Coax length on mag mount question

[Redbeard U812]

I'm out of here.

Here we have a failure to communicate.

 

[Slowmover]

If you guy's are implying that's a ground radial, that is incorrect.

How can it be a a ground radial when it attaches to the hot side of the antenna???

I’m being facetious. A joke about radials with a mag mount.

“Possible”, not the same as realistic.

As it all comes back to not having 18’ of coax in the first place. (Same dead end).

.

 

[Slowmover]

What's a Zing Ring? Does it look cool with 97 chicken lights and chrome?

See thread.

 

[The DB]

I know nothing about them other than having seen them. I know they’re not the same as the 3/8x24 “ground planes” that attached to the hot, and not the same as those that mount above the coil on a Wilson antenna...

Will it act as a ground plane for a CB antenna? No. It will try, however it is far to small. Its simply not large enough for any real currents to develope. Perhaps if they had some form of loading on the "radials" to get them to the right electrical length, but with a radial that short that might be tough, and such loading would be affected by the nearby metal adding capacitance into the mix...

Will it help couple the magnet mount antenna to the metal below? Yes but... With a magnet mount antenna the magnet and the metal below act like a capacitor. The magnet mount is noticeably less than 1/32 of an inch off of the vehicle. A 5 inch diameter circle (from a 5 inch magnet) barely does the job, and as was well documented in the thread above, barely. These "radials" if you will are what, 1.5 to 2 inches above the surface of the vehicle? The further away from the vehicle the less capacitance this device will add, and the drop off is exponential as you move further away from the vehicle. The way to counter this is size, perhaps if they were far larger...

I'm sorry, but I fail to see how this device will make any significant difference at all.


The DB

 

[Slowmover]

Now am serious.
Several sizes from Breedlove.

Attach high quality coax to length.
Bond mobile gear cases, etc

NOW, attach a Texas

 

[2NC995]

A 5 inch diameter circle (from a 5 inch magnet) barely does the job, and as was well documented in the thread above, barely. These "radials" if you will are what, 1.5 to 2 inches above the surface of the vehicle?

I don’t think the presence of an RF issue after shortening the coax is an indication that a magnet barely does the job. I’d actually be extremely surprised if anyone could tell the difference between a magnet mount and a hard mount on the receiving end.

I’ve ran everywhere from barefoot rigs up to 16 transistors, and to be perfectly honest I talked more Dx early on with a single final radio and a magnet mount hustler antenna than I ever have since, and long before I even knew where to buy an amplifier or a decent antenna and mount.

 

[Slowmover]

I know less than nothing . . . and mag mounts create problems, IMO.

Though I’ve used them with success on the basis of a single trip of under a week each time. I expect them to be “different” each time where all else is the same. They are that. Not reliable, (again, IMO).

 

[2NC995]

Now am serious.
Several sizes from Breedlove.

Attach high quality coax to length.
Bond mobile gear cases, etc

NOW, attach a Texas

Holy cow, Slowmover.

I give up.

 

[S&W357]

I took this picture Thanksgiving Day...on hood of son's truck. He's a bear hunter. Said it was a "tracking device extended range antenna" to locate his Hounds Is that what it is? I don't know.

Proof of being Bear Hunter...guy with mustache on right side of pic is my son...one of them!

 

[The DB]

I don’t think the presence of an RF issue after shortening the coax is an indication that a magnet barely does the job. I’d actually be extremely surprised if anyone could tell the difference between a magnet mount and a hard mount on the receiving end.

I said something that largely agrees with your statement back on page 1...

If you have a large enough magnet, this is accurate. But if the body of the vehicle (or whatever part of the vehicle the antenna is mounted) is inadequate to be a ground plane, you can still have currents on the braid, even if you have a better mount than a magnet mount.

I also said in the post you quoted that CMC's form even on solid mounts in vehicles, so we are in agreement that magnet mount antennas aren't the only antennas with such a problem.

What I haven't seen anywhere is a direct comparison between a magnet mount antenna and solid mount version of the same antenna. All I have is a thread from years ago on this forum where we attempted to calculate the capacitance, and by extension the impedance, of a 5 inch diameter magnet mount. It wasn't enough to act like a direct connection, but it wasn't that bad, assuming we were accurate with our calculations.

The problem is the car's chassis normally presents a higher than optimal impedance as it is. The additional impedance from the magnet mount only make that naturally high impedance worse.

Further, as I mentioned in a previous post, a magnet mount antenna can have common mode currents simply because the feed line is outside of the vehicle's chassis as it is run away from the antenna.


The DB

 

[2NC995]

Said it was a "tracking device extended range antenna" to locate his Hounds

That’s a new one. Never seen anything but yagis...hand held or vehicle mounted.

Cool pictures of your son’s hunt!

 

[S&W357]

Yeh, same here. New one on me as well.

 

[Unit 75]

Will it act as a ground plane for a CB antenna? No. It will try, however it is far to small. Its simply not large enough for any real currents to develope. Perhaps if they had some form of loading on the "radials" to get them to the right electrical length, but with a radial that short that might be tough, and such loading would be affected by the nearby metal adding capacitance into the mix...

I believe it being advertised as a "ground plane kit" is technically incorrect, but that is what they call it. DB is correct that the "radials" are too short. Either way, this THING is somewhat acting as a trap by soaking up some of the mismatch current by TRYING to act as a ground plane, albeit poorly, because the radials are not long enough to be 1/4 wave resonant. But knowing that the shield part of the coax is used as a counterpoise and that shield part of the coax is attached to a bracket (inside the Wilson 5000 mag mount base) that connects to the outside of the SO-239 connector that the coil screws on to and those short radials do make contact to the outside of that same SO-239 connector tells you those short radials ARE now a part of that counterpoise. They may not be efficient by any standards but they are still connected.

 

[Road Squawker]

... edit...
The problem is the car's chassis normally presents a higher than optimal impedance as it is...
a magnet mount antenna can have common mode currents simply because the feed line is outside of the vehicle's chassis as it is run away from the antenna.


The DB

What many people miss, is that the vehicles body itself is one plate of a cap....mounted on 4 rubber insulators, the other plate is the actual earth directly below the vehicle.

.. and my mobile (NOT a mag mount) has the feedline choked off with a ferrite right at the feedpoint, outside the vehicles body.

As far as a 5" "counterpoise"being effective, it all comes down to "is it a significant length, compared to a full wave length"?

5" on 27 MHz...... not
5" on 2 meters.... yes

 

[2NC995]

All I have is a thread from years ago on this forum where we attempted to calculate the capacitance, and by extension the impedance, of a 5 inch diameter magnet mount. It wasn't enough to act like a direct connection, but it wasn't that bad, assuming we were accurate with our calculations.

I’ve seen this done elsewhere, I think. You mention accuracy in the calculations which is always going to be the issue. A lot of assumptions have to be made about paint thickness, etc.

I’ve seen a lot of different materials used to replace the vinyl on the bottom also, and I don’t think folks always understand that they’re playing with the values of a capacitor.