This article explores the performance of an unloaded vertical as a multi-band HF antenna.
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.
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 compromise solution. Generally, one expects 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.
The models here are based on a vertical of 20mm diameter aluminium tube mounted at ground height, and a system of shallow buried radials.
The performance of real radial systems is very dependent on soil parameters. The models in this article assume a earth system that contributes an equivalent bulk resistance of 10 ohms at the feed point, which is realistic for moderate to good soil and a radial system of 16 or so shallow buried radials.
All the models 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 at low frequencies do not achieve the low losses shown here for a L Tuner.
This section explores a 10m high vertical of 20mm diameter aluminium tube mounted at ground height, and a system of shallow buried radials, and the following variations in feed line and impedance transformation:
This configuration is a basic way of adapting and connecting a vertical antenna to the transmitter.
The configuration results in high VSWR operation of the transmission line at most frequencies, causing high transmission losses. Tuner losses are insignificant, assisted somewhat by the transmission line losses. The antenna system has unacceptable losses on all but two bands (40m and 15m), and the antenna is probably only suitable for use on the 40m band because of the dominance of high radiation angle lobes on the 15m band (and above).
The performance of the previous configuration can be improved using a lower loss transmission line. Fig 2 shows the system losses using 15m of RG213 feed line.
The configuration results in high VSWR operation of the transmission line at most frequencies, causing high transmission losses. Tuner losses are insignificant, assisted somewhat by the transmission line losses. The antenna system has unacceptable losses on all but four bands (40m, 30m, 15m and 12m), and the antenna is probably only suitable for use on the 40m and 30m bands because of the dominance of high radiation angle lobes on the 15m band and above and above.
This configuration uses Belden 8222 70Ω twin-line which (although obsolete) was promoted in this application in a recent Amateur Radio magazine article (Jan 2006). It appears its use was inspired by the myth that open wire line is necessarily better than coax in any high VSWR application.
The use of an open wire feed line in this application is certain to result in significant imbalance in feed line current and radiation, but the extent and effects of radiation will depend on the routing of the feed line and ground parameters. This model ignores the radiation from the feed line (based mainly on the idea that power radiated anywhere is not lost and arguably useful), but models the power lost loss within the feed line.
The Amateur Radio magazine article mentioned earlier used a feed line length a little less than half that used here, so the feed line losses in that case would be just a little more than half of that shown here (feed line loss is proportional to length ONLY when VSWR is low), eg at 3.5MHz the loss in 7m of 8222 would be 6.3dB against the 11.9dB shown here for 15m length.
It can be seen when comparing Fig 3 with Fig 1 that the 8222 feed line line loss is a little worse than RG58C/U and delivers acceptable losses only on 40m (with losses of 2.3dB). At 3.5MHz, the loss is 16dB which means that 2.5% of the transmitter output power is radiated. It does not qualify as an efficient multi-band antenna, nor as as acceptable single band antenna.
Poor performance at lower frequencies is a result of:
As an illustration of why a system view must be taken, consider an approach to "fix" the high line loss in Fig 1 by using a much lower loss line. At 3.5MHz, using Andrews LDF6-50 Heliax, the transmission line loss is reduced to less than 2dB. That seems to be an improvement, but is it? The impedance now presented to the tuner (0.38-j10.55Ω) would result in tuner loss of close to 10dB for a typical commercial T-Tuner with 200pf variable capacitors, giving a total system loss of 4.1 (ground loss) + 1.8 (line) + 9.7 (ATU) or 15.6dB which is 4dB worse than the RG58C/U configuration.
As the electrical length of the vertical increases (eg at higher frequencies), the radiation pattern spreads into multiple lobes, and more of the power is radiated at relatively high angles, which may be undesirable, especially on the higher bands.
A compromise design to cover 80m through to 20m is to choose a greater length that is not more than about 0.6λ at 20m for best radiation pattern, and the additional length will increase radiation resistance on the 80m band for improved efficiency. A length of 13m has been chosen for this improved configuration.
The power delivered to the base of the antenna is divided amongst three main equivalent RF resistances, they are:
The first two of these are dissipative, that is the energy delivered to them is lost as heat. Energy delivered to the radiation resistance is converted to a radiated electromagnetic wave.
This model uses 20mm aluminium tube, and the radiator conductor loss at 3.5MHz is 0.2% of the power delivered to the feed point, or 0.01dB. The equivalent RF loss resistance of the radiator is too small to significantly affect the results and can be approximated as 0Ω. Such an antenna will usually require a larger tube diameter to be self supporting and to survive winds, in which case the losses will be even lower.
As explained earlier, a fixed 10Ω equivalent series earth resistance is used to approximate a typical good earth system.
Fig 4 shows the modelled radiation resistance on a logarithmic scale.
Not that at frequencies where radiation resistance falls below about 30 ohms, earth loss degrades performance significantly.
The radiation resistance varies from 0.8Ω at 1MHz to more than 1500Ω around 11Mhz (See Fig 4).
The radiation efficiency at the antenna base (ie the power radiated to the power lost as heat) is Rr/(Rr+Rc+Re) where Rr is the radiation resistance, Rc is the radiator conductor RF loss resistance (approximately 0Ω), and Re is the equivalent earth resistance (10Ω for the models). Radiation efficiency at the antenna base varies from a low of 7% at 1MHz to almost 100% at some frequencies. Losses in Re and Rc are shown as "Ant Ground" in the blue area of the graphs below.
