Modifying an Alpha-Delta DX-EE 40-20-15-10 Meter Dipole for 30-17-12-6 Meter WARC Bands

I am currently using an Alpha Delta DX-EE parallel dipole for the 40, 20, 15, and 10 meter bands with rather great success.  It is installed in my attic at about 16 feet above ground.  Since installing the antenna in April 2013, I have worked 76 countries (including India twice) and 49 states as of December 2013.  This is the second QTH that I have installed one of these antennas and they work great!  In fact, I like it so much I purchased a second one that I thought I could modify to resonate on the WARC bands.

This antenna has basically 3 elements. The longest wire is for 40 and 20 meters and it accomplishes this by means of a trap coil near each end.  For 20 meters the coil self-resonates and presents a high impedance which shortens the wire to a 20 meter dipole.  At 40 meters the coil is a low impedance coil of wire which adds enough length to the 20 meter section along with the end stubs to form a shortened 40 meter dipole.  Hanging below the 40/20 meter dipole is a 15 meter dipole and a 10 meter dipole which are separated by plastic spreaders.

My thinking was, if I could figure out how to change the trap coils to resonate on 17 meters; I could shorten the 20 meter dipole to 17-meters; shorten the 40 meter end stubs to 30 meters; shorten the 15 meter dipole to 12 meters; and shorten the 10 meter dipole to 6 meters.  The following steps detail how I converted the DX-EE to work for 30, 17, 12, and 6 meters.  It is divided into parts: construction, installation and tuning, and results.

Part I: Construction

First, let's take a look at the SWR curve for my original DX-EE across the entire HF spectrum:

Figure 1 Original DX-EE SWR Graph

As you can see, I have it tuned nicely for 40, 20, 15, and 10 meters but the WARC bands are useless.  Even though a good tuner will make your radio happy on these bands possibly, the line losses will be too great to be usable.  We'll take a look at the SWR curve for the new modified antenna later on.

To begin, I built up the antenna on the ground by using some rope and hanging the antenna between two nearby objects so the antenna was up off the ground about 3 feet so I could easily work on it.  I roughly built the antenna per the manufacturer's instructions but I left everything loose so I could adjust the spreaders and wire lengths easily.

First, I cut down the 10 meter portion of the antenna for 6 meters.  I estimated approximately where the wire needed to be trimmed to but I used a Rig Expert AA-54 analyzer and just trimmed off 6" at a time on each end until I got the antenna to resonate just below 50 MHz.  I then just folded the end back on itself about 3 inches to trim the antenna for about 51 MHz.  When measured from the nearest bolt on the center insulator to the end of the 6 meter element, the length should be 55" before folding about 4" back on itself.

I repeated the above steps for the 15 meter element and trimmed it down for 12 meters until it resonated at about 24.75 MHz and then folded the ends back onto the antenna so it was resonant on 17 meters and secured with zip-ties on both.  When measured from the nearest bolt on the center insulator to the end of the 12 meter element, the length should be 115.5" before folding about 4" back on itself as shown in Figure 2.   NOTE: Once the antenna is raised, all elements will need to be made a bit shorter as the resonant frequency drops on all bands and you'll want to do fine-tuning once installed in its permanent location.

Figure 2 Extra Length Tie-Wrapped On Itself

The next task was to understand and modify the trap coils.  Each trap coil is essentially a parallel-resonant R-C circuit that at resonance, looks like an open-circuit with respect to the 20 meter portion of the antenna and basically "disconnects" the remaining length of the wire.   I wanted to verify that the traps were indeed resonant around 14 MHz before I began to modify them.  So, I completely removed the coils from the antenna by removing the wire from the connection nuts and the cutting them free from the antenna wire.  Since I needed to shorten the 20 meter section, they needed to be cut off anyhow.  Using my Rig Expert AA-54 analyzer, I constructed a small (~10 cm diameter) one-turn loop as suggested in the manual using some 14 gauge wire soldered to an SO-239 chassis connector which I attached to my analyzer using a male-to-male PL-259 adapter.

Figure 3 One-Turn Loop

By placing the loop over the coil, I was able to measure the resonant frequency and the SWR curve..  Notice that this antenna uses only an air-core inductor with no capacitor.  The distributed self-capacitance of the coil windings provides sufficient capacitance to form the parallel R-C network and to cause the coil to self-resonate.

Figure 4 Measuring Resonant Frequency of Coil

The original coil design from the manufacturer yielded the following SWR graph showing that indeed, the coil is resonant in the 20 meter band.

