A 43 Foot Horizontal Doublet at 80 Feet

Almost six years ago I designed a 43 foot doublet that I mounted vertically for 20 through 10 meters use.

But, apart from its use in a couple of contests, it languished, and weather (or squirrels eating its ropes) eventually brought it down.

I recently became interested in the digital FT8 mode, and, in an attempt to improve my ability to make contacts on the higher bands (compared to my horizontal full-wave 80 meter loop at about 40 feet), I decided to resurrect the doublet and to mount it horizontally this time, rather than vertically.

I've described the design methodology of the vertical doublet in an earlier blog post:  k6jca 43-foot vertical doublet.  And I will recap that methodology, as well as discuss another way to approach doublet design, later in this blog post.

But first, let's get to the actual antenna itself:

A 43-foot Horizontal Doublet for 20 to 10 Meters:

From my earlier design work, I knew that I needed a long run (about 118 feet!) of Wireman 554 Ladder Line to transform the doublet's feedpoint impedance into something in the range that my tuner could match.

Given that the general rule-of-thumb for wire antennas is "the higher the better", and given the required length of the ladder-line transmission line, I needed to get the antenna as high as I could so that I wouldn't have too much excess ladder line gathered under the eaves of the house.

Using a pair of conveniently placed pine trees and my home-brew antenna launcher, I was able to raise the doublet to 80 feet above ground.  Below is a view from the ground of the doublet (wires enhanced using a Paint program).  (Other wires in the picture are part of my full-wave 80 meter horizontal loop (at a much lower height)).

Below is a side-view of the doublet.  You can see it just below the top of the picture (wires enhanced using a Paint program).  

Below are the EZNEC screenshots showing the important information.

First, EZNEC's main screen:

Note that the "Alt SWR Z0" is set to 360 ohms.  This is the approximate impedance of Wireman 554 Ladder Line (a value that is used in programs such as SimSmith and TLDetails).

And the wires defining the antenna.  Note the very short wire 2.  The source is placed at the center of this wire.

And here's a view oft the antenna:

Below are two SWR plots of the antenna's feedpoint impedance.  The first plot is for a Zo of 50 ohms.  As you can see, this would be a poor multi-band antenna if we were feeding it with 50 ohm coax.

This next SWR plot is the antenna's feedpoint impedance relative to the Zo of the 554 Ladder-Line (i.e. 360 ohms).  At first blush, still not a great multi-band antenna...

...but, with the addition of 118 feet of Wireman 554 Ladder Line, SWR (relative to 50 ohms) starts to look much better!

Note that ladder-line, although less lossy than coax, is not lossless!  The "light purple" line shows the transmission-line loss in dB.  Loss on 12 meters is approaching 2 dB.

Doublet Radiation Patterns:

For the doublet mounted at 80 feet:

20 Meters:

17 Meters:

15 Meters:

12 Meters:

10 Meters:

If you compare the "beam widths" of the Azimuth patterns for the different bands, you can see that the higher the frequency, the narrower the beam-width.  This narrowing of the beam-width may or may not be desirable, and illustrates one of the drawbacks of doublet antennas, compared to, for example, fan-dipoles -- as the azimuth beam-width narrows, the doublet becomes less useful for making contacts over a broad swath of area.

Doublet Design, Method 1:

I'll discuss two methods of doublet design.  The first method is the method I describe in my earlier blog post:  k6jca 43-foot vertical doublet. 

This is a multi-step method.

Step 1.  Model the antenna in EZNEC and plot SWR relative to the transmission-line's characteristic impedance (Zo).  In the case of Wireman 554 Ladder-line, this would be 360 ohms.

