Phased 40M Wire Verticals

Written by admin On August - 15 - 2010 5 Comments

About the 40M twin-vertical antenna array

Since early 2007, I have had two 40M wire verticals suspended from a rope between two pines in the forest on the slope below our back yard.

The wire verticals are 1/4-wavelength (about 33′ tall at 7.050 mhz).

Each vertical has four 33′ raised radials each, feedpoints 10′ to 15′ off the ground. The verticals are spaced about 30′ apart, and in switchable end-fire mode they aim at about 30 degrees (northeast to Europe), or 210 degrees (southwest to the deep South Pacific and California).

Early experiments to reach Europe

Initially, I fed the array at the south vertical, and had a 3/4-wave phasing line of 75-ohm RG6 running to the north vertical. This produced a single end-fire pattern to the north (Europe), but this effect was only slight. I suspect the actual velocity factor of the RG6 required an adjustment to the length of the 3/4-wave phasing line (removing 2.5 feet).

I was also able to feed both verticals with 1/4-wave lines for a bi-directional broadside (US/JA) pattern.

In May 2007, I ran an exact 3/4-wave phasing line of RG-6 between the verticals, feeding at the south end, supposedly to end-fire at EU just east of north. Exhibited some gain, but never worked much to EU.

In 2008, I changed the feed of these verticals to use the Christman method — each vertical fed by 84-degrees, and a 71-degree phasing loop inserted between the antenna feedpoints at the common control box. All coax is 50-ohm RG-58 for now. Relays gave me switchable end-fire patterns to the north or south (for either EU or W6 coverage).

Basic schematic for end-fire in two directions
Note: This diagram does not show a second relay used to create a broadside pattern (simply shorts across the A and B terminals, so each antenna is fed by the same length of feedline). If you want to keep each antenna’s radial system isolated from the other antenna, use a DPDT relay instead — which will let you switch both the shield and the center conductors of each feedline.

See notes on my two-element 80M verticals for details of the phasing and relay setup.

Christman Phasing Calculator

Use the online Christman phasing calculator

January 2009 — Interaction problems with half-square array
I had to remove the 40M vertical array in January 2009, however, as these verticals were too close to my 40M half-square array. They severely skewed the pattern of the half-squares when beaming East to the U.S. and Canada.

Here’s how skewed the half-square antenna pattern was, when the 40M twin verticals were in place:

And here’s the half-square antenna pattern to the East with the 40M twin verticals removed:

Christman phasing calculator

Older versions of the Internet Explorer web browser may not run the handy calculator.

For all other visitors, the calculator will easily determine the lengths of your phasing and feedlines.

Use the calculator

May 2009

With the installation of a SteppIR yagi with 30M/40M dipole, I have decommissioned the two-element 40M verticals for now. Plan is to eventually resurrect the array as a three-vertical triangle array for 40M. I am currently working on a three-vertical triangle for 80M to test the concept.

February 2010

I have two major and seemingly perpetual deficiencies for contesting: 40M access to Europe, and 160M to anywhere. I’m working on addressing both. The first order of business in this pursuit was to resurrect the two-element 40M vertical array and get it working properly, firing NE and SW.

I pulled the verticals back into the air in January 2010, but they didn’t work very well. Same old story. Seeing as the array has never achieved the performance I expected, my next suspect was the Christman phasing lines. Perhaps I had cut them to the wrong lengths using the old analog noise bridge.

I had to wait until a warm spell in February to get outdoors and bring the two 84-degree feedlines and 71-degree phasing line inside. There, I checked all three coax lines with my new Autek VA1 RF analyzer. Turns out that they were all too long — as much as 5 inches too long on the phasing line, and about 3 inches on the feedlines. Enough to make a significant difference in the antenna’s gain and F/B.

Using the noise bridge had brought me “close” to the required lengths, but not close enough. The VA1 allowed me to cut them precisely… well, I probably should have cut the cables an inch or so too short to compensate for the extra length of the relay leads inside the switch box, but for now I think I’m “close enough.”

Here’s how I cut them to the correct length:

Connect the antenna analyzer (VA1 in my case) to one end of a cable,other end left open. I manually held the coax centre and shield against the analyzer’s connector, so there was no extra length presented by a PL259 — even a 1/4-inch of extra length makes a difference.

Trim the coax until the analyzer indicates minimum Z impedance — this is where the line is precisely 1/4 wavelength (90 degrees) long, as follows:

  • For 7.050 mhz, the 84-degree feedline is 90 deg. at 7.554 mhz.
  • For 7.050 mhz, the 71-degree phasing line is 90 deg. at 8.937 mhz.

