Heavy duty UHF CB radio antennas and why cheap can be much better.... Answered
If you not a fan of normal road cars then you might get some 4WD and go off road every now and then.
There are even those doing trips to remote locations that take a few weeks.
An essential piece of equippment these days is a UHF radio.
In a convoy you might get away with a handheld one if the cars are not too far apart and the terrain suitable.
Most peole however opt for a permanent installation in the dash or roof console.
After that decision comes the hardest and often costly part: What antenna to use....
If you trust certain online reviews and manufacturer claims then one thing becomes quickly obvious.
If you need a sturdy and powerful antenna then you have to pay top dollar.
Some come with heavy spring bases claiming to protect the heavy fibreglass rod if you hit a bush or trees branch.
There is always a compromise between weight and stability.
And trust me, on the right corrugations you prefer not to look at your antenna....
So how are these expensive antennas made?
As I don't have any own pictures and don't want to steal them from the net: Please search for the terms used if you don't know them already.
To answer this let me go to the other types of antennas you can get for your UHF radio.
There is the famous rubber ducky - a stubby antenna best suited for short range on difficult terrian.
Then we have the fibreglass whip antennas that have more or less coils integrated or even consist of one single coil with different spacings in sections.
Good for normal road use, not so much off road as they won't tolerate too much vibration, they often snap off the base screw.
Most people now just ignore the short whip antennas of 15 to 30cm length as they usually only come with a gain of around 4.5-6db.
I will explain later why that might be a misconception.
The last ones are those steel whips with one or two "loading" coils.
These coils electrically shorten the antenna.
They also provide a matching to the 50 Ohm required for most transmitters and antenna cables.
Usually they are designed to be 5/8 Lambda antennas.
A good antenna for just receiving, like when using a scanner is not critical in length.
Transmitting however means you need a proper Standing Wave Reflection or SWR ratio.
For this the antenna needs to be tuned.
To be of any good use your antenna needs to match the transmitting frequency.
This works best if the antenna length is at 1/2, 1/4 or 5/8 of the corresponging frequencies wavelength.
Also called Lambda if you look for antennas.
In the good old 27-MHz days we were used to quite long antennas in our backyard, for the car we then opted for 1/4 of this - usually around 2m in length.
For UHF however 1/4 wavelength already means you are down to about 16cm...
Going for 1/2 is a good thing here as it is still quite short at about 34cm.
Real difference when it comes to these Lambda factors is the radion angle produced.
You can imagine a 1/4 to look like a huge donut with no hole, about 25° for the radiation angle.
At 1/2 this will be flattened out to around 20°.
A 5/8 Lambda ntenna can get as low as 16°.
Imagine it like a flashligh that has adjustable focus.
The light source is of fixed output as your transmitter.
The lens does the job of the antenna.
If you make the beam more narrow then the light intensity of a certain areas at a certain distance will increase.
Means for the radio you get a longer distance your signal can travel with enough energy.
This however comes at a price!
Imagine you are at the bottom of quite stepp mountain and your mate is up on the top about 2km above you.
A high gain antenna with a narrow radion pattern might not even reach up there, while a short stubbie with just 3db still has a chance due to the more spherical radiation.
Also explain why low gain and with that short antennas work best in hilly terrain...
Back to the thick ones...
As you can see you can basically hide any type of whip antenna into a fibreglass rod.
But most of the are as said "ground independent", means unlike your normal whip they don't need the spring or foot to be of low resistance to your car's body.
How does that work?
Well, exactly like these ground independent whip antennas with a spring base or metal pole base work.
You know the earliest antenna was a dipol - look it up on Wiki ;)
For our CB radios that means you have a metal rod or spring that is about 1/4 or in some cases 1/4 of the wavelenght long.
This is connected to the shielding of the coax cable and provides the required ground for the mounted antenna.
The big difference is that only too often a dirt cheap dipol is hiding in your expensive fibreglass rod ;)
The complete antenna might bring over 2kg on a scale, but the actual thing allowing you to receive and transmit is a few grams of coax cable...