Step 2: (as shown in the image, above)  Pick a frequency in every ham band that you'd like the antenna to operate in and note the angle of the Reflection Coefficient (Gamma).  In the image, above, I chose 14.1 MHz for 20 meters.  And Gamma's angle at this frequency is +77.4 degrees.  (And the impedance that the Reflection Coefficient represents is 

To bring this impedance closer to 50 ohms, I will add a transmission line whose characteristic impedance is this Zo (360 ohms).  As I lengthen the transmission line from a length of 0, the Reflection Coefficient, as measured at the other end (not the antenna end) of the transmission line, can be plotted on a Smith Chart.  This value will follow a clockwise path around a "Circle of Constant SWR" as the transmission line is lengthened.  (Note, following a circle assumes the transmission line is lossless.  It isn't, but for ease of calculation I will assume that it is).

Eventually the Reflection Coefficient will rotate to intersect the x-axis on the left-hand side of the Smith Chart.  This represents the impedance value that a transmitter (or more appropriately, the antenna tuner in front of the transmitter) will see.  In this case, adding enough transmission line to rotate the reflection coefficient by 257.4 degrees (i.e. 77.4 + 180 degrees) gives us close to a 50 ohm match!

(Important note (that I will make use of): If I extend the length of the transmission line by 1/2 of a wavelength (scaled with the transmission line's Velocity Factor), I will rotate the Reflection Coefficient 360 degrees around the Circle of Constant SWR, ending back at 58.3 ohms.)

And I will calculate the amount of feedline necessary to create similar matches on the other bands.

Then, I will add successive amount of 1/2 wavelengths of line to each frequency, and tabulate the resulting lengths.  I am looking for a transmission line length that is common (or close to common) on all bands of interest.

I wrote a spreadsheet to do this calculation for me.  You can see that a feedline length of around 118 feet rotates the reflection coefficients for all bands around the Smith Chart to give a fairly good match (the green boxes in the table, below):

Notes on the above spreadsheet:

1. The values in the "Transmission Line, Half wavelength (feet)" cells equal c*Vf/(2*frequency), where c is the speed of light in feet/sec, Vf is the transmission line's velocity factor (0.93 for 554 ladder line), and frequency is in Hz.  Note that this is the length of transmission line required to rotate 360 degrees around the Smith Chart. For 14.1 MHz and 554 Ladder Line, this length calculates to be 32.45 feet.
2. The "Length to First Minimum Z (feet)" is amount of rotation required to rotate the Reflection Coefficient clockwise to lie on the x-axis on the left-hand side of the Smith Chart.  This angle will be less than 360 degrees, and thus the length will be less than the "half wavelength" calculated length (see above).  In the example of 14.1 MHz, this value equals ((77.4 degrees + 180 degrees)/360 degrees)*(32.45 feet), where 32.45 feet is the length of transmission line that will rotate the Reflection Coefficient 360 degrees around the Smith Chart.
3. The "Length to Next Zmin (feet)" cells simply add an additional half-wavelength of transmission line each time.  In other words, rotating 360 degrees around the Smith Chart each time.

Calculating Transmission Line Power Loss:

Part of a design should include estimating power loss in the transmission line.  If I know the antenna's feedpoint impedance (from the EZNEC SWR plot), the type of transmission line (e.g. Wireman 554 Ladder Line), and the length of the transmission line, I can use AC6LA's "Transmission Line Details" (TLDetails) program to calculate power loss.

Below are the calculations for each band:

20 meters (ladder-line loss 0.715 dB):

17 Meters (ladder-line loss 1.164 dB):

15 Meters (ladder-line loss 1.551 dB):

12 Meters (ladder-line loss 1.896 dB):

10 Meters (ladder-line loss 1.833 dB):

Of course, this is only the loss calculated at one frequency in each band.  Power-loss will vary across a band as SWR varies.  But this value gives you an idea of how much loss there can be, even in "low-loss" ladder-line.

Also, as an interesting point, note that the True Zo of Wireman 554 Ladder Line is slightly complex, not real, and equals 360.466 - j0.443 ohms (see lower left-hand side text box).

Doublet Design, Method 2:

Design method 2, discussed above, requires a number of separate steps and it involves writing down intermediary values to be input later into other programs (such as antenna feedpoint impedance, from EZNEC, into the Transmission Line Details program).

It also relies on assumptions, such as the transmission line being lossless, for my Excel Spreadsheet.

