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Mobile 102 Inch Whip w/spring
- Thread starter GLM927
- Start date
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- Tags
- cb antenna cb antennas triple
As an example, Larsen uses Teflon strip on the bottom of their mags. Others use whatever is cheap that month. Ground straps work for me and I'm not bored.
A few notes on brandon7861 and SuperLid's conversation.
Starting with Brandon's side, I just looked up his numbers for the capacitance of a triple magnet mount, and assuming the triple magnet mount has three inch magnets, they are close to accurate (if anything I would say he rounded the surface area up to some degree).
Also, talking about the ground straps coming from the magnet mount, he is right, there will be some additional (I would call it stray) parallel capacitance, and he even understands the difference between series and parallel in this case, and is using it correctly. This is something a lot of people have trouble with. I wouldn't consider the amount that he calculates to be significant, as it is well within the tuning range of most antennas, but it will exist.
One area I would argue against what he is saying is the power limits. In many cases, running 700 plus watts is possible, but I have also seen installations that a little over 100 watts has arced through the paint. Perhaps there were defects in the paint? Or perhaps the magnetic dust that accumulates under the magnet and swirls around dug into the paint to that degree? Or maybe some other cause entirely? I'm not sure, but in any case I hesitate to recommend more than 100 watts through a magnet mount setup.
The other area I would differ with him on is to say that most magnets used are barely adequate for CB use. You really do need to get to a triple magnet mount to be adequate. The one exception to this would be Sirio's Antenna MAG 145 PL.
Now for SuperLid's side...
Why would you even run grounding straps on a magnet mount? Doesn't that defeat the whole purpose of said magnet mount? Even if you can get some increase in performance, at this point you may as well go ahead and drill a hole for a permanent mount as you will either have to do so for the ground straps, or they will be so long as to be counterproductive anyway, especially if you have three of them running in three different directions... Perhaps I'm visualizing what you are saying wrong, but the way it was written, its not surprising that you are getting push back, and I'm surprised that it is so little at that.
Also, they do make conductive adhesive, and some companies even advertise it for their metal tapes and films, so I wouldn't worry about the adhesive on such mounts being non-conductive.
Capacitive coupling is a powerful tool if used properly. If you have enough capacitance, adding ground straps will make no difference, and in fact can be counter productive. The quad magnet mounts with four 4 inch diameter magnets will do very well surprisingly far into the HF spectrum, to the point that you will have far more losses having to deal with an antenna that has been shortened to a significant degree just to fit in a mobile setting...
The DB
Starting with Brandon's side, I just looked up his numbers for the capacitance of a triple magnet mount, and assuming the triple magnet mount has three inch magnets, they are close to accurate (if anything I would say he rounded the surface area up to some degree).
Also, talking about the ground straps coming from the magnet mount, he is right, there will be some additional (I would call it stray) parallel capacitance, and he even understands the difference between series and parallel in this case, and is using it correctly. This is something a lot of people have trouble with. I wouldn't consider the amount that he calculates to be significant, as it is well within the tuning range of most antennas, but it will exist.
One area I would argue against what he is saying is the power limits. In many cases, running 700 plus watts is possible, but I have also seen installations that a little over 100 watts has arced through the paint. Perhaps there were defects in the paint? Or perhaps the magnetic dust that accumulates under the magnet and swirls around dug into the paint to that degree? Or maybe some other cause entirely? I'm not sure, but in any case I hesitate to recommend more than 100 watts through a magnet mount setup.
The other area I would differ with him on is to say that most magnets used are barely adequate for CB use. You really do need to get to a triple magnet mount to be adequate. The one exception to this would be Sirio's Antenna MAG 145 PL.
Now for SuperLid's side...
Why would you even run grounding straps on a magnet mount? Doesn't that defeat the whole purpose of said magnet mount? Even if you can get some increase in performance, at this point you may as well go ahead and drill a hole for a permanent mount as you will either have to do so for the ground straps, or they will be so long as to be counterproductive anyway, especially if you have three of them running in three different directions... Perhaps I'm visualizing what you are saying wrong, but the way it was written, its not surprising that you are getting push back, and I'm surprised that it is so little at that.
Also, they do make conductive adhesive, and some companies even advertise it for their metal tapes and films, so I wouldn't worry about the adhesive on such mounts being non-conductive.
Capacitive coupling is a powerful tool if used properly. If you have enough capacitance, adding ground straps will make no difference, and in fact can be counter productive. The quad magnet mounts with four 4 inch diameter magnets will do very well surprisingly far into the HF spectrum, to the point that you will have far more losses having to deal with an antenna that has been shortened to a significant degree just to fit in a mobile setting...
The DB
Is it possible that when you had 100w arc through your paint that either the paint was thicker or the conductivity to one of the foils was bad? Any decrease in capacitance will increase the reactance, and the more reactance there is, the more voltage will be across it. That 60 micron figure was an average based on a quick search, but paint can be thicker. I just wonder if the situation you experienced had an underlying cause or if that can happen under ideal conditions due to something I am overlooking.
