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Author Topic: How does this full-wavelength spiral loop antenna work?  (Read 2846 times)
JAHAM2BE
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« on: May 12, 2014, 02:09:34 AM »

I was wondering how the following compact antenna, that contains more than a full wavelength of wire wound into a compact planar spiral, can be resonant at 7 MHz.

Article about the antenna: http://wb9rim.home.insightbb.com/Petlowany/Spiral%20loop.html

Image of the antenna: http://wb9rim.home.insightbb.com/Petlowany/100_1807.jpg

A couple of questions:

1. Some 4nec2 experiments with planar spirals of area about 1 square meter indicated self-resonance with only 3 or 4 turns. So what's happening when we pack a whole wavelength of wire into that space? Does this mean it's operating at some sort of second or third resonance?

2. If a full wavelength of wire is used, doesn't that mean there will necessarily be current phase reversals along the length of the wire, causing weird radiation cancelling effects?

3. Are there any recommended modeling programs to investigate such structures? I think Sonnet Lite is supposed to be pretty good at simulating planar spirals with high accuracy, but I have no experience with it. On the other hand, I think NEC-2-based programs don't work well with this kind of geometry with tight, concentric spirals, close and parallel thin wires, etc.
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W5DXP
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« Reply #1 on: May 12, 2014, 05:53:57 AM »

I was wondering how the following compact antenna, that contains more than a full wavelength of wire wound into a compact planar spiral, can be resonant at 7 MHz.

A well-known rule-of-thumb tells us that to wind a helical monopole that is electrically 1/4WL long takes about 1/2WL of wire. The reason is that the fields from each loop of wire couple into the adjacent loops of wire and induce currents in the adjacent turns of the coil, i.e. some of the photons take a short cut from turn to turn. Ramo and Whinnery, of Fields and Waves ... fame call it the "helical sheet" effect. Dr Corum, of RF Coils ... fame calls it the "sheath helix" model. My personal rule-of-thumb is that it takes about half the time for a signal to travel through a helix as it would if the signal was traveling along the wire of the helix. A helix is also known as a "slow wave structure". Dr. Corum's article is available at:

http://hamwaves.com/antennas/inductance/corum.pdf
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
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« Reply #2 on: May 12, 2014, 04:14:13 PM »

This essentially a self-resonant inductor.  Resonance is established based on the inductance of
the coil and the self-capacitance between the turns:  these have more effect than the actual
length of wire used.  Well, in some cases...

It actually isn't easy to create a good model of such a loop using any of the NEC2 modeling
programs due to the close spacing of the wires.  I got 3 turns at 2m on a side by shortening
the turns in increments of 10cm and starting with 20 segments on the first 3 wires.  That
allowed me to reduce the number of segments for each successive wire, keeping the
segments aligned in each of the wires.  (I gave up after 3 turns as it was a lot of work.)

This gave me 12 sides varying between 1.5 and 2m each, so just a bit over 20m of wire.
Sure enough, there is a resonant point at 14.3 MHz, where the impedance is .003 + j0 ohms.
But that's not likely to be useful as an antenna with an R value that low with some other
means of matching.

I also found a high impedance point around 15m and one with a more useful impedance
(well, up to 5 ohms anyway) somewhat below 28 MHz.  There was another bump where
R increased to 100 ohms around 28.3 MHz, but the reactance remained positive in that
range, so it wasn't resonant.  However, it might be operating in a mode that could be
utilized with some adjustments to the loop length, or with the addition of series
capacitance.

With just 3 turns and comparatively wide spacing, this won't show the same effects as
a complete spiral because there isn't a lot of difference in the sizes of the turns.   In any
case, simply squeezing a full wavelength of wire into a small space is no guarantee of
making an efficient antenna.  My 2m wide model antenna on 20m showed -19dB of
copper losses in #14 wire (but only -3dB on 27.5 MHz with an SWR better than 10 : 1,
so using it on a harmonic isn't necessarily a bad idea.)
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JAHAM2BE
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« Reply #3 on: May 12, 2014, 08:43:39 PM »

My personal rule-of-thumb is that it takes about half the time for a signal to travel through a helix as it would if the signal was traveling along the wire of the helix.

I'm not sure that I'm parsing that statement correctly. Reordering your sentence with the intent of preserving the meaning, I get: "if the signal was traveling along the wire of the helix [then] it takes about half the time for a signal to travel through a helix".

Does this mean that, for instance, if we have a full wavelength of wire stretched out in a straight line, and a signal takes t seconds to travel from one end to the other of that wire, then if we wind that wire into a helix, the signal travels faster through the helix, taking only about t/2 seconds?

If so, then it's not clear to me why
A helix is also known as a "slow wave structure"
because it seems the signal is traveling faster, not slower, through the helix...?

