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Part 8 - Aerial Summary

This page was written to summarise the knowledge I've gained in compiling the previous pages.

It has had the guiding hand of Kurt N Sterba 
of World Radio Online to correct some of the mistakes I made, I hope I've done Kurt's comments justice but refer to the Please see the December 2011 edition article for the full review.

If I'm missing a logical step or I make a mistake, please let me know but I hope it fills in the gap I certainly see exists.

Its not intended to rubbish manufacturers or stop anyone using low efficiency aerials. On the contrary, RF is such a good traveller it will often make the most of a poor system and there are many people who can attest to that. Chasing the most efficient system can be a total waste of time, if you are happy with your lot then be happy - if you are not then something here might strike a cord and you might put your efficiency up 20-30% - good on you too.

This is intended to clear up the gumph that is written about SWR and make a statement for aerial efficiency which is never metioned in adverts. 

Most people have a low radiated power. QRP operators generally use very efficient aerial systems (that are low cost) and the 100 Watt plus crowd (me included) often use very inefficient (and high cost - not me!) aerials. Those who use 100 Watts and cheap efficient aerials are obviously the big-guns!!

When transmitting, it is what leaves the aerial system that is important. I hope this puts a smile on those who already know it and helps those who don't, reassess thier setup and opt for a cheaper, more effective solution.

The facts.....
  • A resonant aerial is NOT more efficient than a non-resonant aerial. Efficiency depends on radiation resistance and loss resistance, if we use a resonant aerial and then change the frequency, neither of these change and the efficiency is the same for a non-resonant frequency! What is harder to do is to get the power into the aerial in the first place.
  • There are many non-resonant aerials that perform very well indeed and these should not be neglected!
  • A resonant aerial has no capacitive or inductive reactance. 
  • A resonant aerial's impedence is therefore resistive and rarely 50 Ohms (close for a half wave-dipole free of obstructions - this is where the low SWR/resonant aerial confusion comes in.
  • A resonant aerial will not always be a match for a 50 Ohm feed line
  • An unmatched aerial and feedline (i.e. not the same impedence) will produce a reflection at the junction of the the aerial and the feedline
  • The reflection will interact with the incident wave to generate a standing wave
  • The standing wave will register on an SWR meter as a ratio higher than 1:1.
  • Using an ATU to tune (it does tune and match) an aerial for minimum SWR is makng it resonant,  Kurt explains that the the Tuner DOES tune the aerial (not quite if there are line losses though). When we use the ATU to match the system we effectively cancel out the the reactances at the antenna feed-point, no reactances leaves just the resistive element and that is the definition of a resonant aerial!
  • A high loss feeder will attenuate (reduce) the RF to the aerial and any reflected RF (created due to a mismatch) on its way back to the SWR meter. In the SWR meter the incident RF will be high as it is measured leaving the transmitter, will reduce to the aerial and reduce further returning to the SWR meter where it is measured as reflected RF. This reduced value of reflected RF will show a low SWR value that is not a correct value.
  • A low SWR (without the use of an ATU), therefore, is not an indication of resonance.It is an indication that the aerial and feeder impedances are matched for improved power transfer.
  • Improved power transfer is an indication of improved aerial performance, it is an indication that the aerial is receiving as much possible RF power as is possible. However, the radiated RF is dependent on the aerial's efficiency.
  • The Aerials efficiency is defined as = Radiation Resistance / (Radiation Resistance + Wire Resistance + Return resistance)
  • Radiation resistance is a function of the aerials length compared to a wavelength and can not be affected in any other way
  • Increasing the Wire Resistance or Return Resistance reduces efficiency.
  • A high Radiation Resistance reduces the effects of the Wire Resistance or Return resistance on reducing efficiency
  • Only by INCREASING the Radiation Resistance or REDUCING the Wire Resistance or Return Resistance can you increase efficiency
  • A half wave dipole mounted 0.25 wavelengths above ground has an efficiency approaching 90%, this where its efficiency peaks
  • Copper wire has a low resistance - steel has a high resistance - increased surface area reduces resistance (at RF). 
  • All Aerials need a return path back to the transmitter to be effective
  • The lower the resistance of this return path the greater the efficiency
  • The return path should 'capture' as well as the radiating element radiates
  • A dipole has two equal elements so the 'return' element 'captures' as well as the active element radiates.
  • A vertical aerial uses the ground as the return element and this is generally a poor 'receptor' (salt water marshes are excepted!)
  • A vertical aerial may use radials as the return elements and these can work well but several may be needed to counteract the ground losses
  • A vertical aerial mounted high can escape ground losses but still requires an efficient return path to the transmitter. These are provided by radials.
  • Any vertical aerial that states 'no radials are required' has a low efficiency as it has a high return loss.
  • Any mag-mount aerial has a low efficiency as it also has a high return loss [However, If you crash your car, the Mag mount will turn into a very efficient missile!]
There is more to add - let me have your pearls of wisdom

Questions you might ask yourself:
How much of my transmitters power is getting to the aerial?
  • Do I have low loss feeder?
  • Do I have a matched feeder and aerial?
How efficient is my aerial in transmitting that power?
  • Does it have a low RF resistance?
  • Does it have a good return path to the transmitter?
Ask yourself these questions:
  • How does a HF mag-mount aerial return RF?
  • How does a vertical with no Radials return RF?
Remember - the outside of the coax is not an option. Also, DC power leads are not an option ("my mobile is earthed to the car via the cigarette lighter so it's got a great RF return" is not a correct response!)