High SWR reading with CB radio

Nissan11

Nissan11

Well-Known Member
Joined
Nov 29, 2005
Location
Marston, NC
I have an old galaxy 959 that I had 'peaked and tuned' by a CB shop near Fayetteville last year. I finally got around to mounting it in my truck yesterday. I mounted a new firestick II to the top of the bed rail behind the cab with 1/3 the antenna sticking above the top of the cab. I have struggled and struggled to get the SWR meter to within an acceptable range. With the coax cable going straight from the radio to the antenna and the antenna in my hand held above the top of the cab I get a very low reading which barely measures on the meter. With the antenna anywhere else on the truck I get readings around 3.5.
I tried grounding the aluminum antenna mount with a wire which made no difference. However, intetestingly when I took the plastic isolator from the top of the mount and moved it to the under side I got fairly consistant readings of 2.25.
This does not make sense to me since my understanding is the antenna is not supposed to be grounded to the mount. Can anyone offer any suggestions for ways to get a better reading?
Also, the tip of the antenna is screwed all the way down. The further out the adjuster screw is the higher the SWR reading. If I put the rubber cap back on the antenna the meter jumps from 2.25 to 3.5 which also does not make sense to me

I am attaching pics of the antenna mount.
 

Attachments

  • 20190120_121041.jpg
    20190120_121041.jpg
    56 KB · Views: 297
  • 20190120_121019.jpg
    20190120_121019.jpg
    60.3 KB · Views: 326
  • 20190120_121004.jpg
    20190120_121004.jpg
    58.6 KB · Views: 301
Last edited:
If the antenna you are using is not a NGP antenna with the appropriate coaxial to supply the ground, then the antenna should have a very good ground to the vehicle.
 
Sounds like you have it installed right. So no "loops" in the coax? If so you should zig zag it instead. If you have much slack you can loop it right where it goes into the antennae mount and zip tie it, it can change your readings. If you are on FB try asking here CB radio show and tell !
 
Loops don't affect SWR. Antenna ground needs to be grounded to the chassis, unless it's a ngp antenna with all the matching parts.

Behind the cab like that isn't a good location. The cab affects propagation. But it shouldn't have a huge effect on SWR.
 
I'd scrape some of the bed liner off under the bracket if you could.

At the very least, run a 10 gauge wire from one of those mount bolts to the frame under the bed with ring terminals and star washers. That way you are 100 percent sure you have a good ground.

Sent from my E6910 using Tapatalk
 
I just looked at the pictures again, you need the plastic on top. Heres my setup on my f250. I doubled the bracket as a spacer.

The coax adapter should be grounded to the chassis and the center post should not. If you have an ohm meter you can check that at the cb end of the coax to check the cable and mount. The screw part should be grounded on both ends measuring with the meter, close to 0 ohms. The center pins should not have any resistance (open circuit).

The plastic cap will change the swr. And I've seen a difference if the antenna is too close to sheet metal like the cab. I couldn't get a good swr on my trail rig behind the cab so I moved it back on the exo cage towards the rear. Back there the swr is <1.5
18bc504d4ab4e6fec58c0c891da12a83.jpg


Sent from my E6910 using Tapatalk
 
Doea your plastic isolator go all the way through the mount hole? Mine only goes half way through. You have no plastic part of any kind on the bottom of the mount?
 
How old is the coax? Pretty much every time I have gotten high SWR's I had a bad spot in the coax.
 
There are three different tests with an ohm meter to check the coax. Disconnect both ends of coax. Check continuity between the center of each end. Should have continuity. Check the outer part of both ends the same way. Again should have continuity. Then check between center and outer. Should not have continuity.

With the antenna mounted as it should be (insulator on top) and coax disconnected check for continuity between the bracket and the bottom metal part of antenna just above the insulator. Should not have continuity.

On the bottom of mount, the outer part of coax fitting should have continuity with the bracket but not the antenna.
The center part should only have continuity with bottom of antenna.
 
Blue CJ said:
There are three different tests with an ohm meter to check the coax. Disconnect both ends of coax. Check continuity between the center of each end. Should have continuity. Check the outer part of both ends the same way. Again should have continuity. Then check between center and outer. Should not have continuity.

With the antenna mounted as it should be (insulator on top) and coax disconnected check for continuity between the bracket and the bottom metal part of antenna just above the insulator. Should not have continuity.

