Attic antenna grounding?

[prcguy]

Please do a Google search and study the effects of ground radials on a vertical monopole (ground plane antenna).

Anywhere current flow and radiation is discussed it will be mentioned the radials are the return current path for the vertical element but because the currents are distributed equally and flowing in opposite directions within the radials, radiation is canceled in the radials leaving only the vertical element to radiate.

Otherwise please give us your theory on how it works, which would result in a ground plane radiating in both vertical and horizontal polarity at the same time.
prcguy

Those 'equal and opposite currents' are only of opposite polarity from the currents in the 'vertical' element. The current in those radial are of the same polarity, not opposite each other. Just because a radial may be 180 degrees away from another one doesn't mean the current in those two radials aren't of the same polarity. Those currents may be flowing in different directions, but 'direction' of the flow has nothing to do with polarity in that aspect. All of those radials are connected to the same point. The current from that point is opposite the polarity of the other 'terminal' of that antenna, the vertical element. That's the same for a dipole not just a 'groundplane' type antenna with radials, no matter how many radials there happens to be, or if they are elevated or in the ground.
And like it or not, radials do radiate. Both sides of a dipole radiate.
- 'Doc

 

[LtDoc]

Try measuring that radiation. Make it easier, use a full circle (solid) groundplane/radials, put that measuring device under that 'solid' radial system. If those radials don't radiate, there should be no reading. That's not 'theory', it's fact. Dispute/rationalize it to your heart's content. Still won't change it. Nothing is 'canceled' in those radials.
- 'Doc

 

[prcguy]

My point of view is from published theory and I believe to be factual. I cannot locate any trusted source of information to collaborate your statement, so what's a girl to do but dismiss it?

What measuring device do you recommend? If I were to wast my time measuring this (which I'm not) I would use a rotatable linear polarized dipole in the far field and measure vertical vs horizontal radiation from an elevated 1/4 ground plane with an RF choke at the feedpoint to keep any RF off the feedline.

If your statement is true I would get equal or near equal pickup in both vertical and horizontal pol. But in real life that would be impossible because the radiation from ground radials on a 1/4 wave ground plane are nearly completely canceled and will not radiate (much) in the horizontal plane.

The result in free space would be horizontal pol radiation being a good 30dB or more down from vertical pol. That's a 1000:1 ratio. The result on an antenna range should get close to 30dB of isolation but I predict at least 25dB.

Now if I had an elevated 1/4 wavelength vertical element and a single horizontal element (call it one ground radial), I would have a half wave inverted V dipole on its side and the measurement I described above would pick up equal vertical and horizontal radiation when looking broadside at the antenna.I would also want an RF choke at the feedpoint to keep RF off the feeedline and skewing the measurement.

I'm sure we're each focusing on some silly nuance of the topic and not understanding each others point of view. However I would suggest you do some research on RF current flow, cancellation and lack of radiation in the radials of a 1/4 wave ground plane so we're on the same page.
prcguy

Try measuring that radiation. Make it easier, use a full circle (solid) groundplane/radials, put that measuring device under that 'solid' radial system. If those radials don't radiate, there should be no reading. That's not 'theory', it's fact. Dispute/rationalize it to your heart's content. Still won't change it. Nothing is 'canceled' in those radials.
- 'Doc

 

[AA1LL]

I made a small sketch using graphics of two ground plane antennas I found on the web which should clarify this whole thing. Let's remember we're all just hams here, respecting each other, and trying to figure something out together by each contributing what we can offer.

The red arrows in the attached drawing show the direction and location of currents on two types of ground plane antennas. On the right the radials are perpendicular to the monopole. This is the configuration which results in all the fields from the currents on the radials nearly canceling at a distant point. On the left side is a ground plane antenna with its radials drooping slightly, which is actually preferred. In this case the radials DO "radiate" since the vertical component of their currents add vectorially to produce a field just like the monopole itself. Note the horizontal component of electric field still (mostly) cancels though, like the antenna on the right. I say MOSTLY because there is still a small amount of horizontally polarized radiation due to non complete cancellation from the field due to the radials, even on the right angle radial antenna. The fact that the radials of the left hand antenna radiate is borne out by the observation that sloping the radials downward increases the radiation resistance from about 36 ohms to something closer to 50 ohms.

