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©2018 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.


Latest page update: 2 November 2018


under construction

INTRODUCTION


To be covered:

  • Background, my local conditions (previous dipole + height; max span on terrace; < 70% of full size ( about 2x3.5 m) --> noticeable less efficient; ability to cover 30-80 or 30-160), DK2DB, DF7XH.
  • 4NEC2 modeling, analysis
  • model (downloadable); tapering
  • 20-10m or 20-6
  • horizontal/vertical radiation pattern for each band and for height 2/4/6m, poor/average/good ground
  • 3D radiation pattern, same
  • SWR sweep, impedance sweep, impedance at select freq in each band
  • Construction
  • parts list (tubing, self-tapping screws, feedline connection, DIN3015-1 clamps, center insulator, mast stub, mast clamps)
  • tapering decision
  • tooling, drilling tube ID, sanding down OD
  • Rotor, incl. adapter stub
  • Feeding, antenna tuner/coupler/matching
  • losses
  • Measurements:
  • miniVNA sweeps at height 2/4/6m
  •  impedance at select freq in each band
  • radiation pattern with Web-SDR or Web-KiWi
  • Suitability for 6m (50 Mz): not tested at my QTH (in France. depends on dept)


DESIGN

My local constraints: temporary installation, hand-crank mast; --> light weight, manageable for 1 person. I cover 40/80 with my mag loop and am not interested in the 30m band.

Dipole antenna systems become noticeably less efficient when the span is less than about 70-75% of a standard half wavelength dipole. In other words: for a tip-to-tip span less than ≈0.35 λ. Less than that, and losses in the feed line and in the impedance matching unit ("tuner", coupler) become significant.

At 14 MHz (20 m band), this is equivalent to a span of 7.5 m (≈25 ft). So I want my dipole to have close to the maximum span that I can install. Given the size of my terrace and the placement of my antenna mast, I am limited to a dipole with a maximum leg length of 3.4 m (≈11 ft). I.e., a tip-to-tip span of 6.8 m (≈22.3 ft) or 65% of ½ λ at 14 MHz. It will (have to) do!

This dipole will have some directivity  in the 20 m band, and (much) more so in the 17, 15, 12, 10, and 6 m bands.

Minimize wind load (incl. stress on rotor/rotator)

Examples: DK2DB commercial (no longer in production) incl. remote symmetrical tuner near the feed point, seen at Ham Radio many years ago, also advertized. DF7XH clone with improved tuner (continuously adjustable vs 5-band switch of DK).

 

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Evaluation of a dozen tapered aluminum tubing configurations with 4 or 5 elements per dipole leg

(Excel spreadsheet: see ref. 3)


I converged on config nr. 3 (which happens to be the same as DF7XH). It has a 2x3.4m=6.8m (22.3 ft) span. Each dipole leg comprises 5 aluminum tube sections, with decreasing diameter: 20-16-12-10-8 mm. All 12 configurations have roughly the same total cost for the aluminum tubing.



SOME MODELING

First, let's have a look at the radiation pattern of a dipole antenna in "free space". That is, far away from all objects and materials that can couple with the dipole or that can otherwise affect the generation and propagation of the antenna's electro-magnetic radiation. This 3D pattern has a doughnut shape, with the the dipole poking through the doughnut hole. All cross-sections that include the entire dipole, look like a figure-8.

Radiation pattern of a horizontal dipole in free space

©2018 F. Dörenberg N4SPP


The above image was generated with the excellent 4NEC2 freeware simulation package. The model of my tapered dipole is provided  as ref. 2. Interesting and beautifully colorful as the above 3D pattern may be (especially if you like doughnuts, hihi), it is not the pattern of an HF dipole antenna in a realistic installation on earth, even if the earth would be homogenous and without any objects anywhere near the antenne. The images below show the 3D radiation pattern of the same horizontal dipole,  for frequencies in the 20, 17, 15, 12, 10, and 6 meter bands, and at installation heights of 2.5, 4, 6, and 10 meters (8, 13, 20, and 33 ft). Ground type is assumed to be "moderate" (conductivity and dielectric constant). They were generated with the same 4NEC2 model as above.

I selected these particular installation heights, as they represent the (low!) height of the 2x6.5 m wire general purpose dipole that I had for many years at the corner of my terrace, the 6 m hand-crank mast that I recently acquired, a height in between the latter two, and the 10 m height of the next-size-up telescopic mast. 

