Antenna Length & Resonance - Please Educate Me

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Jul 31, 2016
As a novice in the world of radio, I want to understand antennas. Can someone explain antenna resonance as it pertains to electrical length? I am having a hard time wrapping my head around the concept.

....Theory tells us resonance is when the capacitive reactance and inductive reactance are equal. This happens when the antennas electrical wavelength matches the desired operating wavelength/frequency

...... My concept is that antenna ‘resonance’ is akin to a tuning fork humming – the EMF wave excites and vibrates the element. Picturing a sinusoidal wave, the wave’s length matches the antennas element perfectly and it becomes resonant. As the element becomes shorter or longer, the wavelength doesn’t match the electrical length and resonance moves elsewhere.

....Yes, mostly, although it is not the physical vibration of the wire....

Let’s talk small lengths, such as a ½ wave 10 meter (28.3 MHz) dipole @ 16.5 feet. What actually happens when the antenna is a full, 3/4, 1/2, 5/8 or 1/4 wavelength? What is ‘resonating’ and is ‘resonance’ a product of the electrical length? ?
If yes, what specifically about the electrical length makes an antenna resonate?

....Again, it is when the capacitive reactance and inductive reactance are equal. If the physical length is such that the capacitive and inductive reactance are not equal (if the antenna is too short it is more capacitive and if is is too long it is more inductive) such as a just a random length antenna, capacitance or inductance can be added to the antenna as needed.

.... For example a physically short CB antenna will have added inductance somewhere (base loaded, center loaded or top loaded). The goal is for the electrical impedance at the feed point be as close as resistive (resonant) as possible and this impedance match the impedance of the feed line and the transmitter/receiver.

Does a ‘normal’ resonant 1/2 dipole perform with a greater efficiency than a coil loaded dipole? If they are electrically the same length, but a portion of the element is a coil, how and why does it perform in a different way?

....Yes, there are losses associated with the coil (inductor) and antenna capture area (larger is better in general terms).

How does a resonant loaded dipole (or other) antenna work when it is considerably shorter than normal? For some designs, the coils are very tight with little or no spacing. Why not make a super coiled physically short 160 dipole? One could easily coil a 250 foot long 160 meter half wave dipole into a compact antenna – right?

....In simple terms the losses would increase and the usable bandwidth would likely decrease. Do a search for 80 meter mobile antenna theory and look at the losses and efficiency. It is not necessarily a pretty picture.

All of this is to understand how to make an electrically long MW receiving antenna. Instead of a long wire, why not load it up to make it electrically longer for the purpose of resonating more efficiently?

....Physically shortened antennas are not uncommon, but the more loading used to shorten the antenna the greater the losses, lower the efficiency and narrower the bandwidth. Again the smaller the antenna with regards to the wavelength in terms of the wavelength of interest, he smaller the capture area and the less well the antenna will work.

....All of the above is not to say that a physically short antenna, with or with out loading to achieve resonance will not work. An antenna that is less than a quarter wavelength long, even with out loading to make it resonant may work okay as a receiving antenna although the mismatch loss and other factors may come into play and not help reception.

....There are other "gotchas".

Antenna resonance for Dummies – that’s what I am looking for …


Sep 19, 2012
You must have started the reply before I change the text. I moved it to a different forum for greater visibility. I am sorry about that.
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