Mag mount SWR problem with extra cable

[ModelTrainGeek]

I picked up a used CB yesterday cheap with a magnetic mount car antenna, then got a SWR meter today. and have th antenna on the balcony rail of me apartment.

If the coax from the antenna is connected directly to the SWR meter I get a usable match.. about 1:1.5, but if I add a extra cable (that I have going out though a a small hole by the door beside that satellite cable) the SWR goes up. about 3.5 or 4 on channel one, and about 2:1 on 40, is right at 3. I have adjusted the whip on the antenna out as far as it goes and still stays in.

So what am I over looking that causing the SWR to go bad? I have checked for shorts or opens on the line, and using a good coupler to connect the cables. and ideas?

thanks

 

[datainmotion]

I'm sure someone will come along and give the technical details, but most pre-cut lengths of coax (whether attached to a mag-mount or just the coax and base) are a specific length for a reason. That reason is that the coax is actually part of the electrical length of the antenna system as a whole.

If you start adding to or removing from a pre-set length, your SWR will be high no matter how much you adjust the mast portion of tha antenna.

 

[jim202]

Most of the antennas that are being used as a mag mount require the metal of the roof of the vehicle
that they are mounted on as part of the antenna. Basically the roof is the ground reflector part of
the antenna. By mounting it on just the rail of the balcony, your not providing that much needed
part of the antenna to function. Who knows what the SWR will measure.

Jim

I picked up a used CB yesterday cheap with a magnetic mount car antenna, then got a SWR meter today. and have th antenna on the balcony rail of me apartment.

If the coax from the antenna is connected directly to the SWR meter I get a usable match.. about 1:1.5, but if I add a extra cable (that I have going out though a a small hole by the door beside that satellite cable) the SWR goes up. about 3.5 or 4 on channel one, and about 2:1 on 40, is right at 3. I have adjusted the whip on the antenna out as far as it goes and still stays in.

So what am I over looking that causing the SWR to go bad? I have checked for shorts or opens on the line, and using a good coupler to connect the cables. and ideas?

thanks

 

[Drowsiness]

Most of the antennas that are being used as a mag mount require the metal of the roof of the vehicle
that they are mounted on as part of the antenna. Basically the roof is the ground reflector part of
the antenna. By mounting it on just the rail of the balcony, your not providing that much needed
part of the antenna to function. Who knows what the SWR will measure.

Jim

That.

Do you have a bunch of extra cable when you attach the extension?

 

[ModelTrainGeek]

That.

Do you have a bunch of extra cable when you attach the extension?

Yes, the extra wire is mainly to have it to go outside through a small hole and hook up the antenna, so the coax on the antenna itself is original and uncut. Without the extra coax, the cable on the antenna just reaches the desk I have the radio on, but then I can't fully close the patio door letting bug and weather in.

 

[GregFromOregon]

You say the coax on the ant. is uncut, but I'm wondering about the cable you have going through a "small" hole. did you cut the end off so you could put it through a hole and the crimp a new connecter on? Or are you closing the door on it in the door jam? Either of those could be the culprit. Since you need some metel for reflection and your cable is to short, maybe try a small metal 30" round patio table that you can get closer to your desk so you don't need the extra piece of coax. Just a thought.

 

[Drowsiness]

You say the coax on the ant. is uncut, but I'm wondering about the cable you have going through a "small" hole. did you cut the end off so you could put it through a hole and the crimp a new connecter on? Or are you closing the door on it in the door jam? Either of those could be the culprit. Since you need some metel for reflection and your cable is to short, maybe try a small metal 30" round patio table that you can get closer to your desk so you don't need the extra piece of coax. Just a thought.

Good thinking there.

My concern was too many coils in any excess slack he may have. Sometimes people buy too much and think "I will just bunch it up here" or they coil it all up and tie it together. Unless things have changed in the last few years, I was taught long ago to avoid doing that.

Just a thought.

 

[NE1C4NSC4N]

Good thinking there.