On the bottom of mount, the outer part of coax fitting should have continuity with the bracket but not the antenna.
The center part should only have continuity with bottom of antenna.
Click to expand...

There IS continuity between the outer part of coax and the antenna.
What does that mean? All other tests were good.
 
Somewhere in your setup the (most likely in the antenna mount) you have a short. The center part of the coax to the antenna has to be isolated from the ground. Without having hands on, my best guess would be the adapter piece of the mount either has an internal short or is not arranged correctly to prevent the short.
 
Number 1 ... it is a BAD thing to transmit ..... check swr ..... at that power level while holding the antenna. Microwaving your brain.
2- you body is changing the swr. It's called near field effect, among other things.
3-the mount needs to be WELL GROUNDED to the truck bed. Scrape off the bedliner and paint. Wouldn't hurt to ground the bed to the frame.
4-might not hurt to clean the threads on the stud and the antenna ..... they look pretty grunged up.


Now, since the adjuster is all the way down and the swr get worse as you go out, the entire thing is too long. Needs to be shorter.
Try it without the extra nut between the antenna and the stud if it will work then adjust it.
 
Lizooki, first of all you have to transmit in order to measure SWR. It means Standing Wave Ratio. It is the ratio of the power that is transmitted by the radio unit and the amount of power that is sent out by the antenna. An SWR of 1:1 means that all power sent by the radio is transmitted out of the antenna. Any SWR other than 1:1 means that some of the transmitted power is reflected back to the radio and not transmitted by the antenna. You have to key the mike to evaluate SWR. Also, a peaked and tuned radio is still probably well under 10 - 15 watts carrier. No microwave brain problems should occur. Even "big" radios pushing 1000+ watts would not be a problem to be around.
 
Thanks for the help guys. I went back to the shop that worked on the radio and talked to the owner. After a discussion I bought a wilson 1000 magnetic mount antenna from him. With that thing slapped to the roof and with no tuning it BARELY makes the SWR meter needle rise, which is about 0.10
I wish I had another 2ft of coax, though. The 18ft cable barely allows me to run the mount to the very rear of the roof after snaking it from the dash, under the carpet and through a vent in the back of the cab.
 
Nissan11 said:
Thanks for the help guys. I went back to the shop that worked on the radio and talked to the owner. After a discussion I bought a wilson 1000 magnetic mount antenna from him. With that thing slapped to the roof and with no tuning it BARELY makes the SWR meter needle rise, which is about 0.10
I wish I had another 2ft of coax, though. The 18ft cable barely allows me to run the mount to the very rear of the roof after snaking it from the dash, under the carpet and through a vent in the back of the cab.
Click to expand...
That's great to hear! I've heard good thongs about those Wilson 1000.'s
 
Blue CJ said:
Lizooki, first of all you have to transmit in order to measure SWR. It means Standing Wave Ratio. It is the ratio of the power that is transmitted by the radio unit and the amount of power that is sent out by the antenna. An SWR of 1:1 means that all power sent by the radio is transmitted out of the antenna. Any SWR other than 1:1 means that some of the transmitted power is reflected back to the radio and not transmitted by the antenna. You have to key the mike to evaluate SWR. Also, a peaked and tuned radio is still probably well under 10 - 15 watts carrier. No microwave brain problems should occur. Even "big" radios pushing 1000+ watts would not be a problem to be around.
Click to expand...

I'm well aware of SWR stands for ..... and that you have to transmit to measure. Well, not really. I have equipment in my radio room that allows me to measure swr without transmitting.
Go back and read #1 .... you missed a word. Holding. Transmitting RF at any power level while holding an antenna is not a good idea.
Yes, "microwaving your brain" might have been a bit melodramatic, but rf exposure and rf burns are a very real thing.
That's the reason handhelds have coated antennas on them.
And "big" radios pushing 1000 watts aren't the problem, but being exposed to the rf from the antenna is.

BTW- OP done a good thing with the Wilson 1000 ..... good choice!
 
The 18 ft coax will only allow me to reach the very rear of the roof. Is it worth it to get a longer cable so I can reach the center of the roof?
 
Nissan11 said:
The 18 ft coax will only allow me to reach the very rear of the roof. Is it worth it to get a longer cable so I can reach the center of the roof?
Click to expand...
I'd say leave well enough alone. You want the antenna in the center of the vehicle not necessarily the center of the roof, only for the best signal propogation.

Sent from my E6910 using Tapatalk
 
Back
Top