Some of the power developed in the radiation resistance as used here
is lost in reflection of waves from the ground so the total power in
the far field will be somewhat less.
The feed point impedance is not a suitable load for most transmitters, nor is the feed point usually immediately adjacent to the transmitter, so further losses are incurred in transforming the impedance to a suitable transmitter load impedance and conveying energy from the transmitter to the antenna feed point.
This configuration transforms the feed point impedance to suit the characteristic impedance of the transmission line in order to minimise feed line loss.
The loss in transmission lines is increases above the specified matched line loss when they are operated with VSWR greater than 1. The exact increase depends on the line parameters, the load impedance and frequency.
To manage transmission line loss, this configuration uses a remote antenna tuner located at the base of the antenna so that the transmission line operates at close to unity VSWR at all operating frequencies. Modern automatic tuners are ideal in this application, and very convenient when fully integrated with transceiver controls and logic.
This strategy yields transmission line loss of less than 0.36dB over the design range 80m to 20m. This is a marked contrast to line losses of up to 7.6dB over the same frequency range in Fig 2.
Fig 5 shows the performance of the better configuration.
Loss is less than 1dB on bands 40m to 20m, and 3dB on the 80m band.
An antenna similar to the better configuration described above has been in use at my Narooma location for over a year with satisfactory results. The antenna tuner was an Icom AH-4 remote automatic antenna tuner which integrates with the IC706IIG transceiver. The earth system was somewhat abbreviated (due to physical constraints) and measurements indicated an RF earth system resistance of around 20 ohms on 3.6MHz which would degrade efficiency at that frequency by a further 3dB than shown in Fig 5.
Making QSOs is not evidence that an antenna performs as well as it could or should under the circumstances!
A correspondent asked whether the feed configuration of the ZeroFive "43 foot MULTI-BAND 10 to 160 meter vertical" works better than the remote tuner configuration described above. The Zero Five 43' vertical is a 13.1m high unloaded ground mounted vertical with shallow buried radials, and so the antenna is somewhat similar to the 13m vertical described above.
The ZeroFive "43 foot MULTI-BAND 10 to 160 meter vertical" is fed at the base with a Array Solutions AS-200-T 4:1 balun rated at 5kW from 1.5MHz to 30MHz. As both the antenna and the feed line are unbalanced, it is not obvious why a balun (as opposed to an un-un) is used, nor do the installation instructions show the connections.
Fig 6 shows the internals of the balun and its connections courtesy of ES1TU. The left hand terminal is to the base of the vertical, and the right hand terminals are to the ground system. In this case, the coax feed to the shack is buried along with one of the green/yellow earth conductors.
A voltage balun is quite unsuited to this application. This arrangement puts the coax shield at a voltage approximately half the voltage at the the base of the vertical. This will drive common mode current on the coax outer, undesired feed line radiation, common mode losses, modification of the feed point impedance and conducted RF entering the shack depending on the feed line route. The outcome will be variable, depending on the path of the coax (eg topology, burial, soil types) and coupling with nearby conductors (eg radials, equipotential bonding, lightning conductors).
The manufacturers do not explain how this works or is intended to work. The configuration defies sound design principles.
In answer to the correspondent's question on the Zero Five / Array Solutions feed arrangement; no, an ideal 4:1 voltage balun and ATU at the tx is not likely to be as good as the remote ATU configuration, and the recommended 4:1 voltage balun is not well suited to the application.
A 4:1 current balun does not drive the coax with common mode current in the way that a voltage balun does.
The following models assume it is an ideal 4:1 transformer with very high common mode impedance (ie a current balun).
Fig 7 shows the modelled loss for the 13m vertical with 15m of LMR-400 feed line (as recommended by ZeroFive) and an ideal 4:1 balun at the feed point to an ATU adjacent to the transmitter.
Loss is less than 1.4dB on bands 40m to 20m, and 4dB on the 80m band. The loss on the 160m band is 12dB which questions rating of a similar antenna as suited to 160m. Note that at 1.8MHz, the feed point voltage at 1.5kW would be 8000V RMS which is impressed fully unbalanced on the output terminals of the balun.
System loss with a practical transformer is likely to be worse than modelled here with an ideal balun, especially at extreme impedances (VSWR>5) which occur in the 160m, 80m, 30m, 20m, and 12m bands .
The extreme impedances encountered are likely to be well beyond the capability of the internal tuner in most transceivers, and so an external high power low loss tuner is advised. Losses in most commercial T-tuners will be higher than the L match modelled, especially on the lower bands (80m, 160m).
A 4:1 unun would work similarly to the current balun case above.
The supplied 4:1 voltage balun could be converted to a 4:1 unun. Referring to Fig 6:
There may be benefit in installing a common mode choke (1:1 current balun) on the coaxial feed line outside the radial field if common mode current entering the shack is a problem. Note that burial of the feed line would usually reduce common mode current entering the shack to a safe level.
|1.02||22/11/2008||Updated Zero Five / Array Solutions balun discussion.|
V1.01 31/05/09 16:00:43 -0600 .
Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.
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