Figure 5 Unmodified Coil SWR

I then began removing turns from the coil until I found it to be resonant for the 17 meter band.  You need to remove 19 of the original 47 turns leaving a total of 28 turns.  I found that by playing around with the extra wire that is bunched up by the screws, and adjusting the angle of the final winding, I could "fine-tune" the resonant frequency a bit.  The new modified coils now have the following SWR curve:

Figure 6 Modified Coil SWR

Below is the original coil (left) versus the modified coil (right) with 19 turns removed.  I removed 10 turns from one end and 9 turns from the other so the coil remain centered on the form.  In this figure, you can easily see the small squiggle of wire between the screw and bolt that can be bent/squeezed to fine-tune the coils.

Figure 7 Original versus Modified Coils

Next, I cut down the 20 meter section of the antenna for 17 meters until it was resonant well below the 17 meter band because the wire would become much shorter once I added the coils back on.  I found I needed to use about 9" of the end of the 17 meter portion to connect the coil the same way the manufacturer did with the 4.5 wire twists and enough remaining length to bend back to the connection bolt.  On the other end of the coil I just wrapped a short piece of scrap wire through so I could reattach my suspension rope. Notice the plastic wrapped around the coil to protect it until it is installed.  Also notice I nicked up the wire a bit with my needle nose pliers adjusting the wire "squiggle", be careful!

Figure 8 Coil Installed On End of 17 Meter Section

After attaching the coils and rehanging the antenna, I checked the resonance frequency and found that it moved closer to the bottom of the 17 meter band as expected.  Once the coil is installed as shown, if you measure from hole that the wire goes through this coil back to the nearest center-insulator bolt, you should measure about 153".  Once installed, the length will be shortened more to fine-tune.  I was also happy to see that the addition of the coils did not change the 17 meter resonant frequency.  I checked this by testing the SWR with the antenna wire unconnected from the bolts first and then again with the antenna wire attached to the bolts as shown above.  This makes sense because the coil should look like an open-circuit at 17 meters and ideally should have little to no effect.  So far, so good!

At this point, I decided to arrange and fix all my spreaders in place.  I choose NOT to use the extra 14 gauge wire supplied from the manufacturer.  This is the 3rd DX-EE I have had and those wires are hard on the fingers to install and difficult to move/adjust later.  So I used a method of crossed tie-wraps and decided I would use the extra wire for the 30 meter end stubs of the antenna.  I used fairly large (1/4" wide) tie-wraps.  If you need to slide them around a bit after installation, just snip them off, move as needed, then replace tie-wraps.

Figure 9 Tie-Wraps Used To Hold Spreaders In Place

I also decided to make some additional spreaders from the tie-wraps by attaching a tie-wrap to one wire and the crossing tie-wraps to attach to the other 2 wires.  This seems to maintain the space between the elements during installation.

Figure 10 Additional Spreaders Made From Tie-Wraps

Lastly, I cut the extra wire provided by manufacturer in 2 pieces to form the 30 meter end stubs and connected each to the outside ends of the coils in the same fashion as shown in Figure 8.  I then connected the end insulators to the wire and began trimming the 30 meter stubs until I got it to resonate at about 8.5 MHz.  These end stubs for 30 meters, as measured from the coil to the installed insulator are 22".  These ends are easy to trim down once installed so I wanted to leave plenty of extra wire.  This completed the initial construction.  Now to install in the attic and tune.

Below are the final dimensions for the antenna as described above.  These should work well for any application as a starting point.  Since I determined all these lengths experimentally and made sure that all elements are slightly too long (resonant frequency is a bit too low) you should be able to just build to these lengths directly.  The initial DX-EE, as it comes from Alpha Delta, seems to leave about the same amount of extra length on each band so I am confident that modifying another DX-EE using the modifications below would result in a working antenna once installed and tuned a bit.

Dimension	Band	Cut Wire To Length	After Installation
A	6 meters	55”	51” (a)
B	12 meters	115.5”	111.5” (a)
C	17 meters	162”	153” (b)
D	30 meters (C+D)	37”	22” (c)

Coil requires removal of 10 turns on one end and 9 turns on the other.

(a) Assumes 4" are folded back on element after outermost spreader.
(b) Assumes 9" is required to attach coil.
(c) Assumes 9" is required to attach coil and 6" required to attach end-insulator.

Figure 11 As-Modified Dimensions

Part II: Installation and Tuning

As mentioned earlier, I have to install my antennas in the attic due to HOA restrictions.  I personally do not mind, as it keeps the antenna out of the blazing Arizona sun.  I'm sure that the UV would eventually do some damage.  I developed some tricks I use for attic installation which you may find helpful if you are also confined to attic antennas.  If not, then use whatever installation technique you would like but the tuning instructions will remain basically the same.

To install my DX-EE dipoles, I screw three eye-hooks into the rafters: one is for the center insulator and the other two are for the antenna ends.  For the center insulator, I use a spring-loaded clip to first attach the dipole to the center mounting position.  You can purchase all the items described in the following sections at your local home store.  

