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Discussion Starter #1 (Edited)
Hello everyone......Am trying to figure out how these antennas actually work:

1) What exactly do NARODS do? (if they are not a reflector or director?)
2) Why do the GH type and Centipede type antennas have a zigzag shape? how does that help them pick up the EM radiation better than a straightwire?
3) In terms of reflectors, why isnt the signal just absorbed by the reflector, instead of it bouncing back?
4) Why doesnt most of the signal go through the gaps in reflectors made of chickenwire/fencing wire/rods etc, and keep on going?

Thanks!
Schoolbus
 

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1) What exactly do NARODS do? (if they are not a reflector or director?)
They lower the SWR of the GH in the vhf-hi range so that the antenna is usable in the vhf-hi range, by means of the close proximity of the NARODs to the GH stubs.

2) Why do the GH type and Centipede type antennas have a zigzag shape? how does that help them pick up the EM radiation better than a straightwire?
3) In terms of reflectors, why isnt the signal just absorbed by the reflector, instead of it bouncing back?
Some of the signal is absorbed by the reflector and it resonates too. (just not as well as the driven element resonates) Some of the signal bounces back and some of the signal passes above and below the reflector, missing it. Some of the signal hits the reflector on a skew and is reflected backwards on an angle or in a curving path like putting english on a cue ball. The same can be said of directors.
The signal isnt just one straight beam like a laser, but rather a fairly wide swarth, depending on the frequency.

4) Why doesnt most of the signal go through the gaps in reflectors made of chickenwire/fencing wire/rods etc, and keep on going?
Depending on the gaps of the reflectors, a lot of it does. Once the gaps get below about 1/10 th of a wavelength, the reflector is catching pretty much all of the available signal.
 

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Trial and error in the late 19th century resulted in the Chireix-Mesny design of the WWI era featuring zigzag elements. If you are looking for the theory behind that you would need to have a background in the study of physics and/or electromagnetism. Are you in that group? :)
 

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1) What exactly do NARODS do? (if they are not a reflector or director?)
They induce current at their resonance in the nearby driven conductor, which lowers the overall radiation resistance at that resonance. Therefore, they are usually used to broaden the VSWR pass band at a frequency different (almost always higher) from the driven dipole. The magnitude of this effect depends on the separation between the elements. This separation is small enough that it pushes the NEC-2 simulator near its limits. NEC-4 is better for this, but it still is subject to careful modelling and skepticism. A far less common purpose for these types of elements is to shape the radiation pattern.

These elements are most commonly called "open sleeves" in the antenna research literature. Unfortunately, most technical discussions are in copyrighted literature, most of which will be accessible if you have a subscription to ieeexplore or have access to large technical university library. In recent editions of the ARRL Antenna Book, there is a chapter by Gary Breed (K9AY), who prefers the term "closely coupled resonators". A few editions further back there was a chapter by Roger Cox (WB0DGF) on open sleeve antennas. Here is a link to one online research paper that has some introductory history and references, but may not be as explanatory as you would like:
http://www.jpier.org/PIER/pier58/07.0509031.Li.G.pdf
(Undoubtedly there are better online refs for explanations, but I didn't want to spend too much time searching for one just now.)

Patents are readily available online, but most are more than a bit harder to decipher. I can send you a bunch of patent numbers and/or literature references if you want them.

L.B. Cebik (W4RNL) (R.I.P.) once suggested a master/slave driver terminology for the case where the two resonant frequencies were relatively close (in the same band) and primary/secondary drivers when they were tuned to separated bands (i.e. broad-band vs. dual-band). (Apologies if I have his two terminoligies reversed.) Over the years, many authors have used their own terms, which makes text searches more challenging.
2) Why do the GH type and Centipede type antennas have a zigzag shape? how does that help them pick up the EM radiation better than a straightwire?
The basic resonant dipole is about a half wavelength long. Longer dipoles resonate on every multiple of a half wavelength, but only the odd resonances have a (desirable) low impedence. These longer dipoles have more gain (more half wavelength radiating sections), but the current phase reversals along the dipole create wave cancellations in the radiated patterns. In the early 1900s there were various attempts to avoid these cancellations (e.g. the Franklin antenna). In the mid-1920s Henri Chireix came up with the zig-zag where the current reversals would be at right angles to minimize the pattern cancellations without resorting to a complicated feed system for all those half wavelength radiators.

