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End-Fed Dipole
  
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KDøETC    End-Fed Dipole
                                                                                                                                                                                                                                                   Ground Snake Antenna

On the right is the current version of my end fed dipole where I use a toroid choke to reduce the bulk and improve the effectiveness over an air core choke. From left to right:  feed-line extension with common mode choke, feed line, common mode choke, coax antenna arm, single wire antenna arm, throw line.


     Note please that in the photo of the antenna the wires are flaked not coiled for storage and Velcro bands are used to secure the bundles. 
If one tries to coil the wire the result is a twisted, tangled pile of wire that takes a long time to straighten out, untwisting the coils one put in earlier.
Flaking is fast and easy and allows a quick, almost trouble free deployment of the antenna.  Watch for possible hitches toward the ends if one happens to form.
End Fed Dipole
This is my present common mode choke made with an FT240-43 toroid.
The choke is a critical part of this antenna design since it defines one arm of the dipole.

*One reference to choke balun construction can be found here: http://www.karinya.net/g3txq/chokes/

Another here: A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing Revision 5a 5 Jun 2010
© Entire Contents Copyright 2007-10 The Audio Systems Group, Inc., except Appendices 2, 3, and 4, which are property of the cited authors, and product data, which is copyright by Fair-Rite Products. All Rights Reserved
by Jim Brown K9YC
Audio Systems Group, Inc.
http://audiosystemsgroup.com

 

 

8Turn Choke

In the August, 1991 QST, available on the ARRL archive, is an article on the end fed dipole titled: RFD-1 and RFD-2: Resonant Feed line Dipole. 
This began my experimentation with this type of antenna.   
The diagram on the right, from the ARRL Antenna Handbook 20th Edition describes how if the shield of the coax is severed RF will flow on the outside of the braid back down the coax.  This common mode current can be stopped by a choke appropriate for the frequency of operation.  This defines one leg of the dipole.  The other leg is the single wire soldered to the center core.

Common Mode Current
End Fed Dipole Sketch

 Discussion:

    I was attracted to the end-fed dipole because it is lightweight, compact, easy to hang and take down, and with a tuner  performs well on more than one band. 
I have heard of versions made with RG-174 but I have not tried that.  

    I pull my end-fed dipole up into trees using a throw weight and slick cord available through tree climber supply catalogs.
With and under-hand toss this system works for about a 30 foot maximum height.  It is compact and easy to carry.  The joint between the coax and the single wire needs to be strong and capable of sliding through tree branches without getting hung up.  I twist and solder the wires, then seal with multiple layers of shrink tubing. 
    I have not figured a way to streamline the choke.  When I pull the antenna up I just hope that the coil can get high in the tree before a branch stops it. 
Update:  October 13, 2012

      Today I took the rig out to Park Point on Minnesota Point (Duluth, Mn) for an afternoon of  "Radio in the Park" using the end-fed dipole.  I found a comfortable place to set up along the shore where a large Cottonwood tree,  mostly free of leaves, provided tall support for the antenna.  Now, this year I have been using a 6 foot walking stick, cut from an invasive Buckthorn bush, during my hikes in the woods.  A hiking stick I find gives me added stability on rough trails and helps on steep accents and decents; plus, with a notch carved in the small end it makes a great throwing tool for my weight and throw line.  With a bit of practice I was estimating I could about double the height I could throw the weighted line up in a tree.  So today with that large Cottonwood in front of me I hooked the weight, eyeballed the branch I wanted and let fly with a two-handed toss.  First try the weight goes over the proper branch and makes it to the ground and I grab the weighted end and pull the dipole up into the tree.  The chokes do not get hung up allowing the end to be pulled to the highest point at which time I notice the bottom end is hanging free in the air.  That makes the top 65 feet up!  Not bad for archaic, stone age technology applied to the 21st century. 
    I had a number of fine contacts that afternoon on 17 meters with 59 plus signal reports.  20 meters was good also, I contacted a few PA QSO operators then moved up. Then later tried 15 and 12 meters successfully.  The end-fed dipole is a good, versatile antenna capable of operating flat-top, sloped or vertical.  Two days earlier I had set it up at Lake Place Park where no tall trees were available.  I laid the antenna over bushes about 5 feet off the ground which worked well for contacts to GA, MS and CO and DE

    I have compared the End-Fed to my 14 foot ground mounted vertical and to my car mounted vertical on a number of occasions and it works about as well, sometimes better, especially over rock where a vertical loses out because of poor soil conductivity.   If there is not a tall tree handy I lay it on bushes, maybe 4 or 5 feet off the ground and it works well.  I have even laid it on the snow, continuing a qso that started with the antenna high in tree then taken down and I laid on the snow.  My contact did say the signal strength went down some.  

    I use an LDG auto tuner in my portable kit because so far the end-fed dipoles do not show a low swr on the band it is cut for. 
I quit trying to get the swr down since I like the multiband capability the tuner gives me so a modestly high swr does not make much
difference as long as the radio is able to feed it. 



Last edited 10 Nov 2012
by Steve Balliette
KD0ETC

.


 Ground Snake Antenna:  
    Due to the strong RFI at my home QTH, an eight unit condo, I have experimented with a variety of receive antennas to try to overcome this problem.  With a background noise level of S6 to S9+ at home and in many innercity parks making contacts is difficult.  Calling CQ is poor operating etiquette if one has little chance of hearing a reply, and one may be interfering with someone on the frequency and not know it.  Answering a CQ or joining an existing QSO has worked for me in these conditions. 
    QST has had articles on a low noise antenna, the snake, that I have used in my backyard and in a variety of parks.  I have constructed it using a 6 foot length of RG-8x and a 15 foot length of wire attached to the center conductor.  The braid being attached to a couple of radials of about the same length.  When it works you get low or no noise and some signals can be heard quite well.  With my FT-857d I use an MFJ send/receive switch allowing me to compare the reception of the ground antenna to whatever I have in the air at that time.  Sometimes the snake is a significant improvement.  Sometimes not.
    I have tried a number of different lengths of antenna wire and grounds, but still have no parameters developed on which to base a successful design.  At the condo, since the noise source is the building, I have found that a long coax lead away from the building works to lower the noise level.  I currently have a well performing vertical 100 feet away from the condo that if mounted 20 feet from the building has a high noise level.  In an innercity location, like Lilliput Park with its ambient S9 RFI a long coax lead to the antenna wire does not make sense to me, so I use a length long enough so the open braid end can have a set of radials attached and not get tangled in my feet while operating.  Six to ten feet of coax then seems enough.  I have never used a balun of any kind on this antenna, but that may improve it.  How long should the single wire be?, what direction should it be oriented? is one wire all that is needed or would two or a fan array like multiple radials improve reception?  Lots of work to do yet.
    Note: On one occasion I made a surprising DX contact using the snake antenna.  I set up just to listen in a downtown Duluth location, less than a block from the Federal/ County/ City complex where I did not want to erect my vertical and perhaps have the police come to check me out.  So I laid down the snake, 15 feet of wire over the grass with the end 2 feet into a bush 2 feet high.  On 20 meters I heard many strong stations, quite a few DX and not being one to just sit there I tried matching the wire to the FT-857d with my MFJ Travel Tuner.  I was able to get the swr below 2 and put my call into a pileup with a Scandanavian station.  On the second call he picked me up with a good signal report.  I have not tried that again, but perhaps if you do please let me know how it works.  Thanks, Steve KD0ETC