Best portable base cb antenna

[CowboyCody]

That's good information, I was wondering about that. Thanks

 

[dd18]

Coils

Homer,
How far below the bottom radiator should the coiled choke go?
As close as possible or within 12".
thanks,
Dave

 

[HomerBB]

As close as possible is best.

 

[W5LZ]

Would a foot really make -that- much difference?
- 'Doc

 

[HomerBB]

Probably not, Doc, however, reply it doesn't matter and sooner or later it doesn't matter at all to folks. It is best done as near as possible given the high voltage on the ends of the dipole are likely to create CMC.
I am not sure of this, but I am thinking that if the coax is radiating for an additional foot on one end it would seem to have taken the length of the dipole to uneven measurement on the one side from the other. So why tune again for the added coax length that radiates when that length can just be choked out where it begins and the antenna remains equal on both sides.
Correct me if I see it wrong. Convince me and I'll believe it.

 

[CowboyCody]

If it is too close wouldnt the high voltage point of the antenna start to saturate/effect the field created by the choke , which may in turn reduce its ability to stop the CMC. Or am I over thinking things or way off base?

 

[Marconi]

Here is a quote from W8JI at: RF in shack with vertical and longwire antennas

A dipole, even though a balanced antenna, also has problems with common mode currents when fed with an unbalanced feedline. Some feedline lengths can cause problems, while other lengths might eliminate need for a balun. There is no universal magic length to minimize common mode currents because the required length to minimize common mode varies with the routing, grounding, and surroundings of the feedline. If we have a specific case, like a vertical feedline hanging in free air from a dipole and running straight down to earth and grounded at Earth's surface, then the magic length is 1/4 or any odd quarter wavelength with no correction for velocity factor required!

I am curious, what does the high voltage ends of a balanced center fed dipole have to do with CMC?

 

[Shockwave]

I just thought I'd share something I noticed using ferrite chokes on the coax at the feedpoint of a gamma matched Yagi. I placed 10 one inch beads on the antenna end of the coax right at the connector and RFI was reduced through the attenuation of common mode currents. First tests were just the PC speakers in the room. Loud enough to cause feedback in them at normal volume levels without the chokes and improved to the point where there was no feedback at full volume but my audio could be faintly heard with the chokes.

At this point I noticed that the scope I'd been using to monitor my transmissions was having problems triggering with a locked trace. It was sensing RF by just having a coil of wire wrapped around the coax to form an inductive pickup to the CMC on the coax. I now had to increase the gain on the vertical amp one click on the scope attenuator to produce the same deflection on the scope. Another good sign that led to more thought.

We want to choke right at the antenna in all cases where the coax is not intended to serve as a counterpoise or become part of the antenna. This would include the vast majority of antennas with some exceptions like the Astroplane for reasons I won't get into here. My next step was to key the transmitter and get just below the antenna with a field strength meter coupled to the coax the same way as the scope was.

When I moved the coil pickup 1/4 wavelength down the coax (away from the antenna) I saw the RF voltage peak on the coax. I immediately wished I had placed another 10 ferrite beads on the coax here before installing the connectors and antenna. None the less I was able to reach the ones at the connector and found that moving 3 of them down 1/4 wavelength gave me the best CMC attenuation.

Now I don't hear anything on the low tech RF detectors (PC speakers) in the room. When I tried to find my carrier on the scope, after a few more clicks increasing the vertical gain, I found a fully modulated signal while my transmitter was unmodulated. It was the local 5KW AM broadcast station 3 miles away! Their signal on the braid of my coax was now stronger than my own CMC inside the shack.

 

[Marconi]

I just thought I'd share something I noticed using ferrite chokes on the coax at the feedpoint of a gamma matched Yagi. I placed 10 one inch beads on the antenna end of the coax right at the connector and RFI was reduced through the attenuation of common mode currents. First tests were just the PC speakers in the room. Loud enough to cause feedback in them at normal volume levels without the chokes and improved to the point where there was no feedback at full volume but my audio could be faintly heard with the chokes.

Since 10 ferrite beads mitigated the problem somewhat, do you think 15 or 20 beads could have made more of a difference?

At this point I noticed that the scope I'd been using to monitor my transmissions was having problems triggering with a locked trace. It was sensing RF by just having a coil of wire wrapped around the coax to form an inductive pickup to the CMC on the coax. I now had to increase the gain on the vertical amp one click on the scope attenuator to produce the same deflection on the scope. Another good sign that led to more thought.

