Tuesday, January 9, 2018

Antennas on the Big Tower: How They Play

I now have sufficient experience with my new tower and antennas that I can begin to see how they perform. This is important in forming my plans for the future. None of the antennas on the tower is intended to continue for the long term. As a recap these are the currently deployed antennas:
  • XM240 2-element short 40 meter yagi, rotatable at 47 meters
  • TH6 tri-bander, rotatable at 44 meters
  • Explorer 14 tri-bander, fixed northeast (Europe) at 34 meters
  • 40/80 meter fan inverted vee, broadside to NE/SW at 32 meters
  • 160 meter t-top wire vertical with 8 x 30 meter radials, 20 meters from the tower
Heights are correct to ~1 meter. All main runs are Heliax (3 x LDF5, 1 x FSJ4) plus LMR400 to the lower antennas and RG213 to the 160 meter antenna. Coax from the outdoor 8 x 2 switch are LMR400 Transmission line loss is very low and not a significant factor for any of the antennas.

Power has been 200 watts (FTdx5000) in daily use, and 5 (KX3), 100 or 150 watts depending on the contest and entry category. My first take on performance was after CQ WW CW, before the TH6 and XM240 were connected. I expand upon that here with more on-the-air experience and with all antennas in operation.

Note on S-units

Do not interpret S-unit differences in the discussion below in decibels. The S meter on almost every amateur receiver is not linear with respect to decibels per S-unit and even nominal decibels per S-unit are not the same among receivers. The "standard" 6 db/S-unit is mostly a fiction. Even the more typical quote of 4 db/S-unit is not reliable.

I therefore do not attempt to estimate decibels from S-units when comparing antennas. In any case signal strength over time on any ionospheric path is too variable for the purposes of good precision.

So don't look for quantitative measurements here. My observations are informal indicators of relative performance. This is nevertheless useful. Exact antenna measurements on sky wave signals are hard to do. Really hard.

160 meters

The wire t-top vertical performs exceptionally well for such a simple antenna. It shows what can be done when you have a 43 meter tall support to work with! It's been used in 3 contests so far -- CQ WW CW, ARRL 160 and Stew Perry -- and casual DXing. I am pleased its performance. But how good is it?

The best test so far was the Stew Perry Top Band Challenge. Many stations with good antennas operate with QRP since there are significant bonuses to both parties in the QSO and the points multiplier is especially rewarding on long DX paths. Looking at the results so far I did well in the category despite being on for only 5 hours. I worked as far as eastern Europe, a similar distance to other QRP stations with good antennas.

In daily use I find that I can crack small pile ups even though many or most stations are running high power. Most hams have difficulty putting up an effective antenna on 160 meters and it shows. With just 8 radials a simple near full size vertical is very competitive. I now have 62 DXCC countries worked on top band from casual and contest activity.

This antenna is especially useful as a base line. It allows me to calibrate performance against others with better antennas and therefore make an informed plan for a permanent antenna. More receive antennas are needed before I increase power to a kilowatt since I can see that I will only attract callers that I would otherwise not hear. That's a good problem to have.

80 meters

This band is where I believe I have seen the smallest performance improvement. It's the same inverted vee I had at lower height, now combined with a 40 meter inverted vee for dual band use. The previous apex was 19 meters compared to the current 32 meters. It certainly works better although the improvement is incremental.

Horizontal antennas on 80 meters need to be higher than where I have it. Consider that λ/2 is 40 meters, and at its apex height of 32 meters the inverted vee's effective height is likely no better than 25 meters, or only a little more than λ/4.

A proper comparison cannot be made until I at least have the driven element of the 80 meter array operational as an omni-directional antenna. For now I have to be satisfied with good, not great results. It has been good enough to work VU and other difficult to reach DX with just 200 watts.

40 meters

This is a band where height makes a substantial difference. Right now I have three antennas on 40 meters, the two on the big tower and the multi-band inverted vee at 19 meters on the smaller tower. It is only on a small number of nearby paths that the lowest antenna is better so I will restrict the discussion to the high inverted vee and rotatable XM240 on top of the big tower.

Before sunset the inverted vee is often stronger towards Europe though not always. It's important to switch between the two to find what works best. Once it is dark there is no question that the yagi is always the best choice. Since the F/B is poor it can also be used most of the time to work Europe and North America without bothering to switch to the inverted vee, even though domestic signals may be stronger on it. Short paths are already strong enough except for the smaller stations.

On intermediate length paths such as to Europe and the Caribbean the yagi is at least 2 S-units better. Where it really shines is on the longer paths. For example, long path to east Asia in the early evening can be better than 5 S-units better on the yagi. This is no surprise. With 100 to 200 watts I have had little difficulty putting countries such as BY, JA, 9V, YB, UA0, HL and others in the log. Working 6O6O was more difficult but it's in the log. South America and the Pacific, and the Antarctic, are typically 3 to 4 S-units better on the yagi.

Perhaps the biggest problem with the yagi is that the prop pitch rotator turns slowly: ~0.6 rpm. You really have to think whether it's worth the time to catch one multiplier. Most of the time it's faster to use the inverted vee and have to call several times to get through.

30 meters

While I did not put up a new 30 meter antenna I have one nevertheless. Just as the 80 meter inverted vee worked well on 30 meters, the third harmonic with a tuner, it still works the same at its new height and form as a 40/80 inverted vee.

Unsurprisingly at 32 meters height it compares favourably to the multi-band inverted vee up 19 meters on the shorter tower. The difference ranges from 1 to 3 S-units depending on path and distance. Noise pickup is lower since it is much further from the house. I now rarely use the lower antenna on this band.

