Here I look at why a dipole should be installed on an aircraft and why your aircraft ground-plane makes the difference in the reception and transmission of your communication. A good ground-plane is only one thing that contributes to a healthy installation.

First, I look at an antenna without a ground-plane. Simply put, a piece of wire will give you reception. Snip off a piece of wire 56 cm long, for resonance right about in the middle of the airband.

SWR curve of a piece of wire
Impedance – 70 ohms !!

So yes it works but the SWR and the Z are on the edge of the scale.

A vertical ‘dipole’ as this one, as expected, will have a Z of about 70 Ω. At resonance we need the Z as close to 50 Ω as we can and the SWR below 2.

A correctly installed ground-plane helps achieve the 50 Ω and the radio communications will be at their best. The length of aerial and (size of-) ground-plane determine the frequency the antenna is tuned to!

It can be set to resonance at the middle of the airband 127Mhz , or SOS frequency at 121.5Mhz, or any frequency in the airband you prefer.

That angle of the aerial to its ground-plane also plays a significant part when it comes to the impedance you radio sees; As does the size/expanse of the ground-plane itself. These are some of the the needs for an efficient dipole so that the ground-plane and aerial are matched and resonate correctly with each other.

So this begs the question; Is the ground plane correct on my aircraft? – It could be, but according to most of the aerial suppliers …

“Make it as big as possible!”

antenna suppliers in general

…. we will measure this empirically (at least my way!)

Next up we will be building an antenna from scratch.


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