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2x3.4m dipole installed at height of 2.5 m (≈8 ft) over moderate ground

(3D radiation patterns for the 20-6m bands)

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2x3.4m dipole installed at height of 4 m (≈13 ft) over moderate ground


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2x3.4m dipole installed at height of 6 m (≈20 ft) over moderate ground


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2x3.4m dipole installed at height of 10 m (≈33 ft) over moderate ground


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2D patterns for 14.1, 18.1, 21.1, 24.9, 25.8, and 51 MHz - each at a height of 2.5, 4, 6, and 10 m

(non-normalized view - full size zoomable view is here; click here for normalized view; note: different gain scales above 21 MHz)


We can draw some general conclusions from the above patterns (which apply in general to all horizontal dipoles):

  • For installation heights below 1/4 λ, most of the radiation is upward. The 3D "blob" is only pink at the very top, and has minimal horizontal directivity in the two directions that are broadside to the dipole. This is a "cloud warmer" antenna, good for local and regional communication. But I am interested in DX....
  • At around 1/4 λ, the 3D "blob" begins to flatten and horizontal directivity becomes more pronounced. The oval upward pink area of the 3D "blob" begins to split into two distinct pink areas. Both are broadside to thedipole and have a relatively high take-off angle.
  • At around 0.5 λ, the blob resembles a flattened "free space" blob.
  • For height above 0.5 λ, an additional lobe begins to appear in the vertical direction. This lobe becomes more pronounced as height increases.
  • Around 0.8 λ, this upward lobe splits into two high-angle broadside lobes. This angle decreases as height increases. There are sharp nulls between these upper lobes, and the lobes below them. Also, small side lobes appear, in the direction of the two dipole legs.
  • Above 1 λ, 1.5 λ, 2 λ, etc., additional upward/broadside lobe-pairs appear, and all the those pairs become flatter as height increases further.
  • The number of lobes in each of the two broadside directions ≈ 2h / λ.
  • 0.3 λ really is about the minimum installation height for obtaining a reasonably low take-off angle. I now have a 6 m (20 ft) hand-crank mast. At full mast extension, the top is at a height of around 0.3 λ for frequencies in the 20 m band, and - of course - more than that at higher frequencies.

The next figure shows that, for installation heights below 0.5 λ, the "quality" of the ground does not seem to impact the shape of the 3D radiation pattern a lot. However, when soil conditions cause high ground losses, effectively less power is radiated. So, ground type does impact the overall size of the blob.

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Impact of ground "quality" (poor/moderate/good) on the radiation pattern at 14.1 MHz for various installation heights



CONSTRUCTION AND INSTALLATION

The legs of my dipole are made of aluminum tubing. I used a tubing made of the commonly available aluminium-magnesium-silicon alloy (Al-Mg-Si) type 6060 T-?. This is quite similar to alloy type 6063. Instead of using a single tube section for each leg, The legs are tapered towards the tip, by using five sections of tubing with decreasing diameter. The section lengths and diameters are as derived in the "design" section above. From feedpoint to tip:

  • 50 cm of tubing with an outer diameter (OD) of 20 mm and an inner diameter (ID) of 16 mm; i.e., 20x2 mm tubing.
  • 90 cm of tubing with an OD of 16 mm and an ID of 12 mm; i.e., 16x2 mm tubing.
  • 90 cm of tubing with an OD of 12 mm and an ID of 10 mm; i.e., 12x1 mm tubing.
  • 90 cm of tubing with an OD of 20 mm and an ID of 8 mm; i.e., 10x1 mm tubing.
  • 60 cm of tubing with an OD of 8 mm.

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The aluminum tubing - lengths per configuration nr. 3 in the table above


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Staggered diameters of the aluminum tubing sections


Each section (other than the 20 mm OD section) is inserted in the next-size-up section: the 8 mm OD section into the 10 mm OD section ( = 8 mm ID), 10 mm OD into 12 mm OD ( = 10 mm ID), and 12 mm OD into 16 mm OD ( = 12 mm ID), and 16mm OD into 20 mm OD ( = 16 mm ID). Insertion depth is 10 cm (4 inch).

Note: unlike a pipe, a tube is defined by its outside diameter. The ID of a tube is not manufactured as accurately as its OD. Typically, the tube ID is a bit smaller than the OD minus twice the nominal wall thickness. To be able to insert the 16 mm OD into a 20 mm OD / 16 mm ID tube, I had to drill the latter 16 mm ID to 16 mm over the desired insertion depth (in my case: 10 cm).

Required tools:

  • 16, 12, 10, and 8 mm drills (for increasing ID to nominal)
  • Use sharp (new) drills.
  • Only end of the 10, 12, and 16 mm OD tube sections must be drilled tonominal ID (the ends into which an other tube end will be inserted).
  • Both ends of the 20 mm OD tube sections must be drilled (to be able to install an optional center insulating rod, for stabilizing the construction)
  • The ends of the 8 mm OD tube sections need not be drilled, as no smaller tube section is inserted into them.
  • Bench vise (to firmly hold the drill while hand drilling tube ends to nominal ID)
  • Visegrip-style locking pliers (D: Klemmzange, F: pince-étau).
  • Large flat file for metal (for slightly reducing OD of inserted ends of aluminum tubes + to remove burrs from clamping the tubes with the locking plier)

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The tools


No, do NOT use an electric drill for this. Don't even try! Fix the correct drill into the bench vise, clamp the tube section with the locking plier, and slowly turn the tube onto the drill by hand, using the pliers as a lever. Back off after every couple of turns. This is easiest when continuing to turn in the same direction. Repeat several times (back off completely each time). You will note that the tubing gets quite hot from friction. Once this is all done, chamfer (bevel) the ID of the drilled tube end with a larger drill - again by hand.

