Polarisation
How is the polarization of an antenna described?
By the orientation of the electric field.
A vertical antenna radiates a vertical electric field; a horizontal dipole, a horizontal one. The magnetic field is perpendicular to it, and by convention we name the electric one.
It matters because mismatched polarisation costs you signal — typically 20 dB or more between vertical and horizontal at VHF over a line-of-sight path. Which is why:
- FM repeater operation is vertically polarised. Handhelds and mobile whips are vertical, so everything else is too.
- Weak-signal SSB and CW work on VHF is horizontally polarised. Yagis are usually mounted horizontally.
Two stations doing the right thing for their mode, on the same band, can be nearly deaf to each other. That is not a fault; it is polarisation.
Length, resonance, and the 19-inch whip
An antenna is resonant when its physical length matches a convenient fraction of the wavelength. Start from the formula you already have:
At 146 MHz, wavelength = 300 / 146 = 2.05 metres. A quarter of that is about 0.51 metres — roughly 20 inches.
Why is a 19-inch-long vertical antenna often used on 2 meters?
It is a resonant quarter-wave.
The 19 rather than 20 is the end effect: a real antenna behaves as slightly longer than its physical length, so the practical cut is a few percent short.
Which direction does length move resonance? Shortening a dipole increases its resonant frequency. Shorter antenna, shorter wavelength, higher frequency — the same inverse relationship as always.
Loading: cheating on length
Sometimes you cannot fit a resonant antenna, and a mobile HF whip is the obvious case: a quarter wave on 40 metres is ten metres long.
Which of the following describes a type of antenna loading?
Electrically lengthening by inserting inductors in radiating elements.
A loading coil adds inductive reactance, cancelling the capacitive reactance of a too-short antenna and bringing it to resonance at a physical length it has no business being resonant at.
It works, and it costs you. A heavily loaded antenna has low radiation resistance and high losses, so it is resonant and inefficient at the same time. Resonance and efficiency are different properties, and confusing them is why people are disappointed by short mobile antennas.
Handheld antennas, and why they disappoint
What is a disadvantage of a handheld radio transceiver’s short flexible antenna compared to a full-sized quarter-wave antenna?
It has low efficiency.
The stubby “rubber duck” is a helically loaded antenna, shortened the same way a mobile whip is, and paying the same price. Swapping it for a proper quarter-wave whip is usually the single biggest improvement available to a handheld — more than any amount of extra power.
And a related trap:
What is a potential drawback of using a handheld VHF transceiver inside a vehicle that lacks an externally mounted antenna?
Signal strength is reduced due to the shielding effect of the vehicle
A car body is a metal box. Sitting inside one with a handheld is operating inside a partial Faraday cage. An external antenna — even a magnetic-mount whip on the roof — is transformative in a way more power is not.
Directivity and gain
A beam antenna concentrates signals in one direction. That is the whole definition, and it leads straight to what gain means:
What is antenna gain?
The increase in signal strength in a specified direction compared to a reference antenna.
Read that carefully. In a specified direction, and compared to a reference. A passive antenna adds no energy — it takes radiation away from directions you do not care about and puts it where you do. Gain is redistribution, not amplification.
The reference matters: dBi compares with an isotropic radiator, dBd with a half-wave dipole, and they differ by 2.15 dB (dBi = dBd + 2.15). Manufacturers naturally quote the larger number.
Of the common types, a Yagi offers the greatest gain — a driven element plus a reflector and one or more directors, all of which shape the pattern.
How a dipole actually radiates
In which direction does a half-wave dipole antenna radiate the strongest signal?
Broadside to the antenna.
Perpendicular to the wire, in both directions, in a shape like a doughnut with the antenna through the hole. Off the ends it radiates almost nothing.
This is immediately practical: a horizontal dipole strung east-west favours north and south. Which way you hang the wire decides who hears you.
And a 5/8-wavelength whip, popular for VHF and UHF mobile, has more gain than a quarter-wave — it compresses the pattern toward the horizon, which is exactly where the other stations are.
Check yourself
- Your handheld cannot reach a repeater from inside your car. Name the two cheapest fixes.
- A dipole is resonant at 7.2 MHz and you need 7.15 MHz. Longer or shorter?
- An antenna is advertised at “9 dB gain”. What is missing from that claim?
Answers
- An external antenna on the vehicle — the body is shielding you — and replacing the rubber duck with a full-size quarter-wave whip.
- Longer. Shortening raises resonant frequency, so lowering it means adding length.
- The reference. dBi or dBd? They differ by 2.15 dB, and gain is meaningless without saying what it is gain over.