In the upper right-hand corner of the picture below, you can see the center insulator attached with the spring clip to the eye-hook.  Also notice the home-made choke-coil that I made with a section cut from a cardboard concrete form.  The antenna on the left is the original 40-10 meter DX-EE which is using a purchased choke which is made with about 4 feet of coax covered with ferrite beads and then covered with shrink-wrap.  Also of note are two close-proximity air-handlers, foil-covered A/C ducts, and the rather short separation between the two antennas.  The roof is covered with tar-paper and concrete roof tiles.  This is all obviously not ideal but you can still make these antennas work quite well.

Figure 12 Attic Installation

For the antenna ends, I tie the supplied black Dacron rope to the insulators; run the other end through a small Quick-Link;  attached to a 25 lb. force spring; attached to another spring-loaded clip; which finally attaches to an eye-hook.

Figure 13 Antenna End Attachment Detail

I find that by having the ends spring loaded, it makes the attachment/detachment of the antenna much easier when it comes to tuning the 30/17 meter top element.  To shorten the antenna, I disconnect from the eye-hook and then make antenna length adjustments as needed.  I untie the rope from the Quick-Link, attach the spring then feed the rope through the Quick-Link and make the antenna taught noting the position on the rope where a new knot should be tied.  I remove the spring from eye-hook, tie a new knot, and then stretch the spring and reconnect to the eye-hook.  This may all be a bit of overkill, but I found it easier than trying to tie a knot while trying to keep the antenna taught, especially if you need to bend yourself back into some tight spot.

Once the antenna was installed in its final location, it was time to test the SWR and adjust the antenna.  I actually had a 40 meter folded dipole installed exactly where the new WARC-band antenna is installed.  That antenna basically was useless compared to the 40-10 meter DX-EE so I removed it and figured I could use the existing eye-bolts for this antenna.  I was a little concerned that removing one antenna and putting up another so close to the existing 40-10 meter dipole may have affected its tuning so I also checked its SWR.  I found that it was slightly de-tuned now on the 10 and 15 meter bands.  The center-frequencies had moved slight up which meant those elements were just a bit too short now.  I would just need to shorten the amount of wire that was folded back by about an inch (to make antenna longer) for those elements to get the SWR centered again within the band.  This just proves that surrounding antennas and metallic objects can affect attic installations so try to keep the antennas as far apart as possible and away from metallic objects as much as possible.

I now checked the SWR on the new WARC-band dipole.  As expected, the resonant frequency for each band had moved slightly lower and would need to be shortened.  I started with the 6 meter element first.  I ended up cutting off about 3" of wire on each end and still had about 4" wrapped back on itself.  I would not suggest cutting wire at all because it is possible to end up being too short.  If you feel there is getting to be too much wire wrapped back on the ends and want to cut some off, only cut an inch or so off each end at a time and check the SWR each time to see the affect.  I was able to get the lowest SWR on the low-end of 6 meter which is where I wanted to be.  Since 6 meters has a 4 MHz bandwidth, you'll need to decide what portion of then band you'd like to operate.

I repeated the same steps for the 12 meter element and found that I needed to shorten the elements by about the same 3" as I did for 6 meters.  Your results will vary based on your antenna's environment.

Now for the hard part!  I tested the SWR for 17 meters and the center-frequency was about 500 KHz too low now.  Based on my experience tuning the 40-10 meter dipole, I figured I needed to shorten the element that runs from the center-insulator to the coil by about 12" as a starting point.

Now, at this point you might be realizing how much of a pain-in-the-butt this is going to be.  Do I really need to remove the coil from the antenna by untwisting all that 9" piece of wire, straighten the wire, cut off 12", and then re-install the coil?  What if I am now too short?  If I do this instead an inch at a time how many times will I need to do this?  I asked myself all these questions and the answer luckily is NO!  I learned by my experience with folding the ends of the other elements back that the RF does not seem to care that there is insulation on the wire but only seems to care about the resulting length.  What if I was to introduce a bend in the wire in between the center-insulator and the coil?  Would this work?  Well, I did it on my other antenna and it DOES work!  If you need to remove 12"  then make a "Z" fold in the wire with each leg of the "Z" being 1/3 the distance or 4" and secure with zip-ties as shown below:

Figure 14 The KB7IUV "Z"-Fold

Here's how it looks on the antenna itself.  Make sure the zip-ties are tight (I actually install 3 once I have finished) so the tension on the antenna does not collapse the fold.  Leaving some amount of  loop at the ends help to keep the wire from slipping (don't bend them completely flat).  A normal amount of tension seems to be no problem.

