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Antenna grounding

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  #17  
Old 03-01-2012, 11:33 AM
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That shorted 'stub' is an electrical 1/4 wave so shows up as a very high impedance or 'open circuit' at the frequency it's designed for. That means at the frequency of use, the 11 meter band, it's like the thing isn't there. At other frequencies/bands or at DC it will act as a short circuiting of the center conductor to ground (where that shield/braid is supposed to be going). Ought'a work just dandy for a single band antenna, not so well for a multi-band antenna, or excursions farther away from the design frequency.
See where that's going?
- 'Doc
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  #18  
Old 03-01-2012, 12:33 PM
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Quote:
That means at the frequency of use, the 11 meter band, it's like the thing isn't there. At other frequencies/bands or at DC it will act as a short circuiting of the center conductor to ground (where that shield/braid is supposed to be going).

And provides a path to ground for your antenna.

Good Post Guys.

73
Jeff
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  #19  
Old 03-01-2012, 12:50 PM
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on a antenna like the sigma 4 or a wolf .64 that has less than 1 ft of element (gamma) connected to the center conductor and the other 22 to 32 ft connected to ground , is there a benefit to this grounding stub ? since the entire antenna is already at DC ground other than the short gamma ....

and .......... can it help folks with bad station noise ? or is it just to help with lightning strikes ?

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  #20  
Old 03-01-2012, 12:53 PM
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A short circuit at DC works quite differently than it does at RF. Consider it a strategically placed needle in the coax. The 'needle' is on another sort of side line to the coax. You use a Tee connector. Radio to one, antenna to another, and the 6-foot shorted line to the third. It gets installed permanently in the line, and can be in place for transmit or receive.

Depending on the source of noise it may assist. Anything that is being generated at 27MHz will not be stopped. However, if it is a static discharge problem within the antenna itself, the antenna will now be fully DC grounded and any static discharged. It won't do anything for reducing noise coming from a pole transformer, vacuum cleaner, etc.

Transmission line - Wikipedia, the free encyclopedia
Zin = Zo^2 / Zl

The equation states input impedance Zin equals the transmission line impedance (squared) divided by the load impedance. With the far end shorted, you get division by zero and Zin is extremely high, essentially open circuit.

What you will build is a quarter wave transformer. In this case, having one end shorted will appear as an open circuit at the other. As far as your radio is concerned the shorted piece of line is not there. Best way to confirm it is to build one and measure your SWR. You won't notice any difference either on transmit or receive.

The six foot length is measured from the center of the Tee connector to the first place where the inner and outer conductors of the coax meet. RG58 and RG8 both have a velocity factor of 66%. A quarter wavelength at 27MHz is 9 feet, so the shorted line should be .66 x 9 = 6 feet.

On the second harmonic, the line is now a half wavelength long. The line will appear as a short circuit for the second and other even number harmonics. You can confirm this by building a shorted line section that is 12 feet long for 27MHz. The SWR meter should peg and the receive signal strength should drop considerably when it is installed.

The 6-foot line length is for 27MHz only. At the FM broadcast band it is considerably shorter, about 18 - 22 inches depending on frequency. Move to 14MHz and the shorted line becomes about 11' 7".

I'd like to try out one of the surge suppressors you mention. Without having one in my hands it's difficult to form an opinion. I've learned that there is a lot of useless BS given in advertising and promoting a product. It would likely cost more than the 6-foot piece of coax and a Tee connector, but would be more suitable for a station that operated over a much wider range of frequencies.




Quote:
Originally Posted by BOOTY MONSTER View Post
"For CB, get yourself a Tee connector. It gets placed along your transmission line somewhere. On the third output connect a 6 foot piece of RG58 or RG8 cable with the far end shorted and soldered together. It will ensure the entire antenna is placed at DC ground."

can you explain how that works a little more please ?
and how does that not act like a needle stuck in the coax ?

also , what's your opinion of Morgan’s M-300 Series arrestors ?
Morgan Manufacturing | IMPULSE SUPPRESSORS, ARRESTORS

thanks .
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Last edited by vo1ks; 03-01-2012 at 01:08 PM.

