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An unloaded elevated vertical as a multi-band HF antenna

This article explores the performance of an unloaded elevated vertical, base matching and feed line as a multi-band HF antenna system.

Introduction

At higher frequencies, as the electrical length of the vertical increases, the radiation pattern splits into multiple lobes and more of the power is radiated at relatively high angles. Such a pattern may be undesirable, especially on the higher bands where DX may be the preferred target. The upper limit to the usefulness of an unloaded vertical might well be caused by this diversion of radiated power into high angle lobes and development of intervening nulls.

The lower limit of usefulness of an unloaded vertical is set by the challenge of radiating most of the available transmitter power using an antenna with very low radiation resistance. This challenge is about minimising system loss, and the key elements of system loss are:

Elevation of the antenna above ground and the use of elevated tuned radials as an alternative to a ground mounted antenna with buried radials, or radials lying on or close to the ground. Adequate decoupling of the support mast and feed line by the tuned radials should ensure low current flowing from the mast to ground, and hence low ground loss.

Location of the impedance transformation (ATU) at the antenna feed point ensures low feed line VSWR and the lowest transmission line loss for typical feed line lengths.

Coaxial transmission line is most suitable for this application, and at 15MHz with load end VSWR<1.5 (max VSWR typical of auto ATUs), losses are less than 1dB for RG58 up to 15m length, or RG213 up to about 40m length.

Practical antenna conductors will have insignificant losses at most frequencies.

A system view

Components of an antenna system interact with each other in a complex way, and it is important to analyse the entire antenna system (radiator, earth, transmission line, balun, ATU etc) to obtain a correct understanding of how the system works overall.

Acceptable loss

Real antennas are a compromise between performance and practical limitations or economies of implementation.

Each implementer must make a judgement of system loss that is acceptable to their own compromise solution. Generally, implementers expect to accept higher losses in a multi-band antenna system as part of the trade-off for frequency coverage. For average situations, it should be possible to implement multi-band HF antennas with not more than 3dB of system loss on any required frequency. For the purpose of this article, 3dB is regarded as the maximum acceptable system loss, that is at least 50% of the transmitter output power is radiated.

Tuner loss

The theoretical antenna system models used for this article use an L Tuner with practical Q values for practical least tuner loss. Other tuner configurations (such as the popular T Tuner) will usually exhibit higher loss. Many commercial T Tuners use small variable capacitors, and with extreme loads encountered at low frequencies will deliver worse losses than shown here for the L Tuner.

13m high elevated vertical

This section explores an elevated vertical with these key characteristics:

Fig 1: Structure of vertical, radials and mast

The radials system consists of paired resonant radials in opposite directions for each of the bands 80m, 40m, 30m, 20m arranged with the same slight downward slope from the feed point at 6m height to a height of 4m at a distance of 20m from the feed point. Fig 1 shows the arrangement.

Other model assumptions are:

Fig 2: Antenna radiation patterns
3.6MHz

7.1MHz

10.1MHz

14.2MHz

Fig 2 shows the modelled radiation patterns of the antenna on the 80m, 40m, 30m, and 20m amateur bands. The patterns are very close to omni-directional even though there are only two radials on each band.

15m of RG58C/U feed line from the tx to an ATU at the antenna base

This section explores the system losses using 15m of RG58C/U feed line from the tx to an ATU at the antenna base.

Fig 3: 15m of RG58C/U feed line

The configuration is a basic way of adapting and connecting a vertical antenna to the transmitter. The tuner is remote from the transmitter, so an automatic type is likely to be more convenient. System losses are shown in Fig 3.

The antenna system has acceptable losses on 80m, 40m, 30m, and 20m bands.

The question arises as to whether adding 160m resonant radials would solve the loss problem on 160m. The greatest cause of the high system loss at 1.8MHz is the loss in the ground resistance caused by high current on the support mast, a consequence of a lack of decoupling the mast. A pair of tuned radials would solve the decoupling issue and reduce the ground loss, but the feed point impedance is just 4-j600Ω which will drive around 3+dB of tuner loss. The system loss does not meet the acceptable criteria, so the antenna is modelled and described without 160m radials. Whilst the antenna will work on 160m with degraded performance, it should be kept in mind that half or more of the transmitter power would be dissipated in the ATU which may damage the ATU.

15m of RG213 feed line from the tx to an ATU at the antenna base

Fig 4: 15m of RG213 feed line

The performance of the previous configuration can be improved using a lower loss transmission line. Fig 4 shows the system losses using 15m of RG213 feed line.

The antenna system has acceptable losses on 80m, 40m, 30m, and 20m bands.

Feed point voltage

The feed point voltage (at the base of the vertical) is of interest in the design or selection of a tuner to withstand the operating voltage.

Fig 5: Feed point RMS voltage for 100W

Fig 5 shows the feed point RMS voltage for 100W. Tuner elements should be sized to withstand the peak voltage plus a margin for safety. The tuner should withstand a peak voltage of at least twice the voltage shown in Fig 5.

Analysis

Poor performance below 3MHz is a result of:

The large spikes in loss are the result of a lack of decoupling of the mast causing mast currents flowing to ground.

Poor performance above 15MHz is due to changes in pattern where the radiator exceeds about 0.6λ.

Conclusions

The following conclusions are made:

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Changes

Version Date Description
1.01 25/04/2007 Initial.
1.02    
1.03    

V1.01 12/02/09 01:05:45 -0700 .

Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.


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