SWR through the roof...help!

Gallowbraid

Gallowbraid

Adventurist
Picked up a Kenwood TM-281A, got everything wired up using a Firestick 2M 1/4 wave antenna (not the 5/8 wave that seems to be more popular). Trying to get everything tuned up and no matter what adjustments I make to the antenna length using the built in adjustment screw the SWR remains off the scale. The antenna is mounted off the bracket for my passenger ditch light.

2m.jpg


I've done the following troubleshooting:

- Double checked all the connections to make sure nothing is shorted, all the connectors check out fine, no shorts in the antenna connection.

- Run an additional grounding wire from the antenna mount to the chassis of the vehicle to make sure there's a good ground.

- Removed the adjustment screw completely to "shorten" the antenna. SWR still off the scale.

Any suggestions? My current thoughts are:

- The mount is hanging off the side of the bracket away from the hood, perhaps there's not enough ground plane?

- The antenna is within 2 inches of the aluminum housing for the ditch light, perhaps there's interference or signal being reflected?
 
Anyone you know locally have a CB that you can use to verify the cable/antenna
install? Maybe you have a bad unit? If no dead short and you have a good ground you shouldn't be endangering their unit by trying . . .

And another after-thought, assume you have the cable installed correctly to the antenna mount - I know the firestik cables that come pre-terminated are very specific in how they get mounted with the nylon washer etc.?
 
zeeeter said:
Anyone you know locally have a CB that you can use to verify the cable/antenna
install? Maybe you have a bad unit? If no dead short and you have a good ground you shouldn't be endangering their unit by trying . . .

And another after-thought, assume you have the cable installed correctly to the antenna mount - I know the firestik cables that come pre-terminated are very specific in how they get mounted with the nylon washer etc.?
Click to expand...

Good suggestion.

Conveniently I also have a CB installed using an identical mount/connector. I'll try swapping them in the morning and see what happens. My CB is tuned to a perfect 1:1 so I know that mount/connection/cable is good. I did use the Firering connectors for both the CB and the 2M connection and set them both up the same way so that *should* be good. The Firering connections are certainly specific, get one thing backwards or leave out a single washer and they simply ground out and won't work.
 
Seems to me you need a bigger ground plane, the antenna should be the highest point on the vehicle. Your signal could reflect back from under the roof and the pillar arm next to it and back towards the antenna.. The antenna is within 2 inches of the aluminum housing for the ditch light that also could be the problem Also how did you tune the CB radio? What did you use? Also remember that the ham radio operates at a high power and freq. then the CB radio. If the mount is tuned for the CB radio great, You had to tune the antenna to the cb and mount, not sure you will get the same results. Remember you are now working on a VHF high band channel so the game of SWR and antenna placement changes. That is why most Law Enforcement vehicles and fire have the antennas on the roof or trunk mounted and clear of obstruction. And run anywhere from 35 watts to 125 watts. Remember buy a cheap antenna and NEVER BE HEARD FROM AGAIN. Just my 2 cents worth.
 
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truck lfit install 022.jpg
If you look at my truck you will see most of the antennas clear metal, I call clean air for a good signal. The antennas on the bumper show a good 3/4 or more of the antenna above the hood of the truck. right side is a Low band 38-54 mhz, the left side has been replaced with a longer Larson antenna for 2 meter ham again 3/4 of the antenna is above the hood and a good 3-4 foot from any metal. And of course the roof mount antenna for VHF 147-174 mhz and side door which is for a 800 mhz radio. All antennas 1.1 except the glass mount is a little higher being on glass. All mounts are MNO from Motorola and cable used is RG142 high power coax except the 800 has it's own special cable. Even the CB antenna shares a MNO mount and the same 142 coax. Get the antenna high and try a mag mount and see what you get. What do you have to loose except a high SWR. Hope I helped a little.
 
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Is the little insulator in place on the 3/8" antenna mount that isolates it from the bracket?
 
Thanks for the suggestions guys. I think the placement is the first thing I'm going to tackle...the antenna is just way too close to that light. I purchased a 5/4 antenna and it arrived today. Using the same wiring and mount placement I received the same results as before. I also added a spring to my CB setup (which was at 1:1) and it is now at 1:3 despite readjusting the antenna 6 ways to Sunday. I did all of this in the dark tonight after cleaning out my shop. I'm going to tackle things tomorrow in the daylight and move the 2M mount back behind the cab with the new antenna clearing the cab.
 
By any chance are you trying to match the SWR in a garage. You need a clear area away from trees,electrical line ect and this may sound funny doors closed.
 
Nope, not in the garage. No power lines around (they're buried), doors closed, trees are about 10 feet away. Without driving a couple miles away from the house I can't get much further than 20 feet away from trees. Just got back from a grocery store run, about to mess around with the antennas, wiring and placement and see what happens. I'll post my results in a bit.
 
A couple things come to mind. What are you using to check SWR? If you are using the same meter that you use for your CB, that is the problem. Meters are built for a specific band of frequencies, and CB is around 27 MHz, where 2M is 144 to 148 MHz. Can't use the same meter, unless it is adjustable/selectable.

The other is that a quarter wave antenna needs a good ground plane under it, and your first location has none. As Richard mentioned, the middle of a steel panel is best. Half wave antennas don't require a ground plane. The ground plane is not the same as electrical ground and is essentially the other half of the antenna.

Also, the CB spring will play havoc with the SWR, because you are effectively changing the length of the antenna, unless there is a direct contact through the spring to the mount.
 
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