To summarize, here are the problems I have with my half sloper:
- The antenna has a significant horizontally-polarized component in many directions. Despite the use of the tower as a major component of the antenna it seems that the wire can often dominate the far-field pattern. Much of the antenna's radiation is at high angles, which is not what I want.
- There are substantial ground losses which severely cut into its efficiency. I am comfortable claiming this based on its on-air performance, feed point impedance (presence of series ground loss resistance) and the software model. NEC2 often has been reported to underestimate losses from real ground, yet even so the modelled losses are substantial.
- Antenna bandwidth is modest. It is cut to resonance within the CW segment (3.5 to 3.6 MHz) and does poorly in the SSB segment (3.8 MHz). I've tried the FT-1000 MP's ATU and an external antenna tuner with poor results. I can match the antenna for SSB, but the matching and SWR loss seem quite high. While adjusting feed line length might help, I'd rather have a broadband match.
As I move lower in frequency my options become increasingly constrained by my available supports and property. Just moving a factor of 2, from 40 down to 80 meters, I can pretty much rule out all horizontal antennas. They will either do no better than what I have or will destructively interact with may high band antennas. Interaction is unacceptable since 80, for now at least, is less valuable for QRP contesting and for DXing.
Regardless of the antenna I choose, horizontal or vertical, ground losses must be addressed. The ground directly under all my antenna is poor, not the medium ground I typically use in my models. I model that way to make the results meaningful for most readers of this blog. My lot is backfilled with sand (septic tile bed) over shale. Soil is only 12" (30 cm) deep.
Do not be deceived by advertising for commercial vertical antennas with "no radials" designs and that have elaborate matching networks, or any home-built antenna that does not require radials, such as a delta loop or half sloper. You cannot so easily dismiss ground interactions, and the inevitable loss.
Radials are needed, although I don't want them. There is too much traffic in my backyard to have wire lying on the surface, even if it is worked into the grass and weeds. Burial is out of the question at this QTH since it would tear up the lawn for what is likely to be a brief deployment. A vertical would have to use the tower that is centred in the yard since it is the only location where radials of any kind can be placed.
As I said, my options are limited. Whether I like it or not I have to go vertical and find a way to put down some radials. This is the only way I can hope to improve low-angle radiation and reduce ground loss. Reducing local noise (QRN), which can be quite severe during the evening, is out of the question since I have no space for a directive receive antenna. With QRP that's rarely a problem since I am far more concerned with being heard. A separate, directive receive antenna is of little value for the same reason.
My objectives for the 80 meter antenna:
- Low-angle radiation suitable for DX. I only need enough high-angle radiation to make contest contacts with the nearby US northeast and midwest.
- The minimum radial field to reduce ground loss to an acceptable level.
- Feed system to achieve a 50 Ω match from 3.5 to 3.8 MHz, for the tower, top loaded with a tri-band yagi, and assorted cable runs.
There are so many variables to determining the resonant frequency of a vertical constructed from a tower and yagis that it is often best to just go out and measure it. Unfortunately this requires that the radial system be in place. For a small number and length of radials the radials play a substantial role in determining the resonant frequency. With more and longer (λ/4) radials the radials tend toward non-resonance, leaving the monopole itself as the principal tuning variable.
Calculation can get us close, or at least close enough to guide the design of the radial system and feed network for a tower with a yagi on top. There is a formula for this in the 1st edition of ON4UN's Low-band DXing, page II-32:
L = 0.38f ( H + SQRT( S ( 1000 - H ) / 500 )
L is the electrical length in degrees (λ/4 = 90°), H is the height in feet, S is the area of the yagi in square feet, and f is the frequency in MHz. This formula is not in the 5th edition, having been replaced by a graph for a more restricted range of figures. My guess is that this was done to discourage readers from applying the formula to extreme cases where it is inaccurate. Therefore only use the formula as a rough estimate.
