Permanent 160 Meter Antennas Under Consideration

When you have a tall tower it is quite easy to put up a high performance wire antenna for the low bands. My 160 meter antenna is a good example: a catenary tied off at the 40 meter level support a slanted T-top vertical that performs remarkably well. In the most recent Stew Perry TBDC in which I operated QRP I was able to work numerous Europeans, with my best distance slightly over 8000 km. Imagine if I had more than the present 8 × 30 meter radials.

Because it's a temporary antenna it will not see further improvement. I would like to do better on top band with a better antenna. For almost all of my ham career I had no antenna for 160 meter so I am now making up for lost time. My contest scores depend on it, as does my pursuit of DX.

My antenna is temporary because it has to be taken down from mid-spring to late summer -- a minimum of about 4 months -- due to the incompatibility of radials and hay harvesting. It is not only a better antenna I need but preferably one that is permanent. These two requirements -- better and permanent -- are not easily accomplished. I will be happy to have a better part-time antenna and a modest though permanent one.

To this end I am actively investigating options. In this article I'll run through what are, so far, the best of them. I think it is worth taking the time to blog about it because there are sure to be others in a similar situation. The ideas may be of wider interest. But to be clear, this is primarily about my station, my operating objectives and my constraints.

Independent full size vertical

This may be the ideal solution. It requires a new tower of at least 30 meters height, well separated from the two big towers and receive antennas. The former requirement is to ensure enough structural strength for a physical ¼λ vertical, which is 40 meters high. The latter is to avoid destructive interactions and coupling which would distort the radiation patterns of the vertical and the receive antennas.

Separation ought to be at least 1λ, which is 160 meters (500'). That's a lot! Fortunately I have the space. Otherwise it may be necessary to dynamically de-tune towers when operating on 160 meters. Some do this on transmit, but if the array is directive you need to do it on receive as well; that is, full time.

My neighbour would likely be annoyed by this antenna since it takes at least 1 acre out of hay production unless I undertake the herculean effort to bury 2000 meters of radials. The area is trebled or quadrupled should I go further and make a 4 direction, 3 element vertical yagi out of it of the same type as my 80 meter array, just as K3LR originally designed. Again, I do have the space were I crazy enough to go for it.

Limiting the tower to 30 meters height would keep the guy stations out of the forested areas at the east side of my property. A 10 meter high stinger makes completes the monopole. The radials can extend into the forest, with some difficulty. The difficulty is multiplied for a yagi.

The areas where it can go (see the site map) are to the semi-enclosed areas east and southeast of the 150' tower or north of the 80 meter array. Elsewhere are swamp, forest, power lines or other towers and antennas. Feed lines would be very long, which is expensive and inconvenient though not a significant loss risk at 1.8 MHz. Many hams with the land put their 160 meter antennas far away to avoid all these problems. My receive antenna field is close, probably too close, to the east area.

Then there's the expense and maintenance for a tower. In sum, I really dislike this option. My ambitions for 160 meters are not grand enough to justify it.

Shared radial system

The 80 meter array has a large and connected radial system that covers a full acre. The radius is ~25 meters. Although this is a little short for 160 meters there is a lot of copper on the ground. Sharing the 80 meter antenna is an option I've discussed before and tentatively rejected because of the challenge of preserving performance on 80 meters.

There is one more option to consider now that the antenna is up and I know what I have to work with. By placing a switchable coil at the top of the tower, electrically inserted into the stinger, there would be negligible degradation of 80 meter performance and 160 meter performance may be acceptable.

The stinger will need to be electrically isolated from the tower. A thick wall fibreglass tube or solid rod would have the necessary strength. There are enough wires in the control cable to accommodate a signal to switch in a matching network and to operate a vacuum relay to switch in a coil at the top of the tower. A vacuum relay is mandatory due to the high voltage that high up the monopole.

According to the model the coil will have a substantial inductance of 105 μH to be electrically equivalent to a ¼λ. The higher up a monopole the inductor is placed the higher its value needs to be. It is important to design the coil with the greatest feasible Q to minimize loss. The inductor has a reactance of 1200 Ω at 1.825 MHz, giving an ESR (equivalent series resistance) of 6 Ω for a Q of 200, or 3 Ω for a Q of 400. Recall that R = X / Q. Since the higher Q can be difficult to achieve for a coil this size I will assume with the lower value.

