Tuesday, November 12, 2013

Comparison of 40 Meters Wire Antennas versus Height

As I concluded in the earlier comparison of my delta loop and a reference inverted vee at the same apex height of 15 meters, the delta loop is the overall winner. This is fair even though the inverted vee would outperform the delta loop in the broadside direction by a small amount -- a little more than 1 db -- because in all other directions the delta loop wins. If you can only put up one 40 meters antenna at this height, you should expect the delta loop to win. Just be sure to feed it for vertical polarization.

If you able and willing to put up two inverted vees orthogonal to each other or a turnstile inverted vee at the same apex the question can be reconsidered. The only significant disadvantage is the longer run of transmission line compared to the delta loop, since the feed point for the inverted vee is at the top of the support versus close to the ground for the delta loop. The matched loss of even a long run of RG-213/U at 7 MHz is quite low so the only real issue is the cost of the additional cable.

Having established that the length of transmission line is a minor concern I now want to consider height. In particular, if the apex of the inverted vee is raised at what point does it become the victor of the performance competition? That's the question I want to deal with in this article.

No, I am not going to actually put up any of these antennas! It is sufficient to do some modelling, here in the shack where the cold wind doesn't blow. The only antenna work I am doing in November is sealing joints and tensioning cables and wires.

I ran the models through EZNEC and produced the following graph. The broadside gain was calculated for each model in 1 meter height increments from 10 to 25 meters.

I ran 3 wire antenna models through EZNEC. A dipole requires more than one support but I threw it in as a reference with which everyone should be familiar.
  • The delta loop is fed λ/4 from the apex. The azimuth pattern is similar at all show heights, with the gain off the ends approximately -3 db from the broadside direction.
  • I chose 10° elevation angle to compare gain since this approximately the median angle for longer, DX paths. My interest is DX so I don't care about higher angles except to lower the gain there, if possible, to reduce QRM from W/VE. Below 20 meters height the dipole and inverted vee have a substantial amount of gain at higher angles, whereas the delta loop attenuates those angles.
  • The choice of ground has less impact on the horizontal antennas. The delta loop, being vertically polarized, is sensitive to ground quality in both the near and far fields.
  • The interior angle I selected for the inverted vee is 120°. That seems a reasonable median point between the delta loop (60°) and the dipole (180°). More acute angles lower the gain, mainly due to the lower average height. As designed, the inverted vee tracks about -1 db gain versus the dipole, until lower heights where the differential increases.
  • The feed point impedance and resonant frequency for all of the antennas is sensitive to height. Each should be cut (by modelling or otherwise) for its installation height.
You can see where the delta loop shines when mounted at 15 meters, which is how I've built it. However it only wins in a tight range of heights. Below 14 meters it cannot be installed since the lower wire would be below ground. Some people choose to distort the loop shape or slant it to make it fit but that is not optimum. If that is a real problem a vertical dipole of some sort would be a better choice.

Although any 1 meter step up or down results in a small change in gain, these add up. For the dipole and inverted vee the gain change is approximately 0.6 db/meter through most of the modelled height range. For example, go up 5 meters and the gain increases ~3 db. The similar figure for the delta loop is only ~0.1 db/meter.

While I do not have a dipole or inverted vee for 40 meters in my current station, I did have one in the late 1980s. Its apex was about 17 meters, about 2 meters below the top of the tower and a TH6DXX. My recollection is that it slightly outperformed the delta loop, however I never had them both up at the same time.

Soon after I turned the dipole into a 2-element inverted vee yagi, switchable to either broadside direction. That antenna worked very well. Perhaps sometime this winter I'll rebuild the model of that antenna and write an article about it. The original model was in done with MiniNEC-based ELNEC (the precursor to EZNEC) and so would benefit from a redesign.

Dipoles and inverted vees are good choices for 40 meters yagi elements. The same can be done with delta loops although they are "bulkier" and tend to have bigger side lobes. Of course if you can have only one antenna for this band that might not be so bad.

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