How to Wrap a Current Balun for 160 Meters!
Yet another project from N3EAQ..... This time I'm making a home-brew current balun for my 160 meter Vee Beam!
As you may have noticed from some of my other postings I work 160M quite a bit. My best and favorite antenna is my "Vee Beam" This is essentially a 160M dipole that has been mounted "high" at the apex/feed point (in my case ~30' off the ground), and extends out for ~135' per leg in a 45 deg. angle from the center line (i.e. 90 deg.) and terminates on an insulator suspended about 6 to 8' off the ground. Now, in a perfect world, a Vee Beam will terminate on a resistive load to ground, but I've found that mine (and others who've used this pattern), have great success with no end terminations.
In most cases the Vee Beam is fed with a coax to a simple "connection block", i.e., no balun, just a coax to direct wire feed block. These are available from many mfg's. In my case I'm using an Alpha-Delta, since it includes the lightening arrestor "fuse link".
Lately I've been thinking this antenna may work better, be a bit more broad band, have less reflected power to the shack, etc., etc. If I feed it with a 1:1 current balun. Mind you, I'd have NOT thought about it if my friend KB3NAA, Jim, had not brought it up recently. (Yes, he's using a Vee Beam too, along with Pat, W3PO, and BOTH are enjoying GREAT success with the design I describe. (and not just on 160 either!).
Rather than buy yet another overpriced 1:1 balun on the market, that usually burns up in no time at 160M when we use "legal limit", I decided to go out and "roll my own", using an Amidon T400-2 Toroid (Yes, only 1 R, from TORUS aka Ring). The T400-2 (RED) from Amidon (Amidon is undoubtedly just a reseller, I'll bet it is really from "Micrometals"...) Is a IRON-POWDER (that's why it's coated in RED), RF Toroid, with the following characteristics:
Outside Diameter = 4" Inside Diameter = 2.5" Height = 0.65 Al Value 180 [Where this is micro-henrys per 100 turns of wire] In other words 1.8 uH per turn.....
There is also a T400A-2 Toroid that is twice as thick and gives an Al value of 360 (or 3.6 uH per turn, i.e. twice the inductance per turn). ALSO, the Bigger/Fatter the Toroid the greater the power handling capability! So the T400-2 is good for about 1 to 2Kw, so the A-2 is good for about twice that. Remember that as you apply RF to this balun/inductor it will cause the wire and core to HEAT. The bigger the core, the better the power dissipation capabilities! THAT'S exactly why the off the shelf baluns tend to smoke at low freqs. and high power! Too much heat kills them!
OK, so now you have a T400-2 toroid, now we need to wrap it with #27Glass Tape. Every article says to do this, but none tell you how! You can get the tape from Amidon, at Home Depot, or wherever, it's up to you. Here's how I do the "wrap"..... I peel off about a 1/4 to 1/2 inch and wrap it back on itself. Then I roll a bunch of turns on top of that, and cut it off. Now I transfer the wraps to the core, and repeat, until the core is covered.
See pics below:
Wrap the Kasbah......
Your T400-2 is tape wrapped. Time for the windings!
For the 1:1 balun start by cutting three equal lengths of your winding conductor and mark the ends A-a B-b C-c. This will avoid any confusion when it comes to soldering the connections once the balun has been wound. Itís always better to cut the winding conductors too long as itís easy to trim off waste but impossible to add to the length!
You can attach the start of the winding to the core by using a small cable tie to keep it in place Ė you need to keep a firm tension on the wire as youíre threading through to keep nice tight windings.
Simply wind all three conductors together around the core, keeping them tight and evenly spaced . The windings must not cross over. See pic. below:
I personally prefer enameled copper wire for this, as itís easily obtained from EBay. For 1:1 baluns on a T-400 core I use 18 or 12 AWG. The next important step is how many windings are required to make the balun. In the case of this core, I'm starting out with 14 turns. That should be about 25uH....
I know I just got done saying to use Enameled wire aka "Magnet Wire", but for the purpose of this tutorial I selected 3 pieces of wire from a length of household wiring I had laying around. The main reason was to add color to the wires so as for it to be easier for you to follow along. In this case we have:
White, Black and Bare as our Color code...
Where I arbitrarily deemed Write to equal A-a, Black to be B-b, and Bare as C-c. The result is shown below:
I Started with the leads extending about an inch or so from the core and wire tied the wires to the core. then added the 14 wraps as shown.
Then I wire tied the end, and cut the remaining wire off about 4 inches from the core.
I deemed the short leads as A (White-start/short), B (Black-start/short), and C (Bare-start/short) and the long leads as a (White-finish/long), b (Black-finish/long), and c (Bare-finish/long).
I then twisted, soldered and cut them as in the diagram above...
1. A to b - or - White (start/short) to Black (finish/long) - twisted it, soldered it, trimmed it. The connection can be seen at the 7 o'clock position.
2. B to c - or - Black (start/short) to Bare (finish/long) - twisted it, soldered it, trimmed the BLACK. The connection can be seen at the 10 o'clock position.
3. C - or - Bare (start/short) DO NOTHING.. The wire can be seen at the 8 o'clock position.
4. a - or - White (finish/long) DO NOTHING.. The wire can be seen at the 9 o'clock position.
As you can see I added labels to the leads... They are as follows:
Connect "2" above - the long BARE wire - Coax Shield Connection
Connect "3" above - the short BARE wire - Side 1 of the antenna
Connect "4" above - the long WHITE wire - Side 2 of the antenna AND Coax Center Conductor
Here are some more views of the finished product, and a quick test with the LC Meter... Guess what, almost 25uH exactly!!
Coming NEXT, "Making and Mounting the Balun in a Box for Outdoors"
That's where I'm at right now.... Will let you all know how it goes, and update from here as I complete the project!
Mike - N3EAQ