The UHF band just so happens to be about 3 times the frequency of the VHF-hi band. But, the zig-zags hurt (raise) the impedence/VSWR at the fundamental (half wavelength) VHF-hi resonance. The NARODs bring it back down so as to minimize the signal lost to the impedence mismatch with the transmission line (cable).
3) In terms of reflectors, why isnt the signal just absorbed by the reflector, instead of it bouncing back?
Both happen. The currents induced in the reflectors reradiate (accelerated electrons always radiate photons), and the induced currents will be higher at resonant frequencies, which are tuned by their lengths (much like a trombone).

HTH
 

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The UHF band just so happens to be about 3 times the frequency of the VHF-hi band.
Yep, and the vhf-hi band is about 3 times the frequency of the the vhf-low band, which a lot of antenna designs of the 1960's took advantage of. :)
 

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Although the three TV bands are harmonically related, there is a mismatch in their bandwidths:

Code:
..freq.(1x)...50........60........70........80........90.......100...
..freq.(3x)..150.......180.......210.......240.......270.......300...
..freq.(9x)..450.......540.......630.......720.......810.......900...
...............|.........|.........|.........|.........|.........|...
VHF-lo.(1x)....|...#..2..|..3.....4|...*....5|....6..#=|===FM====|========
VHF-hi.(3x)....|.......#7|8.9.0.1.2|3#.......|.........|.........|...
UHF....(9x)....|.#.......|.......*.|.......#.|[email protected]|........$|...
...............|.!.....!.|....!....|.!.....!.|........!|........!|...
..freq.(9x).....473...522....585....648...695........805.......887...
...(UHF ch).....(14)..(22)...(33)...(43)..(51).......(69)......(83)..
(Apologies for the lousy text graphics; @ and $ mark the previous upper limits of the UHF TV band.)
The bandwidths are 48%, 22%, and 39%.

Unfortunately, Channels 5 and 6, which are the most common VHF-lo channels still in use, are outliers. So is the Channel 16 in my local market.
 

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has some introductory history and references
That first reference they cite, Very High Frequency Techniques, McGraw-Hill Book Company 1947 rings a bell. (the other references dont) It sounds like a book I probably/would have read when I was UHF DXing in the late 60's and early 70's. (I couldnt have resisted a title like that back then, no way) I guess pages 119 - 137 must have stuck into the back of my mind, heh.
Of course, the interaction of one wire on another wire goes back over 150 years to Maxwell (or his professor ??).

Every now and then I remember the combo to my 7th grade locker for some strange reason, so Im also filled with totally useless info, heh.
 

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That first reference they cite, Very High Frequency Techniques, McGraw-Hill Book Company 1947 rings a bell.
Apparently that was the earliest "full" discussion published after the war. It seems that there was a lot of reluctance to publish even unclassified work in the field during the war. If you look at IRE journals from the war years, you will see the (seemingly desparate) editors encouraging more paper submissions. I suspect that time pressures to complete war related work also hurt the rate of paper submissions, beyond the understanable concerns over their sensitive security nature.

The UF library has a copy in storage, but rather than go to the trouble of having in retrieved, I bought a used copy online (about a year and a half ago).

There are some patents prior to the war that use "closed" sleeves. The patent (2239724, filed in May 1938) for the Lindenblad antenna that was mounted atop the ESB before the war (and supposedly the only antenna ever to appear on the cover of a major non-technical magazine, Forbes during 1945) included: "In order to maintain a constant ratio of resistance to low reactance in one modification this antenna is arranged so that the shell of the concentric transmission line connected thereto comprises a portion of the radiator... The antenna, as a whole, is preferably fed at the transition point between the sleeve and the central extension portion of each quarter wave radiating section."

Of course, the interaction of one wire on another wire goes back over 150 years to Maxwell (or his professor ??).
There would have been earlier work, but IIRC Faraday gets the major credit for that one. Maxwell met him, but my understanding is that they never worked together, certainly not in the sense of being professor and (grad) student. Faraday was much (40 years) older. Maxwell's great advance was "unifying" electrical and magnetic forces mathematcially, though our present vector formulation was redone by Heaviside. Maxwell also contributed to the second great triumph of 19th century physics, connecting the macroscopic "laws" of thermodynamics with the microscopic kinetic theory and statistical mechanics (especially the Maxwell-Boltzmann distribution). But, I digress...

