| Author |
Message |
    
neil carrick
| | Posted on Wednesday, January 01, 2003 - 3:13 pm: | 
|
Happy New Year Everyone!!!!!!!
I was wondering what any ones thoughts were on grounding a mag mount antenna to the chassis by way of a heavy ground strap affixed to the base of the antenna where the ground plane kit goes(wilson 5000) or the gutter clip on the base of a K 40.
Would this have the effect of a regularly grounded antenna and thereby reduce the heat travelling back thru the ground portion of the coax when using Extra power?
I recently had a coax between the radio and amp that heated up and melted the insulation out from between the leads.
Thanks for any ideas you may have.. Neil |
Bigbob
| | Posted on Wednesday, January 01, 2003 - 4:10 pm: |
|
MY GAWD HOW MUCH POWER U USING,WOW?Seriously a wilson is rated pep not avg. power.I use a wilson 1000 trunk lip on a copper super mega-mount,it sits very close to body and has very good capacitive coupling,the normal mag.mounts hold the taped coil about 1.5 inches above body,mine is only 1/2 inch,thus over 2 mHz have less than 1.5 to 1 swr end to end very flat AND I have a 4 gauge 2" long ground strap attached with stainless scerws to top of vehicle.All you need is a large amount of metal beneath it for a ground plane,it doesn't need a battery ground. |
neil carrick
| | Posted on Wednesday, January 01, 2003 - 4:56 pm: |
|
I have a small? hy-gain afterburner+ hooked to it and it is just a mag mount so it gets all of its ground thru the outer braid of the coax.
If it was a solid mount would it not be grounded directly to the chassis by the mount? |
Bigbob
| | Posted on Wednesday, January 01, 2003 - 5:06 pm: |
|
Another thing wilson antennas will handle about 1/3rd their rating on am,1000=300,5000=1700,iffy!the coax they supply on their mag.mounts is rated 500?watts rms(they ARE cb antennas,you know)go with rg213 or equivilent and mount it direct to body or do like I did.A trucker freind had 2 5000s cophased,first he melted the primary coax then the harness,changed both for heavier coax,then proceeded to split the coil housings.He was running a 2950 pushing a ts500 driving a 5000 watt tube amp the antennas could only handle about 2500,so POOF,VERY EXPENSIVE amp replacment,get my drift.E-mail Tech671 give him all the details and he'll give you the skinny on this. Bigbob |
Bigbob
| | Posted on Wednesday, January 01, 2003 - 6:23 pm: |
|
It is capacitively coupled to the body,the body metal is the bottom half of the antenna like the radial arms of a 1/4 wave ground plane antenna the closer the radiator is to the ground plane,without touching it,the more broadbanded it is and the easier it is to match.The coil is tapped by the center conductor approximately 2/3rds from top,the bottom of coil is attached to the coax braid this inturn is physicaly attached to a thin foil film the size of the magnet,the film is 2-sided seperated by a very thin sheet of paper,a capacitor is what it is ,the braid on one side the magnet/body on the other side,hence the ground plain.Radio waves are A.C. NOT D.C. thus they can ground thru a capacitor,otherwise none of this would work.Do not attemt to modify the mag mount to do so would ruin your rig.The radio waves should not "see" the chassis if they did it would raise hell with all your electronics,if they do it is due to an inadequate ground plane(very smmall amount of metal),or loose connections or broken braid or center,on a wilson mag mount center and braid should test 0 ohms.THE center and braid should test shorted. |
neil carrick
| | Posted on Wednesday, January 01, 2003 - 7:30 pm: |
|
OK...I see the antenna is set up differently than a regular solid mount. Can you tell me what may have caused the coax to heat up like that then? |
neil carrick
| | Posted on Wednesday, January 01, 2003 - 8:43 pm: |
|
OK...I have been checking around and it looks to me as if the K40 antenna is simply the same antenna in either application... just with an optional magnet mount to adapt to the trunk lip mount. It seems to be the same part # for the coax assembly. http://www.cfrobbins.com/toban/ground.htm
this site says that if you have a poor ground on, say a roof rack mount or similar that you must run a cable ground from the chassis to properly ground the system. (in a ground plane antenna setup) of course. Keeping in mind I can only find one coax part # for the K40.
So can you tell me if the K40 is the same as the Wilson 5000 magmount or different in its capacitance grounding..as it was the K40 I was using when the coax melted down on me.
Also,(on the wilson) is there a deterioration in the performance when the coating under the magnet gets scuffed up over time?
|
neil carrick
| | Posted on Wednesday, January 01, 2003 - 9:05 pm: |
|
I see on this site as well that wilson is advocating a floating ground kit for no ground systems..although this appears to be a balanced counterpiose that is not connected to a ground at all.
http://www.wilsonantenna.com/fgk.htm
Could it just have been a poor coax that heated like that ? I have cut on end off of it and stripped it out to find it was a crimped factory fitting that was on a short jumper that I think came with an SWR meter at one time.
Thanks...Neil
|
Bigbob
| | Posted on Thursday, January 02, 2003 - 8:14 am: |
|
ALL mag mounts are pretty much standard 17 feet of coax,that said,K-40 in the past used a disc capacitor between sheild and ground to help tune out reactance works good at 4 watts but is terrible at more than 100 watts,remember"cb antenna",I don't know if they changed that practice or not.Save your self a lot of headaches if possible get a ball mount and a 102 inch whip it'll handle any thing you poke at it,I know it's impossible if you lease,but it's fool proof,hey mega-mount,102 whip,rg8-x coax,I ran that,it works great,just stay off two tracks,stays on just one hell of a racket. |
|
