Grounding for INDOOR antennas???

[dustul]

Hey everyone--I have a grounding question that I can't seem to find a good answer for. And before you tell me I have to ground because of lightning please read that I am indoors here!
I just put an antenna up in my attic so that I can improve my VHF/AIR/HAM band reception. It is near the top of my attic (but indoors) and connected with at most 30 feet of RG-58U coax. My concern is that the coax needs to run near some power and computer LAN lines--no way around that as the scanner shares a desk with the pc. I am getting interference in the HF and VHF low range because of the other lines near the antenna--and I have read that a lot of common-mode interference gets to the antenna and into the signal if the coax is un-grounded.

Is there a reason to / benefit for grounding the antenna of a receive-only setup like a scanner? I grabbed a test lead and tried to ground at the scanner with no noticeable benefit--is there a difference between grounding at the scanner vs. the antenna too?

I'd appreciate any advice you can give me because running unnecessary grounds will be a royal pain on that side of my house!

 

[KB7MIB]

Wirelessly posted (Mozilla/4.0 (compatible; MSIE 6.0; BREW 3.1.5; en )/800x480 Samsung SCH-U960)

You can try solving the commom-mode interference at it's source with ferrite coils. Do a web search on "common-mode interference" and you should find info on how to solve it.

Another option would be to upgrade your coax to one with better shielding. Cheap coax won't have as good shielding as more expensive brands. Shielding percentage should be in the high 90's. Cheap coax typically has less than 90%.

I would do both, personally. But start with the ferrites.

John
ARS KB7MIB
GMRS WPXJ598

 

[n5ims]

Change out the RG-58 to some high quality (Belden for example) RG-6 Quad Shield. Yes, I know that 58 is 50 ohm and 6 is 75 and scanners specify 50 ohm cable, it won't matter on a receive-only setup. The RG-6 QS is way better shielded than RG-58 (even cheap 6QS vs quality 58) and should help shield the noise from the coax passing nearby the noisy lines (this should solve the coax picking up any noise, but will still pass any noise that the antenna picks up, of course).

The ferrite core chokes (like Snap-Together Ferrite Choke Core - RadioShack.com) will also help as stated above and is also a good idea. I'm not sure that grounding will help in your situation.

 

[LtDoc]

The simplest way of putting it is to keep things where they are supposed to be and not let things 'leak' in or out. With coaxial feed lines that means good shielding. With 'computer' lines, that means good shielding also.
Gounding the shield of those feed lines gives things (CMC or RF) on the outside of those feed lines someplace to go other than your receiver. Various ways of doing that. Those ferrite 'beads' are one way.
For computer cables (LAN) things get a bit more complicated/expensive. A lot of those type cables are not shielded. Keeping the radio cables and computer cables as far apart as possible is also a good idea. Not always possible though.
RG-58 coax has a bad reputation primarily because it's shielding usually isn't the best in the world, especially the lower cost cables or common cables (wanna guess who sells that sort of cable?). Selecting a name brand manufacturer usually means better shielding. Less than about 97% isn't the best in the world for cables subject to being 'near' RF sensitive thingys. What's the guarantied best distance to keep things separated? Something on the average of a 1/4 mile is a sure bet. Anything less means there's going to be a possibility of leakage in/out.
Keep in mind that it isn't just the coax's problem, but the LAN cabling too...
- 'Doc

(Typical computers just are never going to be best friends with RF producing devices. They will always require some 'adult supervision' to make them get along with radios, or throwing lots of money at the problem. Fun, ain't it?)

 

[dustul]

Thanks for your feedback everyone. I ultimately decided to pull the entire scanner setup out of my office and drop the antenna line into my kitchen area -- even put a nice BNC jack plate in the wall there so I can do whatever I want with it.

One more question though: If I am not having any specific interference issue, is there still a benefit to grounding the coax at the antenna? My only easy grounding option is to run a ground wire halfway across the house to the power/cable/phone grounds, but I'm worried I'll do more harm than good with almost 100 feet of copper stringing across the attic.

 

[John_S]

At most scanner frequencies, any grounding with the intent to improve rf performance is pretty much useless. And any ground leads with lengths of more than just a few feet would be useless anyway. The only reason to have a ground system with a scanner is to provide lighting protection. Me...I don't trust any protection... unplug.

 

[dustul]

Thought I'd share some final results. I am using RS RG58U purchased as a 50 ft package with PL259 connectors on the ends.

My first test was to put a BNC adapter and look at the RSSI display while monitoring a nearby P25 control tower. Roughly 625 RSSI on the scanner compared to 730 with a stock antenna. NOAA weather radio on VHF showed slight improvement to the stock rod--and I went from 2 to 3 channels.

Then I put a BNC coupler directly on the cable--cutting off the PL259. VHF showed little improvement, but the P25 on 769 Mhz went up to 725-750 RSSI.

Then I trimmed the cable down to under 25 feet with the BNC coupler on the end. In addition, I installed a BNC wall-plate so I could disconnect when not listening. This adds two extra BNC connectors and a patch cord. P25 channels are up to 750 or so reliably with 99% decode rate, I am at 4-5 WX channels on VHF. Most notably, I went from 2 LWIN digital towers to an easy three--which goes up to 6 on a good night!!!

Ultimately excellent results for a $29 antenna only 15' off the ground and in an attic. I want to get a nice solder-on BNC and see if it improves things any compared to the push-on RS had in stock.
And um . . . by the way . . . I played with grounding the coax to the electrical system grounds. No change.