This article discusses a very popular HF antenna with hams, the non resonant dipole centre fed with two wire line. Some ‘experts’ call this a doublet, but their distinction is not captured in the IEEE Standard for Definitions of Terms for Antennas which considers doublet and dipole as equivalent.
Whilst these antennas can work well, remembering that all antennas “work” and “any antenna is better than no antenna”, is a deeper understanding useful?
As a basis for discussion, an NEC-4.2 model of a 60m centre fed dipole at 15m over ‘average ground’ (σ=0.005, εr=13) and 30m of lossless 400Ω VF=1 feedline was built. It is almost λ/2 at 160m, so might appeal as potentially useful down to 160m.
Above is the geometry of the example antenna.
Above is a Smith chart plot of Zin to the feed line from 1 to 30MHz, the cursor is at 1.9MHz, and this impedance 14.4-j181Ω will be used in the following discussion.
Note that the Smith chart is normalised to Zref=400Ω as this gives a better picture of transmission line effects in this scenario. Small adjustments to feed line length will rotate the locus CW and CCW.
Let’s try W9CF’s T Tuner calculator to find a possible match solution for parameters somewhat similar to the MF-949E ATU. Well, I used Ql=100 (some experiments suggest Qli @ 1.9Mhz as high as 140), and 30µ though the MFJ-949E might not quite make that inductance.
Note the modelling assumes stepless adjustment of L and C, but the MFJ-949E and many others used a switched tapped inductor and that leads to slightly higher loss than this model indicates, possibly one dB or more. Whilst continuously variable inductors might seem better, they tend to be lower Q.
Above, a match is found and ATU loss is 2.5dB… not wonderful… but the VSWR meter on the rig looks real good.
Let’s create a simulation of the matching range of the ATU, and for completeness, loss of the match. Again assuming generously that Ql=100, and 208pF caps.
Above is the model. The coloured squares a sample points in a sweep of values of the matching components C1, L1, and C2. Green is loss less than 3dB, Yellow 3-6dB, and red greater than 3dB.
Above, zooming in on the Smitch chart and the path for a match as 1.9MHz.
Not much of the chart is green, but this should be viewed in conjunction with the Smith chart of the input impedance looking into the feed line. The point at 1.9MHz is plotted, and it is in the green zone… loss is actually 2.5dB (see the model screen above), so yes, it is green, but some might consider it pretty shabby.
Realise that changing the feed line length rotates the first Smith chart map, and for the same reason, the data point / path plotted above. Feed line length could be varied a little and remain in the green zone.
Readers may have seen advice that if a match cannot be obtained, increase or decrease the feed line length and try again. There is sense in this as can be seen, but finding a feed line length that works for all bands might be a challenge, or at least tedious.
Another issue that arises is that discussed at Avoiding flashover in baluns and ATUs. It may be prudent to exclude areas to the right of the G=0.005 circle (passes through the point labelled 2k/500µ) to avoid flashover (though in this case, a lot of that area is outside the match range of this example ATU.
Improvements
First improvement is the ATU. Though sellers represent T matches with 200pF capacitors as suitable for 1.8MHz, a T match with 500pF capacitors has wider matching coverage AND lower loss.
Above is the match loss chart reworked for 500pF capacitors. Note how much more of the chart area is green, but particularly how much more of the area that coincides the the Zin Smith chart is green.
Now green is a pretty crude measure, the ATU loss in this (500pF) case is 1.6dB, 0.9dB lower than the case with 208pF… just from using larger caps. A better coil will improve things.
Conclusions
The article does not document a design, but is rather a discussion of some of the perhaps hidden issues of this type of antenna system.
The common non-resonant dipole, or even one that is resonant on one band, fed with an arbitrary length of two wire line and T match ATU can be an effective multi band antenna system, but the ATU may hide some poor performance.
The case studied was chosen to expose the problems that are most apparent on the lowest bands that such an antenna system might cover.
Assessing the performance of an antenna system by VSWR alone is a limited view, more so when an ATU is used, they can mask performance issues.