Attic antenna grounding?

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Signal6

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Hello everyone, I looked through the forum and could not see a similar post but if I missed it i apologize. So here is my question, I am installing some antennas in the attic of my home and am worried that they wont perform without some sort of an RF ground. Have some of you come across this issue or are there specific antennas that don't need an Rf ground? Just as an idea I need antennas on VHF/UHF TX and RX and 800/900 TX and RX. Thanks in advance any ideas thoughts or comments this is not a strong subject for me, and pictures would be helpful if anyone has some of previous attic intstalls.
 

LtDoc

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That list of topics ought'a keep you busy for a while! :) And something to think about while perusing them is that most commercially sold antennas supply their own "RF ground", have two connections to them. One connection is typically thought of as the 'hot', and the other is though of as the 'ground'. If there aren't two connections then no current flows, there's no radiation. The 'catch' to that is that the second connection may not be what you think it should be (a separate part?) things can get confusing. Those "no ground" antennas are a good example of that sort of thingy.
Don't you just hate answers like this??
- 'Doc
 

Signal6

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Thank you

Wow thanks guys. I read several threads on attic installation but I was not able to find anything on grounding so those look like they are the ones I may not have been able to find. Also interesting comment on antennas providing their own ground can you elaborate more on that this is one concept I am not familiar with. Thank you in advance
 

zz0468

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...Also interesting comment on antennas providing their own ground can you elaborate more on that this is one concept I am not familiar with. Thank you in advance
It's not so much an antenna providing it's own ground, so much as it's providing it's own counteroise. Many types of antennas require a counterpoise, which is, in essence, a mirror of the radiating element.

A whip antenna on a car uses the car body as a counterpoise. A ground plane uses a set of horizontal radiators as a counterpoise. A dipole has two equal length wires, one is a counterpoise, and so on. Other antennas, a ground mounted vertical like you see at an AM broadcast station uses buried radials and the ground itself as a counterpoise.

So, an antenna that has it's own counterpoise, like a groundplane, doesn't need a ground for performance. A ground connection is required for safety.
 

John_S

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At those frequencies, a proper RF ground is pretty much useless. As far as protective grounding goes, if the lightning is close enough to be a problem to an indoor antenna, you shouldn't have the antenna connected. Disconnecting is still your best protection...period. Not always practical, I agree, but what sucks worse?... Taking a few minutes to disconnect or coming home to a charred mess on your bench...and I've seen this, but not on my bench. I think it's best to leave any protective grounding to cases where it's impossible to disconnect... just my 2 cents.
 

Signal6

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Great info

Thanks again guys that all make sense and thank you for everyone's input and information not only did you help answer my question but I confirmed alot of theories I had in my head as well as learned some things. This is a great group on here. I'm still open to more information if someone has something else to add. Thanks again.
 

LtDoc

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I think 'elaborating' on that 'RF ground' thingy has already been done. It's basically the 'other half' of the antenna. that 'other half' can be lots of things. The house wiring for one. The biggy with that is that if the house wiring, or plumbing, is used, then there will be consequences. Both 'halves' of an antenna radiate. They swap places every half cycle (RF is AC), so the 'hot' side turns into the 'ground' side, and the 'ground' side turns into the 'hot' side every half cycle. So, that 'ground' is going to radiate too. How'z that gonna affect things? Eeooo, nasty thought huh? :)
- 'Doc
 

Signal6

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Hmmm

Now that is a crazy thought. Well i guess I have learned that I need to brush up on grounding and antenna theory I have learned alot here thank you everyone.
 

prcguy

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For a dipole or a wire antenna using house wiring or plumbing as a counterpoise "the other half radiates" would be a true statement but for a ground plane with more than two ground radials or a whip in the center of a car roof, etc, the other half does not radiate. The second or more ground radial cancels out the RF current and it does not radiate.
prcguy


I think 'elaborating' on that 'RF ground' thingy has already been done. It's basically the 'other half' of the antenna. that 'other half' can be lots of things. The house wiring for one. The biggy with that is that if the house wiring, or plumbing, is used, then there will be consequences. Both 'halves' of an antenna radiate. They swap places every half cycle (RF is AC), so the 'hot' side turns into the 'ground' side, and the 'ground' side turns into the 'hot' side every half cycle. So, that 'ground' is going to radiate too. How'z that gonna affect things? Eeooo, nasty thought huh? :)
- 'Doc
 

LtDoc

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I think you might think about that just a little more. Where does the other half cycle go if it isn't radiated? :)
- 'Doc
 

prcguy

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There would (or should) be equal and opposite current in the vertical element during both halves of the cycle on a 1/4 wave vertical ground plane.