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  #21  
Old 03-01-2012, 01:13 PM
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Consider your house to have two ground systems. You have an antenna ground and your AC ground. Your transmitter and other radio equipment will act as a jumper between the two. If there is a ground surge coming on the AC line it will try to dissipate through the antenna ground. Similarly a tower surge or lightning strike will attempt to dissipate through the AC ground. Your equipment between them could pass a great amount of current and potentially become damaged. You need to create a very low impedance path directly between the two grounds, and a high impedance path through your equipment.

The third plug in your AC cords is not suitable for this job...

I've summarized my thoughts, and included grounding documents and broadcast radio station design information in this document.
http://members.rennlist.org/warren/grounding.pdf





Quote:
Originally Posted by IRock817 View Post
This may be lame but in a way it is tied in!
All my equipment is earth grounded through the third plug on the cords!
Before I get thrashed for that comment, I am going to add a couple more rods and bond everything together including the electric company's rod!
My problem is that the electric company's ground rod is on the other side of the house which is about seventy-five too eighty five ft away from my antenna and rods!

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  #22  
Old 03-01-2012, 01:31 PM
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thanks vo1Ks
i'm not gonna pretend i understood all that , but i think i kinda get the gist of what you're saying .

coax is a interesting critter . i constantly read that coax lengths don't matter and don't really change things , to just keep it as sort as necessary to reduce unnecessary loss . but certain lengths , sometimes in a certain place , can have a noticeable or even needed effect . i'm just a chicken bander , but i do appreciate you pork butts sharing what you've learned .

"If there is a ground surge coming on the AC line it will try to dissipate through the antenna ground. Similarly a tower surge or lightning strike will attempt to dissipate through the AC ground. Your equipment between them could pass a great amount of current and potentially become damaged."

if the coax is disconnected from the components in the shack will that keep the damage from your examples from happening ?
and the electrical 1/4 wavelength stub being discussed will prevent or reduce the damage in you're scenarios with the coax connected to the shack components ?

thanks for the link to your pdf page . i'll read and re-read it a few times and get what my limited knowledge of the hobby allows me too .
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Last edited by BOOTY MONSTER; 03-01-2012 at 01:54 PM.

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  #23  
Old 03-01-2012, 01:44 PM
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I started playing with Chicken Band 30-ish years ago, did lots of reading, and moved on from there. It's only once you start to learn that you realize how little you really know. I've always tried to share knowledge with others. Sometimes it goes over well, other times not.

Transmission lines are a fun topic. Sometimes length can be critical to get proper signal phase at the antenna, it can be used for impedance matching and many other things. In terms of the right length between an antenna and transmitter, the best I've heard is that it needs to be the right length. Too short and it won't reach, too long and it gets in the way.

Get yourself a Tee connector and try the 6-foot shorted stub.

If your equipment is entirely disconnected and put away in a closet there is no way for current to flow through it. Just unplugging the connector may not solve your problem. You still have the transmission line coming into your house, possibly running next to copper pipes, AC wiring, telephone lines, etc. Your transmitter may be ok, but the power supply, maybe your computer, and other items are still at risk. No guarantees, but a properly grounded station is much more likely to survive a direct lightning strike without significant damage. However, improperly grounded equipment and building are in grave danger.





Quote:
Originally Posted by BOOTY MONSTER View Post
thanks vo1Ks
i'm not gonna pretend i understood all that , but i think i kinda get the gist of what you're saying .

coax is a interesting critter . i constantly read that coax lengths don't matter and don't really change things , to just keep it as sort as necessary to reduce unnecessary loss . but certain lengths , sometimes in a certain place , can have a noticeable or even needed effect . i'm just a chicken bander , but i do appreciate you pork butts sharing what you've learned .

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  #24  
Old 03-01-2012, 01:52 PM
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BTW , in case no one warned you ... be careful answering my questions ...... you'll only encourage me to ask more of them

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