Radiation Resistance, SWR and Resonance

posted in: Info | 0

Radiation resistance is that part of an antenna’s feed point resistance that is caused by the radiation of electromagnetic waves from the antenna, as opposed to loss resistance which generally causes the antenna to heat up. The total of radiation resistance and loss resistance is the electrical feed point resistance of the antenna. For a half wave dipole antenna the radiation resistance will vary between approximately 60 and 90Ω for all practical heights on the HF bands. For Antennas with Traps or loading coils loss resistance in these components will add to the feed point impedance But will not alter the radiation resistance of the antenna.
SWR… It is unfortunate that many Amateurs do not understand SWR. The reason it seems to have become the most important parameter for an antenna system is that it is the most easily measured parameter. Prior to WWII and the availability of low cost coax cable Amateurs did not know or even care about SWR. The most common feed line was the 600Ω open wire line and antenna systems with standing wave ratios of 10:1 would not have been uncommon. Solid State transceivers with fixed output impedance that will not tolerate high SWR have only added to the many erroneous myths that are propagated by the internet. Reasons for low SWR are often false and too many times cited as the single defining parameter regarding the performance of an antenna. Low SWR is no indication of an antennas performance it is simply a measure of how closely the antenna impedance is to the feed line impedance. Often the antenna with the best SWR is nothing more than a good dummy load. Antennas with poor ground systems, lossy loading devices or high resistance conductors or connections will almost certainly provide a flat SWR curve across the band. Antennas that are electrically short in length will have low radiation resistance as well as narrow bandwidths; if they do not then they will without doubt be very inefficient.
Resonance …does a Resonant Dipole Radiate Better than a Dipole off Resonance? The answer is No both antennas will radiate all the power delivered to them; the only issue will be the difficulty experienced trying to feed the off resonant dipole. A typical half wave dipole on a wide band such as 80 meters will if resonated near the band center will exhibit an SWR of about 2:1 at each of the band edges with impedances in the range of 40 + j70 and 60 + j70 respectively. Losses will be negligible when operated with SWR in this range. Your solid state transceiver may be happier if you use an antenna tuner but this level of SWR and even up to 3:1 will not add any significant losses. By the way the reflected 25% power with the 3:1 SWR will not be lost it, will be re- reflected back and forwards down the transmission line until it is all radiated or lost in heat in the coax.

See http://vk8da.com/how-much-power-gets-lost-due-to-swr/
Please keep in mind Amateurs work on Bands and an antenna will only be resonant (Resistive with no reactive component) at one single spot frequency.