I'm not really expecting this antenna to be a super performer, but I'm just trying to understand it. It was rather odd to me that with so many turns and a full wavelength of wire that the small spiral would be resonant at 7 MHz (as I said, my simulated antenna of roughly similar dimensions was already self-resonant at 7 MHz with far fewer turns and - presumably - far lower inductance).
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N4JTE
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« Reply #4 on: May 12, 2014, 10:44:18 PM »

Quick answer; build it and see who shows up on your radio!!
Bob
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WX7G
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« Reply #5 on: May 13, 2014, 03:28:40 AM »

How does this loop work? The "antenna" is the coaxial cable shield; it is doing most of the radiating while the spiral structure is a "common-mode excitation device".

To confirm this using EZNEC I built a helix having a small pitch. The resulting helix has a diameter of 3' and a height of 1'. Driven by itself the radiation is vertically polarized. Add a 33' wire off the feedpoint (this represents the coaxial cable shield) and we now have horizontally polarized radiation that is 10 dB above the vertically polarized radiation. Add a common-mode choke over the coax at the feedpoint and the antenna performance will drop 10 dB.
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JAHAM2BE
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« Reply #6 on: May 13, 2014, 03:53:56 AM »

How does this loop work? The "antenna" is the coaxial cable shield; it is doing most of the radiating while the spiral structure is a "common-mode excitation device".
Ah, good point, I hadn't thought of that. So it's sort of like half of a TAK-tenna? Smiley

On the topic of common-mode excitation devices, I've been considering building such an antenna - intentionally designed to excite the coax shield - if such a common-mode excitation device could be built small and efficiently. Assuming an apartment balcony situation where grounding is difficult and any antenna must be on or stuck out of the balcony, would it be better just to make a base-loaded vertical (with the radiator angled out of the balcony as far as possible), or is there some benefit to the alternative approach of sticking a common-mode excitation device at the far end of the radiator, thus elevating the feed and allowing the coax to radiate? In other words, is there any way that a common-mode excitation device (like this spiral loop antenna, the TAK-tenna, the Isotron, and EH antennas) can perform better than an equivalently-long (and equivalent-diameter) wire configured as a vertical and base-loaded?
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W5DXP
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« Reply #7 on: May 13, 2014, 05:27:40 AM »

If so, then it's not clear to me why
A helix is also known as a "slow wave structure"
because it seems the signal is traveling faster, not slower, through the helix...?

Sorry about the confusion. The "slow wave structure" refers to the length of the structure, i.e. the length of the coil, not the length of the wire making up the coil. For instance, my 75m Texas Bugcatcher loading coil is a "slow wave structure" with a measured VF=0.0172 at 3.83 MHz.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WX7G
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« Reply #8 on: May 13, 2014, 06:35:27 AM »

How does this loop work? The "antenna" is the coaxial cable shield; it is doing most of the radiating while the spiral structure is a "common-mode excitation device".
Ah, good point, I hadn't thought of that. So it's sort of like half of a TAK-tenna? Smiley

On the topic of common-mode excitation devices, I've been considering building such an antenna - intentionally designed to excite the coax shield - if such a common-mode excitation device could be built small and efficiently. Assuming an apartment balcony situation where grounding is difficult and any antenna must be on or stuck out of the balcony, would it be better just to make a base-loaded vertical (with the radiator angled out of the balcony as far as possible), or is there some benefit to the alternative approach of sticking a common-mode excitation device at the far end of the radiator, thus elevating the feed and allowing the coax to radiate? In other words, is there any way that a common-mode excitation device (like this spiral loop antenna, the TAK-tenna, the Isotron, and EH antennas) can perform better than an equivalently-long (and equivalent-diameter) wire configured as a vertical and base-loaded?

For a short (<90 deg) top loaded vertical where it's fed does not matter. But the fact that it is top loaded makes the current more uniform thereby increasing the current-area and increasing the radiation resistance. So, a top loaded short wire such as an Isotron can work quite well but with most users not understanding how it all works it can be hit-or-miss. What is almost always missing is a good RF ground system.
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WX7G
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« Reply #9 on: May 13, 2014, 11:46:51 AM »

And there are installations where top feeding a vertical makes sense. For example, from a 2nd floor apartment a top loading structure (like a mobile whip) can be mounted on a balcony. A "ground" wire - which is really the dominant radiator - is routed to a ground rod or it can stop just short of ground. The mobile whip is then fed through a current balun. With the wire grounded the antenna is a top loaded vertical with elevated feed. With the wire not grounded the antenna is an asymmetrical vertical dipole.

In this example the common-mode current excitation device can be a screwdriver antenna.
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