How "directional" can a radio transmitter be?

Assuming sufficient LOS and power is there a way to shield an antenna so transmissions can only be received say within a few degrees of the compass direction where the transmitting antenna is pointed.

Example to clarify: I have a portable CB with this antenna, I want to be able to point this antenna at a point 300m away and that another CB say 10-15m away from that point would not pick up that signal.

Could something like this be achieved by simply embedding a transmitter in a steel pipe and pointing it to the spot in question? Not so much worried about optimizing range so much as making the radio broadcast as narrowly directional as possible.

I cant really divulge the specific application because its a product idea and I do not want to post in a public forum.


Here’s how to calculate it:
It’s about radar but the principles are the same.

Depending on how narrow you want it to be, you might need to use a rather large antenna and a rather high frequency.

However, if the point is to prevent others from listening in, it would likely be much easier to use cryptography or spread spectrum techniques.

Aiming for something easily carried by hand

More a desire to not interfere with other nearby radio sources, especially those near the transmitter.

Like a Pringles can?

Would a parabolic dish act as a directional transmitter?

Not possible.

What frequency is your radio at? A quick Google says CB radios are 30 MHz, which means a wavelength of 100 m. To get any kind of directivity, you’ll need an antenna larger than that.

If you want a handheld antenna, say 1 m size, then your wavelength must be smaller than that. Or a frequency greater than 300 MHz. That’s simply to get in the ballpark of plausible.

That brings up an interesting question. How do CB radios manage to tune into a 100m long wave with an antenna that’s perhaps 3m long?

The critical point (and one described on the page MichealEMouse links to, is that you only get tighter beam width with increased size of the antenna, and just how tight the beam is is a function of the ratio of wavelength to size. A 27MHz CB is going to be impossible to manage - the wavelength is about 11 metres, and even with a UHF CB, you have big problems.

300m to 10 metres, is 30:1, about 2 degrees. You will very roughly need an antenna 30 times the wavelength - which is about 65cm. 20 metres. You are not going to be handholding this.

The antenna need not be a paraboloid, a Yagi gets it directivity with length, but there is no such thing as a free lunch. A 17 element Yagi 3.2 times the wavelength long (So over 2 meters) only gets you a beam that is 20dB down at +/- 30 degrees. A helical (aka Pringles Can) will do no better.

There is nothing that says the antenna has to be close to a full wavelength long. It intersects the EM field, and will develop changing voltage and current gradients along its length, no matter what length it has. How long it is will effect how much of the field it intersects, and also what the feed impedance is. The latter needs the receiver input stage to match to effect best coupling, but even a stub of wire will intersect some of the field.

The impedance matching is often effected with an inductor at the base of the whip (base loaded) but other options exist - for instance an inductor part way up the whip.

The reverse is true for transmitting. A shorter antenna still works, but the impedance issues must be managed, and there will be scant directionality.

“Laser transmission” is what came to my mind when I read “directional”.

They already have MASERs
Can they make a coherent, directional beam of even lower EM signal?

ok would it matter if all you wanted to do is jam/disrupt radio in the path of said directional broadcast…a “static gun” for lack of a better term.

frequency range around 70Mhz

So, you want a jammer that will jam A while not being picked up by B which is close to A?

Even with a directional antenna, you’d emit sidelobes which could be picked up by B. Unless you turned down the emitted power but then your jamming wouldn’t be much good.
Maybe a scanned array radio could be that selectively directional but you’re not going to be MacGyvering that.

Get a pair of Cobra 48s. They’re 40 channel CBs, BUT you can operate them in single sideband or AME (I think that’s what it is). If you and the person you want secure communication with are both on the same channel on upper or lower sideband, I don’t think anyone with a standard AM CB can hear it.

Well since I obviously lack the technical mojo to play in this field, I’m a computer guy not so much an electronics/radio guy, might as well just throw it out there.

Recent discussions of drone spying/tresspassing and a couple stories I read about drones dropping packages into prison yards made me wonder about the practicality of a jammer gun of sorts that might disrupt/disable/drop a quadcopter without the stray bullet problem of shooting one down. I am aware that private use of such a device is a HUGE no no from the FCC’s perspective. I was thinking it might be interesting to try and craft out some kind of “RF shotgun” of sorts that might be very sale able to correctional facilities for such situations. Sounds like an interesting engineering challenge, but from the sounds of it not practical in any meaningful way.

[terminator]I would like a 70Mhz Maser rifle in the 40watt range. :D[/terminator]

Wavelength at 30 MHz is about 11 meters, not 100. You would have to be down around 3 MHz for 100 meters.

And any antenna will pick up some sort of a signal at any RF wavelength. The best signal comes when the antenna is a fundamental multiple (1, 2, 4 times) or fraction of the wavelength (1/4, 1/2, 5/8 even), and is impedance matched to the receiver.

With some loading coils, a Yagi antenna can be pretty compact for 10 (or 11) meters. There are common 4 or 5 element Yagi’s commercially available off the shelf for up to the 20 and 30 meter band that fit in a back yard. A typical 4 element Yagi has a directional gain of about 8dB over a simple dipole, with a pretty decent front to back ratio.