Think about this a little more, if the ground plane radiated you would have a dual polarity horizontal and vertical antenna, not a vertical.
prcguy

I think you might think about that just a little more. Where does the other half cycle go if it isn't radiated? :)
- 'Doc
 

LtDoc

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"Equal and opposite", yep, but then, that 'equal and opposite' current is also in that counterpoise. Right?
- 'Doc
 

prcguy

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A single counterpoise wire would make a dipole and you will have equal and opposite currents if the main radiator and counterpoise are the same length and decoupled from the feed line.

If not decoupled some of the current will flow on the feed line and there may not be equal and opposite currents in the main radiator and counterpoise but the combined counterpoise and feed line currents should add up to equal and opposite.

In a typical 1/4 wave ground plane with 3 or 4 radials the ground current propagates from the feed point out each radial and back. Since the RF currents are divided and traveling opposite directions they cancel and there is no radiation from the radials. Same basic thing with a 1/4 wave whip on a car roof, the roof does not radiate because of RF currents traveling on the skin in all directions simultaneously.

It's always bugged me when I read the Buddipole manual suggestions for setting up the antenna as a vertical. They recommend a single counterpoise wire, arggg! That turns the vertical into an inverted V dipole on its side and since the counterpoise wire is a full 1/4 wavelength working against the super shortened Buddipole the counterpoise would radiate better than the short loaded vertical section. It would, except now it's laying on the ground or running very low and parallel the the ground and is very lossy at best.



prcguy



"Equal and opposite", yep, but then, that 'equal and opposite' current is also in that counterpoise. Right?
- 'Doc
 
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AA1LL

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Hi, Guys,
I think what prcguy is saying is, say you have a short vertical with only two horizontal radials in free space (like a ground plane antenna) with the two horizontal radials opposing each other. Then at a distant point the fields due to currents on those two radials cancel and you're left with the field due only to the current on the vertical section. Now add more radials, keeping them symmetrically spaced around the feedpoint. As long as you have symmetry you wont get any radiation from the radials which becomes a "counterpoise." You can experiment with this with NEC2--it does happen. It makes sense too because the direction of current for any 2 radials is opposite at a distant point, resulting in zero radiation.

Hopefully all the currents from the radials sum up to equal the current on the vertical section so this return current travels down the inside of the outer conductor to match the current on the center conductor. This results in no currents on the outside of the coax but just to make sure and prevent coupling, you would put several large ferrites over the coax anyway especially if it is coming off at an angle. The ferrites create high impedance regions to reduce current on the outside of the coax.
 

prcguy

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A much more through explanation than I attempted, thanks.
prcguy

Hi, Guys,
I think what prcguy is saying is, say you have a short vertical with only two horizontal radials in free space (like a ground plane antenna) with the two horizontal radials opposing each other. Then at a distant point the fields due to currents on those two radials cancel and you're left with the field due only to the current on the vertical section. Now add more radials, keeping them symmetrically spaced around the feedpoint. As long as you have symmetry you wont get any radiation from the radials which becomes a "counterpoise." You can experiment with this with NEC2--it does happen. It makes sense too because the direction of current for any 2 radials is opposite at a distant point, resulting in zero radiation.

Hopefully all the currents from the radials sum up to equal the current on the vertical section so this return current travels down the inside of the outer conductor to match the current on the center conductor. This results in no currents on the outside of the coax but just to make sure and prevent coupling, you would put several large ferrites over the coax anyway especially if it is coming off at an angle. The ferrites create high impedance regions to reduce current on the outside of the coax.
 

commstar

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A Serious Comment

Doc/PRC/et al.,
This is a really great exchange.

I feel like I am about to learn something....appreciate the civil tone.

Staying tuned for the resolution.

No sarcasm, serious.

Mike
 

LtDoc

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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
 
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