"Secrets" about CB/UHF antennas on your car Answered
Despite more and cellphone and internet coverage mobile radios are still a favourite of people in remote areas or just loving to explore by 4WD.
And for most the complex task of starting with this great hobby ends by buying a radio and antenna.
Of course the raio goes it fits best and that is fine but what about that antenna and while at it what type of antenna do you need?
Let's start with the great myth that so called rugged or heavy duty antennas are really worth their money.
Durability and stiffness is their main selling point or better was.
At highway speeds they won't bend and flex around like your whip antenna.
In the bush they won't swing all the way down onto the painted parts of your car either.
But then again, a more solid steel whip on a spring base would do the same - but won't look as cool or proffessional...
Inside those plastic or fibreglass rods is a dipole antenna, in rare case you get a 5/8 configuration or even a normal steel whip with a pipe around it...
Means in terms of reception or transmission quality they are not a tiny better or worse than any other antenna out there.
Come down to well it is tuned and where the radiating part is located.
There are often obvious choices to put an antenna, like that nice mount or your nudge- or bull-bar.
Or the good old boot clamp in the back.
I have even see 4WD's with the antenna mounted onto the rear bumper :(
Why is it so important to place the antenna correctly?
Place on the front corner of your car it is not only quite low but also partially covered by the metal parts of your car - limiting where the antenna can properly radiate.
If you ever got some half decent training on a tiny handheld UHF radio then you remember to seek higher ground if you can't get anyone to hear you.
The same is true for the antenna on your car.
You want nothing obstructing it.
People with a long spring base often think of it as just an elevated foot.
Most of these however are "ground idependent", which means they actually form a dipole with the antenna you screw on the top.
Having this "pole" below bonnet hight is quite bad for your reach and reception quality already.
Mounting just a normal whip on a direct foot here means you might have better luck with a handhelp from inside the car...
The best would be right in the center of your roof, the highest and most centered point.
This provides not only the optimum radiation pattern but also give you that little edge in terms of higher ground.
How much gain do I need and how long should my antenna be?
Gain is quite relative if you ask me as it far more important to mount the antenna in the best possible place.
A 3DB antenna on the roof will often provide better reception and range than a 9DB mounted on the bullbar.
Assuming you have the best feasable location than to simplify it:
The further you can see the higher DB you want if reaching far is the main objective.
For general use a 4.5 - 6DB antenna is always good.
In hilly terrain range does not matter that much, here you want the outgoing signal to be as strong as possible and with a shape that allows better coverage by being more like a sphere.
For extreme cases it can mean on a 9DB antenna your friend that is just over that little bump ahead is not visible and with that can't hear you.
On a 3DB antenna however the signal is strong enough to reach that blindspot at a short distance.
The overall length of an antenna can be deceiving for UHF frequencies.
What matters is where the antenna is radiating from.
In most cases it will be the top 15 to 30 cm of your antenna.
For a dipole or ground independent antenna you often need to include the entrire base mount.
For the good old 27MHz bands we always had our SWR meter at hand and tried to get the best tuning.
Since the big jump to UHF the commercial antennas come pre-tuned and are claimed to be good to go.
That means that are within acceptable performance to fit almost all installation locations.
In terms of SWR reading it means that 1 over 3 is still perfectly fine.
Would have been an outcry on 27Mhz though in my times.
What is true though that there is not that much real difference to notice between a near perfect 1 over 1.1 to 1 over 3.
If you would bother to a distance test it might be less than 200m you gain on the near perfect antenna.
For the reception it does not even matter all, so why bother anyway?
On the much lower frequencies a really good SWR does not only mean you can get your signal much further out there but also that your transmitter is happy.
Modern ones are now all digital and have ways to protect and compensate for bad antennas or cables.
Together it means we could just forget about these few extra meters and move on.
The stress on the transmitter in your UHF radio however is still there ;)
The reflective energy from a badly tuned antenna has to go somewhere and that is usually back into the transmitter.
The bit that lost directly to your antenna, as said, does not matter too much with the overall limited range of the UHF frequencies.
I did quite a lot of experimenting with my own and commercail UHF antennas, so a network analyser and SWR meter was a requirement anyway.
In terms of output power a good SWR reading means you get what your transmitter is capable of and set for.
With an SRW reading between 2.5 and 3 however a 4W radio might only actually transmit 3-3.5W.
With a bad mounting and an not so optimal cable it might go below 3W!
Distance is not so much affected by this as we now know, but the loss in power on 3DB antenna in hilly terrain can make the difference between being heard and your signal getting lost in static on the other end.
Repairing a blown transmitter often costs more than a simple SWR meter for UHF, so why not add it to the Xmas wish list? ;)
In some case you want two antennas.
Be it for two different DB ratings on the same radio or for totally different frequencies.
High and centered is still best here but you should keep the antennas as far apart as possible, preferable at different heights as well.
On a single radio it does not matter too much but right next to each other the unused anteanna is like these beams on your TV antenna on the roof.
Unlike the directional and watned features in a Yagi antenna the unused element means we change the radiation pattern.
In the worst case creating a blindspot from which direction we won't get any signals.
As a rule of thumb let them be apart at least twisc as far as the wavelenght, so for UHF over 65cm.