Why Do Some Antenna Need the "Extras" and Some Don't?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K2SOF, Apr 8, 2017.

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  1. K2SOF

    K2SOF Ham Member QRZ Page

    Hi everyone! So my question is this...I've noticed some antenna have need of radials..or counterpoises...also others have need of ground wires...So How does one know which antenna need to be grounded? or which ones need radials? Like , you could tell me that magnetic loops don't need radials..but how do you know this? and why? What is it about certain antenna design that require ground wires, or radials, and others don't?Just for an example, a magnetic loop antenna, I haven't read anywhere or seen videos where it shows it needs to be grounded or have radials attached. I'm assuming mag loops don't need either? But why then do half-dipoles need them?
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    That's really your answer right there. Ground radials or other RF return/grounding/counterpoise is the other half of the antenna. Some antennas are inherently balanced like dipoles, doublets, Yagis, Hexbeams, loops, etc. Others build half the antenna and need an RF return as the other half of the antenna. This is most obvious with antennas like a quarter or 5/8 wave vertical that's obviously missing half the antenna but any antenna needs an RF return path or a source and sink for charge carriers to balance any currents you drive into the radiating element.

    So if the antenna is built with both halves then there's no need for radials or counterpoise. But for half antennas as you put it, you need the other half.

    There can be good reasons to add ground plane (not usually connected electrically to the feed line) under balanced antennas like loops. It can help reinforce ground reflections and sometimes reduce ground losses. So you may see recommendations to put something like a mag loop antenna above some screening, same thing for some NVIS antenna designs. Any antenna benefits from ground reflection gains (which is one reason operating near salt water is so useful) and some designs really benefit from enhancing these ground reflections but that's different than simply balancing currents in the antenna by providing the missing half. BTW, that second use of ground screen or radials to enhance ground reflections and reduce ground losses is the original use of the term 'counterpoise' it wasn't really describing the missing half of the antenna but it's often used that way by hams these days.

    -Dave

    P.S. There are some inherently unbalanced antennas that are marketed as not needing that other half. Some actually are balanced in ways not obvious, for instance what looks like a conventional quarter wave vertical without radials but is actually a vertical dipole in clever packaging.

    Others tell you you don't need radials or counterpoise (in the RF return or current balancing sense) but will rely on the coax shield to carry common mode currents that effectively work as the missing half of the antenna. In some cases that can be fine and isn't noticed by the operator but in others it can create troubles like RFI in the shack or sensitivity to local electrical noise sources. But in the end all antennas need to balance any currents flowing into their radiating elements, if something is not provided to supply those balancing currents (e.g. radials or an inherently balanced antenna design) then the antenna will find something to fill that role and that something is often the outer shield of the feed line coax via common mode current.
     
    Last edited: Apr 8, 2017
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  3. K8ERV

    K8ERV Ham Member QRZ Page

    I am inherently unbalanced. Maybe I need some radials-------

    TOM K8ERV Montrose Colo
     
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  4. AA7QQ

    AA7QQ Ham Member QRZ Page

    There have been a few tests that show a mag loop (STL) can benefit from radials 2-4X the diameter.

    Ed
     
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  5. K2SOF

    K2SOF Ham Member QRZ Page

    Thanks for the great answer k7trf! So there is no way around the math and physics of it... Just more clever ways to find another subtable missing "piece" of the antenna. So cool...and very clever idea with using the coax threaded metal jacket as part of the antenna...

    Do magnetic based antenna, in general, need less grounding than an electrically based antenna? I'm actually considering getting a mag loop for myself and grounding it properly probably is t feasible... I'm in an attic environment
     
  6. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    All antennas must be dipoles, in the physics sense, despite their common name.

    Meaning that the antenna must have two poles in some way: Current which flows from the transmitter out to the antenna in one direction must return to the transmitter in the opposite direction. It can't just disappear into the universe (due to the law of charge conservation).

    It's easy to see this with a wire dipole or a loop. But with some antenna designs it isn't so clear, like an end-fed antenna. But physics will not be fooled and there are most assuredly two poles. The apparently missing one might turn out to be the coax shield, or the wiring in the house, or you with a handheld, but it's there.

    And if the second pole is not deliberately designed, there can be problems such as poor performance, RF voltage in the shack, worse RFI in the house, etc.
     
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  7. NH7RO

    NH7RO Ham Member QRZ Page

    Actually, using the coax shield as the other part of the antenna isn't so cool or clever, Steve; it just happens in situations where there is no other return path. A J-pole antenna is one example, an end-fed half wave is another. Best to avoid these situations since as others mentioned it can create RFI/noise havoc in the shack.

    As for STLs (small transmitting loops which is a far more accurate term than "magnetic" loop) the grounding you ask about is more of a ground plane concept than a RF or electrical ground---it's easy to get them mixed up hence all the perpetual confusion about grounds, grounding, lightning protection, proper ground planes and so on.

    To answer your question about STLs more precisely; they do not need to be "grounded" but if they have a ground plane of radials underneath them they will benefit from that to some degree (but still not necessary for good performance).

    I live in an HOA condo complex and cannot put up antennas, basically---but I get by by setting up my loops out in the back yard out of view of most of the other residents and operate on a temporary basis OK. I worked about 50 countries in the last contest a few weeks ago using my 7-foot diameter STL for 40M and a 3.5-foot diameter loop I built for 15-20 meters---had an absolute blast and both loops were only 5-7 feet above the lawn about 20 feet from my shack window. STLs can work well in small spaces and if an attic is all you have at your disposal don't despair---they might be what you need to get on the air well enough to enjoy ham radio to its fullest despite obstacles like that.

    Here's a highly recommended 32-page article written by the loop guru himself, Leigh Turner, VK5KLT who can explain it all much better than I; he also refers to the misnomer of "magnetic" loop and why that is so, too (only magnetic properties commence in the near field---far-field radiation is in the electrical plane as with all other antennas).

    https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-mag-loop-2.pdf This will really give you a good "grounding" on these fine little antennas and how and why they can do what they do. The key is to build them properly for best performance as they are highly susceptible to losses within the system. Check out my QRZ page for pix of ones that I've built and peruse this forum for "QRO STL" and "7-foot diameter STL" if interested in learning more about building one yourself---it is a rewarding experience!

    73, Jeff Totally loopy over loops
     
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  8. AI3V

    AI3V Ham Member QRZ Page

    Let's back up a bit.

    Way back before there were radio transmitters some guy named James Clerk Maxwell came up with this crazy idea that the magnetic and electric fields of a alternating current are inseparable.

    Iow there is no such animal as a electric antenna or a magnetic antenna.

    Every single antenna ever made is a electragnetic antenna.

    Every Single Antenna ever made.
     
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  9. AI3V

    AI3V Ham Member QRZ Page

    After you read about Clerk,

    Tale a look at the antenna some dude named Hertz used to prove Clerks waves existed.

    And if you still have some time, take a look at that Italian fellow Marconi's skyhook.

    Rege
     
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  10. W6OGC

    W6OGC Ham Member QRZ Page

    Should one ground their loop?
     

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