Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Mobile Grounding?  (Read 672 times)
N5BX
Member

Posts: 4




Ignore
« on: July 29, 2007, 07:23:09 PM »

Greetings.  I've just recently gotten my Wrangler all hooked up with a Kenwood D700 and GPS.  I'm noticing that I'm not having much luck hitting the APRS digipeater, which I should be able to hit just fine.  I have a Comet SBB-2 antenna and a special aluminum mount that hooks the antenna to the spare tire mount.  I'm wondering if I have a grounding issue (not confirmed yet, but will hopefully confirm this week with an SWR meter).

Does anyone have a recommendation for creating a better ground plane on a vehicle where you are not drilling through the roof?  Would a ground wire to the chassis help?  If so, where should I connect the wire?  Maybe some small radials?

Thanks for any help you can give, I'm a little bit of a noob here, and have not explored much with antennas and grounding for mobile vehicles.

73,

N5BX
Logged
K0BG
Member

Posts: 9867


WWW

Ignore
« Reply #1 on: July 30, 2007, 05:45:07 AM »

It isn't a DC ground issue, it's an RF ground issue. More correctly, a lack of a image plane under the antenna. There are several VHF antennas which will operate fairly efficiently with a minimal image plane, but yours isn't one of them. You might look for a Larsen OM150. Both AES and HRO carry them.

This assumes you've checked the SWR, and the coax connections, etc.?

Alan, KØBG
www.k0bg.com
Logged

N5BX
Member

Posts: 4




Ignore
« Reply #2 on: July 30, 2007, 06:12:41 AM »

Thanks Alan!  Out of curiosity, are there any dual-band (70cm/2m) antennas that work well without RF Ground planes?  And is there a way to retrofit the antenna to have a ground plane?
Logged
KD0AFK
Member

Posts: 245




Ignore
« Reply #3 on: July 30, 2007, 06:32:34 AM »

Diamond makes a dual band mobile antenna that is an NMO mount that would be installed in the center of your roof, or you can make your own end fed dual band jpole out of aluminum rod that could be mounted to the spare tire mount.
Logged
KE3WD
Member

Posts: 5694




Ignore
« Reply #4 on: July 30, 2007, 11:11:42 AM »

If you don't want to or can't punch a hole in the roof, why not consider going with a good quality dual band magmount up there?  

OR one of the clampon or in door suv mounts from places like Comet, you may be able to mount the antenna you already have on one of those, placed somewhere more toward vehicle center.  And it will put the base of the antenna more towards the top of the vehicle's ground plane if not above it.  

If height is the issue, and parking garages, etc. then consider adding a motorized fold down mount, Comet, Diamond, etc.  I had one on the old SUV in the late 90s, of course the day came when I forgot to push the button and sailed on in to the parking garage anyway dontcha know...  

Mounting to the spare tire mount may yield a good looking VSWR reading, but likely will wreak havoc with the desired omnidirectional pattern of the vertical antenna.  


.
Logged
K0BG
Member

Posts: 9867


WWW

Ignore
« Reply #5 on: July 30, 2007, 06:13:44 PM »

The biggest problem with mag mounts are, most insurance companies will not cover them. If you doubt this, contact yours, and ask.

Alan, KØBG
www.k0bg.com
Logged

N5BX
Member

Posts: 4




Ignore
« Reply #6 on: July 30, 2007, 07:00:34 PM »

I left out one very critical detail.  I have a soft top Wrangler.  Apologies...

With that said... I'm considering the JPole option, but I'm wondering if I'm being crazy by thinking I could get a dual band?  Maybe I could mod an existing antenna to act like a JPole?
Logged
KE3WD
Member

Posts: 5694




Ignore
« Reply #7 on: July 30, 2007, 09:21:48 PM »

>>The biggest problem with mag mounts are, most insurance companies will not cover them. If you doubt this, contact yours, and ask.

Alan, KØBG
www.k0bg.com <<

LOL.

Um, yeh, right, I'm gonna call my auto insurance company over the loss of a forty dollah VHF/UHF magmount antenna...


Now this is where it gets good, Alan comes back with the worst-case scenario of a magmount antenna damaging someone else's proppity and you gettin' sued and losin' ever' thang you evah owned and throwed in jail, too.  

Here's a plan:  wrap the coax around the luggage rack twice before runnin' it through the car door.  That'll retain the leetle sucker and choke some of the RF off the coax at the same time.  