HOLD ON A MINUTE!! Some will say now...
My whip is 70cm long and my expensive heavy duty one with 9db is 2m long - how does that work with your wavelength theory??
Well, it is not my theory, just a fact ;)
Imagine a 1/4 Lambda dipol, then it would be all up around 35cm long.
And funny enough, that is about the length of a $180 heavy duty stubby if you just tak the rod itself.
Go 1/2 of Lambda and you get an overall length of the dipol of around 65cm - add the metal rod making the crew that holds the fibreglass rod and you have the common 70-75cm heavy duty antenna...
Anything above this length usually is either just a long rod with noting above the 75cm mark or simple has the 75cm long dipol made from the coax cable at the top with the antenna cable going down the otherwise empty rod.
Ok, I got it, either 1/2 of the wavelength or 1/4, so about 35 or 16cm long.
Ground independent we add either 1/4 for the spring base and rod or 1/2 for the longer ones.
And how again does it work with the gain of an antenna?
If you trust Wiki then it comes down to the radiation pattern.
These heavy duty antennas usually come with around 6 or 9db, the short ones with 3-4.5db.
These values might give you an indication about the theoretically possible distance you can transmit but nothing about the terrain it is suited for.
Common rules of thumb created by those selling antennas and radios is that you a high gain antenna on flat terrain and a low gain antenna in a hilly area.
Around 4.5-6db seems to be the golden ratio here as these anteannas are equally bad for both extremes in terms of terrain options.
What you really would need to know is the actual radiation patterns in a three dimensional plane.
A straight whip or dipol as a more or less donut shapead radiation pattern.
However, location affects this!
Mounted in the middle of the roof it is closest to perfect, while at the corner of your bumper bar you will distord the donut and also block parts out with the body of your car.
This is why for this type of mounting elevated antennas are prefered.
Makes no sense to have a 16cm long stubbie mounted so the top is still lower than your bonnet...
Any antenna with a loading coil (or several) or top load will have a distinctively different pattern.
We speak of so called "lobes".
If you see it in 2D then for example a 1/2 lambda straight whip will look a bit like the infinity symbol.
A 1/4 Lambda of the same style looks more like two ping pong bats joined without the handles.
Those with loading coils or linear arrays made from coax cable however can produce multiple, prefered lobes.
Usually they are in the 4-6db range and claim to be "universal" or as "allrounder".
Here you get a quite narrow main lobe of 12-16° with one or more but much shorter lobes going upwards at about 10-30° depending on the configuration.
At short range, like in hilly terrain both lobes overlap while you get a dead area at greater distances.
You can sometimes notice that when you are on a low level talking to someone up high.
There are cases when with a bit more difference in angle to each other (in terms of height and distance) the signal jumps up a few numbers.
You just went from the dead zone into the lobe ;)
With just a db value for the anteanna but no details about the actual design, heavy duty antennas can fool you badly.
In mayn cases a 1/2 Lambda straight whip on the roof will outperform a costly, heavy duty antenna mounted to your bullbar.
This is the reason why the expensive ones are the biggest cheat - they just elevate a quite small antenna above your roof line.
And since it is heavy it needs a big spring and you hope it will not break if you hit something on a narrow track.
All while the thin stainless stell whip with the cheap magnetic base just flexes under all obstacles with no damage at all.
Plus, if you really get into the thick jungle you can just take the magnet off until you are through LOL
Why is a SWR and power meter still an important tool to invest into?
Those remembering or still using 27-MHz radios only know too well why you need a proper SWR and power meter.
With the lenght of the antenna at these low frequencies and affecting factors new the antenna proper tuning is a must.
The bandwidth of the channels also means you have to tune the lowest and highest channel so the are basically even, anything else and the old guys would scream "UNACCEPTABLE!" ;)
Especially it you want to get the last out of your system without going illegal.
For some reason we accepted the claims that an SWR reading of around 1:2 is fine and acceptable.