And, power dissipation is calculated at a single point in a band.  Calculating additional points is additional work.

Design Method 2 simplifies the process and gives more information (e.g. power loss across a range of frequencies).

The first step requires creating a "Touchstone"-compatible file of the antenna's S11 values (this would be in the form of an .s1p file).

Although there are versions of EZNEC which can generate s1p files, the version I own is not one of those.

So, instead, I use the free antenna-modeling program "4nec2" to create the s1p file.  Unfortunately, this is not as easy as simply creating an antenna model and clicking a button.  4nec2 requires that you first calculate the NEC output-data as a frequency sweep.  Then you need to open a Smith Chart window, and then, from this Smith Chart window you need to export the Smith Chart data as a Touchstone file (you have four formatting choices, I use "s-par (Magn)" (i.e. s-parameters, magnitude (and angle)).  

But wait, you still aren't done.  The Touchstone file is written as a .txt file.  You need to find this file and rename its extension from .txt to .s1p.  (Note that on my computer, 4nec2 always writes this file to the folder:  C:/4nec2/plots).

When the .s1p file has been created containing the antenna's S11 data, you can load it into SimSmith and then model how a transmission line can bring your antenna's impedance down to an acceptable SWR (that a tuner can match), as well as calculate how much power will be lost in the transmission line.  The SimSmith screen-capture below shows how this is done:

Notes:

• SimSmith really simplifies finding an appropriate length of feedline (no EXCEL spreadsheet required!).  Simply insert a transmission line between the load (defined by the antenna's s1p file) and source, select the type of transmission line (e.g. 554, dry conditions), and then increment the length of the line up or down while watching the SWR nulls move with respect to frequency on the SWR plot (I use my keyboard's up/down arrows to do the incrementing).  When they look good, note the transmission line's length!
• You can also plot transmission line power-loss across the frequency range.  Simply set SimSmith's Generator Model to xMtch (as shown, above).  This will define a generator that is a perfect match for the impedance it sees (thus removing loss due to mismatch), so that any losses in the network between source and load are due solely to the network itself (in this case, the network being the transmission line).  You can see loss plotted, above (light purple dotted line).

Adding Additional Coax to the Transmission Line:

At my QTH I need to bring the transmission line in through the side of the house and then to my operating position.  Rather than run ladder-line through the wall, I run a 20 foot piece of RG-142/B coax from my operating position, through the wall, and out to the eaves of the house, where the coax connects to the ladder-line.

This coax will add additional loss which we can calculate using SimSmith.  Below is the model including 30 feet of RG-142/B:

Loss through both transmission lines totals to be 1.43 dB.  Of this value, there is a loss of 0.716 dB in the coax, leaving the reminder of the loss (0.714 dB) as ladder-line loss.  (Note that the value of the transmission line loss (0.714 dB) is essentially identical to the loss calculated earlier (in Method 1) using the "Transmission Line Details" program (in that case, the loss was 0.715 dB).

And note how the coax causes the loss to be significantly worse as we move away from minimum-SWR on the higher frequencies.

Measured S11:

Below is an S11 measurement of the Doublet antenna at the transmitter-end of the run of RG-142/B coax (30 feet?) that connects the end of the ladder-line transmission line to my transmitter.  VNA is an HP 3577A.

Comparing Measured S11 versus SimSmith Simulation:

Below is a plot comparing the measured SWR of the Doublet antenna system (calculated from the S11 data captured by the HP 3577A VNA) to the SimSmith Simulation of a 43 foot doublet fed with 118 feet of Wireman 554 Ladder-line and 30 feet of RG-142B/U.

As you can see, the two are quite close, despite there being some amount of uncertainty as to the actual wire lengths that I cut for the doublet as well as the lengths of ladder-line and coax (although coax length ought not change the position, with respect to frequency, of the SWR nulls).

Standard Caveat:

I might have made a mistake in my designs, equations, schematics, models, etc. If anything looks confusing or wrong to you, please feel free to comment below or send me an email.

Also, I will note:

This design and any associated information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.