A reasonable assumption of 5 Ω ground loss for the full 80 meter array's radial system at 1.8 MHz the loss antenna loss is -2.5 db. The additional coil loss is -1 db. These values are rough estimates. The pattern shows the impact of these losses. The capacitance hat in the model has negligible affect on the required coil inductance due to its small size. A larger hat affects 80 meter performance because it comes too close to the parasitic elements. It may be best to remove them entirely.

It is no surprise that the SWR bandwidth is poor. There is a span of 35 kHz with an SWR below 2. An L-network in the model optimizes SWR at 1.825 MHz, which is close to the centre of the band segment of most importance to me.

It is entirely possible to place the coil lower or entirely forego it by adjusting for the low impedance in the matching network at the base. In either case the antenna efficiency will be lower due to the coil and network loss, and the effectiveness will be lowered due to most of the antenna current being near the ground. A coil up higher put more of the current up higher. It's just that the latter is more difficult to build and switch.

Of course the antenna cannot be concurrently used on 80 and 160 meters. This is a problem in contests were there are no other antennas for these bands. But for the benefit of a year round 160 meter antenna it is of interest to me despite this constraint, the poor bandwidth and the loss.

Between two towers

I have written previously about the presence of an interaction between my current 160 meter antenna and the 150' tower from which it hangs. The physical height of the tower and mast is ~47 meters. The electrical length will be substantially more due to the top loading of the yagis, at least 55 meters. Although not resonant at 1.8 MHz it does distort the pattern a small amount by acting as a weakly active reflector. The estimated gain reduction towards Europe is approximately 2 db.

This is a significant amount for working stations in that direction, and there is little recompense in gain towards the opposite (southwest) direction. I would like to correct the problem. Regardless of what I do the antenna must be temporary due to haying during the spring and summer. But I'd rather have a good temporary antenna than a mediocre one.

To begin this exercise I placed a wire vertical centred between the two towers, which are 60 meters apart. The 30 meter spacing is ~0.18λ, a little more than is ideal for yagi spacing. The towers are modelled as 50 meters high with poor grounds of 25 Ω, representing the lightning ground with no radials. The resistance lightning sees is lower, but the near field of a 160 meter antenna involves a large area of lossy soil.

The antenna radial ground loss is 10 Ω, representative of a mediocre radial system, on the order of what I have on my current antenna: 8 × 30 meter radials. Using MININEC ground simplifies modelling of ground loss.

With this configuration the loss in the tower grounds is -0.7 db  and -1.0 db in the antenna ground, for a net gain of 1.4 dbi. The azimuth pattern is almost perfectly omni-directional. Improved tower grounds increase efficiency while the azimuth pattern becomes slightly asymmetric, with a broadside gain ~0.5 db better than in end fire. My towers are aligned approximately northeast towards Europe to facilitate wire yagis should I choose to do so at some future date.

With the electrical length increased to 55 meters the gain increases ~0.15 db as interaction decreases. This is negligible. The 2:1 SWR bandwidth in both cases is 100 kHz, without a matching network. With a better radial system the impedance would fall and require a simple L-network. However that does not change the SWR bandwidth.

This antenna is simpler and has somewhat better performance than my existing T-top wire vertical. All it requires is a catenary rope between the 40 meter levels of the towers. It would in actuality have to be at least 1 meter lower to avoid the rotation loops for the yagis on the 140 meter tower.

With sag in the centre of the span the true height would be around 36 to 37 meters, requiring a short capacitance hat suspended from the rope or dealt with at the feed point matching network. Gain loss for the slight length reduction is negligible. Were I to have a catenary rope to support a low band yagi it would be at a lower height and therefore require a longer capacitance hat for the 160 meter vertical.

Exploiting interactions

I am pleased that placing a wire vertical between the towers produces a better omni-directional pattern than with just the one tower. This is cause less by the symmetry of being between two towers than the increased distance (30 m) between antenna and tower. The base of my 160 meter antenna is only 20 meters from the 150' tower and the upper arm of the T-top gets to within ~10 meters. Modelling confirms the distance sensitive interaction.

With a 0.18λ spacing it is natural to think about a 3 element yagi switchable northeast and southwest, along with the omni-directional mode. It would require putting radials, switches and loading elements on the towers. For an antenna that requires the radials to be rolled up each spring that's a lot of work. Further, the antenna may preclude using the antennas on both towers while operating in yagi mode on 160 meters.

Is it worth it? Maybe. At the very least a model is justified to explore what is possible. Several decibels on top band can go a long way to improved contest scores. Implementation, if it occurs, won't happen until at least 2020. I have more than enough to do in 2019.