Sunday, July 12, 2015

"A Winning Antenna Design" or "A New OCF Carolina Windom"

This antenna is based on a design by K4IWL and published in 2005 in an article named "A Winning Antenna" from a newsletter called "QRP Expressions." I have not seen any other issue of that news letter during my search of the Internet. I attempted to contact K4IWL but received no response so I do not want to republish the article by I can link it here.

It is similar to a Carolina Windom or an Off-Center-Fed design but I really like the radiation pattern that comes from the 10 feet of coax that hangs down from the 4 to 1 balun to a choke or line isolator. I will not rehash the design as it is included in the link above.

I did not make the 4 to 1 Balun as described in the article but rather used one from KI6J. Stu offers them as kits here. Those little kits are hard to beat for price. You can wind Stu's balun to be a 1 to 1, 4 to 1 or 9 to 1. I ordered another one for another linked dipole I am thinking about.

I used BNC connectors that I already had on hand and a piece of PVC pipe that was a shipping container for a whip antenna in its past life. It is 1 inch OD so it fit the washers I planned to use as end caps. I did not have screws and bolts to attached the BNC connectors to the washer so I used brass nails, solder and some glue.

The glue I used throughout the project is JB Weld Plastic Weld and I cannot say enough good things about it. It does require setup time but it secured everything I needed it to, metal and plastic.

Washers, BNC Connectors and brass nails.

Ferrites to be used for the line isolator (12 inches worth)

These numbers do not mean much to me but this is what I used

RG-58 U used for the line isolator/choke (I used about 14 inches)

Washers with the holes drilled out


Ferrets and coax going in to the pipe

The end connector going together

The ends are glued to the pipe

Perfect fit

Finished product


The 4 to 1 balun kit


together

add a 25 foot wire and 41 foot wire, 10 feet of coax and a feed line and it is ready to go



I tried it in the backyard and it worked fine. It is flat on 20 and about 1.7 to 1 on 30 meters and tuned all bands 80 through 6 meters no problem with an Elecraft T1 tuner. Of course the disadvantage is it takes some space to put this thing up. It is heavy at 1 pound 12 ozs and you need to be able to support it. However, on the right summit this thing should work very well.

I hope to get it up to a summit soon. I am positive it will out perform anything I have used in the past.  Thanks to K4IWL for the article/design and Stu, KI6J, for the balun kit.

If you plan to make this antenna please check this link as there is some additional information about the design.

Good luck

72
Frank
K0JQZ

UPDATE 16 July 2015: After getting some good advice (see comments) I went back and made a 4 to 1 Voltage Balun using a T106-2 (red) core toroid and 26 bifilar turns of 20 AWG enameled magnet wire. This is different that the original design but is recommended on the link I included above. I have not tried it yet but tested the balun and it seems to be flat at 50 ohms. I will update this entry when I test it.



Toroid wound and installed

The final product

UPDATE 18 July 2015: I was having all kinds of problems with the 4 to 1 balun in the small tin so I decided to use a pill bottle which did three things. Made it more light weight, gave me more room to work with and finally, it allowed me to attach a rope support and to tie off the antenna wire making it easier to adjust if needed. This helped a lot. I also decided to try David's other suggestion and use a 1 to 1 curruent balun instead of the ferris and coax. This saved a lot of weight and made the overall package much smaller. 

Pill Bottle Balun

Another view

1 to 1 current Balun

inside

I strung this up in the backyard and it performed well. I worked a few SOTA stations then decided to test another antenna. More to come.

Thanks David VE7EZM/AF7BZ

72
Frank
K0JQZ

6 comments:

  1. Frank, I would do two things differently.
    (1) I'd use the 4:1 balun design in the original article, not the one from KI6J.
    (2) I'd use a 1:1 current balun instead of all those toroids slipped over the coax 10 ft below the 4:1 baun.
    Here's why. The KI6J 4:1 balun is a current balun, designed both to have a 4:1 impedance transformation ratio and to do a good job of suppressing common mode current. The 4:1 balun in the original design is a voltage balun and it will do a lousy job of suppressing common mode current. Normally you want to do the best job of suppressing common mode current that you can. But not here. The common mode currnet on the 10 ft section of coax is what gives you the vertically polarized radiation, particularly helpful on 15 meters! Now below the line isolator you DO want to suppress common mode current, and an ordinary 1:1 current balun made out of a suitable toroid will do that far better than a bunch of toroids slipped over the coax, and at lower cost. Why? with the toroids slipped over the coax all in a row, each toroid has in effect one turn through it. With an ordinary 1:1 current balun as for example in the KI6J design, you can have many turns through one toroid.