I recommend using the simple online Christman calculator to get the correct frequency.

In my set up, I’m using ring terminals on each end of the coax lines, and these only add a tiny bit of extra length to the finished cables. Using PL-259 connectors would add more length, so take that into account.

Propagation to Europe hasn’t been very good lately, but tonight I was hearing more European countries than I’ve heard ever before. There is pronounced front-to-back when switching from NE to SW. The corrected phasing arrangement has dramatically improved how the array works.

For now, I will try out the 40M vertical array in the upcoming 2010 XE RTTY and CQWW WPX RTTY contests.

This spring, I may actually take one more attempt at trimming the coax lines a tiny bit more to allow for the extra length of the connecting wires inside the phasing box.

August 2010

The vertical wires have been remeasured for resonance at 7.050 Mhz with the VA1 RF analyzer. They are both exactly 10.127m (33.23 feet) tall and SWR minimum for each measured at 1.2:1 at the target frequency — pretty close to a 50-ohm feedpoint impedance, due to the fact that the radials slope down from the feedpoint.

When both verticals are in place, the system SWR rises to about 1.8:1 to the north, and nearly 2:1 to the south. That’s caused by the expected mutual coupling — and no doubt some unwanted radial interaction — when they’re both in the air. The ON4UN’s Low Band DXing (Chapter 11-8, part 3.4.2, Fig. 11-7) suggests the common feedpoint impedance (at the relay) should produce an SWR of 2.3:1 for this array, so I think I am pretty close, taking into account the sloping radial effect).

The book says any suitable matching network could be employed to provide a good 50-ohm feed. I’m going to just match it at the shack end with a tuner, and run only low power into this system (a 2:1 SWR at the relay could burn things with sustained high power, particulary on RTTY).

Care was taken to arrange the radial wires so none of them overlap. Radials of one antenna within a few feet of the other antenna’s radials detuned the combined system quite a bit. All radials in this array slope down from the feedpoints at various angles, depending on where I’ve been able to tie off their ends.

I need to confirm the length of all the radials, as some broke and were spliced back together over the past year or two. If a radial on one of the antennas is significantly off-frequency, that could explain why SWR is worse in one of the directions. Another explanation: some radials slope down more than others, which definitely affects the feedpoint impedance of these elevated antennas (though sloping radials generally offer the benefit of raising the impedance closer to 50 ohms).

Audio Recordings of 40M vertical array in action

Aiming Northeast — Here’s ES3AX (Estonia) on 7.010 mhz at around 0140z on Feb. 17, 2010. You’ll hear him clearly with the 40M array switched to the Northeast (about 30 degrees) then fade away when I switch the array to aim Southwest.

Aiming Southwest — This is HK1KRY (Colombia) on 7.010 mhz at around 0150z. The difference between directions isn’t as noticeable in this one, but it’s still prominent.

It starts pointed at EU, then I switch SW (not directly at HK1) and he comes up quite a bit. He really disappears into the noise when I switch the array back totoward Europe. You’ll hear some fading on the signal, and I think his bearing is actually on one of the rear-facing lobes in the cardiod pattern, so he’s still fairly strong at times even off the back quarter.

Hear how the 80M version of this vertical array sounds


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5 Responses so far.

  1. admin says:

    Hello Edward. These are good questions. First, the phasing system is called Christman phasing, and lots of information about it is available in this online article (PDF).

    The following text is from my page about the 80M vertical array I currently use.

    Phasing with 84-degree and 71-degree Coax Lines

    In an antenna book (Low Band DXing, I think), I ran across a suggestion for “from the book” Christman phasing that could work for typical two-vertical arrays spaced 1/4-wavelength apart.
    While every antenna installation varies, the suggestion I followed said that if you have 1/4-wave verticals on 80M, each with a self-impedance at the feedpoint of about 50 ohms (a reasonable assumption if you have less than ideal radials), the mutual impedance of such an array could lend itself nicely to feeding each antenna with an 84-degree line of 50-ohm coax, and connecting the two feedpoints with a 71-degree piece of coax — the “phasing” line. (This is a replacement for the typical “90-degree” lines to each antenna, and a 90-degree phasing line).

    By shorting the 71-degree phasing line with a relay, the pair of antennas becomes a broadside array.

