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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 ;)


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1 year ago

A little update fresh from my simulator :(
Despite the otherwise quite good features of a double quad antenna, they are a true pain when stacked in ways you shouldn't stack them.
If we just estimate that a duble quad with reflector and two directors gives us an imagined gain of 8.3db than this gain is shredded when you go omnidirectional.
To oversimplify it you could say the 8.3db are divided by four antennas going north, east,south and west.
Turns out this loss is somehow linked to the reflector.
If more than one antenna get the same signal than these signals will be slightly out of phse and the stronger signal is reduced by the weaker signal to some extend.
Using reflectors that go close to the radiation patter solve this a bit but it won't really help much for stron incoming signals.
I did not do any transmission tests yet as I have to assume the same interference will happen here and I still need my radio...

I did not learn how to make the perfect antenna but I did learn a lot of ways how not to design one ;)
Failure is not an option here so after a big brain freeze I came up with an idea that so far I fail to find in any antenna related patent or existing design for a quad stale antenna.
Here is the idea behind that derived from the not so succesfull attempts so far:
Whenever a higher gain is achieved other things like bandwidth or matching ar either compromised or made more complicated.
A lot of these issues are address by the drone community and their often great atnenna designs.
Like omnidirectional antennas with a circular polarisation.
The look a bit like a twisted spiral in ball shape.
For porposes where the signal is only polarised in the vertical plane this however is not of that much use.
After all we won't turn our car upside down or have endless conversations while climing a 70° incline with our winch...
At least not to the max of our reach in terms of distance.
Wouldn't it be good to be able to cover the vertical polarisation with a beam pattern optimised for long distance communication?

The looped quad antenna.
I found some historic documents about using a quad antenna made from 2 or three loops of wire.
I won't go too far into this as it is not really that related to my idea:
A quad antenna already is a directional antenna.
And if we trust simulations than only the area of half a wavelenght around the feedpoint and the opposing side actual do any work.
The areas between them actually compromise the signal by having out of phase currents in them.
Logical conclusing would be to make the square a loop and a quite narrow one so the area of interference is eleminated.
Physics however tell us this is not good as the resulting dipole would be far less efficient.
So the only real way out is a full circle where the diameter is not fully linked to the wavelength but shorter, ideally so that at the tuned frequency the area of interference would be zero.
Needless to say phsics has something against that as well, especially if you require at least a little bit of bandwidth.
But what if the antenna is left the seemingly perfect quad style but as a loop with the reflector between the loops?
A double quad is already a nice 50Ohm match with just a reflector, even better with an added director.
If we would basically fold it then we have two quad antennas that literally should cancel their signals out.
However, if the first forms a clockwise orientation from the lets say the connection to the core, then we can make a counter-clockwise orientation on the other side of the reflector.
Being that close together the possible interference from signals reaching both antennas can be neglected - at least if you consider the final goal of another set orientated 90° to the first.
Only problem is that due to the required distance to the reflector the desgin might only be feasable for above 1MHz.
Adding the basic concept of the double quad to this would mean we get a doublesided "infinity" antenna.
Imagine the infinity symbol of just draw it ;)
Start at the center and go up to the right to form the first loop of the doublesided double quad.
When you come back to the center you would then continue the loop on the OTHER side of the reflector.
When back to the center again you stop and have the infinity symbol split onto both sides of the reflector.
You do the same again to form the second infinity symbol.
You end with four endpoints.
If you look through the design from one side than you have two on either side of the center and those two are either side of the reflector.
You connect the points through the reflector to get just two in total, one ither side of the center.
Does not really matter which one you connect to the shield of the cable and which one to the core.
What does matter though is that the resulting antenna is not a match for 50Ohm!
Also that the signal received by both antenna halfs will be reduced by the weaker signal.
But unlike before we now have a "clean" cancellation instead of interference signals out of phase.
To match the 50 Ohm and to cover the other half of the circle we need a second set orientated 90° to the first.
Currenty I am only as far using two flat antennas instead of true spherical one...
Either way, two sets require to use a length of 50Ohm cable between them to join them.
This needs to be an odd multiplicator of 1/4 of the wavelenght to come closer.
It might be required to do this in a coil form with about 6 to 10 turns but I am still struggling to eastablish the correct numbers just by simulating the design.

A double quad antenna has an overall lengths 2 Lambda, or twice the actual wavelength it is tuned for.
Using all up 4 such antennas gives us an overall length of 8 Lambda!
And if we trust official handbooks than this is about as worse as you can go.
But: Is it really?
In a threedimensional design forming an endless infinity symbol with all up 8 loops you can utilise what Tesla called the bifilar winding.
For each winding or loop you would have one that goes exactly the other way around.
The result is no inductivity at all and all stray effects cancelling each other out as well.
The magnetic field however is 4 times stronger compared to a single coil with the same overall number of turns....
A resulting antenna would effictively have zero impedence like a dead short but only in terms of the transmission signal.
Means the signal has no loss at all in the antenna - if you knidly exclude the physical things like material and such.
For the transmitter however it would appear as a full wavelength antenna at about 100 Ohm.
Means stacking two of them brings more gain and again a good match down to 50Ohm.
So far for theory at least....
An antenna coiled in a 3D infinity symbol could be considered a fractal antenna and as sch is subject to the shortening effect.
For a 2D fractal on top of your speed factor this is about 0.95-0.97.
In a 3D fractal this could be as high 0.6 though.
Apart from this tuning problem there is also the problem of how radio waves travel.
We like to simplify whenever possible and that made us think that polarisation is a thing that is only twodimensional.
The wave is eithe going up and don or left to right unless it comes as sprial.
And said spiral is only too often explained as the wave literally spinning around the transmission axis.
In reality is more closer to you throwing a stone into a calm lake - it hits the water and you get circular waves expanding outwards.
What we interpret for our antennas to visualise it is like taking a vertical slice out of the lake surface.
And for a horizontal orientation the lake surface would be vertical ;)
You see the problem I hope...
A single hitting the antenna so this slice is fully utilised is best.
And since with distance the radius increases the stone throw creates we don't even need a satellite dish shape antenna rods, a simple wire will do fine.
However, anything that hits the antenna wire from other than opimum angles will cause problems.
For example if you would mount your standard whip antanne horizontal instead of the prefered vertical orientation you loose about 3db in gain.
You can not though turn a vertical antenna into a different horizontal angle unless it is a directional antenna, like the Yagi you have on your house that points to the TV station.
In theory a 3d infinity loop antenna would cancel all incoming signals out as there is always an opposing current in the opposing loop on the other side.
Wound with two loops each in a bifilar fashion however would mean those effects are cancelled out twice !!??
Nothing times nothing still makes nothing!
A coil, especially a bifilar one however also creates a magnetic field.
Would that field then be received the the other loops or increase/decrease the magnetic fields in the other loops?
Considering the current flow with the corresponding magnetic fields we might as well end up with a rotating magnetic field that spins at the frequency of the received signal...
The antenna would be self resonant with basically no electrical losses and act like an amplifier for the signal.
From a tranceiver side of things such an antenna would provide the correct electrical match of 50Ohms while appearing to have a negative resistance for the signal.
Sadly physics will render most of this useless one way or another I guess.
Does not mean I would try it anyway ;)