Per this formula, my tower plus mast height of 15 meters, with an Explorer 14 at the top, has an approximate electrical length of 105° at 3.65 MHz. It should therefore be resonant below the band edge. This is close enough to λ/4 to allow a simple matching network. I like that.
However there is more involved in getting a good match on a ground mounted vertical. With a small quantity of radials the radial length contributes to the resonant frequency. With more and longer ones the radial system becomes non-resonant. I will have a few short ones, so it matters.
There are ample resources in the amateur literature about ground loss and the mitigation of ground loss. One in particular I like is the 5th edition of ON4UN's book. On the internet, one good place to look is the series of articles by N6LF. Read them if you have or intend to build a vertically-polarized antenna for the low bands. I will only say a few words here that are specific to my situation on 80 meters.
Z = ( Rrad + Rgnd + Rant ) + jX
The resistance term R in the antenna feed point impedance Z is composed of 3 series resistances:
- Rrad: Radiation resistance
- Rgnd: Ground loss
- Rant: Conductor and ESR (equivalent series resistance) loss in the antenna and matching network
The only way to manage loss outside of the near field (far field ground reflections) is to move to a better QTH! This is outside of my control for the time being, as it is for most hams. What I hope to manage is near field ground loss close to the antenna. This is where the radials come into play.
Using EZNEC, I built a model of a vertical with two grounds.The first ground, out to 8 meters distance from the tower, is poor (0.002, 10) to account for the sand fill and bedrock. From 8 meters outward I use a medium ground (0.005, 13). The intention is to approximate near-field ground losses without distorting the far-field pattern. That's the best I can do, knowing that nearby buildings are within the near field. But then that's true for any low band antenna on my property. There is a significant change in ground loss when the media parameters for that inner ground are varied.
Matching network components should be selected for their low ESR. It is a mistake to only pay attention to the component values and their maximum voltage and current ratings. Transmitting capacitors (fixed or variables) and high-Q coils are best. The loss is there at all power levels so don't take shortcuts if you, like me, operate QRP.
The radial system is unlikely have more than 8 radials. Their maximum length is limited by the size of my backyard and fixtures. Toward the south I can go quite long. North toward the house allows lengths up to 15 meters, and shorter where I have the deck and and landscaping. The main limitation is east and west, where the maximum possible length is 7 meters. Although my lot is ¼ acre it is only 15 meters (50') wide.
With this constraint my choices are to go long where I can and short where I cannot, or to choose equal but short lengths for all radials. The question is which does better? Radial asymmetry is something I've dealt with before, its good and bad points. Once again, EZNEC helps to answer this question.
In the view at right (with currents plotted), X is east and Y is north. My lot is 15 meters wide along the X axis. My house is about 15 meters north of the tower. Going south, there is about 25 meters of available space.
In this first model wires #2 and #4 are 7 meters long. The diagonal wires are 10 meters long and wires #3 and #5 are 15 meters long. It turns out this is a poor arrangement.
Most of the radial current is in the 6 longest radials, with almost no current in the 2 short ones. When the current is so low their effectiveness is quite poor. Snipping those two wires from the models had a negligible impact on ground loss (-0.1 db). The azimuth pattern is omni-directional for all reasonable arrangements of 6 or 8 radials. It is just ground loss and resonant frequency that are effected.
When the two longest radials are reduced to 10 meters length the current becomes more equalized and ground loss is slightly reduced. The relative current flowing in the 7 meter long radials is also higher. I decided to proceed through the rest of the modelling with this arrangement.
Antenna feed and matching network
For the typical shunt-fed tower it is necessary to use a gamma or omega match since the tower is not isolated from ground. The combination of electrical continuity into the concrete-embedded tower base comprises a Ufer ground. My tower is isolated from ground, though not in an ideal fashion. The preserved wood base is a poor conductor, even when wet, and its ground contact area is less than 1 square meter.
It may be worth an experiment to feed the tower directly. If the feed point impedance at resonance comes reasonably close to 50 Ω I'll take it as an indication that the tower's ground isolation is acceptable (see below).