    I suggest you try it both ways. In addition to making SWR measurements on all bands, try to check current on the outside of the lower portion of the coax (below the isolator) each way. You can do the latter by slipping the feedline through a toroid (once) and having a lot of turns of wire wound through that toroid (as a secondary). Terminate the secondary with a moderate sized resistor; 50 ohms will probably be fine. If you have 7 turns on the secondary than the primary (one turn) will only have 1 ohm or so inserted. Place the toroid at various locations along the coax feedline and measure; if there is current on the outside of the coax shield, it will vary as you move along the coax feedline. Keep the power low. Measure the RF voltage across the 50 ohm resistor. In addition put up a short test antenna several wavelengths away from the modified Windom and measure field strength, both with the short test antenna horizontally oriented and with it vertically oriented. You should be able to measure the votlage at the test antenna with a diode, capacitor, and DC voltmeter or milliammeter.

    If you do this I predict that particularly on 15 meters the vertically polarized field will be a lot stronger if you use a 4:1 voltage balun than if you use a 4:1 current balun, and that the current on the outside of the coax below the isolator will be a lot lower if you use a 1:1 toroid current balun than if you use a bunch of toroids all in a row.

    David, VE7EZM and AF7BZ

    ReplyDelete
    Replies
    1. David, thanks for the suggestions. I did research the different balun types and to tell you the truth I got a little confused but the way you explained it sounds much better. I will have to see if I can find some more wire to wire one.

      I will give it a go and see what happens. That is half the fun. Thanks for 1. taking the time to read my blog and 2. providing great suggestions.

      72
      Frank
      K0JQZ

      Delete
  2. This comment has been removed by the author.

    ReplyDelete
  3. Frank, when you wrote "I was having all kinds of problems with the 4 to 1 balun in the small tin so I decided to use a pill bottle which did three things. Made it more light weight, gave me more room to work with and finally, it allowed me to attach a rope support and to tie off the antenna wire making it easier to adjust if needed." it did four things, the three you mentioned plus it removed metal from the immediate vicinity of the 4:1 voltage balun. It's never a good idea to put a balun near metal since capacitance between the windings on the balun and the metal can upset things. But with a 4:1 voltage balun (which normally I would avoid like the plague, but as I indicated there's good reason to use one in this design) the balun is really an autotranformer and there is a lot more flux in the core than there would be with a 1:1 current balun feeding an already well-balanced load. (In the latter case the flux is due only to common mode current). So any flux leakage is going to interact with the metal case as well. Several people recommend plastic balun cases.

    Did you ever make the common mode current tests (below the the 1:1 current balun; you'd expect and want common mode current above the 4:1 voltage balun), and the field strength tests (with balun arrangements done both ways, and with field strength meter short antenna both vertical and horizontal)?

    Envying you your cool Colorado WX. Out here on the coast where it's supposed to be cool and wet it's been hot and dry almost all summer -- not conducive to hiking at all!

    ReplyDelete
    Replies
    1. I am unable to make the common mode current test because my batteries died in the analyzer. They lasted a long time but I am getting fed up with replacing 10 AAs. I am looking at rechargeable batteries on amazon. I think I can go a few days without it. Maybe next weekend. Thanks for the comments and advice.

      Delete
  4. Hello Frank, I'm following your 'New Windom' iterations keenly and will likely build one after learning of your latest tests per some of David Ryeburn's suggestions- which seem to make sense to me. I'm a new ham, low on the curve, so your blog is great stuff. I've been collecting amber colored pill bottles for some time- just too nice to toss, now I may have a use for some of them!
    Thanks,
    Oliver
    KC2OZL

    ReplyDelete