- what you gain and what you lose

These days it seems that broadband is the flavour of the month. Nearly everyone’s wish list includes the shortest of antennas with the widest of bandwidths. This is especially so with shipboard antennas where the space available for mounting seems to be decreasing in proportion to the proliferation of new systems that need to accommodated. However, in the mad rush for bandwidth, you may find that in going wideband you are giving up more than you gain. ...

While broadband and electrically small antennas may look attractive on the surface, they are by necessity compromises and usually don’t work as well as tuned antennas.

Due to fluctuations in the reactive component of the antenna impedance, SWR changes with frequency. When the rate of change is slow, the rate of variation in the SWR is slow too, giving a wider bandwidth over a given SWR rate. Moving away from the optimum SWR, higher or lower in frequency, will raise the SWR. For these reasons, bandwidth is traditionally determined by a specific VSWR figure.

VSWR can be calculated by measuring the forward and reflected power and using the above formula :

So, VSWR plots for tuned antennas will be quite different than those for broadband types.

Another measure of antenna bandwidth is its Q. The antenna Q is regarded as a measure of antenna selectivity or being tuned. At the point where an antenna is tuned or resonant, the antenna appears largely resistive (combined loss and radiation resistance). This is because the capacitive and inductive reactances cancel each other out.

Antenna Q is determined by measuring input resistance and reactance at a frequency close to the resonant frequency. Measurements should be read at the input terminals of the matching network or antenna tuning unit as this will be as significant as the selectivity of the antenna itself in determining bandwidth in terms of SWR on the line.

While Q measurements are not dependent on feed line impedance and band of operation, they will not always give a precise result. Due to the variation in radiation resistance as the frequency changes, measurement at the equivalent positions above and below the resonant frequency (where the reactance at the feed point goes through zero) can often give different values. This can be complicated further where antennas are designed to be resonant at more than one frequency.

Generally speaking, a lower Q matching network will give a wider bandwidth. However, if there is mismatch between the antenna and the line, the matching network will need to use large values of reactance and the Q will be too high. However, determining the actual bandwidth of an antenna in terms of VSWR does not guarantee its performance efficiency - or its usefulness to your given communications requirement.

HF Broadband Antenna 2-30 MHz

With any broadband antenna you will also need to know how efficient this bandwidth is. Typically efficiency falls off at band edges as the SWR increases. Losses, which can add up to several dB, can occur in the matching network and also in the transmission line. When losses are low the efficiency is high.

With greater resistive or ohmic losses in the matching network, you may well get a wider bandwidth but this will be at the expense of efficiency.

While an antenna specification may quote a bandwidth of 20 MHz at 2:1 VSWR, this does not mean that the response will be adequate at the frequencies you will be using. The SWR will vary across the range and the design of the matching network will determine the response.

Taking a look at two different matching networks for a shortened HF broadband wire antenna will show how varied they can be. The choice will naturally be frequency related.

VSWR HF Broadband Antennas 2-30 MHz

The rate of change of the SWR will depend on the antenna design and generally will be faster with antennas which have been reduced in length from the optimum for the required frequency. So, an antenna that has the required length will always perform better. This is especially relevant at HF or lower, where required antenna lengths vary greatly with frequency. This can be seen in the VSWR plots for the same HF broadband antenna, one at the correct length, the other at 60% of the required length, where the SWR is much greater on the lower frequencies.

Length & VSWR HF Broadband Antennas 2-30 MHz

The suitability of the bandwidth also needs to be taken into account. Impedance matching, gain, polarisation, radiation characteristics can all be vital factors in determining whether an antenna can deliver what you need. These factors are especially important below VHF where impedance and gain figures and radiation patterns can change quite rapidly over the bandwidth. Even high gain systems at VHF and UHF can be significantly affected.

There is no point in having good gain and low impedance when there is no lobe available at the elevation and direction you require. Conversely, the front to back ratio of a beam can fall off rapidly outside a given bandwidth and so will the gain. The directionality of end fed antennas can vary greatly over a given frequency range where the actual pattern shape, gain and impedance remain substantially the same.

VSWR HF Broadband Antennas 2-30 MHz

That said, for antennas that are a half wavelength or less, normally impedance performance is the limiting factor.

But what is an acceptable VSWR figure? A tuned antenna with a small bandwidth can give you the much vaunted 1.5:1 ratio over 7-10 MHz at VHF or UHF but this is unlikely to be achievable with a broadband HF antenna covering 2-30 MHz.

At 2:1 VSWR the system efficiency is still 89%, having a loss of around 0.5dB. At 3:1 VSWR, system efficiency is reduced to 75%, but the decrease in received signal strength is undetectable by the human ear. Aircraft antennas operating in the VHF band usually have a specified VSWR upper limit as high as 2.5:1. Therefore the 2:1 VSWR 0.5dB loss would seem to be an acceptable value to determine bandwidth for most antennas.

VSWR and audio efficiency

Above 3:1 VSWR may indicate a poorer performance or a fault in the antenna system . However it may also be due to extended bandwidth being required, with or without size restrictions, and be quite acceptable, such as with military applications.

So, when buying a broadband antenna, you often need to decide between a wider bandwidth with a higher VSWR and a narrow bandwidth with a lower VSWR.

Obviously in antenna design where bandwidth exceeds 1% of the carrier frequency, bandwidth is a primary consideration. For good broadband performance, the variation in impedance between the centre frequency and the outer edges of the band needs to be kept as low as practicable.

As a general rule, where the impedance variation is the same, the bandwidth capabilities of the antenna will be better in antennas with larger radiator diameters than those with thin wire radiators, as these have much greater selectivity. Antenna elements should be very large at low frequencies, decreasing in size as the frequency is increased.

With extreme bandwidth, it is important to avoid selectivity in the feeder coupling network. Conservative matching networks with minimum total stored energy are indicated. If changes in impedance levels at coupling points are not minimised and there are standing waves on the transmission line, these will also add to the selectivity of the system.

The actual choice of antenna can also influence bandwidth. A dipole, for example, does not have as large a bandwidth as a folded type.

One point that is sometimes overlook is that of interference. With a broadband or wideband antenna, interference at a particular frequency can affect reception across the whole frequency range. If your frequency or frequencies do not actually conflict with the interference, it is often much more effective to use tuned antennas, which will in effect help to screen out the unwanted signals.

On the other hand, in installations where there are a number of systems, each with transmit and receive antennas, like one finds nowadays on military vessels, there is more opportunity for dissimilar antennas to interfere with one another or other ship systems and degrade performance.

Of course looking at practical considerations, more antennas take up more space, require more spares and may cost more than multi-band types, but the risk of failure will be limited to individual systems.

So, next time you are planning a broadband system, it will pay you to look further than the quoted VSWR over the bandwidth. It will be well worth your while to take a much wider view to ensure you know exactly what you are getting. Moreover, today’s range of antennas offers a wide selection of types to suit different situations.

It may be that a multi-band type or ones that combine multiple antenna types to maximise performance over an extremely wide bandwidth like the Moonraker BRX receiving series , will give better results. If you need to make compromises, make sure they are not the wrong ones.