As a practical matter, the vertical pattern of these ground plane antennas is greatly effected by the metal pole you mount it on and the coax line it is fed with. Liberal use of ferrite is recommended to keep currents off the coax. If mounted in the attic keep it away from other conductors such as wiring, gutters, anything metal (old bicycles, swords, rifles, etc.) that may be in the attic. Obviously the higher the better. Height is number one. Snow and ice may be less of an issue than you think. Shingle material and tar paper are probably not going to help at all since the organic material they are made of is probably lossy at VHF/UHF.

I sure hope this helps and let me know offline if it needs clarification.

 

[LtDoc]

You are only showing 1/2 of a cycle with those illustrations. Now, if you reverse those arrow to show the other half of a cycle, the radials (either case) are doing the radiating and the 'monopole' is the 'ground' (negative) part of the antenna. RF is still AC. If one terminal is '+' the the other terminal is '-' otherwise no current flows. When that current reverses during the 2nd half of the cycle those terminals are reversed. What was '+' before is now '-', and the '-' is now '+'. If there is no radiation from the '+' terminal then the '-' terminal has to radiate or there's no radiation at all. Unless when the "monopole" becomes '-' there's no radiation, and the antenna only radiates for 1/2 of each cycle. Some how, that just doesn't make much sense, does it. So, those @#$ radials DO radiate! Think it through for yourself. And those currents in the radials do not 'cancel' each other. They can't. They are all flowing in the same direction, -away- from the same originating point. That idea is a misapplication of vector addition.
- 'Doc

 

[prcguy]

Since the current in the (multiple) radials flow from the feedpoint outward to the ends of the radials in different directions, the currents are flowing in opposite directions, they cancel and do not radiate. I'm not making this up, that's the published theory.

Let me ask you why a 1/4 wave ground plane is not considered a dual horizontal and vertical polarity antenna?
prcguy

You are only showing 1/2 of a cycle with those illustrations. Now, if you reverse those arrow to show the other half of a cycle, the radials (either case) are doing the radiating and the 'monopole' is the 'ground' (negative) part of the antenna. RF is still AC. If one terminal is '+' the the other terminal is '-' otherwise no current flows. When that current reverses during the 2nd half of the cycle those terminals are reversed. What was '+' before is now '-', and the '-' is now '+'. If there is no radiation from the '+' terminal then the '-' terminal has to radiate or there's no radiation at all. Unless when the "monopole" becomes '-' there's no radiation, and the antenna only radiates for 1/2 of each cycle. Some how, that just doesn't make much sense, does it. So, those @#$ radials DO radiate! Think it through for yourself. And those currents in the radials do not 'cancel' each other. They can't. They are all flowing in the same direction, -away- from the same originating point. That idea is a misapplication of vector addition.
- 'Doc

 

[ridgescan]

My point of view is from published theory and I believe to be factual. I cannot locate any trusted source of information to collaborate your statement, so what's a girl to do but dismiss it?

What measuring device do you recommend? If I were to wast my time measuring this (which I'm not) I would use a rotatable linear polarized dipole in the far field and measure vertical vs horizontal radiation from an elevated 1/4 ground plane with an RF choke at the feedpoint to keep any RF off the feedline.

If your statement is true I would get equal or near equal pickup in both vertical and horizontal pol. But in real life that would be impossible because the radiation from ground radials on a 1/4 wave ground plane are nearly completely canceled and will not radiate (much) in the horizontal plane.

The result in free space would be horizontal pol radiation being a good 30dB or more down from vertical pol. That's a 1000:1 ratio. The result on an antenna range should get close to 30dB of isolation but I predict at least 25dB.

Now if I had an elevated 1/4 wavelength vertical element and a single horizontal element (call it one ground radial), I would have a half wave inverted V dipole on its side and the measurement I described above would pick up equal vertical and horizontal radiation when looking broadside at the antenna.I would also want an RF choke at the feedpoint to keep RF off the feeedline and skewing the measurement.

I'm sure we're each focusing on some silly nuance of the topic and not understanding each others point of view. However I would suggest you do some research on RF current flow, cancellation and lack of radiation in the radials of a 1/4 wave ground plane so we're on the same page.
prcguy

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[Nap]

How about you tell us first what type of antenna do you have on your mind?

As there are two different issues here, one is
- the "ground plane" like in a horizontal, continuous or discrete metal surface under a vertical antenna
and the other is
- "grounding" like in providing an electrical connection to the earth.