I know you have modeled more antennas than I have ever even seen, so I really appreciate your thoughts on it. I am tempted to put my 8 pill on my mag mount this spring when the ice melts off the truck, but I think I will test the capacitance and put some HV DC across it before I give it any RF.
I hadn't really considered the voltage on a capacitor changing with the changes in dielectric thickness. When it comes to the effect of inductance and capacitance, my knowledge comes more from studying their effects on the antenna directly rather than working with the devices themselves.
As far as thickness goes, while the voltage goes up, the amount of voltage needed to ark through more dielectric would also go up. I simply don't know which would go up faster in such a situation.
This is also assuming the thickness of the paint is even under said magnet. If it doesn't have an even thickness, will there be enough of a variation for the thicker part to drive up the voltage enough for the thinner part to be susceptible to such arc through? I would think offhand that that would require more of a difference than I would expect to see in an automotive paint job, but that is just speculation on my part, I am not an auto paint expert.
In the end I just don't know for sure why it happened, so speculation is all I really have. It happened in my earlier days in the hobby, literally several decades ago at this point, so I really didn't know any better at the time, and I definitely don't remember all of the details now. All I can say is I also know a guy that runs 1500 watts through a triple magnet mount on the trunk of a car, a manually adjustable Wolf River coil, and a 102" whip with great success to 40 meters (and perhaps beyond?). That being said, he doesn't just leave the magnet mount on the vehicle when he isn't using it either, and he is the type of guy to take good care of the car and the mount so as to not have any kinds of problems...
I guess in the end it is a matter of if the person running said magnet mount is willing to properly take care of it, or if they plan to let it sit in the same place for years while the magnetic dust that collects underneath causes more damage to the vehicle than drilling a hole would... Unfortunately I've seen more than my share of the latter, so I assume that is the case until the person demonstrates otherwise...
The DB
As far as thickness goes, while the voltage goes up, the amount of voltage needed to ark through more dielectric would also go up. I simply don't know which would go up faster in such a situation.
This is also assuming the thickness of the paint is even under said magnet. If it doesn't have an even thickness, will there be enough of a variation for the thicker part to drive up the voltage enough for the thinner part to be susceptible to such arc through? I would think offhand that that would require more of a difference than I would expect to see in an automotive paint job, but that is just speculation on my part, I am not an auto paint expert.
In the end I just don't know for sure why it happened, so speculation is all I really have. It happened in my earlier days in the hobby, literally several decades ago at this point, so I really didn't know any better at the time, and I definitely don't remember all of the details now. All I can say is I also know a guy that runs 1500 watts through a triple magnet mount on the trunk of a car, a manually adjustable Wolf River coil, and a 102" whip with great success to 40 meters (and perhaps beyond?). That being said, he doesn't just leave the magnet mount on the vehicle when he isn't using it either, and he is the type of guy to take good care of the car and the mount so as to not have any kinds of problems...
I guess in the end it is a matter of if the person running said magnet mount is willing to properly take care of it, or if they plan to let it sit in the same place for years while the magnetic dust that collects underneath causes more damage to the vehicle than drilling a hole would... Unfortunately I've seen more than my share of the latter, so I assume that is the case until the person demonstrates otherwise...
The DB
It is because I didn't want to drill holes in my roof. The installation was considered temporary to me.
Tarheel antennas makes a magnetic mount with (6) magnets on it & it will hold a full size screwdriver antenna in place going down the highway. They are Not CHEAP but they are beyond Serious for a magnet mount. They don't advertise the mount so you must communicate with them by email if you think you can afford it & are Man Enough that you can remove it if needed. It's a BEAST for a Serious antenna.
Tarheel doesn't build those mounts anymore. I've looked over their website and contacted them. I know Dan (Mr coily) builds a beautiful automatic 4 magnet tilt and raise mount with remote.. as of right now no to many carry any more then a 3 or 4 magnet mount. Trust me I've been looking.
They had three mag mounts on Fleabay a few months back. The mounts are good, but the feed through us garbage and needs to be redone.
They brought one to Orlando Hamcation several years ago & they said they sold very few of them but some did take advantage of them. I'm putting a LIL TARHEEL HP on my pickup this coming week so at 1 lb 9 ozs I don't think I'll need one. I'm replacing the Four Hustler antennas, 6 meter 1/4 wave Larsen ,Duplexer, LDG RT-11 automatic tuner because I miss having all of the bands & 6 meters on one antenna & I never get on 75 meters from my mobile even when I had antennas for it. I hate having the ICOM 7000 not talk where I want to talk when I want to talk. I had a TARHEEL II several years ago & I was sorry that I sold it but a very good friend has the LIL TARHEEL HP for a great price & I have the extra Cash so I'm buying it. {:>) Best wishes on your continued search.