To begin the analysis I added a radial system to a vertical monopole in EZNEC. The radial system has 8 radials of 10 meters length, except for the east and west radials which can only be 7 meters long (see discussion above). Since the radials in this configuration affect system resonance, the monopole must be tuned once the radial lengths are set.
The SWR plot above has the monopole adjusted to an electrical length that is resonant at 3.6 MHz.Notice that the feed point impedance is 60 Ω. This is 23 Ω higher than the 37 Ω of a ground-mounted vertical with an ideal (zero loss) radial system. Assuming this reflects reality (which is unlikely) the near field ground loss would be approximately -2 db. This does not include other environmental loss, such as nearby houses and ground reflections beyond the near field. The true ground loss almost certainly will be greater.
Ground loss is undesirable. Yet if it can't be avoided we can at least use it to our advantage. As the cliche goes, when life serves us lemons, make lemonade! Here we find that the ground loss makes it possible to achieve a broadband match to 50 Ω coax without a matching network. This is not unlike some commercial tri-band yagis where the trap loss permits a direct match to 50 Ω coax even though the radiation resistance may be half that value.
All we now must do is add a series capacitor (between the coax centre conductor and the tower) to compensate for the inductive reactance due to the tower's resonance at a lower frequency. The capacitor would be adjusted for minimum SWR at the selected centre frequency of 3.6 MHz. Tuning can be done with a variable capacitor, and then substituting a fixed capacitor of the required value.
Should direct feed result in a higher SWR (due to actual ground loss or excessive ground interaction at the tower base), a gamma match is the next best bet. The estimated requirement is a gamma rod (or wire) about 7 meters long, a series capacitor to tune the gamma match and tying the radial system to the tower base.
In the unlikely event this is insufficient, an omega match would be required. Were I to attempt to shunt feed the tower on 160 meters, an omega match would certainly be required since the electrical length of the tower would only be 53° at 1.85 MHz. The additional capacitor would add loss, though it would be small in comparison to higher ground loss of the small radial system.
In an ideal vertical installation the coax transmission would be buried to minimize antenna coupling, and would have a substantial common mode choke at the base of the vertical. My situation is far from the ideal, and that may be okay. The coax will have to run overhead, in parallel with all my other cables out to the tower, to avoid damage to the cable and to people.
Some amount of coupling and therefore common mode current is going to be unavoidable. Even were I to follow the ideal for this one antenna, there would still be coupling to the several cables running up the tower to the rotator and other antennas. I have that very situation today with the loaded half sloper antenna for 80. There is coupling, though not enough to be a problem. Running a kilowatt would change my opinion, but that won't happen.
In all likelihood I will run the coax overhead, with all the other runs, and have it dip down toward the feed point at the tower base. Since I will be using a 130' (40 meter) length of RG-213 there will be enough spare coax to wind a coaxial choke. That choke must not be "scramble wound" because the inter-turn capacitance would render the choke ineffective. However, I am unconvinced that a properly wound coax choke is worth the trouble because I know there will be coupling back to the shack on the other cables. This decision will be deferred until I build the antenna.
I'll be removing the half sloper when I am ready to install the second 40 meter inverted vee. I plan to do that in the next two weeks (early September). When that is done, and the septic tank is pumped, I'll be ready to proceed with the 80 meter vertical. Allowing time for experimentation and a further delay toward the end of lawn mowing season, I should have radials in place and the antenna ready by early October.
|Elevation pattern of my tower vertical with the ground, |
radial system and feed system discussed in this article
Based on models alone, it appears I can expect at least 3 db low-angle gain improvement, and probably more, in comparison to the half sloper. Some of that comes from the switch to vertical polarization and the rest from reduction of near-field ground loss by using radials, poor as those radials must be.
I may have to roll up the radials when not in use, at least until early November when people traffic in the yard is no longer a factor. Then it'll be safe until spring thaw.
Once the antenna is built, tuned and I have some performance observations from on-air use I will follow up on this article. You can then compare my modelling alternatives with my final choice of feed and radials.