Call Search

New to Ham Radio?
My Profile

Friends Remembered
Survey Question

DX Cluster Spots

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

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
Author Topic: moble ant. swr  (Read 1103 times)

Posts: 1

« on: August 13, 2003, 07:42:36 PM »

i bought a mfj magnetic mount 2-m 70-cm ant and cant get the swr down below about 2:1

Posts: 10

« Reply #1 on: August 13, 2003, 08:11:17 PM »

Hi OM,

I have not used the MFJ mag mount antenna, however with similar units I have found that placement on the vehicle has a great deal to do with the SWR.

Try the center of the top and see what the SWR looks like.  If it is OK there, the antenna is good.  If the SWR stays about the same as, say, on the trunk deck, the antenna may need pruning.

On the other hand, a 2/1 SWR at UHF isn't really that bad, if your rig puts out full power, since the coax run is short.  High SWR simply multiplies the normal loss in the coax.

Try moving up and down the band to see whether the antenna is too long or too short at your chosen frequency.

73 de George/W0AV
Hamming since 1935

Good luck!

73 de George/W0AV

Posts: 3189

« Reply #2 on: August 13, 2003, 09:16:28 PM »

Have you tried the antenna out of the vehicle, say out in the backyard with an HT?

Secondly, are you sure that the metal in the area of the antenna is ALSO electrically connected to other areas of the vehicle?

For example some trunks require a grounding strap to "electrically" connect the truck lid to the body of the vehicle.Similarly, some metal trim on a vehicle may not be "electrically" connected to ground.

Use an OHM meter to determine if you have contunitity in the grounding system.

If your radio isn't properly grounded (not the negative power lead) but from the physical chassis of the radio to the grounded frame of the vehicle.

It is quite possible to have standing waves on your coax line when transmitting if you only rely on the negative power lead for grounding your radio. These type of grounds might run for long distances before actually electrically connect or reach the vehicle's chassis. Therefore in some cases they act more like intentional radiators than actual grounds.

To ensure a good ground to the radio chassis, run a  10ga. cable (negative side of a battery booster cable works best) through the firewall (under dash) and attach it to chassis bolt on the vehicle behind the motor under the hood of the vehicle. Keep the cable run as short as reasonably neccisary. Dont tuck any excess cable under the dash, cut it off instead.

Do not attach the wire on the other side to the hood or to the hinges on the hood. Also be sure the wire will not get heated or be melted by the motor. Keep the wire clear from any parts that would move. And keep the wire away from spark plug wires unless you want a lot of QRM. Lastly, use common sense.

Usually, a heater hose running under your dash will be the best area to "squeeze" the cable through. Heater hoses usually have "putty" around the opening that can be molded by hand after inserting the cable through to keep water leaks out. If that doesn't work search around the steering column for small openings. You may have to peel the carpet back slightly to discover some "hidden" areas.

I would be hard pressed if after that you have an SWR problem.

This will also help reduce any electrical noise generated from today's computer controlled electronics.

Also check your coax connection. sometimes a hairline wire inside your BNC or PL259 connector will maintain the connection to the antenna, but in some cases will resonate the coax as the antenna. This will fool you into believing you are transmitting from the antenna, when in fact it is the coax legnth that is giving you that "close" to 2:1 SWR.

73 Good Luck



Posts: 2193

« Reply #3 on: August 14, 2003, 08:52:37 AM »

I can tell you that the suggestion to ground your radio to the chassis wont do a thing to improve your VSWR at UHF.

Most likely you need to relocate your antenna. Over any assymetrical metal surface (like a car body) there are going to be places that provide a good match and places that dont. Obviously the best place is right in the center of a large, flat section of metal.

I dont know anything about your particular antenna, but if that doesnt work, some antennas have the ability to make the radiating element longer or shorter. Try that too if you can.
Pages: [1]   Go Up
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!