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: Thru glass on a Corvette / SWR issue  (Read 1116 times)
AB5ON
Member

Posts: 36




Ignore
« on: August 09, 2004, 06:00:19 PM »

Mag mount and drilling holes were out of the question.  So I bought a new thru glass 2m/70cm dual band antenna.  The SWR adjusts OK, but ANY amount of movement of the coax makes it go nuts.  What's up with that?   I have found a location behind the seat that seems to work when I coil the coax up tight.  So long as it doesn't move I will be OK.

Any thoughts on what's causing this and how to get it stable??

BTW, I put my Icom 225 in the center console since I have no dash or under dash room available.

73
Rick
Logged
N8EKT
Member

Posts: 371




Ignore
« Reply #1 on: August 09, 2004, 11:27:40 PM »

WHAT BRAND OF ANTENNA?
SOME ON GLASS ANTENNAS HAVE TO HAVE THE COUPLER INSTALLED ONLY ONE WAY.
SECONDLY, KEEP THE ANTENNA AWAY FROM WINDOW DEFROSTERS AND IN AT LEAST 2 INCHES FROM ANY METAL TRIM.
SOME VEHICLES HAVE HEAVY LEAD CONTENT IN THE WINDSHIELD THAT CAUSES PROBLEMS.
LARSEN CLEARLY MAKES THE BEST DUAL BAND ON GLASS.
THE RADIO SHACK, VALOR, AND NUMEROUS OTHER BRANDS OF THE SAME DESIGN WORK POORLY EVEN UNDER THE BEST CONDITIONS.

Logged
AB5ON
Member

Posts: 36




Ignore
« Reply #2 on: August 09, 2004, 11:43:52 PM »

Tnx for the reply.

Don't know the brand.  Picked it up at a Hamfest.

There are defrost lines across the rear hatch where I have the antenna.  But then again the SWR is around 1.2 when it is at it's best.

The coax is running under the trim in front of the rear hatch.  So all the coax within 4 feet of the antenna cannot move and the antenna is not being moved. But if I move the middle of the coax the SWR jumps around.

Could just be a poor design.  I may just bite the bullet and buy a good one.

Logged
K0BG
Member

Posts: 9896


WWW

Ignore
« Reply #3 on: August 10, 2004, 10:17:39 AM »

I'm not surprised with the results you are getting with a glass mount antenna, especially on 2 meters. The best of the best  (Larsen) is still problematic as you have found out.

Larsen does make a loaded half wave for 2 (and 440) which requires no groundplane, but requires that holes be drilled.

About your only alternative is a frame mount and you'll have to make the mount.

Alan, KØBG
www.k0bg.com
Logged

WB2WIK
Member

Posts: 20666




Ignore
« Reply #4 on: August 10, 2004, 01:13:53 PM »

I'm not surprised, either.

I've seen exactly the same thing with most glass-mount antennas, regardless of what kind of car they're installed on.  The problem is these "ground-free" antennas are half-waves with a matching device in the little coupler box, and they're not well decoupled at all.  As such, the coax is pretty "hot" (lots of RF on the outer surface of the braid conductor) and moving the coax around will make everything change.  This is one of the drawbacks of using an end-fed half-wave antenna without an effective choke-type current balun.

One thing that does help a lot is if you coil up the coax about four turns in a 2" diameter, tape that tightly together to hold its form, and locate that coil (which is now a choke balun) right under the antenna feedpoint, inside the glass at the rear window.  Unfortunately, that looks ugly and almost nobody wants to do this!

As K0BG said, the other alternative is to use an antenna that's actually grounded to metal on the car, or has an effective ground plane of some sort.  What I used to do with the 'Vette is use a 1/4-wave whip (19") in the center of the roof (through-hole NMO mount) with four 19" copper wire radials attached to the shield side of the coax at the NMO mount, underneath the fibreglas roof, taped to the fibreglas under the roof, and then completely hidden by the headliner.  You can't see the radials, and they aren't actually connected to any "ground" on the car, but they work very well and make the coax completely "cold," so its position doesn't matter.  This antenna also works about 200% better than the glass mount on the rear window, and costs almost nothing.

To assure a good job with the headliner removal and replacement, I used a local auto upholstery shop for their services.  That cost me a dozen donuts and a few cups of coffee, and they did the work for free.  With the right tools and experience, those guys can get a headliner out, and reinstalled perfectly, in five minutes.

