Touching coax cable connection

[Limeybastard]

I just noticed if i touch the coax metal outer threaded area the RX seems to improve. The 2970 radio has about a foot of coax hanging out the back which is connected to a Wilson 1000 magmount to the roof of my pickup. If the connection between the two coax is touched or if I move it to touch an area of thr vehicle chassis rx improves. Normal? Or do I need to ground my coax further? Cheers

Edit. Perhaps this behavior is just the nature of magmount and their grounding not being that great?

 

[fourstringburn]

Magmount…should only be used for temporary installs.

They provide inadequate ground and in an minor accident it will fly off and could cause injury to someone.

Never had RX improve by touching coax connectors, magmount over painted surfaces makes a poor ground, sounds plausible.

 

[Limeybastard]

Magmount…should only be used for temporary installs.

They provide inadequate ground and in an minor accident it will fly off and could cause injury to someone.

Never had RX improve by touching coax connectors, magmount over painted surfaces makes a poor ground, sounds plausible.

Yeah, good point about it flying off in an accident.

So was I right in thinking that a not so ideal ground would be the reason for what im seeing?

 

[vkrules]

Yeah, good point about it flying off in an accident.

So was I right in thinking that a not so ideal ground would be the reason for what im seeing?

Yes thats the problem. The coax is acting as a counterpoise and is radiating. Look up
CMC ( common mode current ).A good earth connection will most likely fix it.

 

[fourstringburn]

Many people on this forum use a "Puck mount" by Breedlove. I use breed love products for different applications. Their products aren't cheap but they are very well made and the best out there for mobile mounts.

https://www.breedlovemounts.com/cb-antenna-puck-mount2.html

Do it right...Drill a hole!

 

[DainBramage]

...so was I right in thinking that a not so ideal ground would be the reason for what im seeing?

Close, but no cigar.........................

think about how "touch screen" controls work.

 

[w9cll]

Their products aren't cheap but they are very well made and the best out there for mobile mounts.

By far the best you can get.

 

[Limeybastard]

I read http://www.k0bg.com/common.html pretty interesting.

The metal pl259 connector is what I'm touching , effectively grounding it further when I notice a difference in my received signal.

The threaded part of that connection is if I'm not mistaken attached to the outer copper part of the antenna coax. So in essence isnt this behavior normal? Isn't it similar to when touching a TV antenna oje sees an improvement nin reception perhaps?

I guess my question is , if I'm not wrong in my above assumptions that a better ground ( plane ) would eliminate this symptom or minimize it further?

Should I prevent the pl259 from restinf on a chassis part inside vehicle?

 

[M0GVZ]

The threaded part of that connection is if I'm not mistaken attached to the outer copper part of the antenna coax. So in essence isnt this behavior normal?

No. The WANTED RF flows on the outside of the centre conductor and the inner of the braid. Unwanted RF, aka common mode RF, flows on the outside of the braid. That common mode RF causes noise on RX and distortion and other issues on transmit. When you touch the connector you provide a path for the common mode RFI which means it doesn't go into the receiver so you get a better receive.

If touching the coax or radio, changing the coax length etc alters something whether that be SWR or receive noise then you've got an inadequate RF ground.

 

[Limeybastard]

No. The WANTED RF flows on the outside of the centre conductor and the inner of the braid. Unwanted RF, aka common mode RF, flows on the outside of the braid. That common mode RF causes noise on RX and distortion and other issues on transmit. When you touch the connector you provide a path for the common mode RFI which means it doesn't go into the receiver so you get a better receive.

If touching the coax or radio, changing the coax length etc alters something whether that be SWR or receive noise then you've got an inadequate RF ground.

Cheers for your kind explanation, appreciate it. All the above was jn receive mode and touching the case didnt change anything, just touching the pl259 connection.

The coax is a Wilson 1000 mag, fixed ( length)coax.

