Tuning & Testing the 80 Meter Vertical

It seems everything I do takes longer than expected. This is essentially a progress report on the construction of a 3-element, 4-direction 80 meter vertical yagi. When last I checked in the driven element (a tower with stinger at the top) was completed. Since only a few radials were installed and the antenna not tuned it was premature to talk about performance.

Before the 80 meter vertical yagi array can be built it is first necessary to get the driven element working as a simple vertical. I have slowly made progress to the point that I now have a very effective full-size vertical for 80 meters. This will also be its function after the array is built for the omni-directional mode. The character of the vertical changes markedly as radials are added.

It is important in a project of this complexity that it be broken into a sequence of steps with testing after each step is completed. Surprises can be investigated and dealt with before they can be obscured by further changes. I see this as an advantage rather than a burden since it is a tremendous opportunity to learn about antennas and propagation.

I'll step through the process as I go from the basic vertical with 4 radials and a monopole to a tuned 34 radial vertical for 80 meters, including what I learned along the way. Turning the antenna into a yagi will take more time. Reasons for the slow progress so far include: lack of 80 meter activity during the summer needed for on-air evaluation; connecting the antenna was inconvenient until I repaired the antenna switch; many other concurrent projects; and, non-radio summertime activities.

4 radials

Despite the stinger being fully retracted the resonant frequency with only 4 radials of 20 meter length the fell well below 3.5 MHz. The radial length dominates the short monopole (~18 m). This is expected since not only are the sparse radials resonant they are electrically much longer than 20 meters due to ground proximity lowering the velocity factor.

I didn't bother to precisely measure the resonant frequency since 4 radials was a transitory configuration. On air the vertical performed poorly when compared to the 32 meter high inverted vee. Both signals and noise were noticably attenuated. The match was good because the high ground loss raised the feed point impedance close to 50 Ω. Loss is in series with the radiation resistance.

With a perfect ground a full-size vertical ought to have feed point impedance of 37 Ω, and can be much lower as the monopole diameter increases, as it does when it is a lattice tower. The estimated ground loss with 4 radials was a minimum of -3 db, based on feed point impedance, but likely closer to -6 db. Measuring ground loss accurately isn't easy and I didn't try.

Some insights can be had even though the ground loss is high. By using perceived SNR (purely by listening) an inkling of how the antenna will perform with more radials can be ascertained. SNR on the vertical was better on the longest paths, such as to PY. The inverted vee SNR was always better before sunset when elevation angles on all paths is higher due to absorption at low angles.

8 radials

The addition of 4 more radials made a dramatic difference. Resonance made a big jump to ~3.8 MHz. This shows rapid progression towards non-resonant radials system as radial count increases from a low number. Efficiency improved so that it was more equatable to the inverted vee on reception tests. The inverted vee continued to outperform the vertical within eastern North America. On longer paths the received signals with the vertical were often equal or better than the inverted vee.

The impedance changed little, only dropping several ohms. Obviously there is more affecting the impedance than simply ground loss. I suspect the main reason is that with radials longer than an electrical λ/4 the current distribution on the radials places the current peak away from the feed point. Lower current at the feed point is associated with a higher impedance, thus partially masking the effect of the decreasing serial resistance due to ground loss. This has amply documented by N6LF in his extensive experimentation with verticals.

12 and 20 radials

I did not do on air tests for these radial count for the aforementioned reason of inaccessibility to the antenna from the shack. The only testing was measurement with my antenna analyzer.

The next 4 radials were not symmetrically interlaced with the existing 8. These radials went to the hubs of the 4 parasitic elements, elements which have yet to be installed. Collinear with these short 10.5 meter long radials is a 15 meter radial, bringing the total radial length to 25.5 meters. The parasitic element radials will all be 15 meters and these systems will overlap rather than employ boundary busses. These radials for the parasitic elements are the only ones bonded to the driven element's radial system.

Once these radials were buried the antenna's impedance was measured. The resonant frequency continued to increase though at the expected slower rate due to the monopole beginning to dominate as the radial system becomes increasingly non-resonant. The resonant frequency was ~3.9 MHz with an impedance in the low 40s.

The next 8 radials are symmetric with respect to the original 8. After adding them the resonant frequency increased to ~4.02 MHz.

Measurement precautions

The photo of the analyzer display shows a lower resonant frequency than what I quote above. This is due to the 1 meter length of transmission line between the analyzer and feed point. It adds several ohms of reactance which lowers the apparent resonant frequency by more than 50 kHz. That is ~1.5%, a not inconsiderable amount.

This is easily simulated using TLW to determine the impedance at the feed point. In this example notice the difference between the source and load ends of the transmission line. The amount may seem trivial, but it is enough to impact the performance of a yagi when tuning the parasitic elements, as I will be doing with this antenna. It is far less critical for single element antennas such as ordinary dipoles and verticals. Better analyzers include a feature to compensate for the transmission line so that you can directly read the feed point impedance.