So, this step was as a result of the first fix with 10 ferrite chokes as seen on your scope, right?

We want to choke right at the antenna in all cases where the coax is not intended to serve as a counterpoise or become part of the antenna. This would include the vast majority of antennas with some exceptions like the Astroplane for reasons I won't get into here. My next step was to key the transmitter and get just below the antenna with a field strength meter coupled to the coax the same way as the scope was.

Are you saying the Field Strength Meter was just and inch or so below and adjacent to the coax?

When I moved the coil pickup 1/4 wavelength down the coax (away from the antenna) I saw the RF voltage peak on the coax. I immediately wished I had placed another 10 ferrite beads on the coax here before installing the connectors and antenna. None the less I was able to reach the ones at the connector and found that moving 3 of them down 1/4 wavelength gave me the best CMC attenuation.

I've heard and your test seems to indicated that CB FS meters typically respond to voltage fields and less at current nodes. When I first tested my FS meters I was surprised that is what I've found as well. So, if this is true, and you saw a voltage peak at a 1/4 wavelength down from the feed point, isn't that what you would expect?

As I note in my earlier post, what good is served by placing a common mode choke at a voltage node?

Now I don't hear anything on the low tech RF detectors (PC speakers) in the room. When I tried to find my carrier on the scope, after a few more clicks increasing the vertical gain, I found a fully modulated signal while my transmitter was unmodulated. It was the local 5KW AM broadcast station 3 miles away! Their signal on the braid of my coax was now stronger than my own CMC inside the shack.

I don't understand the specific functions of a scope nor how it works, but how did you determine this AM broadcast station was what your scope indicated?

Did you know this AM station was a problem before you did the ferrite bead fix?

So, was your problem the CMC's you thought, or was it the RF from the AM broadcast station?

 

[Shockwave]

Since 10 ferrite beads mitigated the problem somewhat, do you think 15 or 20 beads could have made more of a difference?

It could have but I was not willing to unseal the coax connector, cut it of to install more beads and reinstall the connector to test that idea. The fact that moving 3 of them 1/4 wavelength down the coax suggests that more at the feedpoint might not be as effective as choking in both places.

So, this step was as a result of the first fix with 10 ferrite chokes as seen on your scope, right?

You are correct.

Are you saying the Field Strength Meter was just and inch or so below and adjacent to the coax?

The field strength meter I used is modified so that it's shielded and the input is through a BNC connector rather than the small screw on whip that is standard. This BNC connector has a loop of wire approximately 4 inches long that I wrap around the coax to measure CMC. Because the pickup coil on the FSM is a short closed loop, it has a low impedance and is far less sensitive to any RF that is not inside the inductive loop pickup.

I've heard and your test seems to indicated that CB FS meters typically respond to voltage fields and less at current nodes. When I first tested my FS meters I was surprised that is what I've found as well. So, if this is true, and you saw a voltage peak at a 1/4 wavelength down from the feed point, isn't that what you would expect? As I note in my earlier post, what good is served by placing a common mode choke at a voltage node?

Seeing the CMC 1/4 wavelength down the coax line is exactly what I would expect in the event there was still noticeable CMC remaining on the coax line. I was hoping not to see any with just one set of chokes at the feedpoint. After moving 3 chokes down the line, I was not able to place the FSM in the exact same spot but just under the choke the meter showed less deflection with the 3 moved to the new spot.

I don't understand the specific functions of a scope nor how it works, but how did you determine this AM broadcast station was what your scope indicated?

Did you know this AM station was a problem before you did the ferrite bead fix?

So, was your problem the CMC's you thought, or was it the RF from the AM broadcast station?

The scope was loosely coupled to the braid of my coax from the antenna. This was just to sample the modulation for microphone settings. I was able to determine it was the local AM station by the waveform on the scope. The signal was always keyed and modulated. It had to be AM broadcast. Just turning on my AM receiver in the same room quickly revealed who's signal it was I was looking at on the scope because each time the announcer paused between words, the scope showed a clean unmodulated carrier until the second he spoke again.

The AM station never was a problem because I never had to turn the gain on the scope up so much to see my own signal before. The 80 feet of coax to my antenna does allow the braid to become a fairly good receiving antenna for standard broadcast and is why I see them on the scope with the vertical gain cranked up.

CMC was reduced as seen on the scope and PC speaker through the use of ferrite beads on the coax at the feedpoint. Testing showed that it can be more effective to choke in both of the places I mention but the priority is at the feedpoint first.