 

[The_Tinman]

I generally do like additional ground loops, especially in vehicles due to more possible noise. However, the real world sometimes dictates experimenting in particular situations. Try running a wire from your radio case to a good vehicle chassis ground point. Check the SWR and noise floor. If it improves then you have a fix. If it does not improve then you were not a ground source but rather another capacitance coupler. Then change your antenna.

Your comment about touching the radio case changes nothing leads me to think you are just another capacitor.

Good luck.

 

[Road Squawker]

... you were not a ground source but rather another capacitance coupler... you are just another capacitor.

Good luck.

BINGO!

 

[Limeybastard]

I generally do like additional ground loops, especially in vehicles due to more possible noise. However, the real world sometimes dictates experimenting in particular situations. Try running a wire from your radio case to a good vehicle chassis ground point. Check the SWR and noise floor. If it improves then you have a fix. If it does not improve then you were not a ground source but rather another capacitance coupler. Then change your antenna.

Your comment about touching the radio case changes nothing leads me to think you are just another capacitor.

Good luck.

Maybe the paint on the radio case is the reason it doesn't change anything?

Could I not just tie a wire around the pl259 connection and tie other end on the metal of the vehicle?

Bearing in mind the pl259 connection I'm touching is not on the back of the radio but approx 1 foot away due to it having that length of coax pre wired at factory. Basically the radio has a foot of coax hanging out of the back.

I believe when the pl259 rests on vehicle metal it gives me similar results as to when i touch it.

 

[The_Tinman]

Maybe the paint on the radio case is the reason it doesn't change anything?

Could I not just tie a wire around the pl259 connection and tie other end on the metal of the vehicle?

Bearing in mind the pl259 connection I'm touching is not on the back of the radio but approx 1 foot away due to it having that length of coax pre wired at factory. Basically the radio has a foot of coax hanging out of the back.

I believe when the pl259 rests on vehicle metal it gives me similar results as to when i touch it.

Maybe the paint on the radio case is the reason it doesn't change anything?

Could I not just tie a wire around the pl259 connection and tie other end on the metal of the vehicle?

Bearing in mind the pl259 connection I'm touching is not on the back of the radio but approx 1 foot away due to it having that length of coax pre wired at factory. Basically the radio has a foot of coax hanging out of the back.

I believe when the pl259 rests on vehicle metal it gives me similar results as to when i touch it.

1st let me correct my post. I generally do NOT like additional ground loops.

On to your questions. You should touch the radio on the back plate. It is metal and the coax connector attaches to this plate (typically). That is why it was confusing that nothing happened when you touched the radio cabinet. It should be the same as the coax.

You could try many things but without finding the problem. I can’t help but think that your coax is faulty and not making a good connection. But then, I try the easy first. If all of the coax is good and the connections are good then you are just adding to the capacitive coupling of the mag mount when you touch it. As I said, nothing is more proven than a particular situation in the real word. If grounding the connector in some way eliminates the problem, your SWR stays the same or improves, and your TX is good then do it.

 

[Limeybastard]

1st let me correct my post. I generally do NOT like additional ground loops.

On to your questions. You should touch the radio on the back plate. It is metal and the coax connector attaches to this plate (typically). That is why it was confusing that nothing happened when you touched the radio cabinet. It should be the same as the coax.

You could try many things but without finding the problem. I can’t help but think that your coax is faulty and not making a good connection. But then, I try the easy first. If all of the coax is good and the connections are good then you are just adding to the capacitive coupling of the mag mount when you touch it. As I said, nothing is more proven than a particular situation in the real word. If grounding the connector in some way eliminates the problem, your SWR stays the same or improves, and your TX is good then do it.

Ill try the grounding approach, good point about the back of the radio, yes its bare metal and will probably behave the same way as you mentioned.

My current swr is very good, hypothetically speaking, if my swr gets worse if I ground what would that indicate?

Im thinking it might be the factory protective pad under the magmount plus my additional paint protection film applied to the vehicle roof panel , but the quantity is half meter x half meter clear film. Doubt that is the problem.