Windom99 (sau OCFD, sau dipol asimetric) - montata inclinat

Acest articol descrie comportamentul binecunoscutei antene in conditiile mele locale. Literatura prezinta antena ca un dipol asimetric orizontal. Spatiul restrans m-a obligat sa aleg o multibanda si unicul mod in care puteam sa o montez, inclinata la cca. 45 de grade. This article describes the behaviour of the well-known antenna in my local mounting conditions. The books describe this antenna as a horizontal, asymetric dipole. The lack of space have forced me to choose some multibander for the lower bands and the only one way I could mount it was inclined at about 45 degs.

Windom99 este un dipol asimetric, foarte asmanatoare cu antena FD4, diferenta dintre acestea fiind raportul lungimilor bratelor, deci implicit impedanta in punctul de atac. Windom99 are impedanta de cca. 220 de ohm, in timp ce FD4 are cca. 300ohm. Ca urmare, transformatoarele simetrizor (balunuile) au raporturi diferite, respectiv 1:4 si 1:6. Balunurile 1:4 sunt mai simple decat cele 1:6 si au largime de banda mai mare. The so-called Windom99 is an asymetric dipole, very similar to the FD4 antenna. The main difference between them is the lenght ratio of the two sides, thus the impedance at feeding point. The Windom99 has about 220 ohms, the FD4 has about 300 ohms. As a consequence, if fed through a 50 ohm line, the balun transformers have different transformation ratios, of 1-to-4 and 1-to-6 respectively. The 1-to-4 balun is simpler to build than the 1-to-6 and seem to have a wider band.
Aceasta este simularea facuta cu binecunoscutul MMANA pentru conditiile mele de montaj: Capatul inalt la 19m, cel de jos la 4m, unghiul astfel format de aproximativ 45 de grade, sol uscat, citadin. In figura am suprapus diagramele orizontala si verticala.
Dimensiunile "clasice" cu un total de cca. 42m, s-au dovedit a fi prea mari atunci cand antena se monteaza inclinat. In timpul reglajelor, am obtinut dimensiunile reale de cca. 40m, cum se vede si in figura de mai jos.

This is the simmulation made with the well-known MMANA, for my mounting conditions: the upper end at 19m (62ft), the lower end at 4m (13ft), the resulting angle about 45 degs, city, dry soil. In the pics I have overlapped the horizontal and vertical diagrams.
The "classic" dimensions of an overall of about 42m were proven to be too much for resonance. As I've noticed during tunings, the real length if mounted "sloping" are slightly shorter, with an overall of about 40m, as presented in the figure below.
Transformatorul balun 1:4 (vezi schema de mai jos) este realizat pe o teava de plastic recuperata de la tuburile de silicon, cu diametrul exterior de vreo 45mm si lungimea de cca. 150mm. (Mai intai faceti bobinajul si apoi taiati tubul, HI!)
Bobinajul are 2x12 spire (bobinaj cu ambele fire deodata) din fir de cupru izolat in PVC, cu diametrul de 1..1.5 mm, din cel folosit pentru instalatii electrice. Acest balun suporta bine cei 100W ai transceiverului standard. Pentru puteri mai mari se recomanda un balun pe inel de ferita.		 The balun transformer 1-to-4 (see schematic below) is realized on an empty tube of silicone sealant, with the external dia of about 45mm and the lenght of about 150 mm. (First wind the wire, then cut the tube, HI!)
The winding is of 2x12 turns (simultaneous winding with both wires) using Cu wire  of 1...1.5 mm. insulated in PVC, the one used for domestic electric cabling. This balun supports well the "standard" 100W of power from a normal transceiver. For a higher power, the use of a balun on ferrite ring is desirable.

For those who are using the saxon units, see the equivalence of some dimensions, on this link.

Ca orice multibanda, si aceasta antena functioneaza prost in toate benzile, HI! Nu am reusit sa aduc SWR-ul sub 1.3 pe nici una din frecventele de rezonanta. Astfel, am obtinut un SWR de 1.5 la 3550kHz, 1.9 la 3720kHz, 1.3 la 7050kHz si 1.9 la 21200 kHz. In restul benzilor, valorile au fost intre 2 si 3. Transmatchul, ca de obicei, a salvat situatia.

Izolatia antenei trebuie sa fie buna, cu minim 3 izolatori tip nuca, de portelan, sticla sau plexiglas pe fiecare capat.

Cablul este un coaxial de 50 de ohm, de calitate buna. Eu am ales clasicul RG-213.

Reglajul l-am facut ajutandu-ma de o punte reflectometrica de tip MFJ-207. Am fost atent sa tai din ambele capete, in raport de 2/3 si 1/3. De pilda, daca am avut de taiat 1m, am taiat 0.66m din bratul lung si 0.33m din cel scurt, astfel incat sa se pastreze asimetria proiectata a antenei.

 As any other multibander, this antenna works bad in any band, HI! In fact, the SWRs were as listed: 1.5 at 3550kHz, 1.9 at 3720kHz, 1.3 at 7050kHz and  1.9 at 21200 kHz. On other bands, the values were between 2 and 3. As always the transmatch saved the situation.

The insulation of the antenna has to be very good. At least 3 nut-type, porcelain, glass or organic glass insulators have to be placed at both ends.

The cable is a good coaxial of 50 ohms. I have chosen the good old RG-213.

The tuning was helped by the use of the well-known MFJ-207 reflectometric bridge. Beware to cut both ends, respecting the 2/3 and 1/3 ratios. I.e. if you have to cut 1m, cut 0.66m of the long wire and 0.33m of the short wire, to maintain the designed asymmetry of the antenna.
Observatie importanta:
In cazul ruperii antenei, balunul s-ar putea sa loveasca cladirea. Alegeti lungimea ancorelor si respectiv a cablului de alimentare astfel incat punctul de impact al balunului  sa fie pe zidul cladirii si nu pe vreuna din ferestre.		 Important safety notice:
Beware that, if the antenna is broken, the balun might hit the building. Choose the length of the end anchors and feeding cable in such way that the impact point to be somewhere on the wall and not on some window.