My concern was too many coils in any excess slack he may have. Sometimes people buy too much and think "I will just bunch it up here" or they coil it all up and tie it together. Unless things have changed in the last few years, I was taught long ago to avoid doing that.

Just a thought.

Not sure if its the same for Radio/CB, but when using Coax in a TV or CCTV application, its ok to have a loop/coil of exceess in the cable, the one thing suggested is to make the loop diameter at least 12 inches, so the cable isnt bent too much..but that may only apply for TV use.

 

[w8jjr]

Try trimming the second piece of coax to the same length the coax on the antenna. Lots of
the cb antennas have coax precut to 9' lengths. Many times you need to add lengths by 9' or
the precut coax length. 9' - 18' - 27' ect.

 

[k8mcn]

ok, assuming this is a non phased single band mag mount antenna, the length of your coax makes no difference on the SWR if: The coax is 50 ohm coax with good connectors and no breaks---------run a length of coax to a 50 ohm dummy load and the SWR will be 1:1 or close IF the coax is good. If you take an antenna analyzer up to the top of a 100 foot tower and check the SWR, its good, then go to the other end of the 100 foot run of coax, it will STILL be good. I am talking about SWR only--yes you will get loss etc, but the SWR will stay the same assuming all integral parts of the path are stable.

I can almost bet that there some thing wrong ( bad connection, short), in the extra piece of coax--eliminate that and you will be good

 

[ModelTrainGeek]

Thanks for all the replies..

The coax is not be squished in the patio door in any way. it is going though a hole with another line to my satellite dish. whoever made the hole in the first place me it way bigger then needed, so I'm taking advantage of it..

The coax I'm trying to use is cut down from an older 50' cable.. I'd have to measure it to see how long it really it, but off hand I'd say about 15 feet. I had another coax there at first with the same problem, bit it was a fair bit longer 25 feet apx.
It's not looped or anything.. it goes from the desk, down to the floor and behind some furniture then out the hole where I soldered on a new connector, and I checked for shorts, none..

I tried the loax as it is now with a dummy load, and got a 1.4 reading,

I'm wondering it it's somethingto do with the fact it's such a short antenna. It stands about 19.5" tallfrom botton to tip. withe few spirals in the middle, so it probably had a big loading coil in the base.

 

[magicjellybeans]

Extra coax = ng

I'd say the extra coax is bad if when you add it the SWR goes insanely high!
Try using like a k40 on a stop sign ,keep the antenna away from any metal objects...
CB antennas need to be far away from your building.try putting it on a pole if you cant get it away from the building....

 

[KAM101]

The OP says he attached the antenna to his balcony rail so I'm guessing the top of the antenna is too close to the balcony above, that may give a slightly higher SWR reading.

 

[ModelTrainGeek]

The OP says he attached the antenna to his balcony rail so I'm guessing the top of the antenna is too close to the balcony above, that may give a slightly higher SWR reading.

There's nothing above it, I'm on the top floor. The only thing I've been changing in adding the extra cable in line, I've use 2 different coaxes, both RG-58, so the only thing I can think of is either I have two bad cables, or maybe there's a problems with the PL-259 connectors I bought that don't show as a short..

 

[KAM101]

If your coax doesn't have any shorts, my next guess is the coax length , there's some good reading here:
The Ultimate Guide to 11 Meter CB Antennas
it's a controversial subject, some people say size does matter, some say it doesn't
From my own experience I have to agree with w8jjr, I also had some very high SWR readings with odd lengths of coax.

 

[ModelTrainGeek]

Solved !

I solved the SWR problem tonight!

I painstakingly redid the PL259 on the coax going out to the balcony, not change.. The decided to make up a new patch cable to put between the radio and SWR meter, bingo. No more SWR problem. Even though the bad one showed no shorts or opens, it must have not had good enough contact on the shield. So when I have time, I'll de-solder the bad cable, clean up the PL259's and see if I can rebuild it.

Thanks to all that replied.