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Turning the end of a 20x2 mm  tube section onto a 16 mm drill


If you are lucky, the OD tube ends can now be inserted the entire 10 cm into the drilled mating ID tube ends, with a tight fit that requires considerable effort. Most likely, you will have no such luck! You will have to hand file down the outside of the ends that are to be inserted, over a length of 10 cm. This must be done evenly, all the way around the tube end. Again, do not use an electric power tool for this! Power tools remove material to fast, and before you know it, you will have removed too much. And make sure you do not file down the outside of the tube ends that you just drilled, but the opposite end! This filing is not necessary for the 20x2 mm tube sections, as they are not inserted into another tube. If you cannot insert a  tube end more than a cm (1/2 inch), continue filing - careful, or you it will seize and you will not be able to pull it back out! This will take a number of try-and-file-some-more iterations. Once you get close to a - be very careful not to remove to much material! It is OK if you can't insert it the full 10 cm. Once both legs are done, measure their length, and if necessary, cut off  of the 8mm tube of the longer one (yes, not the shorter one, hihi).

Option considered for mounting on plate: 2 line clamps per dipole leg, 1 line clamp per dipole leg + solid insulating rod inserted into each leg end.

Options considered for connecting feedline to end of 20cm tubes: stainless hose clamps (easily adjustable/removable), machine screws + washers + lug + lock nut.

Additional installation material:

  • For securing overlaping tubing sections - options:
  • small stainles steel self-tapping screws (with pre-drilling) or self-drilling screws
  • "blind" pop rivets (2.4x6 mm for 1.5-3.5 mm grip range (total metal thickness), 2.4x8 mm for 3.5-5.5 mm grip range, 3x6 mm for 2.5-3.5, 3x8 mm for 3-6 mm), riveting pliers, and 2.4 or 3 mm metal drill.
  • 25x25x1 cm poly mounting plate (kitchen cutting board)
  • 4x DIN3015-1 hydraulic line clamps (a.k.a. Stauff clamps), standard model, size 3, ribbed, for 20 mm OD tubing
  • 2x hose clamps (UK: jubilee clamps), for attaching the feed line wire to the 20 mm tubing
  • 2x V-bolt mast clamp

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The additional installation material


Standard methods for securing telescoping tubing sections are illustrated below:

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Methods of connecting telescoping tubing sections

(source: adapted from ref. 3)



FEEDLINE AND ANTENNA TUNER / COUPLER / MATCHING UNIT


Options, needs (incl. expected impedance range to deal with), my choice(s), losses.

Multi-band operation --> not coax to ATU/TRX, but symmetrical feedline (window line, twin lead, or ladder line).

The resonance frequency of this antenna is around xx MHz, so it is operated as a non-resonant antenna. Contrary to popular myth: a non-resonant antenna radiates just as well as a resonant antenna. The only (yes, the only!!!) advantage of a resonant antenna is that around its resonance frequencies, it typically has a feedpoint impedance that is easy to match to a coax cable. That's all!

Need common mode choke?

Feedpoint impedance range to be accommodated at the various heights + freq bands (simulated/modeled vs measured)

Resulting impedance ranges at the output of the feedline = impedance ranges at the ATU input.

ATU losses for these impedance ranges.



MEASUREMENTS

miniVNA antenna analyzer sweeps (at feedpoint, after feedline; 20-10 / 20-6, zoom per band, various heights.

Short feedline to auto ATU near base of mast, symmetrical ATU in shack.

Antenna currents.

Horizontal pattern via WebSDR/KiwiSDR or with local beacon.

Photos of installation, incl. w miniVNA via Bluetooth/WiFi.



ANTENNA ROTOR / ROTATOR


Obviously a "rotary dipole" must be rotated. My antenna, including mounting hardware, weighs about xxx kg (≈ xxx lbs). This includes a 40 cm long POM/Delrin/acetal rod with 35 mm diameter as "top mast" stub. Wind load.

My requirements --> light "TV antenna" rotor will do. Decided to get a Channel Master 9521HD, which is an improved model 9521A. It sells for $145 in the USA (2018 pricing). Ref. datasheet etc.


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Channel Master CM9521HD TV antenna rotor/rotator system


The lower clamps accommodate a mast diameter of up to 2 inch (5 cm), but the top clamps will only accommodate a 1.75 inch (4.5 cm) diameter mast stub (3 ft (90 cm) max).


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Azimuthal map, centered on my QTH - to quickly determine antenna bearing direction


Map tool? e.g., https://ns6t.net/azimuth/azimuth.html



REFERENCES



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