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ALL BAND DIPOLE Mk2

Improved All Band HF Doublet was constructed and refined from the experience from my previous All Band HF Doublet for use with a Z-Match Antenna Matching Unit. 2011


Photo#1 All Band HF DoubletWith a 1064m2 (1/4 acre) town block with a depth of 60mtr (200') I intended to improve on my previous All Band HF Doublet with an antenna that would be suitable for all the HF amateur bands, including the so called WARC bands and including the 160 metre band. The antenna system should also be useful for other HF services i.e. broadcast, military etc. With a newly installed mast positioned to achieve an Inverted 'V' of 42mtr in length and achieving an apex of 11mtr above ground the new All Band HF Doublet should experience significant performance improvements, particularly on the lower bands. I had seriously considered constructing something different as the All Band HF Doublet had be done and there many alternatives, but the flexibility that this configuration offers for the simplicity of construction I ultimately decided to go with what I had learnt and go with a proven performer.

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 crossarm  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 twin heliax (Explained in detail below) that is run into the shack to a Z-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.

 

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.

Features

While the antenna was principally the same as the previous All Band HF Doublet there was a key new design challenge which was how access the balanced ladder line to the radio room located within a steel clad building. Ladder line is a very efficient transmission line particularly when high SWR is likely as in the proposed configuration; however the ladder line must be kept well clear of any metal structures and prefers to be run in a sweeping manner avoiding sharp bends. The solution was to terminate the twin ladder line conductors into a twin section of coax cable for the run into the shack. With the shields bonded together at both end of the twin coax section producing a section of shielded balanced 100ohm transmission line. The use of coax in this way can introduce  high losses when operated at high SWR particularly on the higher HF bands, it is therefore important to use the best, lowest loss coax available and keep to section as sort as is possible. I chose to use 1/2" heliax that was available as the run would be about 3.5mtr in length. While not important that the coax or heliax follow the same route it is critically important that they be of the same length and the shields be bonded at both ends and earthed ideally at both ends, but at least at the station end.

Construction

The inverted 'V' doublet was constructed using 42mtr of standard 2.5mm2, 7 strand copper earth wire with the PVC insulation removed and glazed porcelain electric fence insulators at the end 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 a small plastic junction box attached at a small sub mast located in the centre of radio room building's steel roof.  The ladder line that is then connected to a 3.5mtr section of twin heliax at the junction box where the shields of the twin heliax are bonded and grounded to the small sub mast, which is electrically bonded to the steel roof. The twin heliax is then routed through the roof space and wall cavities and presented via two SO259 bulkhead sockets where the shields of the twin heliax are also bonded and grounded to the station earth.

The feed-line is then match via a balance matching unit such at the Z-Match antenna matching unit to the typical 50ohm radio antenna socket. It is obviously critically important that the SWR is carefully monitored during the match procedure.

Figure 1  General Layout.

Figure 1  General Layout.

 

Basic All Band Dipole Arrangement

(1)  Inverted 'V' Dipole. (Length subject to available installation space. In this case it was 42Mtr total length)

(2)  450 Ohm Ladder Line. (Can be any realistic length. In this case it was 8Mtr)

(3)  Junction box (Ladder line - twin heliax interface)

(4)  Twin heliax (As short as possible. In this case it was 3.5Mtr)

(5)  Twin SO259 bulkhead sockets

(6)  Z-Match AMU. See Z-Match AMU

(7)  VSWR Meter.

(8)  HF Transceiver.

 

Photo#2 Junction box (Ladder line - twin heliax interface) Photo#3 Junction box (Ladder line - twin heliax interface) assembly Photo#4 Twin SO259 bulkhead sockets
Photo#2 Junction box (Ladder line - twin heliax interface) Photo#3 Junction box (Ladder line - twin heliax interface) assembly Photo#4 Twin SO259 bulkhead sockets

Operational

It is critically important that the SWR is carefully monitored during the match procedure required after band changes and when even moderate changes in frequency are made within a band. This is particularly important on the lower frequency bands including the 20 and 30mtr bands for significant moves and for even minor frequency moves on the 80 and 160mtr bands. See the AIM 4170C antenna analyser displays for the 160, 80,40 and 30metre bands in Fig#2 - 5 below. Note the very narrow band width of 6kHz below a SWR of 1.5 - 1 for the 160m band

The Z-Match antenna matching unit while not exclusively designed for a balanced antenna system is particularly well suited to this configuration. A balanced antenna system requires that each half of the doublet as well as each side of the transmission line be a near mirror image and should also avoid nearby trees and structures in particular metallic structures. When the system is balanced the transmission line will have equal, but opposite current flowing in each line. This will cancel out any radiation or reception on the transmission line.

The transmission line is the main reason for maintaining a well balanced system as it will be prone to radiating and receiving signals as it enters the radio room. Devices such as computers radiate noise which may find its way into the sensitive radio receiver and strong fields around un-balanced transmission feed may interfere with other sensitive equipment.

A real world issue for many if not most balanced antenna systems is achieving this more or less perfect balance. Imbalance is primarily caused by more capacitive coupling to one side of the system than the other. Lloyd Butler suggests a method to counter this effect by simply adding additional capacitance to the opposite side the system. There for I have added this feature to my version of the Z-Match. Which side requires the additional capacitance is a bit of trial and error, but a method to test for balance is to measure the current in each leg simultaneously and observe if they are equal or to simply adjust until locally generated noise reduces. The further the problem noise source is from the antenna system the more likely it will be the antenna and not the transmission line that is receiving it and the less effective the balance capacitor will be.

Anecdotally the All Band HF Doublet is performing well on all bands from 80 to 10m and as well could be expected given its length on 160m. From Northam , Western Australia consistently reasonable contacts have been established with the Australian east coast and New Zealand via the 80m band. All bands from 40m and above have usable world wide coverage subject to conditions. See below MMANA-GAL antenna analyser modelled radiation plots for 30, 20, 17, 15, 12 and 10m bands.

AIM 4170C antenna analyser displays for the 160, 80,40 and 30m bands

Figure 2  AIM 4170C antenna analyser display of the antenna system including the Z-Match at 1.85MHz. Note the very narrow band width of 6kHz below a SWR of 1.5 - 1
Figure 2  AIM 4170C antenna analyser display of the antenna system including the Z-Match at 1.85MHz. Note the very narrow band width of 6kHz below a SWR of 1.5 - 1

 

Figure 3  AIM 4170C antenna analyser display of the antenna system including the Z-Match at 3.6MHz. Note the band width of 25kHz below a SWR of 1.5 - 1
Figure 3  AIM 4170C antenna analyser display of the antenna system including the Z-Match at 3.6MHz. Note the band width of 25kHz below a SWR of 1.5 - 1

 

Figure 4  AIM 4170C antenna analyser display of the antenna system including the Z-Match at 7.13MHz. Note the band width of 70kHz below a SWR of 1.5 - 1
Figure 4  AIM 4170C antenna analyser display of the antenna system including the Z-Match at 7.13MHz. Note the band width of 70kHz below a SWR of 1.5 - 1

 

Figure 5  AIM 4170C antenna analyser display of the antenna system including the Z-Match at 10.125MHz. Note the narrow band width 150kHz (Full 30m band) below a SWR of 1.5 - 1
Figure 5  AIM 4170C antenna analyser display of the antenna system including the Z-Match at 10.125MHz. Note the band width of 150kHz (Full 30m band) below a SWR of 1.5 - 1

 

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 

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 9 May 2011 
 

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