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I'm having a hard time with high SWR with my mobile setup...

B

biz

Member
Dec 16, 2009
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Im running an Galaxy 95T2 with a predator 10k on a tri-mag mount on the roof of my F-350.

im measuring with a radio shack swr meter.

after calibrating im getting a real high reading on the swr meter..... well in the red.

but i know this radio has a built in swr circuit and swr meter.... the radio shows 2.5 swr on average.

ive tried moving the whip in 1/2 inch increments as far as it would go in both directions....

The radio is directly wired to the battery both pos and neg.

if any of you are familiar with the bottom on the tri-mag mount... theres a bolt that holds it all together. I put a piece of 10g wire with a ring terminal on that bolt and ran it back to one of the 4 knobs holding the radio on the mount.

I also added another ground on another one of the 4 knobs and ran it to the frame of the truck about 2 feet away under the dash.

having no luck tuning.... i trimmed an inch off the whip and still it wasnt getting any better...

i dont want to trim more and screw myself.... so i thought i would come here and ask what i should do.
 

its all new equipment....

readings on all channels are in the red....

the needle is pinned.
 
It sounds like you've done some good stuff already, like making sure your power supply is good, although that won't help with antenna match. I'd suggest a few things, though.
1) The supplemental ground leads seem like a good idea at first, but I'd remove them. They're long, and will be affected by the presence and motion of things (like people) in the cab. Also, for now, they'll radiate some signal, which can be picked up by the Field Strength antenna stub on your Radio Shack meter, which will throw off your readings.
2) Ohm out your coax. Remove the antenna from the mount. Make sure you've got zero ohms from the center pin of the PL-259 to the antenna mount stud, zero ohms from the PL-259 shell to the frame of the mag mount, and infinite ohms (no connection) between the PL-259 center pin and shell). It's easy for coax to get pinched and stretched inside the jacket or plug and never show on the outside, but it will show up as bad SWR.
3) Do you hear other stations with the current setup, besides a handheld in the cab or another mobile parked next to you, i.e., is RX OK, but just TX seems bad?
4) Do you have another antenna you can put in this mount to try out? It would be nice to see if the problem follows the new Predator or the rest of the system.
5) Do both SWR meters agree? Also, in case the outboard one is messed up at all, try this: instead of using the FWD/REV switch as usual, go ahead and do a CAL with the meter set on FWD. Unkey, and take the meter out of the line. Now reverse the connections, and put it in backwards without touching the adjustment knob or the FWD/REV knob. Key up the radio and read the SWR. Whatever it reads now is more likely to be accurate. Note: This also works to use CB meters for VHF & UHF, too.... cool :)
5a.) You don't have your finger on the Signal Strength antenna stud, or have the antenna mounted when you do these tests, do you? The signal will swamp the meter.... even without the little teensy antenna, if your finger is on the place where it mounts, you'll couple in enough RF to do the same thing.
6) Try placing the whole mount and antenna in different places on the pickup (not driving, just sitting still) -- like the middle of the hood, middle of the bed with tailgate both open and closed (if there's no shell), middle of one door or the other (with both doors closed). Low band (especially as low as 27MHz) is hugely dependent on ground plane, and anything less than about 8 feet in every direction from the feed point of the antenna is a compromise. So every nearly *every* land vehicle will be a compromise, and we must make the best of it. Magnetic mounts count on capacitance to couple to ground, and this is part of the tuning itself.
7) Here's a cheap way to experiment with the length. Take a foot or so of one of the conductors out of some 12-2 or 14-2 Romex house wiring, the solid stuff with plastic jacket. Strip about 2 inches from one end, and coil it pretty tightly around the top inch or so of the highest part of the Predator. Stick the rest of it straight up. Since it's solid copper wire, it'll do that. Adjust the "official" part of the antenna 4 or 5 inches shorter. Now put the antenna where you want it. It's probably way too long, but do a quick SWR test. Now, though, you can snip merrily, 1/4-in or 1/2-in at a time, much faster and without wearing out some set screw, until you get down to where it's resonant. Measure the whole thing, take the copper wire off, set the "official" adjustment to that length, and you should be good.
If the coax is good.
And you've taken the extra grounds out of the cab.
HTH,
-- Zygoma --
 
double check the mount for proper assembly and id also try another external meter to see if it has the same type of readings . never use coax with the molded 259's on it .

you have a dead short or a faulty meter or no ground .

ive never herd of ohming out coax , but ive herd of checking it for contiunuty with a multimeter . is that the same thing ? even if it checks out it can still be dammaged from being pinched or turned too tightly in its installation . try another run of coax just to check .
 
Ohming out coax is the same as checking continuity in the center conductor and the shield. Also means checking for any leakage between center and shield.

On my quad mag mount with my P10K, the bottom of the center bolt was close enough (without actually touching) that it was causing am SWR problem.
I added longer bolts with a couple off washers between the magnets and the brackets and also used liquid electrical tape to coat the bolt as added insulation.
SWR is now flat all the way across.
 
the tri-mag mount had the coax and the connector already on there when it came...

seemed like i was recieving ok....

you said "You don't have your finger on the Signal Strength antenna stud, or have the antenna mounted when you do these tests, do you?"

how would you test swr without it mounted?
 
ok so heres the deal...

I lowered the output power so my swr meter would be readable and i determined that my antenna was tall...

so I started trimming untill my numbers were closer together and now the best i can get it to is this point.... i had to hack off a total of 6 inches.

I used 2 meters... the one on the radio and the radio shack meter, the one i have is without the little stubby antenna.


Channel 1 readings : 3.4 on the meter , 1.6 on the radio.

Channel 19 readings : 3.5 on the meter , 1.5 on the radio.

Channel 40 readings : 3.5 on the meter, 1.5 on the radio.


I was using a 2 foot jumper between the radio and the meter.

when i disconnected the meter and checked the swr on the radio it was pretty much 2.0 ish across.

I ohmed out the wires by themselves and all connected to the mount/antenna and everytihng checks.

I disconnected the ground from the stud in the center of the mount for these tests. theres still the short ground from the radio to the frame behind the dash connected.

now what.
 
I don't like using any cable that short. The make a lot of sharp right turns to get between those two points. Three feet is the shortest I use. Do you have another cable to use?

You had to cut 6 inches off of your antenna to get the SWR close?
What frequency were you on?
 
If the radio is showing a SWR of 1.4 and 1.5 - but the external meter doesn't show the same - I would beg/borrow/but not steal another SWR meter and jumper to check your system. To be sure...

Whe I had that radio over here, it showed a 1.3 on the radio that agreed with my Dosy meter's SWR reading. But I would NEVER take a chance with a bad SWR. I would go out of my way to make sure first - as you are doing.
 

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