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Message |
    
Legend
Member
Username: Legend
Post Number: 57
Registered: 12-2002
| | Posted on Tuesday, June 15, 2004 - 5:24 am: | 
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Techs... (or anybody) I need some input about this...
Over the years, I've noticed that some people prefer to electrically isolate their base antenna from the metal pole it is mounted on. You know... keep the metal base of the antenna from coming into contact with the metal pole, in order to keep the pole from radiating RF.
* Yes... I know that most people use the pole as their path to the ground rod.
From a TVI perspective, is this the way to go? Obviously, using a choke to keep RF from going back down your feedline helps keep TVI at lower levels, so wouldn't a person experiencing TVI also want to also keep from RADIATING RF as it travels down the pole?
I realize that the reason for having metal to metal contact at the base of your antenna, to the top of the pole, is to provide you with a ground for your antenna. But, would it help to reduce TVI if I were to use a SHIELDED ground WIRE leading down to the ground rod, instead of using the POLE for my path to ground? So that way, I would still have a ground, but the PATH to ground would not radiate RF all over the place.
This is something I have wondered about for a long time, but never really gotten a good answer for. Can some people here give me their opinions please?
Thanks... |
Tech237
Moderator
Username: Tech237
Post Number: 39
Registered: 4-2004
| | Posted on Tuesday, June 15, 2004 - 9:44 am: |
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Firstly it depends on the antenna. Some antennas are designed to be isolated from the support pole and some to be electrically connected to it. In my case all my vertical antennas are connected to the pole and with the excepton of 10m (and then only to my stereo) I have no trouble with interference.
Secondly the connection to ground needs to be a lareg a diameter as possible - remember RF only travels on the outside of the conductor. That is why multi-wire cables are more commonly usewd for RF grounds and solid copper conductors for house wiring grounds.
Thirdly - most mast are mounted in a large mass of concrete which doesn't make the best ground connection, and in fact under the wrong conditions can become deadly. I once saw an Air traffic Control Tower taken out during a lightning storm because the tower took a direct hit and the only ground was via the concrete. Lumps of wet concrete punched holes through the ATC wall and destroyed most of the equipment. Yes, it may well be a rarity but once is once too many.
Fourthly - from past experience I have learnt that more interference is caused by RF on the outside of the coax sheild than by any RF on the mast. Use good connectors, good quality coax, put a low pass filter at the radio and you should not have much trouble with TVI. |
Marconi
Intermediate Member
Username: Marconi
Post Number: 309
Registered: 11-2001
| | Posted on Tuesday, June 15, 2004 - 11:12 am: |
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Legend, you can ground a shield, but you cannot shield a ground. Like 237 notes, if there is stray RF around the feed point, it is going to get on the outside of something going towards the ground, that is just what happens.
TVI and RF on the mast or feed line do not necessarily go hand and hand. TVI can come from the radiation off the antenna or from the radiation off the feeder, or from something else reflecting the RF. How would you know where it is coming from. Most 11 meter antennas, resonant types, should show isolation at the feed point and thus you should have little or no RF on the feed line, unless it gets on there from the antennna itself after being radiated or reflected off of something nearby.
Personally I believe TVI has more to do with the affects of proximity to sensitive devices. In other words if a sensitive device is close (in proximity) to a source of near field RF then you are likely to see TVI. Only thing you can really do is to move something out of proximity or substantially reduce the source of RF to that space. |
Tech833
Moderator
Username: Tech833
Post Number: 742
Registered: 8-2002
| | Posted on Tuesday, June 15, 2004 - 11:59 am: |
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To reduce or eliminate TVI caused by intermod or self generated intermod, everything the antenna mounts to should be well bonded and grounded. Isolating the mounting structure from the antenna will not help your signal at all, and puts you in danger of being the neighborhood nuisance. |
409
Member
Username: 409
Post Number: 72
Registered: 3-2002
| | Posted on Tuesday, June 15, 2004 - 1:47 pm: |
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Most base type ground plane antennas have DC ground for lightning protection. If you isolate it you may be in for a real shock in a thunderstorm. |
Legend
Member
Username: Legend
Post Number: 58
Registered: 12-2002
| | Posted on Tuesday, June 15, 2004 - 9:46 pm: |
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Which is why I was asking about using a shielded ground. A ground wire with shielding around it. ie.. Like using a piece (or several pieces) of RG-8 coax with the center conducter connected to the base of the antenna, and the other end attached to the grounding rod. That way, you'd still have a ground, but you would not radiate RF off of your mast. |
Tech237
Moderator
Username: Tech237
Post Number: 41
Registered: 4-2004
| | Posted on Wednesday, June 16, 2004 - 8:41 am: |
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Legend,
Once again I emphasise do not isolate the antenna from the mast, use a proper LARGE diameter cable to run to ground and do not use a sheilded one. If this idea was any good it would be used on commercial sites and in 30 years in the trade I have yet to see one using it.
Personally I'd use the largest stranded wire I can run for the ground and if the run was long enough (and it should be as short as possible) I'd even consider using something the size of a battery cable.
To add to Tech833 comment. Several years back I had a welded mesh fence around my yard that caused RFI. To fix the problem I added ground straps at each fence post that not only electrically joined the panels together but also grounded each pane/post. Extreme? Maybe but it worked. |
Legend
Member
Username: Legend
Post Number: 59
Registered: 12-2002
| | Posted on Saturday, June 19, 2004 - 12:22 pm: |
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My antenna mast is made of pot metal, and is telescopic. Should I run a short, thick ground cable from the base of the mast to my ground rod, or should I run a copper ground cable all the way down alongside the mast from the base of my antenna?
I've always wondered if that antenna mast is really even a good conductor, considering it is made of some kind of pot metal and also telescopic. Not to mention the corrosion and such that inevitably forms around and inside the joints. Is it better to run a copper cable directly from the antenna? Or maybe do both? |
Tech237
Moderator
Username: Tech237
Post Number: 49
Registered: 4-2004
| | Posted on Sunday, June 20, 2004 - 9:23 am: |
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Ive seen both methods used successfully. Try using just a ground run ffrom the base of the mast and using jumpers across the joins in the mast to ensure good contact. The if you find that this doesnt work well enough for you run one all the way down. |
Tech237
Moderator
Username: Tech237
Post Number: 55
Registered: 4-2004
| | Posted on Monday, June 21, 2004 - 10:22 am: |
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I repeat DO NOT use a signle solid conductor for RF ground. Use a stranded cable you actually have more area for the RF to current to flow on even if it is the samesize as a solid conductor. RF ONLY flows on the outside of a conductor. |