    Using the handy Christmas phasing calculator, your 84-degree feedlines for 21.200 Mhz would be 7.145 ft or 2.178 m — if you are using coax with a velocity factor (vF) of 0.66.

    The calculator is located here, if you wish to change the velocity factor:

    • Edward Lei says:

      Thx for your quickly reponse, UMM.. I can’t find your calculator, when I click the link above, it come back here, so for the 21.200MHZ, the 84-degree feedline is 2.178 m and the 71-deree phaseline is 1.841 m. Am I correct ?

      can I ask the preformance of this system, is it same as the 90-degree system, eg. 3db in endFire and 1db in broadfire over a single GP?

      Moreover, what is the horizontal half power angle for end fire and broadfire?
      what is the minimum height above the ground?(e.g. Single GP can set very near the ground, (1m is OK), is your system still hold this?)
      what is the vertical radiation angle?
      THX a lot,

      • admin says:

        Hi Edward. The calculator is on this 40M Vertical Array page: — just scroll up from the Comments section and you’ll see it.

        The performance of the system would be the about same as a any two-element vertical array spaced 90-degrees. There is no extra gain over any other phasing approach. The Christman approach simply uses current transformation in the phasing line and feedline lengths — set to be 84 and 155 (71 + 84) degrees long — to produce an end-fire pattern with two 1/4 wave verticals, 1/4 wavelength apart, and 50 ohm coax.

        The spacing of the antennas is still 90 degrees, and I believe the gain is in the order of 3 dB in end-fire, and about 1 dB broadside, as you would expect from a 90-degree spacing.

        The Christman phasing I used is explained in ON4UN’s popular book Low Band DXing — specifically Chapter 11-8, part 3.4.2, Fig. 11-7.

        I do not know what the half-power angles are for end-fire or broadside. The takeoff angle should be very low, as with any well-built vertical antenna with good groundplane or radial field. There’s nothing unusual about this 2-vertical array other than the phasing lines, and switching to allow it to fire in one direction or the other.

        The antenna I have now works very well on 80M. The feedpoints for each vertical are about 1m above ground, with elevated radials.

        I no longer use a 40M version of it, as I have built a 2-element 40M quad instead.

        Here are the calculated lengths for your 21.200 Mhz antenna coax lines:

        Your 71-degree phasing line
        The 71-degree phasing line should be: 6.039 ft or 1.841 m.

        The 71-degree phasing line is 90 degrees at 26.873 Mhz.

        Cut the coax to the suggested length plus a few inches, in case your velocity factor is not quite right.

        Leaving one end of the coax open, set your RF analyzer to 26.873 Mhz and trim the coax until you see minimum Z impedance. You now have a length of 71 degrees at your desired operating frequency.

        Your 84-degree feedlines
        Each 84-degree feedline should be: 7.145 ft or 2.178 m.

        The 84-degree feedlines are 90 degrees long at 22.714 Mhz.

        Cut the coax to the suggested length plus a few inches, in case your velocity factor is not quite right.

        Leaving one end of the coax open, set your RF analyzer to 22.714 Mhzand trim the coax until you see minimum Z impedance. You now have a length of 84 degrees at your desired operating frequency.

        Sorry I can’t provide more technical details. The model I built for 80M is still in use and it has worked extremely well for several years, particularly in contests where it has proven every effective to the US and Asia (the two directions I can point it). It may not work this well for 15M compared to even a simple dipole, but a low dipole won’t give much for low-angle gain, so perhaps the verticals will get you some DX that horizontal wires would not.

        All the best with it.

        • Edward Lei says:

          Hi Bud,
          Thx for your detail info. Ummm.. I have one more question, As I dont have RF analyser, is there any alternative of using SWR meter and 50 Ohm load ?

          • admin says:

            Hi Edward. I don’t know of a technique to measure coax line lengths using an SWR meter and 50 ohm load. The Christman calculator will get you quite close to the proper length of coax for the feedlines and phasing line (assuming you know the velocity factor of the coax cable), but for fine-tuning I don’t think SWR is a suitable parameter — the line lengths are best cut for minimum Z impedance at the frequency of interest.

            For example, the 84-degree feedline with the far end open should show a minimum Z point at 22.714 Mhz for your 21.200 Mhz target frequency. Without being able to read the impedance of the line when cut for 90 degrees at 22.714 Mhz, you won’t know for sure that it is 84 degrees at 21.200 Mhz for Christman phasing.

            Perhaps someone reading this can offer advice about using SWR as a guide to cutting a feedline to a specific length.

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