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ALL BAND DIPOLE Mk3

The Mk3 All Band HF Doublet is the same as the Mk2 version with the inclusion of a balun at the interface from the coax to the open wire feed-line.  This antenna is normally used with a T-Match Antenna Tuning Unit. November 2019


The All Band HF Doublet Mk3 is an Inverted 'V' of 42mtr in length and achieving an apex of 11mtr above ground and is a useful antenna for all bands from 160m to 10m.

Description

The All Band HF Doublet is often referred to as a random length dipole as it is generally as long as the available space within reason, but there are a couple of limitations to the ultimate dipole length. First antenna efficiency will begin to drop off at dipole lengths significantly less than a half wave length for the lowest frequency band to be operated. It is also wise to avoid lengths that produce extremely high impedance to the Matching Unit as it may be beyond its ability to match this impedance. The second example is fairly easily rectified by simple adding or subtracting some length to either the dipole or the feed line, often as little as a metre will do the trick. While the antenna is well less than a half wave length at 1.8 MHz, at just over a quarter wave length it will still give usable access to this band.

 

The completed antenna system consists of a 42 metre centre fed doublet (21 metres for each leg) suspended at the centre and supported by the short 1 metre cross arm  attached at the top of a 11mtr tower. The doublet is fed with 8mtrs of 450ohm ladder line that is then connected to a 3.5mtr section of LDF4-50 heliax with a combination choking balun and step up balun (Explained in detail below) that is run into the shack to a T-Match antenna matching unit.

 

The ARRL handbook presents the results of a comparative study of the All Band HF Doublet constructed as a flat top doublet and as an inverted 'V' configuration. Conclusion was that both configurations offer a practical and flexible antenna with the flat top representing a superior low angle radiation pattern due to its general greater height above ground.  

 

Photo 1 Multi-band Doublet and feed-lines. Other antennas are the 2m band ground plane and 6m Ringo antenna November 2019

 

One of the disadvantages of this antenna system is that it is a balanced system that is each halves of the doublet and feed-line configuration have to mirror the other. Failure to achieve this will cause the feed-line to receive and radiate energy which will result in a distortion of the radiation pattern and also allow the feed-line to pick up stray signals from computers etc as the feed-line enters the radio room. Despite this I have found that this antenna system is reasonably forgiving.  

The un-balanced coaxial cable is connected to the balanced feed line with the combination of a 1:1 choking balun and a 1:4 impedance step up balun connected in series. The antenna, feed lines, balun and antenna set-up is shown in below in Fig 1.

The 1:1 choking balun is to mitigate common mode RF currents on the coax cable, reduces noise pickup on the coax from within the building entering the antenna system and also produce a balanced antenna system that will have a more predictable radiation pattern . 

The 1:4 impedance step up balun is included to more broadly match the range of impedance at the antenna/feed line with the nominal 50-ohm impedance of the coax.  

 

 

Figure 1 Multi-band Doublet, feed-lines and balun configuration

Basic Multi-band Doublet Arrangement

(1)  Inverted 'V' Dipole. (42Mtr total length)

(2)  450 Ohm Ladder Line. (8Mtr)

(3)  1:4 Current Balun. See Balun Guanella Current 1:4 Single core

(4)  1:1 Choking Balun. See Balun Choke HF

       For details of the feed-lines and balun configuration. See HF feedline Choke & Balun

(5)  LDF4-50 Heliax (3.5Mtr)

(6)  T-Match Tuner. See T Match Tuner

(7)  VSWR Meter.

(8)  HF Transceiver.

 

Construction

The 1:1 choking balun is a length of RG58 50ohm coax wound with 14 turns on a single FT240-43 ferrite toroid core. See Fig 2 for the connection details and Balun Choke HF for the 1:1 choking balun details.

The 1:4 current balun has two ideally 100ohm transmission lines wound evenly spaced in the same direction around a single FT240-43 ferrite toroid core. See Fig 2 for the connection details and Balun Guanella Current 1:4 Single core for balun details.

Construction

The inverted 'V' doublet was constructed using 42mtr of standard 4mm2, 7 strand copper wire and glazed porcelain electric fence insulators at the ends and 'V' apex attachments. The apex of the 'V' was attached to a small steel cross-arm located at the top of the 11m tower, fed with 8mtrs of 450ohm ladder line dropped vertically away from the 'V' apex to an aluminium enclosure housing the baluns. 

 

MMANA-GAL antenna analyser modelled radiation plot for 30, 20, 17, 15, 12 and 10m bands.

Figure 6  Modelled radiation plot for the 10m band

Figure 7  Modelled radiation plot for the 12m band

Figure 6  Modelled radiation plot for the 10m band

 

Figure 7  Modelled radiation plot for the 12m band

 

Figure 8  Modelled radiation plot for the 15m band

Figure 9  Modelled radiation plot for the 17m band

Figure 8  Modelled radiation plot for the 15m band

 

Figure 9  Modelled radiation plot for the 17m band

 

Figure 10  Modelled radiation plot for the 20m band Figure 10 Modelled radiation plot for the 30m band

Figure 10  Modelled radiation plot for the 20m band

Figure 11  Modelled radiation plot for the 30m band

Summary

The random length all band doublet represents some clear advantages in cost and operational flexibility within the limitations of the average Australian suburban block. There for if you can have only one HF antenna the random length all band doublet would be a pretty good choice.  

References 

 

WIRE SPLICE - INLINE

Wire splicing for inline joining of aerial wire.

  

WIRE SPLICE TERMINATION

Wire splicing for terminating an antenna aerial wire or guy wire to a strain insulator or thimble.

 

WIRE SPLICE TERMINATION & TAIL

Wire splicing for terminating an antenna aerial wire to a strain insulator with a tail for connecting to the feed line.

 

  

The ARRL Antenna Book.

The 1990 ARRL Hand Book.

The above radiation plots were produced using MMANA-GAL Antenna Analyser software by JE3HHT, Makoto (Mako) Mori at http://hamsoft.ca/

 

 

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Page last revised 22 November, 2019
 

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