, will allow the classical ¼ w ground plane to mimic the radiation characteristics of a typical 5/8 w vertical:

angle of radiation: 15 deg.

elevation beamwidth: 24.8 deg.

nominal gain: 3.1 dBi

Click here to view polar plot

In addition to the advantages of a low radiation angle and a narrow elevation beamwidth, the shorted stub across the feed point acts as a closed loop, thus reducing the antenna's susceptibility to noise and making this antenna an acceptable alternative to a beam.

PRINCIPAL OF OPERATION

A typical ¼ w vertical ground plane is mounted on a ¼ w vertical aluminum mast that is grounded to earth ground. In effect, the antenna now becomes an electrical ½ wavelength. A shorted coaxial (RG-8) stub with a delay of approximately 1/8 wavelength (5' 10¾") is shunted across the feed point insulator, making the antenna the electrical equivalent of a 5/8w element. Four (4) radials made of #12 wire, each 17 ft. in length, are than attached to the top of the mast just below the feed point (see drawing) with their ends angled downward, to give a 50 ohm feed point impedance.

Click here to view VSWR curve

DETAILS
http://www.raibeam.com/wa7rai/20m_vert/20m_vert.html


CONSTRUCTION

The main ¼ w element is constructed of three (3) telescoping sections of aluminum tubing with a wall thickness of approximately .058". The bottom section is 6' long by 1" diameter. The middle section is 6' long by 5/8" diameter, which is telescoped into the 1" tubing by three (3) inches, making it's effective length 5' 9". The top section is 4½' long by ¾" diameter, and is telescoped into the middle section until its exposed length is 4' 1½". The overall length of the main element is 16' ¾" for 14.225 mHz as the center operating frequency.

The main element is inserted and secured into a tubular insulator that is 1" i.d. and approximately 1-5/8" o.d., which is then inserted and secured into the top of the 16' length of 1¾" aluminum mast. The insertion depth for each should be around 2" to 3", thus the insulator's length will be around 4" to 6" long, depending upon the insertion depth used. Note: the mast may be comprised of telescoping sections, but conductivity between sections must be maintained. The mast may also be of a diameter other than specified, but in either case, length of the main element and/or the stub length may need compensation.

The stub is constructed of a length of RG-8 (or equivalent with a .66 velocity factor) that is 5' 10¾" long. One end is attached across the insulator -- with the shield connected to the mast, the inner conductor to the main element -- and on the opposite end, the inner conductor and the shield are connected together and soldered. The stub can hang parallel to the mast but a separation of 1" to 2" should be maintained.

The length of the four (4) radials is 17'. They are attached to the top of the mast with their ends lowered to be 10' above ground, and horizontally, 15' 11" from the center of the mast.

The antenna is fed across the insulator, with the feed line shield connected to the mast where the radials attach, and the inner conductor connected to the bottom of the main element (as shown). It is recommended that a ferrite sleeve on the coax, or a length of feed line coax coiled into a choke, be placed at or near the feed point to suppress common mode radiation.

TUNING AND MATCHING

If necessary, the center operating frequency may be adjusted by varying the length of the top section of the main element. Similarly, the feed point impedance can be adjusted by altering the length of the shorted stub.

Roberto Napoletani (I5JKI)