How Does a Double-Bazooka Antenna Propagate?

SSTJ

SSTJ

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Not TJ related, so maybe this is the wrong sub-forum. But some of you on here know your Ham radio antennas better than I do.

I understand, and can visualize, how a dipole antenna creates a changing electric field and also a changing magnetic field. Combined, we get electromagnetic radiation.

I can also see how a double-bazooka antenna produces a changing magnetic field, by the transceiver causing a change of current. But I cannot see how it creates a changing electric field.

Anyone have any insight?
 
ummmm, thats above my pay grade - you throw power at it and radio waves come out, radio waves come in and get picked up by the radio. If you are really curious I can ask my Dad, he's more of an antenna gu-ru than I am.
 
Disclaimer* I have a masters in RF Engineering and started my career designing antenna arrays for various interesting applications but I'm horribly rusty with HF theory so take what I say with a grain of salt. Dunning-Kruger applies here!

The fundamental concept you might have slightly wrong is you talk about a changing magnetic field and an electric field. They are two sides of the same coin, in a practical sense (outside specialised applications and quantum physical theory which is *way* beyond me) if you have a changing electrical field you must also have a changing magnetic field. This is fundamental to Maxwell's equations and underpins the whole antenna theory - not to mention most of our understanding of light and magnetism, flawed that it is. https://en.wikipedia.org/wiki/Maxwell's_equations

The second thing that might be being misunderstood here (and humble apologies if you already get this) is a fairly common misconception people have that RF behaves like DC . A dead short at DC (like that centre conductor of the bazooka) isn't necessarily a dead short for HF. Its all about where the current is flowing (in this case of course the current is flowing on the outer braid not the centre conductor) and what impedance the antenna is presenting to the transmitter and the frequency of operation. HF antenna design is all about the impedance design not the "radiator" design if you want to horribly simplify it.

The most efficient antenna design possible is one that allows the maximum transfer of electrical energy from the transmitter into an electromagnetic wave going in the desired direction (in the case of HF that's usually but not always a toroidal / donut shape). That maximum energy transfer almost always occurs when the antenna is fully resonant which means the impedance of the antenna including the feeder cable matches the optimum output impedance of the transmitter. The Bazooka antenna, like other designs such as the G5RV antenna use the characteristic impedance of the feeder as part of the design. That's an entire subject for another discussion but its why some designs use segments of 300 Ohm balanced feeders, some use 50 Ohm coax and there are a multitude of other feeder configurations also possible.

It also explains why a BALUN isn't a magical device and isn't always needed in a design - in fact designing HF antennas without BALUN's (Balanced - Unbalanced or ladder line to coax to put it simply) is generally the preferred approach for maximum efficient and power transfer and that's whay you wind up with odd looking designs like the Bazooka that empirically "look weird" when you first see them.

To borrow a phrase from the old days, there is science, there is black magic and then there is antenna design!
 
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Bammo68 said:
The fundamental concept you might have slightly wrong is you talk about a changing magnetic field and an electric field. They are two sides of the same coin,
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Yep, I'm good with that, in theory. But still when I imagine how a dipole antenna works (like this), I can still see/imagine the two sides of the same coin in action. I get how the changing current creates a changing magnetic field, and I get how the current also leads to electrons "piling up" on one side at a time, which creates a changing electric potential, and therefore a changing electric field. With the Bazooka I cannot see the electrical field being generated. But maybe what I'm forgetting is how the changing magnetic field creates one even when there is no "electric potential" being created by electrons piling up on either end of the antenna dipole.

Bammo68 said:
The second thing that might be being misunderstood here (and humble apologies if you already get this) is a fairly common misconception people have that RF behaves like DC .
Click to expand...

I think I'm good with that, but still helpful that you reminded me.

Bammo68 said:
The most efficient antenna design possible is one that allows the maximum transfer of electrical energy from the transmitter into an electromagnetic wave going in the desired direction
Click to expand...

Cool, but I just want to be able to understand how the Bazooka creates an EM field, whereas I can only imagine it creating a fluctuating magnetic field. But again, maybe what I'm forgetting is how a changing magnetic fields creates a complementary changing electric field, even if there is no good-old-fashioned "electric potential" in the way that the electrons pile up at opposite ends of a dipole.
 
SSTJ said:
But maybe what I'm forgetting is how the changing magnetic field creates one even when there is no "electric potential" being created by electrons piling up on either end of the antenna dipole.
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Yea, I'm looking at Maxwell's equations and this is what I was missing/forgetting.
 
KingCarGuyZ said:
I asked, he has no clue either.
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All good! I figured it out, I think. The changing magnetic field (created by the back-and-forth changing current) induces an "electro-motive force", which is effectively the same as the electric 'potential' that's so easy to see and illustrate in a dipole antenna.

I'm re-reading my college physics textbook chapter on Maxwell's equations. Haven't looked at that book in decades.
 
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Just chiming in, Public Safety RF Engineer here. I'll describe how I kind of understand EM communication, put somewhat simply.

I can imagine a stationary electron, this electron is negatively charged and does produce some kind of electric field around it. Stationary, the electric field is static and constant. Now what could happen if you move the electron around, specifically in a sinusoidal fashion. The electric field that extends into space, over time from this electron will also be sinusoidal (and have the magnetic field perpendicular from the electric field). This EM "wave" theoretically extends into infinity, but quantifiably obviously it drops off very quickly, according to the inverse-square law.

What if I connect a bunch of electrons together, in a long line, and produce the same EM wave? The EM wave emitted from a longer line of electrons will have a lot more energy to penetrate space further. And when the line of electrons (the antenna), is a very specific length tuned to the frequency you wish to transmit on, the length of electrons resonant at that frequency which drastically increases the energy of the EM wave emitted into space.

If someone else so happens to have a wire of similar length, pointed in the right direction, the energy from the originally transmitted EM wave will also be received in an antenna where the electrons absorb the transferred EM energy, and translate this as an electrical signal into your radio. And the small amount of power that radios now adays can receive voice and data at is insane. Some radios I work with can receive and decode an intelligible signal at -125dBm, or .0000000000000003 watts of power received, and still receive the message.


I know this may have discussed here already, but RF truly is awe inspiring, and confusing. And I am no master of it and I am constantly learning more about it everyday. Not sure if my simple description made sense, but maybe it will help someone understand better.

My research specifically was in 3-D printed antennas, and using abstract shapes for an antenna to maximize its performance for a limited ground plane; like an antenna mounted on a vehicle.
 
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vito2772 said:
My research specifically was in 3-D printed antennas, and using abstract shapes for an antenna to maximize its performance for a limited ground plane; like an antenna mounted on a vehicle.
Click to expand...

Thanks for sharing. I don't mind hijacking my own thread to say that I'd be interested in learning more about this part, in particular. Got any tips to share about shapes of antennas mounted to TJs?
 
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