Every now and then I remember the combo to my 7th grade locker for some strange reason, so Im also filled with totally useless info, heh.
Your memory must be better than mine!

Cheers
 

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For a description on how passive elements are used to redirect the energy of the active element,
see fol. re Yagi-Uda:
http://en.wikipedia.org/wiki/Yagi-Uda_antenna

Unfortunately, Uda's and Yagi's articles are found in copyrighted IRE articles....
But I'll give you the bottom line....cut and try....cut and try....cut and try.....
 

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Visualization

Can the interaction between the electromagnetic waves and the wires be visualized? Perhaps in a wave pool like fluid dynamics?
 

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Can the interaction between the electromagnetic waves and the wires be visualized? Perhaps in a wave pool like fluid dynamics?
ISTR that one of the open source Unix/X-Windows NEC visualizaton tools animated currents on the antenna elements in some manner or another. I'm guessing that it is less likely that any of the NEC based tools would do what you ask, compared with FDTD simulators. But, I have never even looked at any of the commercial tools (e.g. Ansoft, Remcom, Zeland).

Sorry for not being helpful, but I didn't want you to feel ignored. If you learn anything significant, you might wish to report in the modeling thread. I am slightly interested in modeling parallel plate lenses and prisms, which likely requires an FDTD rather than an MoM simulator. So, I'll be interested if you learn anything in that regard.

Best Regards
 

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Can the interaction between the electromagnetic waves and the wires be visualized? Perhaps in a wave pool like fluid dynamics?
Yes, you can visualize an electromagnetic wave like a wave in water. But when theres a wave in a narrow channel, much less than its wavelength, theres a whole lot of splishing and a splashing goin on, heh.
 

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NO, although I've seen E-M animations and (as a Physics student) played in the wave pool, I have never seen
an animation of how the individual segments on each of the wires interacts with the individual segments
in every other wire to formulate the resultant antenna gain pattern.....NO, it is NOT a simple wave reflecting
back and forth--they are MUTUAL IMPEDANCE effects--something that NEC does by calculating MILLIONS of
individual equations for the upwards of THOUSANDS of individual wire segments....

And thanks to modern technology, NEC now runs on everyday computers & laptops....
 

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.....NO, it is NOT a simple wave reflecting back and forth-
Yes, correct. I was just visualizing the general wave in free air space from the transmitter. Because of the mass of the water and the forward inertia it generates, a water wave hitting a round dipole wouldnt look anything like a radio wave, with almost no mass, hitting a round dipole.
 

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Flcs3,
I am barely able to follow due to my limited knowledge, but, if I understand you correctly, the electrical current in the NAROD induces a current in the driven element, thereby "helping" the current along in that element? But wouldn't the current induced by the Narod have to be exactly in phase with the current being induced by the EM wave used directly by the driven element? I would guess they would be slightly out of phase, no?
Al
 

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I am trying to get a student at my school to build an antenna, can you recommend an antenna that is prefereably directional, that can pick up Toronto signals, if located in Bowmanville? (the simplest one please!)
Thanks,
Post his TVFool report.

But wouldn't the current induced by the Narod have to be exactly in phase with the current being induced by the EM wave used directly by the driven element?
It is, thats why the careful narrow spacing of the NAROD to stub distance.
 

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that can pick up Toronto signals, if located in Bowmanville?
So, Toronto is 242 degrees true ? If he wants channel 9 too, then he should build a GH10n3.
 

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But wouldn't the current induced by the Narod have to be exactly in phase with the current being induced by the EM wave used directly by the driven element? I would guess they would be slightly out of phase, no?
In common EE practice, RF stubs usually are limited to 10 to 20 degrees of phase "length" at the operating frequency. It may not be quite perfect "tuning" but it is close enough, depending on the application, of course. In the case of open sleeves (and NARODs), their spacing to the driven elements are just a few per cent of a wavelength (3% being about 11° of phase), so the phase difference is small. cos(10°) = 0.985 and cos(20°) = 0.940.

In contrast, I've been experimenting (numerically in NEC) with FM cancellation elements, which are half-wavelength resonators spaced a half-wavelength (180° of phase) from the driven element. cos(180°) = -1.0.
 
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