!
Logged
KE3WD
Member

Posts: 5694




Ignore
« Reply #8 on: July 30, 2007, 09:38:40 PM »

>>With that said... I'm considering the JPole option, but I'm wondering if I'm being crazy by thinking I could get a dual band? Maybe I could mod an existing antenna to act like a JPole? <<

Jpole would be kinda long and ungainly on a mobile, no?  It would also have to be made of tubing of such cross section as to be rigid, a couple of whips likely wouldn't work well in mobile config, although you could put stiff insulators between the two, I guess.  

Also likely wouldn't give you as much gain as you think as compared to a standard mobile 1/4 wave antenna.  

That Comet you've got is a short leetle thang.  

Since you've already got the mount on there for a Comet, why not consider one of the longer Comet Dualbanders?

AX-95
DualBand Mobile Antenna
146/446MHz
Gain& Wave:
146MHz  1/2 wave 3.3dBi
446MHz  5/8 wave x 2 5.8dBi
VSWR:  1.5:1 or less
Max Power:  60W
Length:  38"
Connector: Gold-plated PL-259
 
That's pretty near twice the length.  

If the height may be problematic, garages or whatnot:

CA-2x4SR
VHF/UHF Dual Band Antenna
1.5:1 or less SWR:
140-160/435-465MHz
Gain:  3.8/6.2dBi
Max Power:  150 watts
Length:  40 inches
Connector:  
PL-259
Fold-over hinge included
 
Tradeoff in height:

 AX-75
DualBand Mobile Antenna
146/446MHz
Gain& Wave:
146MHz  1/2wave 3.2dBi
446MHz  5/8 wave x 2 5.7dBi
VSWR:  1.5:1 or less
Max Power:  60W
Length:  30"          
Connector: Gold-plated PL-259
 
You've got a whole page full of plug 'n play options to try on the mount that is already installed:

http://www.cometantenna.com/products.php?CatID=1&famID=4&childID=4

And put that short stick up for sale.  


.



Logged
N5BX
Member

Posts: 4




Ignore
« Reply #9 on: August 03, 2007, 06:29:33 AM »

Will check it out!  I saw a post earlier that might also work.  If I turned it into a vertical dipole, and purchased another SBB2 (just exploring...), would that reasonably solve the issue?
Logged
K0JEG
Member

Posts: 650




Ignore
« Reply #10 on: August 04, 2007, 10:56:00 AM »

There are several 1/2 wave mobile antennas out there. Comet SBB-5, AX-75 and AX-95 are all 1/2 wave on 2M, collinear on 70cm. I'm partial to Diamond NR770, but some have reported problems. I use the 770NMO on my car with a trunk lip mount (no good ground), and the UHF mount on a backpack portable setup. Both work very well. I also have a 72" long telescoping antenna for my TH-6a that I extend to a 1/2 wave length when transmitting. Much easier to get a good SWR than with counterpoise or other methods.
Logged
KB0USY
Member

Posts: 33




Ignore
« Reply #11 on: August 05, 2007, 09:16:21 AM »

Mag mounts coming off a a car and hitting some pedestrian or occupant  could kill or maim.This is K0BG's point.(Thanks Allen)
    Buy a Larsen.The best antenna for the money.I have 4 in my family.One time I had one break off at the coil and called Larsen.They sent me a New whip.FREE!!
Logged
KB0USY
Member

Posts: 33




Ignore
« Reply #12 on: August 05, 2007, 09:46:54 AM »

Mag mounts coming off a a car and hitting some pedestrian or occupant  could kill or maim.This is K0BG's point.(Thanks Allen)
    Buy a Larsen.The best antenna for the money.I have 4 in my family.One time I had one break off at the coil and called Larsen.They sent me a New whip.FREE!!
Logged
KE3WD
Member

Posts: 5694




Ignore
« Reply #13 on: August 05, 2007, 05:53:41 PM »

Magmount with two turns of coax wrapped around roof rack isn't going very far when magnet is compromised.

If yer really Anal about it, use some parachute cord.  

This is Mac's point.  

But Mac also operates motorcycle mobile CW...

And used to fly planes with antennas all over 'em.  

And lets not even bring up some of the flotsam and jetsam Mac has floated on the brown and blue waterways.  

Take reasonable precautions, if really concerned about this issue, then punch the roof and mount an antenna or get an antenna mount.  

In the catastrophic failure scenario, even the hard mounted antenna could break loose and become problematic along those line.  

"reasonable"  

Whatever happened to that concept?

Use of magmounts for VHF/UHF grade antennas off and on since somewhere around the late 60s.  

Now, those huge three magnet monstrosities for HF antennas are another story entirely.  Better dig out the rope and straps.  Better yet, punch some holes in the darn vehicle.  Get a trailer mount.  I just break out the bandsaw, hotwrench, tig torch and grinder, fabricate a good mount.  


.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!