When measured at the true resonant frequency -- with regard to the analyzer reading, which is limited to its inherent accuracy -- the resistance part of the impedance is ~40 Ω. This is more reasonable since the "ideal vertical" has a radiation resistance of ~37 Ω. Ground loss is certainly present. As we'll see below even this value is misleading since the ground loss is significantly greater than 3 Ω (40 - 37).

Another factor to keep in mind is the placement of the analyzer, the coax and your body during the measurement. Although all are small relative to the wavelength I have experienced up to ±30 kHz (±1%) variation with this antenna. Don't let the coax drape over the monopole or the radials, don't lean against the antenna or lie on the ground (on top of the radials). In some cases it can help to prop the analyzer on an insulating platform rather than hold it in your hand.

To bury or not to bury

I stopped burying radials after these first 20. It's a lot of work and I found that if I was careful to put little tension on a radial and walk along it to press it down that it was soon hidden under the vegetation in the hay field. Large dips in the field were levelled to help this along. I can mow over the entire radial field with ease. However I caution visitors to step carefully to avoid tripping and ripping them out. After a year even than warning may become unnecessary.

The other concern with leaving radials on the surface is when my neighbour harvests the hay. That equipment is not like a lawn mower and will tear up surface radials. When he did the harvesting this summer he was exceedingly cautious following my line of stakes and brick markers and would not have hit a radial had the parasitic elements used surface radials.

In light of this experience I have decided to stop burying radials. It's a lot of effort that I am happy to avoid.

The temporary run of RG-213 from the antenna was replaced with ~90' of LDF4-50A. It was laid in the trench along with the control cable and covered over. Heliax is rated for direct burial. From the tree line (see top photo) there is an overhead run of RG-213.

34 radials

This may seem an unusual number of radials until you consider the geometry of the radial system. My objective was to double the 16 radials. With the 4 additional ones that connect to the parasitic element base hubs there were 20. When I added the next set I should have only needed 12 more to reach 32. However two of the hubs were not in the required position for symmetric placement. Therefore I needed two more.

Should I eventually go for 64 radials on the driven element (1,280 meters of wire!) those hubs are aligned as required so I will only need to add 30 radials.

With the analyzer attached it is clear that we are approaching the limit with respect to the antenna's true resonance with so many radials creating a non-resonant ground plane. The additional radials increased the resonant frequency by only about 15 to 20 kHz. This is effectively nil since measurement repeatability at this level of accuracy is unlikely.

Of greater interest is the resistance value. Notice that it is 5 to 6 Ω lower than with 20 radials. That's a big improvement that is evident from on the air receive tests. On all DX paths during full dark the vertical now equals or exceeds the inverted vee with respect to absolute signal level and often SNR as well. This bodes well. I have worked several DX stations with the vertical and, while I did not do A-B comparisons, it does seem to get out well.

Notice that the resistance is well below that of the ideal vertical. That ideal is for a thin vertical (with respect to wavelength) such as a wire. For a fat monopole such as mine the radiation resistance is lower. Unfortunately NEC2 does not handle fat verticals very well and with experimentation I could not push a model lower than 35 Ω over perfect ground. Many hams report fat vertical radiation resistance below 30 Ω. Without a viable model I can only estimate based on the trend line that my vertical's radiation resistance is probably no lower than 25 Ω.

For the moment I will therefore assume that the ground loss is ~6 Ω. This approaches my initial goal of 5 Ω. With the high currents (low impedance) when the antennas becomes a yagi I should see a performance improvement by doubling the radials to 64, since I expect that this would lower ground loss to ~4 Ω and perhaps lower. But not this year.

After reaching this milestone I climbed the tower and raised the stinger to its full height. The resonant frequency moved ~200 kHz lower. This is ~75 kHz higher than the objective I set in the antenna design. I am not concerned since the low radiation resistance compels me to build a matching network for the antenna in its omni-directional mode to meet my SWR objective. Originally I planned a matching network to be switched in when the antenna is in directional (yagi) mode.

With the vertical working -- albeit using the rig's ATU -- the inverted vee will come down shortly for relocation to a different tower. It can have a lower apex and do well for short and medium distance contest QSOs and for select grey line DX conditions.

The ropes hanging from the stinger have been untangled and prepared to act as the upper catenaries for the 4 parasitic wire elements. In the coming weeks these elements will be built and installed and full radial systems laid for them. My objective is 16 radials for each parasite this year, possibly rising to 32 next year. Then I'll complete the matching networks and switching system. Unless I get sidelined by other projects this 3-element vertical yagi for 80 meter will be operational this year.