20 meters

Of all bands 20 meters is the most enigmatic when it comes to my current selection of antennas. I have in effect 4 antennas on this band, from the multi-band inverted vee up 19 meters to the TH6 up 44 meters. Path length, direction, time of day and overall conditions determine which antenna works best.

As I've discussed before the Explorer 14 fixed on Europe still does pretty well for working the US since the F/B is not great on 20 meters. Three elements on such a short boom is far from ideal. Even so it can be very useful by limiting the necessity of switching antennas, and of having a second antenna pointed at the US much of the time. The situation will change when I redo the antenna later this year.

Despite the lower height the Explorer 14 almost always delivers the best signals from Europe. On marginal opening the higher antenna does better. The performance difference is 1 to 2 S-units most of the time, but is occasionally greater.

On other short to medium length paths the rotatable TH7 works quite well despite being much lower at 21 meters height. Even to the current 6O6O DXpedition the Explorer 14 and TH6 are about equivalent. In this case I favour the TH6 because the better F/B is better at hearing through the pile up.

This selection of antennas, and nodes in the elevation patterns, make plain that the ionosphere determines the path, not the antenna! On 20 meters 44 meters (2λ) can be too high much of the time.

As for 40 meters it is on the longer paths that the high antenna shines. Whether it's Asia, the Pacific or South America the TH6 is better than the TH7 by no less than 2 S-units and can be another 2 to 3 S-units better than that. On long path to VK and the Pacific the difference is remarkable. Signals pop out of the noise on the TH7 to almost S9 on the TH6.

Of course there is no need to always use the best antenna. Other than for the longest paths I will more often simply use the one closest to the correct direction, turn it and use it. Making the QSO is what matters, not burying the other fellow's S-meter.

Height clearly brings a new level of performance to my station on 20 meters. This bodes well for upcoming DX contests when I expect to increase QSOs and multipliers.

17 meters

There are two unintentional antennas for 17 meters on the big tower: XM240 and 40/80 inverted vee. Both are efficient and present good feed point impedance, although the rig's ATU is required for full power operation. That the XM240 resonates is no surprise since it is widely known that the antenna has a harmonic resonance near 18 MHz. Unlike full size 40 meter antennas the third harmonic resonance (3λ/2) is lower due to the effect of the loading coils.

There is good and bad performance on 17 meters with these antennas. The good is that they're much higher than the multi-band inverted vee up 19 meters. The bad is that the azimuth patterns are complex with multiple nodes and lobes typical of antennas operated well above their fundamental frequency.

As a result it requires switching among the three antennas to find the best signal, and then adjusting the tuner. This is inconvenient though acceptable for a non-contest band. For now I can live with what I have. A resonant gain antenna (yagi) on 17 meters is low priority.

15 meters

Most of the time the TH6 outperforms both lower yagis. It is rarely worse. This may in part be due to the low MUF and therefore low elevation angles on all paths at this time of year and solar cycle. It seems that every 15 meter opening is fleeting.

When the band is open to Europe I will swing the TH6 towards more distant shores and the TH7 towards the west or southwest, using the Explorer 14 for Europe. While not ideal for all openings to Europe it gives me the flexibility to roam all the high bands for multipliers with the TH6 up high and for shorter paths with the TH7 down low. This approach works well in contests.

On longer paths such as South America, the Pacific, the Indian Ocean and east/south Africa the high antenna is superior to the TH7 by 3 to 4 S-units. Compared to the Explorer 14 to the Indian Ocean and east Africa (about the same bearing from here as southern Europe) the difference is smaller, usually less than 2 S-units.

10 meters

My experience on 10 meters with the new antennas is limited due to conditions. When the band has opened to South America, ZS and ZL the high TH6 is the clear winner by 2 to 4 S-units or more over the TH7 at 21 meters height. With no European opening on 10 meters this winter I cannot yet compare the TH7 to the side mounted (fixed) yagi lower down by a full 1λ on 10 meters.

Indications are generally favourable for when conditions do improve, and they will do so as the winter progresses despite the low solar flux. Back scatter signals from US stations appear to be stronger as well on the high yagi. Again, this is as expected.

Where I go from here

The tower is high but the antennas are not the best. It is enough to show the promise but not deliver it, at least not yet. Three more things have to happen in order to put out a dominating signal.

The first is to replace tri-band yagis with mono-band yagis. Depending on the band and boom length an improvement of from 1 db to 4 db can be expected. The price to be paid is clutter on the tower, expense and management of interaction between antennas and with the guys. This is not a job you want to get right the first time.

The second is to stack those mono-band yagis. Depending on height, boom length and spacing the gain improvement typically ranges from 2 db to 4 db for two identical yagi and more by adding more yagis. The problems of expense, clutter and interactions are obviously worse. If one or more of the yagis in a stack can be separated and rotated the challenge is even more acute.

The last is to increase power. Running 100 watts in a contest with this antenna farm is competitive against others with similar power, but not with those using more modest antennas and a kilowatt. There is no easier and cheaper way to gain 10 db than to plug in a box and turn it on. Of course this is no help with receiving well or competing against those with high power and big antennas.

You might also wonder whether one or two S-units matters at all. For casual operating perhaps not. Where it makes a big difference is in contests, putting many more QSOs with smaller stations within reach and cutting through the QRM at the other end, especially within Europe. Your individual interests determine whether the improvement is worthwhile.

These are topics I will come back to later. I have decisions to make before I can proceed with my plans for 2018 and beyond. I intend to be modest in my ambitions. But for now I am on the bands and having fun with what I've built so far.

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