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====== Short-circuited Quadrifilar helix  ======

{{ :cs:antennas:qha:bolidozor_sc-qha_antenna_prototype.jpg?direct&800 |Bolidozor Short-circuited Quadrifilar helix prototype}}

Antenna construction is supposed to tackle a following network limitations: 
  * To increase a sensitivity for objects flying over the area of Czech Republic
  * To increase a gain of antennas used by our stations and thus increase the number of detected meteors
  * To eliminate fluctuating sensitivity towards different orientations of linearly polarised reflections
  * To improve the overall data quality
  * To enable a construction of antennas fields that would make it possible to calculate an estimated direction of signal

===== Parameters =====

NEC model was used to calculate the radiation characteristics and impedance adjustment of antenna for a given band.

The carrier frequency of [[en:graves|GRAVEsu]] is highlighted in the graph by a red vertical line.
{{ :cs:antennas:qha:qha_graves_gain_vswr.png?600 | Antenna adjustement for GRAVES carrier frequency}}

===== Construction =====

Structural parts of the antenna are primarily made of galvanised steel components. Those parts, which are under a higher current load are made of copper tubes.

Antenna’s dimensions were optimised using a [[https://github.com/bolidozor/NEC-models/blob/optimize/antennas/src/SC-QHA.ipynb|computer model]]

==== Dimensions ====

* Helix diameter: 389.9 mm 
  * Helix height : 312.2 mm (angle of inclination 27 degrees)
  * Cutting length of upper cross (??Řezná) : 369.9 mm
  * Cutting length of helix arm : 687.4 mm

==== Material ====

Phasing lines (?? fázovací vedení) of the antenna is made of coaxial cable RG58U. Quaterwave length, in this case, is 295 mm.

Antenna helix is made of copper tubes with outer diameter of 18 mm. The tubes are made of ‘soft’ copper, used in air-conditioning units installations. They are available in reel that makes it much easier to construct the antenna.

{{:cs:antennas:qha:cu_tube_reel.jpg?direct&600|Copper tube’s reel.}}

* 400x Galvanized  tear rivet (?? trhací nýt) 4.8x10 mm

==== Construction ====

{{:cs:antennas:qha:sc-qha_base.jpg?direct&600|The basic construction elements of SC-QHA antenna}}

The antenna is made of four pieces of base squares that serve as a universal ground potential. These squares are screwed together at each side by a four M8 screws. There is a fan washer (?? vějířová podložka) at both sides of the screw. A reason for this is a better conductivity of connection and, at the same time, better prevention against a spontaneous loosening during manipulation with antenna.

{{:cs:antennas:qha:base_contact_point.jpg?direct&600|Connection of sieves in the basis of antenna.}}

Zkratovací kříž (??) at the top of the helix is soldered from 3 pieces of tubes made of ‘hard’ copper. It is probably the most challenging part of the antenna to construct - unfortunately, a copper fitting (?? měděná tvarovka) in the shape of a cross is not available.

The cross is thus made of one longer tube with two shorter ones soldered perpendicularly to it. The shorter tubes have their walls pierced, so they fit precisely to the longer tube. It is also necessary to keep the sum of the lengths of the tubes such, that both arms of the cross measure 370 mm.

{{:cs:antennas:qha:sc-qha_top_tube_cut.jpg?direct&600|Úprava řezu části vrchního zkratovacího kříže ??}}

===== Measured characteristics =====

The assembled antenna underwent a measurement of adaptation (přizpůsobení antény) using a spectral analyser DSA815-TG with [[http://www.ebay.com/itm/3MHz-1200MHz-RF-Reflection-Bridge-Directional-VSWR-SWR-Bridge-Antenna-Analyzer-/181865632141?|3-1200 MHz reflection bridge]].

{{:cs:antennas:qha:antenna_reflection_measurement_setup.jpg?direct&600|The set used to measure reflections on the antenna}}

The antenna is sensitive to the way of connecting the phasing lines. Two basic possibilities can be seen on the following pictures. They differ in the direction of the 
Anténa je citlivá na způsob připojení fázovacího vedení. Dvě základní možnosti jsou vidět na následujících obrázcích, které se vzájemně liší směrem otáčení fáze přívodů antény.

{{:cs:antennas:qha:sc-qha_phased.jpg?direct&600|Fázovací vedení zapojené v rezonanci se šroubovicí}}
{{:cs:antennas:qha:sc-qha_prototype_resonance.png?direct&600|Rezonance prototypu antény}}

Na grafu je vidět rezonance antény na přibližně 160 MHz. Anténa tak rezonuje poněkud výše, oproti modelu. Zatím není známa přesná příčina tohoto jevu, ale je možné, že je způsobena změnou činitele zkrácení vlivem změny teploty koaxiálního kabelu na přímém slunečním světle.

{{:cs:antennas:qha:sc-qha_antiphased.jpg?direct&600|Fázovací vedení zapojené v protifázi se šroubovicí antény}}
{{:cs:antennas:qha:sc-qha_antiphased.png?direct&600|Odrazy měřené při napáječích zapojených v protifázi k vinutí šroubovice.}}

Při opačném zapojení fázovacího vedení se na anténě, žádná výrazná rezonance, která by odčerpávala přiváděnou energii neprojevuje.

====== Reference ======

* [[http://personal.ee.surrey.ac.uk/Personal/T.Brown/research/qha.html|Fázovací vedení na tištěném plošném spoji]].

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