Tuesday, May 6, 2014

An updated EFHW for 17-40m

I've been requested over and over to make a post on the topic of my latest EFHW project.  As seasoned readers of the blog will know, I am a huge fan of these antennas for a number of reasons, primarily that they are incredibly easy and quick to erect.  The big limitation is the operating frequencies are relatively fixed with a single wire antenna.  The PAR design for 40, 20, and 10m works great on those bands, and in a pinch on other bands with a tuner but I wanted something with a good match on all bands I wish to operate on.

I should note that this design is nothing novel, I know other hams are using similar methods to multi-band their antennas so I can't take any credit for the idea.

My antenna solves the problems of my other EFHW antennas by introducing breaks in the wire where I can add or remove length to get on other bands.  These are made by tying fishing snap swivels onto the wire (we're using #26 PolyStealth wire here) and soldering a mini-bullet connector as the mate.  This is best described in pictures:
As you can see, both ends of the wire get a snap swivel which then connect to each other, taking any tension off of the connection and placing it on the wire instead.  Then to switch bands, you just press the bullet connectors together and hoist the antenna back up.  Sure, pulling the antenna up and down any time you want to change bands is a bit of a pain but it uses zero power and has zero loss so you can't really beat that.

Now you could just do that for every band, but after 20m, your wire starts to get pretty long and I had grown very fond of the ~40' wire used on the 40/20/10 PAR antenna.  It is a great length and I don't usually have any problem getting a support string up that high on the first shot.  Any longer though, and things get interesting.  So, I was convinced I'd have to figure out how to keep the antenna short enough for that.

As you know, the PAR design uses a loading coil to shorten the antenna on 40m.  I had to add one to my design as well.  Here's how I managed it:
As you can see, I continued with the snap swivel design but because I wanted to get on other bands, I couldn't just solder the wire right onto the bottom of the coil so I added bullet connectors again.  The snap swivel is held on the coil by a second piece of heat shrink.  It feels pretty solid but there is probably a better way to secure the swivel.  Oh and the coil is about 34uH as we've discovered in our previous work with EFHWs.  I think it is 51 or so turns around 1/2" pipe.  In this case I'm not sure it is even that critical.

So with the coil in place, there's another length of wire and the same snap swivel/bullet gadget between a 30m element and the 40m element.  The total length is about 40 feet again but now I can get on 17 through 40m without any problem.  I'd like to go back and add 15m but I'm hesitant to do that and risk missing the mark and scrapping the whole first length of wire.  I may go that route eventually.

So anyway, that's the new antenna.  The matchbox is unchanged from our previous models and it seems to work great.  I hope others can improve on the design, and if you do, let me know!