DG7YBN / Producing Bent Dipoles for GTV Yagis
  Last Update June 20th 2016




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Blade-Style bent Dipole for UHF
Upright Blade-Style bent Dipole for UHF
Bent Dipole made from Aluminium Tubes
Bent Dipole Online Calculator
Driven Element Center and Box
Using a Pipe Bender
Bending Tool
Adjustment Bench





Bent Dipole

The Bent Dipole transforms from low impedance inside the Yagis structure to a higher impedance suiting the feedline coax.

Scrolling down you may think that its a lot of extra work - BUT - any other transformation like two parallel
Quarter Wave coaxes including water tightening all or employing a Folded DE means work too ... using a pipe bending tool makes things easy!




Blade Dipole for UHF

This dipole is made out of two pieces cut from an aluminium sheet with a jigsaw

New rendered 3D CAD drawings: Click the image to enlarge


 


Why use such a flat dipole?

The 'Blade Dipole' cuts QRM like a hot knife through Butter ... is but an advertiser.

This dipole is applied on the GTV series Low Noise Yagis. Due to their high level of back and side lobe suppresson these antennas are capable of reducing pickup of QRM and QRN against non Low Noise designs by many dBs. The bent dipole is an integral part of such designs. It provides an internal match to a wanted feedpoint impedance of 50 ohms without using an additional transformation member.

When using very thin diameters like 4 mm for the dipole arms tuning becomes quite a twitchy business. For a real world UHF Yagi I also found a short bent rod of 10 x 1 mm aluminium to be difficult to produce. The broad flat outlines of the Blade Dipole ease the reactions of the antenna impdance. Same time I find it easier to reproduced a precise angle in cutting a shape from sheet metal then to bend a short rod.
Finally the make of a Blade Dipole as shown enables to mount the arms on precisely same height as elements mounted through the boom. Having zero offset between height of dipole arms and element plane means being as close a possible to simulated plattern and impedance parameters. This provides an excellent base for building Cross Yagis.

• Summarised

The matching of Blade Dipole and 4 ... 5 mm or 3/16 in elements mounted through boom with the advanced SM5BSZ Boom Correction (BC.exe) applied is an effective on. In my view it produces lightweight, very exact and inexpensive Long Yagis that are relatively easy to build and hard to beat when we compare net weight or wind load in ratio to Antenna G/T.

• Components of the Blade Dipole, exploded view

All the grey is aluminium or steel in case of screws, nuts and washers (galvanized). Stainless is nice against corrosion but has higher resistivty but thay is a marginal detail. I have used screws M5 x 25 so far.

All the white is plastic: You could actually shape the base plate a bit like a rhombus, thinner towards the dipole arms. Materials: You can use a bit of PP, PE, PA, Acryl Glass or PFTE to make the mounting plate for the dipole halfs from. Non coloured plastic ie. 'natural' are to be prefered for RF properties, added UV stabilisers are to be preferred for long lasting builds. This will be a compromise issue. Size is not critical. Around 100 x 30 to 40 mm, t 4...6 mm

• Feeding Coax Cable

The extended coax wires go between the inner pair of nuts. I have extended the split coax ends with 2.5 mm2 copper wire. Make these thick, short and straight so that they are as good inline with the straight inner section of the bent dipole as can be (in imperative)! Any extra length outside the inline will virtually add to the length of the dipole by means of the RF wise full spanwidth

• Cross Sectional Drawing (Source Dubus 4/2014)
Note the screws for mounting the base plate to the boom is fastend in threads M3 cut into the boom tubes wall

Screws are M5 x 25 (boom 20x20 mm) for fastening the arms of the sheet metal dipole,
M3 for fastening plate to boom

Advanced version & drawing

Dimensions

• Base Plate: (a) = 46 mm. (b) = 84 mm, other plate dimensions are up to you but keep it relatively small please. This is for a 20 x 20 mm or 3/4 inch square boom and puts a distance of around 5 mm between boom and start of arms either side.

• Dipole arms: thickness of sheet metal is t = 2.0 mm. I think this is a good willing parameter, so that anything from t = 1.5 mm for a minium of mechanical stability to around 3 mm should work out without changing its flat dimensions.

• Layout of Blade Dipole arms

Every straight or folded dipole has to fit the very Yagi design in length. A Bent Dipole has to fit in length and bending angle. Even though more forgiving in dimensions the Blade Dipole does not make a general exception of that rule. A technical drawing in PDF format showing a layout of a Blade Dipole suitable for the 70 cm GTV 70-14 and GTV 70-19 you find in the download section of the 19 elem. Yagi

For a 403 MHz Radio Sonde tracking Yagi you find a Blade Dipole drawing in the download section of the GTV RS-19

For building other designs with a Blade Dipole dimensions have to be adapted. Mail me and I will help in this matter.

• Symmetrising Member - the coax you see on the inserted photos is a 3/4 λ (which is 3 x 1/4 λ) coax stub made from H2010 foam insulated low loss cable. Its length is 385 mm including the far side of the N-flange bushing it ends in. Find more details on getting the coax length right in section 'Phasing' of this website, here
Note the ferrite core placed close to the split point (photo on right).


→ Place mouse over images to enlarge

 

 
Blade Dipole installed in a GTV 70-19 in close up - lower side Blade Dipole installed in a GTV 70-19 in close up - upper side


Experimenters Version

For any unknown design a bent sheet metal dipole that is adjustable in

• Bending Angle
• Spanwidth
• Position to D1

can be arranged with a few waste pieces of aluminium strips. Once the dipole is adjusted to best VSWR the final shape can be copied to a full sheet of aluminium and the final shape cut out in one piece



→ Place mouse over image to enlarge

 
It does not need to look pretty to do its duty, hi.





