Wednesday, August 2, 2017

Good Antennas Aren't Always Straight: Dipoles and Ladder Line Doublets

New hams are often concerned about how their dipole will look, when it's put up.  They've usually read enough about what happens to patterns to understand that putting the legs in different positions will affect how the antennas perform.  This is all very true.  For example, you may know that an inverted-V dipole will have a somewhat omni-directional pattern, whereas a dipole with level legs, at the proper height, will be direction along the long sides of the wire, and provide a fair amount of signal nulling at the ends.

I want to write about dipoles and doublets in the real world.  In the real world, as opposed to free space, you may not be able to erect a dipole as perfectly as you'd like.  I am lucky enough to live in a building about 5 stories tall, with a flat roof.  In some ways, it's antenna-building nirvana.  In other ways, I have a lot of crap to deal with:  air conditioners on the roof, a large cement walk-up covering the staircase, neighbors who covered about 1/4 of the roof in a garden, lack of building size etc.  This all adds up to me not having a way to fit a full-sized 40m dipole onto my roof.

My solution was to use a doublet with ladder line fed all of the way to the shack, and to not worry so much about shape.  True, my pattern is surely wacky, but I've got an antenna up, and I'm making boatloads of contacts.  Having an antenna is far better than no antenna at all.  There is no point in hemming and hawing over how to put up an antenna perfectly -- just put something up, and start making contacts!  After a few weeks, you'll probably learn that there are some areas you just aren't reaching.  If so, move your antenna around and work areas that you didn't work, before.

Paper Towel Holders as Ladder Line Doublet Stand-offs, Running Along Copper Roof Cap

My point is -- don't worry about making the perfect antenna!  Just make something!  Start learning, ask questions of experts in antenna forums in QRZ or EHam, or on Facebook, and have fun!  If you are using ladder line, and have a balanced input for it, at the tuner -- put up two equal-length wires, feed it to the ladder line, and just tune it!  That's what a doublet is.  I suggest you start out with wires that are at least 33' by 33' (10.06m x 10.06m) in length, to start.  That will allow you to tune basically every band from 40m on up -- SWR won't be workable without an external tuner, and this will be a very lossy antenna, if you use coax, but with ladder line, there is little loss, and it'll work great.  The higher you can get it up, the better.  In fact, this exact antenna is in the graphic, at the top of this page.  I've worked all over the world on it, and had loads of fun.  In fact, it seems to have both vertical DX and horizontal properties, as well as good NVIS performance.

So what is a doublet, compared to a dipole?  A doublet is just a dipole, but as I understand it, the doublet is called a doublet, because of the ladder line feed.  There is no other difference between it and a dipole.  The beauty of the ladder line is that it's extremely low loss.  How low?  As low as some of the most expensive helax cabling you can buy -- the kind you'd spend thousands of dollars installing at your QTH.  In short -- ladder line is a huge bargain.

Here are some great pages to get you started on ladder line antennas:

The problem of working with ladder line is that it isn't as easy to deal with, as coax.  It can flop in the wind.  It needs to be kept at least 12-16 inches away from metal (it can cross metal, for a few inches, with no real problem).  It can change a bit in SWR, when it gets wet (no big issue).   You can also accidentally end up with a resonant length, and find it unable to tune a certain band (to fix, just lengthen it or shorten by 3 or 4 feet, either way).

New hams -- don't worry about making the perfect dipole, or doublet.  Just make sure the legs are an equal length.  If you are building a doublet, then the 4:1 balun in your tuner is good enough to start (but you may find a 1:1 Current balun works better, depending on length, if you plan to add a balun outside the window to a VERY SHORT feed of LMR400 coax, to get inside).  I found a 1:1 Current Balun from Balundesigns is what you want for a high-power G5RV.  If going with a 33' x 33' doublet, you'll want a 4:1 balun, if you decide to place it outside the window, and feed a few feet of coax into the shack.  The length of the wire legs will be the main thing that dictates the impedance.  I used an antenna analyzer to determine which balun was best for my antenna.  To make it easy, for starters -- just use the internal 4:1 balun in your MFJ tuner, if that's what you have.

I just feed the ladder line straight into the house, and to the tuner!  My window happened to have gaps cut for water to seep out of the inside.  You might find you need a feed-through panel, like the MFJ-4602.

If you want a cheaper balun (emphasis on cheap) you can buy them from E-bay or Aliexpress.  I suggest ponying-up for the better baluns from Balundesigns.  They hold their value, and really do work.  You'll have it for years.

If you are making a dipole, just add a 1:1 current balun as your center (not required, but it can help reduce common mode noise, can improve pattern, and provides a nice center feedpoint).  If you are building an off-center fed dipole, you'll want a voltage balun, which will allow common mode current to pass into the coax hanging down (you'll then need to choke the common mode off further down the line, per instructions).  I haven't tried and OCF dipole, so read-up elsewhere.

Get an antenna up -- follow basic design principles, tune properly, and just operate!  Learn what can be improved, and go from there.

Good sources for ladder line:
Amateur Radio Supplies
KF7P's website
The Wireman

Or, just do what I did, and buy an MFJ-1778 and cut off the coax feed.  Good price, plenty of line, and already soldered at the center!  You can trim the wires equally, if needed.

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