WB2WIK/6
Logged
AA4PB
Member

Posts: 13032




Ignore
« Reply #5 on: August 10, 2004, 04:33:13 PM »

I've also used copper foil tape to make a ground plane on the inside of the fiberglass.
Logged
AB5ON
Member

Posts: 36




Ignore
« Reply #6 on: August 10, 2004, 06:23:28 PM »

"One thing that does help a lot is if you coil up the coax about four turns in a 2" diameter, tape that tightly together to hold its form, and locate that coil (which is now a choke balun) right under the antenna feedpoint, inside the glass at the rear window. Unfortunately, that looks ugly and almost nobody wants to do this! "

************************************************
I tried this down near the radio end with some results, but your idea sounds better.  I may be able to get the coil under the halo trim.  Not sure how much room is there, but it is worth a try.  Do you think a transformer laminated core or a toroid would help?

Now that I think about it, a 2" coil of coax is what I used on a copper pipe 2M J-Pole and it worked fine there.

Thanks for the ideas.
Logged
KB1LKR
Member

Posts: 1898




Ignore
« Reply #7 on: August 11, 2004, 10:01:28 AM »

Would a ferrite bead over the coax help any, in leiu of, or in addition to a coil farther down the coax?
Logged
K0BG
Member

Posts: 9896


WWW

Ignore
« Reply #8 on: August 11, 2004, 05:55:38 PM »

Maybe. It depends on what material you chose, the number of turns, and where in the coax it was located. In short, glass mount antennas are an extreme compromise and their shortcomings are hard to overcome.

Alan, KØBG
www.k0bg.com
Logged

AB5ON
Member

Posts: 36




Ignore
« Reply #9 on: August 11, 2004, 07:56:45 PM »

OK FB re: compromise.  Putting a radio on a plastic car kind of dictates some sort of compromise. :-)

Moving the coil of coax up to the coupler seems to have done the trick.  I re-adjusted the SWR and it is much better now and seems to be stable.  Looks are not TOO bad.  Had to leave some slack in it so the rear hatch will open without putting stress on the coax.

May still try to fabricate a mount from the frame under the rear bumper.

Thanks for all the input guys!

73
Logged
WILLY
Member

Posts: 286




Ignore
« Reply #10 on: August 13, 2004, 12:39:20 PM »


If you become too frustrated by it all, and decide to give up -  contact me, and I'll give you a thousand bucks for the 'Vette.  You don't even have to include the radio!


>g<

Logged
WD4HXG
Member

Posts: 189




Ignore
« Reply #11 on: August 15, 2004, 12:24:33 PM »

Did your antenna have the snap metal clip that fastens to the vehicle sheet metal to insure that the vehicle ground is used. The clip connects the shield at the antenna base coupler to the vehicle. If so and you did not use it you need to find a connection for the ground clip. If the Vette does not have any metal close enough to use for the antenna ground plane then consider using a decorative metalized mylar film on the interior roof of the Vette. A section about 38 by 19 inches will work. Connect to the edge midway along the 38 inch side. Use an adhesive that removes and cleans up easily. Alternatively you can use copper foil tape to realize a dipole on the rear window. Rolls of the tape 1" wide sell for about $30.00.

I have tried both the copper foil tape and the thru glass mount antenna. The dipole was much easier to implement, more stable over the long term and cost significantly less then the commercial antenna. I did not run any measurements with lab gear to determine patterns. One unexpected thing was even with the thru glass antenna adjusted for less then a 1.5:1 return loss the dipole yielded reports of less noise in the audio in fringe areas.

I have not experienced difficulty with coupling to the heater elements with either antenna.

Logged
KC8VUK
Member

Posts: 55




Ignore
« Reply #12 on: August 19, 2004, 03:30:30 AM »

  I put a antenna on my 1996, conv, I went to the right rear frame, as there are holes already there, made me a mount to come back to the back of the body, then bent it up and then flat, so I could put a stick anenna on it, run the coax thru a smzll hole thru the rear wheel well, and run it under the trim, to the bottom of the dash board, then mounted the radio there,worked real good!!!
    Billie KC8VUK
  P.S. Make sure you rtv the small hole where the coax comes thru, no one will be the wiser!!!!
 
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!