coil height on mobile antennas ????

[BOOTY MONSTER]

its pretty much generally said/accepted that if a coil must be used on a mobile antenna that the higher the coil the better . its also said that a coil forces or directs more current or signal up the antenna above that coil ........ if those two statements are true then the higher coil is forcing more signal into a shorter stinger . so even though the antenna may be the same overall height and the coil design (diameter/length , # of wraps and spacing) may be the same moving it up it supposed to be better .

the thing that im not getting is that it doesn't make sense to me that if the coil is forcing more signal into the section of the antenna above it that having more stinger/antenna above the coil gives more length for that signal to radiate . and if the coil forcing more signal up the antenna is true doesn't that mean there is less signal below the coil ....... so a top or near the top loaded coil reduces the efficiency of the shaft/antenna below the coil but but the few feet above it is stronger .

do coils really force more signal up the antenna ...... or do they just simply shorten the antenna ?

 

[GnG8d]

...... or do they just simply shorten the antenna ?

Said the way you said it, this sounds logical.

 

[Robb]

Top load coils have no stinger above them

The Ultimate Guide to 11 Meter CB Antennas

 

[W5LZ]

Before you get too wrapped up in that "forcing the current higher", think of it just raising the 'peak current point' of the radiated signal higher in the antenna. It isn't 'robbing' any current from lower, and it's not 'forcing' anything into any part of the antenna higher than the coil. There's no change in the total amount of current being radiated, it's still the same. But since the 'peak' of that current wave is further away from the base of the antenna it doesn't react as much with the typical metal around the base of the antenna. That reaction with the stuff around the bottom of that antenna means that slightly more signal/current get's radiated. That's why a top loaded antenna does slightly better than a center loaded, or base loaded one. And before you 'read' too much into that coil positioning, the differences between them are -very- slight. If you aren't specifically looking -very- hard, you'll never notice the differences.
That any help?
- 'Doc

 

[BOOTY MONSTER]

most of the top loaded mobile cb antennas are fiberglass shafts with wire wrapped around them in a spiral with the spiral tightening up at the top and then a screw or nail for a little fine tuning . the goldenrod 45 was a metal shaft with a coil at the top , but it had a lot of length in the top hat that the two loops made , so was it actually a mid-coil ?
http://www.firecommunications.com/gr45.pdf
anyway , comments on coils are interesting . they run from them almost having mystical abilities to enhance tx and rx to that they can only reduce the performance compared to a full 1/4 WL design .

 

[WR0220]

A loading coil is used on a shortened antenna to cancel out capacitive reactance so the antenna will be resonant. The higher the ratio of reactance to resistance the smaller amount of loss. When the coil is elevated it (as Doc mentioned) improves radiation efficiency. The same principal applies to the wire wound fiberglass whips. The tighter wraps at the top of the whip is the coil. These antennas are often referred to as a helical loaded antenna.

 

[GnG8d]

I have always considered a f/g antenna to be a continuous load, like the illustration above, but if suppose if enough of the winding was at the top, it could be considered "top load"?

 

[Robb]

I had two Wilson 4 ft Silver Load antennas that I gave to a couple of local radio operators. These Silver Loads are the kind that have the continuous coil - with most of the windings at the top. It worked considerably better than the antennas they were using in both cases.

I think it goes to prove that a cheap antenna can be a fine choice - so long as it has some quality to the build. Not saying they are better than a 1/4 wave steel whip - no way. But for what they are, they do work very well. I was surprised to hear the difference - quite noticeable for the better. Beat a K40 - for one.

 

[unit_399]

Like Doc said, loaded antennas (ones with coils) radiate from the top of the coil. But his statement that you'll never notice the difference isn't 100%. The antennas we're talking about here are designed primarily for mobile use. In a mobile, getting the radiating point of the antenna as high as possible is very important. A bottom loaded antenna like the Wilson 1000 will work reasonably well on the roof of a mobile, but put it on the trunk where the roof partially blocks the signal, and it will fall flat on its face. This is one of the reasons why 102" steel whips work so well on mobiles. They also radiate from the top, and they are TALL.
But... for those of us who don't particularly like the looks of a whip, a 5 foot top load is the next best thing.

- 399

 

[W5LZ]

And to maybe clarify it a bit more, that whole antenna radiates, not just the coil or what's above it, but the whole thing starting at where the feed line ends and all the way to the top. That coil doesn't radiate any more/better than a straight piece of wire of the same height, not unraveled length of wire making up the coil, just the wire/mast that would fill in the space where you remove the coil. If that coil is 6 inches tall, a 6 inch section of mast that would replace it would radiate just as well. The only thing that coil does is furnish the required inductance needed to make the antenna resonant.
- 'Doc

 

[TonyV225]

Just remember to work efficiant the coil should be up above metal structures (truck cabs pickup truck bed walls ETC) so up and or away from obsticles.

 

[WR0220]

I have always considered a f/g antenna to be a continuous load, like the illustration above, but if suppose if enough of the winding was at the top, it could be considered "top load"?

If the greatest majority of the wraps are concentrated at the top then yes, it would be considered a top load antenna. The wraps on a countinous load antenna are (more or less) uniform across the entire length of the antenna.

 

[mackmobile43]

The better antennas weld the coil to the upper and lower sections of the antenna to eliminate coil connection problems due to vibration. Some garbage I've seen did have the problem you mention but it was because the coil was pop riveted or screwed in place. The voltage drop across the length of coil is more then any other place on the antenna because of the way the impedance is stepped up by the coil. Raising the impedance raises the RF voltage across the coil. Raising the RF voltage leads to insulator failure when wet or covered in road salt.

If this fact holds true then why does it cause VSWR issues when the coil section
does not have a clear path to the horizon? Not disputing the voltage issue because I'm no expert on the subject but it seems to me that if the voltages where lower in this area of the antenna that it would not be a problem is the coil section was in close proximity to or lower than it metal surroundings.

You've seen that moronic You Tube video bulldawg did with a fluorescent tube that indicates a high RF voltage exposure at the coil section which should indicate high voltage at the coil section>laugh< not.

 

[dudmuck]

Top load coils have no stinger above them

The Ultimate Guide to 11 Meter CB Antennas

The signal engineering page doesnt mention one important point about antenna loading coils.
Its Q or quality factor.
Signal engineering page says that most efficient is continuously loaded. But according to coil Q rules, that cant be true. The hams say long-skinny coils have low Q. and coils with 1:1 length/diameter ratio have the best Q. Reading elsewhere it seems up to 4:1 length:diameter seems ok.

Thats what the fiberglass whip has against it. Also the physical support material has a dielectric constant worse than air, causing poor Q and more loss.

The difference between base-load and center-load coil is insignificant compared to other antenna losses like ground loss, coil Q and radiation resistance.

Antenna Efficiency