Figure 15 The "Z"-Fold In Place

Checking the SWR again, I found I was a bit too short!  I release the tension on the antenna by removing the ends from the eye-hooks (now you see how this becomes handy) and readjust the "Z"-Fold so its about 1" shorter overall which effectively adds back about 3" of overall length.  I needed to repeat these steps one more time to get the center-frequency dialed in.  It's not as easy as the 12 and 6 meter elements but it sure beats removing the coil each time!

It is also worth noting that I also spot check the 12 and 6 meter SWR to see if changing this wire by so much had any affect.  It did slightly on 6 meters, so be sure to check the other bands and tweak as needed.  I think I needed to adjust the 6 meter length by about 1/2".  The affect should be minimal.

The last task is to tune the 30 meter stubs.  Due to the high Q of the coils, the bandwidth on 30 meters is very narrow.  For the original DX-EE this is an issue on 40 meters because the band is 300 KHz wide.  You need to tune the antenna for the portion of 40 meters you wish to work.  Fortunately, the 30 meter band is only 50 KHz wide so the entire band should be workable.

To adjust the length of the stubs, it is just a simple task of unwrapping the wire connected to the end-insulators and adjusting as necessary and loosely wrapping the wire back on itself again.  When I first measure the SWR for the 30 meter section, it was resonating at about 8.5 MHz.  I started shorting the antenna about 3"-6" at a time until I got close to the 30 meter band.  I ended up actually shortening the overall Dimenson D from Figure 11 to about 11" or about 1/2 its original length but better too long than too short.  The adjustments become real tricky as you get very close to the band to try and center the lowest SWR right in the middle of the band due to the narrow band and narrow coil bandwidth.  Once I got very close and was actually just a bit below where I wanted to be, I started just cutting off about 1/2" of wire from the portion that is twisted back on itself.  I found that this actually has a "very-fine-tuning" affect.  I realized that at some point I was "close enough" and considered it done.  I cleaned up all my gear and got down out of the attic.  Now it was time to check my results!

Part III: Results

To take the final SWR and impedance readings, I attached my Rig Expert AA-54 to my PC and used the supplied software.  I used the Windows 7 Snipping Tool to capture the image and save to a JPEG file.  Here are my final readings:

30 Meters

Despite the narrow bandwidth of the coil, the narrow bandwidth of 30 meters allows the SWR to be relatively flat!  Notice the |Z| varies slightly around 50 Ohms.

30 Meter SWR

30 Meter Impedance

17 Meters

Similar results for 17 meters!  At this point, I must confess I was more than little concerned that I could get this antenna to work properly since I had to alter the coils.  Even though Figure 6 showed a nice SWR curve for one coil, the other coil I could not get  tuned as well.  I even noticed that the original coil SWR for 20 meters was not precise for both coils.  It is obvious now that this was not an issue.  The |Z| for this band is even more flat at 50 Ohms.

17 Meter SWR

17 Meter Impedance

12 Meters

Similar results for 12 meters except the |Z| probably varies from about 75 Ohms down to 60 Ohms.

12 Meter SWR

12 Meter Impedance

6 Meters

The 6 meter band looks a bit different.  But remember we're now looking at 4 MHz in bandwidth.  Even so, the SWR is not bad across the entire band except for below 50.4 MHz.  My tuner will easily handle the higher SWR on the ends.  But if you look at the 2:1 SWR bandwidth it is from 51.4 MHz up to 53.6 MHz!

However, the impedance does some strange things in lower portion of the band and becomes a better 50 Ohm match once we get above 51.6 MHz.

6 Meter SWR

6 Meter Impedance

In case you are wondering if the above results are true, especially for the 30, 17, and 12 meter bands.  Looking at the overall band scans for SWR you can definitely see the "nulls" for the WARC bands easily.  In fact, you can see that for 17 and 12 meters the lowest SWR occurs a bit higher in frequency.  But I am not climbing back into that attic and messing around for an 1.1:1 SWR when I already have less than 1.4:1.  It may appear that for 30 meters we are not covering the entire band.  For some reason the software shows the yellow band for 30 meters from 10.0 to 10.3 MHz.  For the graph above for 30 meters I manually set the range to proper band edges.

3-30 MHz SWR Scan

30-54 MHz SWR Scan

Final Comments

I hope I have demonstrated that it is possible to modify a purchased DX-EE antenna and to successfully get it to operate on the WARC bands.  I was immediately able to hear the "difference" in the antennas as the WARC bands now come in much stronger on an antenna cut to their size.  I immediately made two SSB contacts on 17 meters with a stations in Washington and North Carolina with good reports.  I think I have also proven it is possible to make these antennas even work in an attic situation.

If you attempt to do this on your own, please drop me an email and let me know how it worked.  You can email me with any questions and I'll be glad to try an help out.  I am good on QRZ.com.

Good luck and 73 de Mike!