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Optimising a coax fed half wave dipole

One of the simplest antenna system is a half wave dipole fed with a balun and coax feedline.

It has the advantage of requiring very little knowledge and experience to be implemented with a high degree of confidence that it is working very well. As such, it is an excellent antenna for a beginner.

The optimisation objective

All RF energy from the transmitter that is not converted to heat in various parts of the antenna system will be radiated, the objective is to maximise radiation by minimising system losses.

Since almost all of the loss is in the feedline in most practical cases with this type of antenna, optimising performance is really about optimising feedline loss.

To illustrate the techniques, an NEC model of a dipole for 40m, at a height of 10m, and fed with an ideal 1:1 current balun and 30m of RG-58/CU coax is used.

Above is a plot of feedline loss and source end VSWR. Source end VSWR is what would be indicated on a VSWR meter in the shack. It can be seen that the frequency for least line loss is that where VSWR is minimum.

To optimise line loss in this case, we must minimise VSWR.

Minimising VSWR

Whilst we can estimate the physical length of a dipole for a given frequency, the length for lowest VSWR depends on factors that include the wire diameter, insulation, height, environment (ground type, nearby conductive structures, vegetation etc).

So, the procedure is to measure VSWR at a few points within the band, and use that information to find the frequency where it is minimum. You can then calculate a percentage error between that frequency and the centre of the desired operating range, and adjust the length of the antenna by that percentage. If the frequency of minimum VSWR is x% low, then shorten the antenna by approximately x%. It pays to always try a length a little longer than estimated as it is easier to cut more wire off than it is to add wire.

If the minimum cannot be found inside the band, use the VSWR slope across the band as a hint for whether to shorten or length the antenna.

You should expect to achieve a minimum VSWR less than 2:1 for this type of configuration. If you don’t something is wrong, check all connections and components.

Is resonance a better goal

There is a view often expressed in ham radio that resonant antennas naturally work better. It is an appealing concept, but wrong!

Resonance is not a necessary condition for good performance, nor is it of itself an assurance of good performance. For more information, see The importance (or not) of being resonant.

The graph above shows feedpoint resistance and reactance for the modelled dipole, and source end VSWR using 25m of RG58C/U feedline. The dipole is resonant where X=0, so at 7MHz in this case. It can be seen that in fact VSWR is minimum very very close to resonance, due to the fact that reactance changes quickly near resonance whereas resistance changes relatively slowly.

Such an antenna will perform best at around 7MHz, and some incorrectly draw the conclusion that it is because it is resonant whereas the real reason is that line loss is minimised, albeit very close to resonance.

Resonance is not the primary objective, lowest VSWR is, and lowest VSWR is easily measured using a common VSWR meter.

The graph above shows the impedance looking into the 25m of RG58C/U feed line for the same antenna.

Note that X does not pass through zero anywhere near the freqeuncy at which the dipole itself is resonant (7MHz). Using an instrument such as the MFJ-259B at the input of the feedline to adjust the dipole for X=0 (resonance at input) will not optimise the antenna, but adjusting the dipole for minimum input VSWR will deliver lowest line loss in this case.

It happens that in this example, that minimum input X occurs close to the dipole resonance, but if the coax length was slightly different, they would not coincide. Minimum input X is not a good tuning objective.

Where to measure VSWR

There is a commonly held believe that VSWR can only be measured at the antenna feedpoint.

The VSWR(50) (meaning read on a 50Ω VSWR meter) on a practical nominal 50Ω feedline smoothly decreases from load to source as accounted for exactly by the line loss. If you measure VSWR at the source end, the figure will be lower than occurs elsewhere on the transmission line, but the minimum will occur at exactly the same frequency everywhere along the line.

The VSWR at any point on the line (eg at the load end) can be calculated reasonably accurately from the VSWR measured at another point if the line loss between the two is known. VSWR calculator is a handy online tool to perform the necessary calculations.

For example, in the above model, the loss in 25m of RG58C/U at 7MHz using TLLC is 1.1dB. From VSWR calculator, with a source end VSWR of 1.3, the load end VSWR is 1.4.

Is a tuner needed

A tuner transforms the impedance seen looking into the feedline to the preferred load (typically 50+j0Ω). If the VSWR after optimisation above is more than about 1.5, then an ATU may assist in obtaining rated power from the transmitter.

A tuner will also allow developing rated transmitter power away from the lowest VSWR, so can provide some bandwidth extension for operation over wider bands like 80m, but although the ATU delivers the rated load to the radio, it cannot overcome the feedline loss at higher VSWR.

Conclusion

A coax fed half wave dipole can be easily optimised using a transmitter and common VSWR meter. Some transmitters contain a VSWR meter that may be adequate to the task.

The use of a VSWR meter for this task is not a compromise, it is the instrument most suited to the task

Optimised, the efficiency of this antenna system using a long RG-58/CU feedline is greater than 75%, with RG213 system efficiency will approach 90%. Such high efficiency is very hard to improve upon, though of course, a shorter feedline would be even better.


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