Everything you need to know about how an UHF aerial works

ByWes WhitworthMarch 12, 2020
Everything you need to know about how an UHF aerial works

If you’ve got a 4X4, chances are you’ll have a UHF, but how do they work? Here’s everything you need to know about how an UHF aerial works.

While UHF’s are pretty standard things, with the only real decision you need to make when purchasing a new one is how big you want the unit and where to mount it or how many bells and whistles it needs, their corresponding UHF aerial are a very different kettle of fish.

Choosing the right aerial, beyond just the aesthetics of the thing (yep, I’m that bloke who loves the big broom-stick handle jobbies) does get a little involved as to what you want to do with it. For example, you can have different dBi ratings in the aerials for different terrain. This dBi rating will then dictate, to an extent, the size, and material of the aerial you’re going to want or be able to get.

Gone (mostly) are the days where every 4X4 you see has a black piece of fibreglass, that closely resembles a fishing rod blank with a copper wire wrapped around it, hanging off the bullbar. They were just about the best all-round aerial you could get back in the day. It was mostly a one-size-fits-most with them, due to them being set to a much lower frequency; namely 27MHz.

With the introduction and subsequent proliferation of UHF radios (around 477MHz) there were a lot more aerials made available, that were designed for use in different environments. The use of UHF gave better bandwidth (voice and data quality), however due to the higher frequency, the radio waves can punch through the ionosphere and off into space, without the typical ‘bounce back’ that HF (27MHz) would get. But that’s a story for another day.

So, to the nuts and bolts of it – which aerial dBi should I be looking at, and why?

Weight Diag

 What does dBi do in regard to a radio signal?

Imagine if you will, an invisible donut hovering around the tip of the aerial on your four-wheel-drive. Except the hole is really small, the donut is really big, and it’s not made of flour and cinnamon. This donut, based on the dBi rating of the aerial, will be stretched either up and down (vertically), or sideways (along the horizontal plane) – with a higher dBi the donut is flatter but wider, and a lower dBi it’s higher but skinnier.

The different aerials

Measuring aerials gain is generally done in dBd or dBi. We’ll be explaining gain in dBi, simply because it’s easier to get our heads around, and the majority of aerials are advertised with just the dBi rating.

dBi technically means ‘decibels relative to isotropic radiator’, however the important bit for us, is the number. Think of dBi in aerials like a torch with a focusable beam, where the bulb or LED is your 5W UHF, and the focusing ring is your aerial.

The torch is still putting out the same 5W but focus the beam (say 9 dBi) and it goes a long way, only illuminating a small area way over the other side of camp, but you can’t see your feet or the roof of your awning. Go the other way with the focusing ring (say 3 dBi), and you can now see two campsites over, your roof, your feet, and what the kids are up to, but you can’t see the Boobook owl 800m away anymore. That’s dBi in a nutshell.

Weight Diag

High Gain: 6 – 9 dBi

These are usually the broomstick handle style aerials and are often fairly long. They’re great for long-distance on open plains and on the highway where the terrain is nice and flat. The donut on this one is flat, and wide – so it’ll throw a long way with line-of-sight, but put a building, hill or state forest in the way and you’re going to have comms issues.

Medium Gain: 3 – 6 dBi

These fellas are usually the ‘black fishing rod blank with copper wrapping’ whip-style or the three-foot stainless-steel rod with ‘pigs tail‘ in the middle. Anywhere in this dBi range is going to be a good compromise between distance, and terrain/obstructions to line-of-sight. The signal output of this range would closely resemble an actual donut – not too flat, but not perfectly round either – right in the middle. These are the all-rounders.

Low Unity Gain: 0 – 3 dBi

These ones are more often seen in cities and in alpine/mountainous areas. The ‘Unity’ part of the name just means that the aerial pattern radiates in all directions equally. They’re usually short, stubby, and have a heat-shrink coating. This aerial will push a signal quite happily over trees, mountains, buildings, whatever – but you’re sacrificing range to do it. Their donut is essentially perfectly round – It’ll bounce over hills (or 60-storey buildings) as high as it will throw signal horizontally.

180701 Guide To Aerials (9 Of 9)

What UHF aerial do I use?

This will come down to where you spend your time driving. Being an off-road journo, I generally like to keep my options open. I need a lowish dBi aerial for when I’m off in hilly country like the Vic High Country, but then I like to be able to keep in touch with truckies 10km up the road on the highway. With this in mind, and maybe a touch overkill, I’ve got both. I’ve got a Uniden 4.5 dBi stainless-steel whip for in the hilly country connected up to one UHF that usually scans between channel 5 (emergency channel), channel 10 (convoy/clubs channel), channel 16 (general chit-chat channel), and 18 (caravanning channel). Then, I’ve got a big GME 6.6 dBi hanging off the bullbar, that gets me better distance, while still being able to punch over rolling hills on the Pacific and Bruce highways. This second UHF is generally scanning between channel 29 (Pacific/Bruce highway channel), and 40 (general highway/truck channel). This combination covers me for just about any eventuality, so no matter what I’m doing, I’ll be able to talk with the folks around me.