UHF CB radio antennas or scanner antennas anyone? Answered
For a while now I am back to experimenting with various antenna designs for my UHF radio.
Started as something entirely different but who cares LOL
Anyways, we usually have either a standard whip antenna or a vertical dipole design for mobile use, like on your car.
This is a very good design for both types of antennas, mostly due to the short wavelenght compared to the "old" 2m or 27mHz CB radios.
Fun fact for at least AU in this regard is that back in the day everyone jumped onto UHF to get away from the overfilled chatter on 27MHz, now the old 2m band is basically dead except for marine use.
One thing I really miss on UHF though is distance!
Line of sight communication sounds fine at first but then you really have to realise the limits once out and about.
As a result we often have to choose between a high gain antenna or a stubby for going into the mountains or being in a convoy.
Only way out seems to be the golden middle by opting for one of the 4.5 - 6-5DBI antennas.
Neither solution really satisfies me though :(
To get around the limitations and have some fun again when sitting high on a mountain I compared various commercial antennas I had.
You know, pick someone with a weak signal coming in and hope he hears you and has some time for a chat.
Turned out there is not really that much difference in terms of receiving a signal.
Quite huge differences though when it comes to the other side being able to hear you!
So I jumped onto a basic SWR meter and actually checked my so called factory tuned antennas - all but one were way off the charts here.
At least if you are like me and like optimised instead of compromised antenna systems.
A Standing Wave Reflection ratio of 1:2 or even 1:2.5 is seen as very acceptable thing on UHF for some weird reason.
Back in my glory days no one I knew would have accepted anything above 1:1.3 for his 27MHz system, be it mobil or a station at home.
Sure enough, repeating my long distance tests a few days later with tuned antennas resulted in far better results.
An antenna for long distance calls or emergencies?!
The easiest way to get more distance on UHF is obviously getting the antenna as high as possible.
From a nice mountain top you have it way easier than in any urban areas for example.
Explains why so many of the long and sturdy antennas are just simple dipoles on a long pole.
If you ever checked the feedback for these double quad TV antennas you realise they are literally in the same frequency range.
Similar story for all the Yagi antennas for our TV sets that you find on so many roofs these days.
Only problem with them is that they usually don't give a damn about transmitting qualities.
And of course that they are usually providing around 300Ohm instead of the 50Ohm our radio expects.
A TV needs 75Ohm and a balun is used to provide a isolation as well as a matching for the TV.
Needless to say I could not accept this ;)
My first design was made from aluminium foil glued onto some printed and stiched pages for a double quad.
1MHz bandwidth is quite narrow, so the basic square design with about 13.8cm for the sides of the squares was easy.
To my surprise by just adding standard RG58 cable this design was very close to 50Ohm already, I had around 60.
A few glue sessions later I was able to match it to 52Ohm - close enough for test.
Did not want to risk anything with my expensive Icom so I used my cheap Baofeng handheld for the initial tests.
Indoor mind you...
To me disappointment I seemed to be unable to pick up anything.
So let the sanner run and waited...
After being bored enough I decided to pick, starting with the antenna and when I moved it the handheld suddenly had a clear signal.
Desperate as I was I grabbed the radio and waited for the chat to continue - nothing again...
Then it finally hit me: The double quad is quite directional....
Sure enough by just turning it slowly I was able to pick up chatter on various channels.
Once brave enough to risk it I even got confirmation that people about 10km away could hear me just fine.
Time to scale it up a notch...
Next design already had the directional problem included as a design feature.
I added a director in the front and a reflector in the back.
The endless hours of glueing and cutting were replaced by using 12mm wide copper tape and sturdier cardboard.
A test with my car on a little "mountain" and the antenna on 2m pole mounted to roof rack gave me, for the first time ever, conversations with people that were over 30km away.
I am currently trying to make the entire thing omnidirectional.
The omni quad....
If you take two double quad antennas and mount them at a 90° angle you end up with very nice 2-lobe pattern, 4 lobes if you don't use a reflector.
Problem here though is that these designs really mess with your antenna matching.
Not to mention that using it while driving only works at quite low speeds.
One day I will do the final design in stainless steel wire though...
Anyways, using the directional properties to get an omnidirection radiation pattern meant using 4 seperate antennas with a reflector for each of them.
Still left a few dark spots at short range but otherwise really nice for long distance if you don't want to constantly turn your antenna.
Getting 4 antennas down to 50Ohm on the connection to the cable was painful to say the least.
The problem of taking care of distances to be in the sweet spot for the 477MHz range also meant the design ended up to be quite bulky.
One of my lightbulb moments providing good ideas caused me to use one reflector with a double quad either side.
Worked almost fine but again provided totally different values to somehow match if it ever should transmit a signal without damaging the transmitter.
Obvious conclusion was to buy a new roll of copper tape and to try to get four single antennas into one "housing" without stacking them.
Bad idea here was to cross the antennas :(
Better idea was to make 4 pyramids out of carboard and aluminium foil.
Sadly this resulted in a failure because the 4 sides of the pyramid reflect the signal not flat bt at their corresponding angles.
Had to make it really big and use 90° angles, resulting in the signal being reflected like light in these cat-eye reflectors on your bike.
After wasting a few days to create the real thing it turned out to not really work properly for the reception already.
Even close range signals came in really distorted.
I guess the double reflection meant the phase is shifting and cancelling out what the antenna gets.
My last desing idea for testing will be with a different approach.
Instead of 1/4 Lambda elements I will use small 1/64 Lambda elements.
Only about 2cm long means that matching is pain in the behind but if the purpose is just receiving it should be fine.
The reflector will be four flat sides like a box with antennas in front of the sides and the cables joined in the center.
If you like to fiddle with antennas and don't mind trying a weird looking base antenna on your roof I might be willing to write up a short Instructable based on my designs - let me know in the comments....
By the way: it is quite easy to desing this for the common 2.4GHz Wifi bands ;)