PORTABLE HF DOUBLET ANTENNA

Portable HF Doublet Antenna for a frequency range of 3.5MHz - 30MHz. October 2025.

Requiring an easily deployable and reliable antenna for portable campsite radio communication, I selected a multi-band dipole, as I had a proven track record of success with this type of antenna. While end-fed wire antennas had also been used for this purpose due to their flexibility in installation, they often produced inconsistent performance depending on ground type and other site conditions. The multi-band dipole will have an overall length of 30 m, which creates some installation challenges; however, this is considered a worthwhile trade-off. It will serve as a secondary antenna, providing better coverage of the lower frequencies from 3.5MHz to about 10MHz, to complement the primary smaller vertical ground-plane antenna that covers 7MHz to 30MHz.

Various multi-band dipole lengths were modelled using the MMANA-GAL antenna-modelling application. The analysis concluded that a 30m overall dipole length offered suitable performance across the HF bands. This length was chosen because it avoids extreme impedances—in other words, it avoids resonant lengths on all amateur HF bands. The modelling also provided insights into likely radiation patterns and matching requirements.

The antenna is a balanced dipole fed at the centre, with two 15m elements on either side. The feed point is connected to an 8m length of 450-ohm ladder line, followed by a 4:1 balun and a 1:1 choking balun. A length of coaxial cable then connects the system to the antenna matching unit and SWR meter before finally reaching the transceiver.

Fig 1 General antenna, matching and radio arragment.

The below charts indicates the likely VSWR ratios across the HF band with a 1:1 balun (50 Ohms), 4:1 balun (200 Ohms) and 9:1 balun (450 Ohms). The chart clearly eliminates the opportunity to direct connection at 50 Ohms and while 4:1 and 9:1 Balun VSWR levels are far too high for a direct transceiver connection they are much more within the range of the Antenna Matching Unit.

Fig 2 MMANA-GAL antenna model prediction for VSWR ratios across the HF band for various connection impedance. Vertical axis is the VSWR ratio and the horizontal axis is the frequency in MHz.

Fig 3 AIM antenna analyser results for VSWR ratios across the HF band for connection at 4:1 Balun. Vertical axis is the VSWR ratio and the horizontal axis is the frequency in MHz.

Construction

The antenna is simply two lengths of PVC cover thin gauge multi-stranded copper wire of 15mtr each (30mtr total length) attached at the centre with 6mm PVC sheet that is designed for a rope supporting attachment at the top and strain support attachment for the 450 ohm ladder line to the bottom. The Ladder line is approximately 8mtr in length.

The feed-line is then connected to a combination balun of a 4:1 impedance transformer balun and 1:1 choking balun.

See Balun details: HF feedline Choke & Balun-20250901

Photo 1 The dipole centre support made from 6mm PVC sheet with large 11mm rope attachment holes and strain clamp attachment for the 450 ohm ladder line.

Photo 2 Front view of the dipole centre support made from 6mm PVC sheet with large 11mm rope attachment holes and strain clamp attachment for the 450 ohm ladder line.

Photo 3 The dipole end insulators are also made from 6mm PVC sheet with large 11mm rope attachment holes.

HF ladder feed-line and other multi-band HF antennas to coaxial cable choke and 1:4 balun.

See Balun details: HF feedline Choke & Balun-20250901

Radiation Pattern

At an installation height of about 8m above ground, the dipole is approximately 0.09λ high at 80m, 0.19λ high at 40 m, and proportionally higher at the upper bands. This height is considered low for the lower bands and results in a high take-off angle, favouring Near Vertical Incidence Sky-wave (NVIS) propagation on 80m and 40m, while producing more useful DX radiation patterns on the higher bands.

Taking advantage Near Vertical Incidence Sky-wave (NVIS) propagation for 80m and 40m should be consistent with portable camping style operation with longer distance contacts on this antenna and the vertical antenna that will target bands from the 30m band above and are also consistent with proposed needs.

Fig 4 MMANA-GAL antenna model prediction: At 80m a broadside bi-directional pattern with NVIS enhancement due to high radiation angle for a low installation height of 8mtr or less.

Fig 5 MMANA-GAL antenna model prediction: At 40m the antenna develops lobes, giving more DX potential but also nulls off the ends pf the antenna. The antenna still has high radiation angle and is regarded as being mostly useful for NVIS type propagation.

Fig 6 MMANA-GAL antenna model prediction: At 30m the antenna develops more complex lower radiation angle lobes favouring the broadsides of the antenna and with nulls off the ends. The antenna is more suited for long distance at this frequency.

Fig 7 MMANA-GAL antenna model prediction: At 20m and above, increasingly multi-lobed patterns with narrower beamwidths, potentially advantageous for DX but less predictable in coverage.

Fig 8 MMANA-GAL antenna model prediction: The antenna at 17m a four lobed pattern and additional higher angle lobes of the ends of the antenna.

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Fig 9 MMANA-GAL antenna model prediction: The antenna at 15m with a complex four lobed pattern.

Fig 10 MMANA-GAL antenna model prediction: The antenna at 12m a much more complex lobed patterns.

Fig 11 MMANA-GAL antenna model prediction: The antenna at 10m with a very complex multi-lobed pattern

Fig 12 MMANA-GAL antenna model prediction: A 3D view of the antenna at 10m to better display the very complex multi-lobed radiation pattern.

At 5 to 8 m height, performance favours local and regional contacts on the lower bands, and medium to long-distance DX on upper HF. For the higher HF bands the orientation of the antenna could be an important consideration.

Photo 5 The dipole centre support.

Photo 6 The dipole end insulators

Photo 7 Open wire feed connection to coax cable with 4:1 Balun and choke balun.

Photo 8 The on the ground radio equipment with balun connection to the open wire transmission line in the foreground.

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Page initiated 25 August, 2025 

Page last revised 18 October, 2025