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IW5EDI Simone - Ham-Radio

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Tuning Dipoles

Posted: 04 Sep 2021 01:08 PM PDT http://www.iw5edi.com/ham-radio/5159/tuning-dipoles



This is a standard calculation method that can help you while tuning dipole antennas, by adjusting wire lengths.




For my opportunity I will use the metric unit in this article, but you can easily convert to inches.




Example




If you wan to tune a dipole on 7.070 kHz, while the lower SWR is at 6.950
kHz, you need to short the wires, since it is obviously too long. You need therefore to change the wire total lenght in order to move the center band
of 7070-6950 and therefore of 120 kHz.




You need to multiply the difference in kHz (120)by 100 and dividing by the desired frequency (7070) in kHz. You will obtain the percentage of the
total lenght to be cut.




120x100/7070= 1,69%




So, assuming your antenna for 40 meters band, is 21.60 m you will have




21,60 x 1,69 = 36,5 cm




The dipole will have to be shortened of 36,5 cm and therefore 17,75 cm each leg.













This method can be used also when you need to add lenght to your wires, and
can be additionally used to quarter waves vertical antennas, dividing by 2
the final result. Note that on vertical antennas also ground radials need
to be adjusted.




Reference table




If you want to move by 100 kHz



80 meters band110 cm55 cm40 meters band28 cm14 cm20 meters band7 cm3,5 cm15 meters band3 cm1,5 cm10 meters band1,8 cm0,9 cmBand, total length to be shortened, and corrensponding single leg length.






The post Tuning Dipoles appeared first on IW5EDI Simone - Ham-Radio.


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Double Quad for VHF UHF by I5NZR

Posted: 04 Sep 2021 10:33 AM PDT http://www.iw5edi.com/ham-radio/5151/double-quad-for-vhf-uhf-by-i5nzr



By I5NZR Renzo (SK)




This antenna made me know, IW5ARM Luigi Del Turco Rosselli of Pisa in the
70s. He used it in his country house in Fabbrica di Peccioli, making it
rotate on itself with a series of references made from bicycle chains and
gears to vary the polarization. I think he made the first one out of
vineyard wire. I later found this antenna published and developed in
various magazines.




A few years ago, in the early 1990s, I saw it used by RTF (French TV) to
keep contact between the cameras on the bikes and the repeater helicopter
at the Tour de France. A directive version with a curtain reflector and
Yagi directors is used as a TV receiving antenna. I have built and used two examples for VHF and UHF.





The structure of the antenna is very simple. It consists of two identical square loops, whose sides measure a quarter wave (?/4), connected together
at one end to form a double lozenge, as can be seen in the drawing. The
system for calculating the size of the single loop is identical to that of
the Quad.




To calculate quarter wave, use 75 / Frequency, like for all Quad is not necessary to calculate any shortening.













The antenna is fed at the centre, at the point where the two loops (x y)
join, directly with 50 ? coaxial cable. At that point the antenna has a characteristic impedance of approx. 50 ? as it is an array of two Quad
loops, fed in parallel, where each individual loop has a characteristic impedance of approx. 100 ?. Gain is 2.8 Db on the dipole. (1.4 dbd per
loop, 4.5 dbi).





The polarisation of the antenna is vertical when the elements are placed horizontally (as in the drawing) and horizontal when the elements are
placed vertically. Without parasitic elements the radiation is
bidirectional and perpendicular to the antenna plane. The shape of the radiation lobe is identical to that of a single-element quad or dipole and
is the sum of the two lobes emitted in phase by each of the quad elements.





The antenna can be made directive with parasitic elements. The reflector
can be made either with a curtain of only 2 Yagi elements, but 4 or 6 is
much better, or more conveniently with a wide-meshed wire mesh structure:
The Yagi elements of the curtain or the short side of the wire mesh curtain must be between 5 and 7% larger than ?/4.




The long side of the curtain should also be slightly longer than the space occupied by the two loops side by side.
The spacing between the reflector and the radiator, as in Quad antennas, is between 0.10 and 0.20 of ?, depending on whether you want to enhance the
gain or the forward-back ratio, bearing in mind, however, that bringing the elements very close together lowers the impedance of the antenna and is
more difficult to achieve a low ROS. The highest antenna gain with the reflector alone is obtained by spacing the two elements by 0.12 ?.














Any conductors should be Yagi elements and should be calculated using the
Yagi antenna method, both for size and spacing. Ive seen a design with two
rows of directors in the middle of each loop. I think this only serves to complicate things, both mechanically and in terms of antenna tuning. A
single row of Yagi directors in the centre of the antenna is enough to give very good performance: In fact, I could see that such a 5-element antenna offers a higher gain than an equal Yagi element.









An antenna structured in this way for 2 meters can present mechanical
problems due to the size of the radiator and reflector that will always
need a support, but in UHF, where the antenna is self-supporting, certain problems can be easily overcome.





Recently (December 2011) Loris IK5GFC has realized it as a mobile antenna,
with only the radiator placed inside the car, applied behind the headrest
of a rear seat of the FIAT Panda HI HI !




Article by I5NZR (SK) and freely translated by IW5EDI






The post Double Quad for VHF UHF by I5NZR appeared first on IW5EDI Simone - Ham-Radio.

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