Most of the radios lower the power output to protect the transmitter if the SWR goes to far out.
Allows for simple mass production of antennas with fixed cable lengths that are usually well overpriced.
The most expensive bit is the cable itself here...
I had no time to build an analog SWR and pwoer meter that works properly on 27 and 470MHz, so I ordered cheap SW30 from China.
With that I first checked my little collection of antennas then those of some of my friends.
This includes everything from short rubber duckies over loaded and straight whips to heavy duty models.
One thing that was obvious right away: most are far away from an SWR readin anyone with a 27MHz groundplane antenna on a long mast would accept.
And only one heavy duty antenna had a SWR readin of below 1:1.6 for channel 1 AND channel 40!
That one was relatively cheap noname brand.
With that sorted I decided to tune at least my steel whip antennas.
To my utter disappointment they were all just a tiny bit too short - a thing that would have caused a proper 27MHz antenna manufacturer to to keep a large stockpile of his antennas...
I had one though that was longer than needed.
And before you ask: Yes, I tested them on both my elevated bullbar mount AND a direct mount on the roof rack.
Did not change much for the bad ones so I ignored the mounting position for the tuning.
I checked the power once for both channels on my prefered antenna and got 4.4W.
Mind you that one has a SWR reading of 1:2.2 and will no longer be used as I can't be bothered to make it longer.
For some reason I thought I check the SWR and power everytime I cut a bit off the antenna instead of just watching the SWR reading.
At original length I had a SWR of 1:1.9 on ch40 and 1:1.75 on ch1 with 4.6W.
I kept trimming down by about 2mm increments until I got an even readin of 1:1.07 on both channels.
And with every trim the power went up a little bit.
I have a friend that is or better, was just at the brink of being unable to reach from my driveway when he is parked in front of his house.
A radio check after the tuning revealed that instead of coming with a lot of static noise and sometimes cut out I got a solid reading of 2 on his end with a much more acceptable level of noise.
However, I still struggles as badly to hear him...
Funny thing is that test was with just a plain and straight whip of 1/2 Lambda, in my case the tuning resulted in a length of 38cm from the base of antenna screw to the tip.
My fancy 9db high gain antenna that is just over 70cm long did not even reach him while I could hear him slightly better than on my tuned one.
Changing the mounting to the roof rack gave me a clear reception and a signal strength of 4 at my friends end with no noise.
Elevation and nothing around the antenna does matter...
After all this, would I still bother to buy a ready to go antenna for 470MHz?
Only if I had to.
Getting some RG58 cable or re-using it from on old antenna is cheap enough.
A standard screw mount with a grub screw to hold a steel whip sets you back less than 10 bucks, from China even cheaper.
And most will find a soldered on connector on the other end of an old antenna to be salvaged if required.
If you don't have any sring steel wire of about 40cm length then think out of the box ;)
The packing of pillows and such often come with a sring steel wire to make the plasitc floil keep its shape, some old suitaces have thicker wire doing the same.
And if you ask nicely you might get a bristle or two for free from a street sweeping machine at your councils depot ;)
After all we only need a maximum of 40cm to have enough left to trim and tune down.
What is left to do?
Of course some distance tests to check how well such a simple antenna really really works in comparison to commercial models.
I only have one 9db antenna and will check it first to see if on flat terrain there is much difference in distance for transmitting.
Unless this difference is well above one kilometer I will not bother with a high gain antenna like this anymore and instead opt for a longer mast and 1/2 Lambda.
Another thing on the to do list to try a ground plane antenna with topload to squeeze the radion lobe down to under 15°.
The resulting antenna would be quite short here and using an elevated mast is a must have to get over roof level with at leat a few wavelengths of distance to the roof.
I guesstimate that an optimised antenna of this style should result in a distance increase of about 10-15km of flat terrain while being utterly useless in a hilly areas.
Plus, such a design is not really suited for a vehicle going fast on a freeway, so it will go on my house instead.