Vertical Matching Coils

[Robb]

If your antenna is a A99 or an Archer 1/2 aluminum antenna, there is a matching coil built in the base of it to match its impedance. Applying too much power to it, and it is burns out/goes open and must be repaired/replaced.

Where is the formula and construction guide to build my own? If I wanted to take the 1/2 wave and turn it into a 5/8 wave; I will need to change the requirements of this coil as well as the lengths of the elements. I'm not worried about the element length, the formulas are out there in abundance. A balun would work I suppose; but if I wanted to restore the antenna to its orginal form - I would want to rebuild what is there. I just can't find the coil formula and design/build requirements.
Thanks!.

 

[W5LZ]

Rob,
There isn't any 'set' formula for such a thing (that I'm aware of). But you've gotten a taste of what's required to figure it out. It deals with impedance matching, reactances, etc, an impedance matching transformer, sort of. How do you make a transformer variable? Couple of ways. Vary the number of turns in each 'half' of the transformer, or vary that transformer's 'permeability'. (@#$$, another new word!) Rather than go into a long explanation of all that, especially since I'm too lazy, look it up. Not all that difficult to do, 'they' do that with those 'rings' you turn, or the 'screw' in the middle of those 'IF' cans, right?
How 'variable' you want to make it just depends on how 'wide' a difference in impedances you want to be able to match. The wider that variance, the bigger the the thingy gets. (Also why very capable tuners are not tiny.)
If/when you figure out -one- method/formula that works for everything, would you let me know what it is??
- 'Doc

 

[Marconi]

Rob, click on End-Fed Halfwave Antenna by dxzone.com. This article should give you some ideas of what is inolved in end feeding a 1/2 wave element, sort of. You may have to look around a bit to find the end-fed topic.

 

[BOOTY MONSTER]

i certianly cant answer your question , but if youre interested in building a 5/8 base antenna you may want to look at this .
Tips and tricks on how to build your own CB Antennas.

heres a thread by a guy who used it as a guide to build his , but modified it .
http://thequackshack.com/index.php?showtopic=21717
and his atricle .
MY 10-METER VERTICAL Photo Gallery by Sean 12Footer at pbase.com

 

[Robb]

The Imax 2000 EXPOSED!
This link kinda points out what I'm talking about.
Point is, there isn't much info on how to make your own inductor for a given length of vertical radiator. Thanks for the help so far guys! I need to get more info on this, and as yet, I haven't found it yet.

 

[W5LZ]

One 'simple' method is used with some mobile antennas. Lots of coil tap changing to start with, but then you don't have to do it again until something changes (bands, most often). I think there's still a commercial version sold by 'GLA', but I'm not sure about that at all. They are fairly easy to make.
Very basically, it's a coil of wire about 3 - 4 inches in diameter, made up of something close to 16 ga. - 14 ga. wire, maybe 15 - 20 turns. The bottom end of that coil is shaped so that the end turn is slipped under the base of the vertical portion of the antenna, and the rest of the coil sort of 'sleeves' the bottom of the vertical shaft, doesn't touch it anwhere except the base, evenly spaced around the antenna. If using a ball-mount, the antenna is slipped down through the coil, makes contact with that bottom end of the coil, and is then screwed into the ball-mount. Feed line is attached as you normally would with any ball mount.
Adjusting that impedance match is done by running a ground strap to various coil turns till you find the point where the input impedance is 50 ohms. That 'tap' on the coil is usually done first with an alligator clip till the 'right' place is found. Then it can be soldered to that point on the coil, or connected by using a 'tap-clip, or just leave the alligator clip there. Not really much point in leaving tht alligator clip there unless you do some jumping around on different bands, unless you're lazy like me.
This whole thing can be done out of sight at the 'back-end' of a ball mount instead of the coil being around the antenna's bottom shaft. It's just a variable tap coil going to ground from the center lead connection of the feed line. I'm sure there are other methods, but this one has been the easiest one for me to use. It's basically the same method done inside that bottom matching coil on several commercial antennas, just adjustable and 'home-made'. The one made by 'GLA', the Texas BugCatcher guy's company has worked for me for a lot of years, even with something like 500 watts at one time. Is it the absolute, most bestest way of doing it? Probably not, but it sure works, so who cares?
Never tried it on a 'fixed' antenna, but then I gotta tuner that does the same thing for those...
- 'Doc

See for information;

GLA INDEX Page

 

[freecell]

if you knw the value of inductance that is required there are several programs that can tell you the number of turns, the diameter and the spacing based on an inputted value in henrys, (H) millihenrys (mH) or microhenrys (uH)....

you could also feed it directly with 50 ohm feedline through a shorted 450 ohm 1/4 wave matching stub. this applies to the proper feeding of an end fed half wave vertical. the input impedance at the feedpoint of a half wave is approximately 4000 ohms and you're attempting to feed it with 50 ohm coax. the impedance of the 1/4 wave transformer required to perform this function is the result of the square root of the product of the two impedances to me matched. so, 50 X 4000 = 200,000 and the square root is 447. the matching section can be constructed from a shorted section of 450 ohm ladder line and the coax is tapped close to the shorted end of the stub and adjusted for best match.