Milling a set of Blade Dipoles for a Bay of GTV 70-19m at ZS6JON

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(1) milling an arm of a Blade Dipole (2) using the technical drawing - see download section of GTV 70-19


 

 
(3) milling the mounting plates out of a Pertinax board (4) ready to go for driving a 4 bay of GTV 70-19m Yagis


 

 
(5) Blade Dipole installed in a GTV 70-19m - upper side (6) Blade Dipole installed in a GTV 70-19m in close up - lower side


tnx John!

see what it is good for










Upright Blade Dipole for UHF



This is what same time Joszef -Joska-, HA5CW and Kazuo, 7L1TIG came up with:

An upright version of the 'Blade Dipole'. 7L1TIG so far built 3 measured 70 cm Yagis (GTV 70-4m, GTV 70-7n, GTV 70-11w)
with that dipole and his feedback shows it is working nicely as well.

The dipoles at 7L1TIG are made from a strip of aluminuim 10 x 2 mm. Find a very smart, because simple and effective
solution in the photo below. The mounting bracket is a stretch of a plastic profile:

Photo: with kind permission of Kazuo, 7L1TIG

Kazou made a whole set of websites about small DG7YBN Yagis.
A very clear and methodic work. I recommend this to read. Use a free website online translator.

GTV 70-11w with sheet metal dipole
GTV 70-7n with sheet metal dipole
GTV 70-4m with sheet metal dipole









Bent Dipoles made from Aluminium Tubes





The transformation ratio depends on the bending angle given by distance a-b.



Geometry Configurator: from straight tube to bent ends

Bent DE Online Calculator for mechanical production of a bent dipole


Enter Numbers in [mm]

(a) Feed Pos.
         
(b) Backdraw Pos.
         
(c) Mid Section
         
(d) 1/2 Span
         
Insulator Lgth.




                                                                                            Segm. BC [mm]
               











(e) Straight Lgth.
               

Bending Angle [degr.]
               

(a-b) Backdraw Lgth.
               

Tube cutting Lgth.


Add Fraction of specific BC! Use HALF BC on tube cutting length ...



Note: However, the bending angle is crutial for the Yagis Z and needs post adjustment!








Driven Element Center



Example Drawing of how I make the Driven Element Center housing box on a 144 MHz 5 elem. Yagi.

and close up on the DE's center: ... a machined plastic bolt, a pair of self tapping screws 3 x 14 mm and soldering eyelets

                 



The bent DE parts before assembling

Some details and hints

 • Use 10 mm spacer made from plastic
	  
 • Use a small plastic box. Use a material that does not contian black fillers.
   I use ABS plastic boxes made by "Bopla" or Fibox".

 • Use plastic cable glands for feeding the tubes through the boxes side walls and keep them in place
   Glands M16 will be fine for 10 x 1 mm tube. M12 fitts 8 x 1 tube
   
 • M12 equals PG7; M16 equals PG9
   M12 thread is 12 x 1.5 with core hole diam. 12.5
   M16 thread is 16 x 1.5 with core hole diam. 15.2
   Same core hole diameter applies to PG glands but other threads with PG chamfers are uses
   However mounting hole clearance is 12.7 mm for PG7 and 15.4 mm for PG9
   
 • Cut threads in boxes walls if you have a right size tapper tool.
   This will save space the nuts would need and brings down bores height by 4..7 mm.
   See below ...


  Attenzione!     Drill hole for cable glands as low as possible - just scraching the bottom of the box is best.
Since any offset between DE and element plane will deteriorate the elevation radiation pattern


  Attenzione!     Soldering coax ends to cable eye is done BEFORE fastening the tube-piercing screws fully.







Using a conventional Pipe Bender


Even the close to the inner end bends can be done with ease using a simple pipe bender:
Thomas, M0ABA demonstrates this, see the mark on the scale? Which is to get both sides bent similar.

And the result, no problem, bent just like that, even for the 34 mm each side for the 70 cm band.

  

Set of photos on courtesy of Thomas, M0ABA







Bending Tool



Take some wooden board of slightly larger thickness then the to be bent tube.
Here I used 'OSB' industrial plate, t = 15 mm

1.  Cut a square piece of 150 x 150 mm roughly
	
2.  Draw a circle line (radius 150 mm approx.)
    you might use a kitchen dish) and prolong with straight
    lines (blue lines) aside

3.  Cut along that line with your jigsaw so that you produce
    a simple punch and matrix pair

4.  Make two side plates, fit matrix and sideplates,
    bore through and join with long screws.

- Ready -


  Attenzione!     ... mind the aluminium tubes spring back and add reduce radius if needed.



Bending tool ready for action


Make a 1:1 sketch of your DE on cardboard and bend the two tubes, leave inside ends stick out some excess millimeters and cut when all in shape.
This format of bending tool is suitable to bend from 8 ... 15 mm tubes. Use slightly larger radius and hard wood in t = 18 or 20 for 18 mm or 3/4".







Bent DE adjustment bench



Adding high precision by refering to a measurement bench ...



Actually not a big deal to build this bench is a help in adjusting both sides to same angle
in case a post trim is necessary or if you want to do many different bent DEs without taking
to 1:1 cardboard sketches every time.


 	• Base is an OSB plate
	  
	• Mount four blocks of wood enclosing the DE box tightly
	  serving as a center mark

 	• Add rulers left / right side; zero is on hight of DE boxes zero ie. bores


A Semi Professional Adjustment Bench - an outstanding mechanic job done by Jörg, DL2VL




A bent dipole jig - another nice job done by John, ZS6JON

Bending jig by ZS6JON: centre detail











73, Hartmut, DG7YBN



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