Sunday, September 29, 2013

Multi-band Inverted Vee Performance

It's been almost 2 weeks since I put up the multi-band inverted vee, so now is a good time to summarize my impressions of its performance. To briefly recap, this is 4-band inverted vee with parallel wires for 30, 20, 17 and 15 meters, and that also resonates on 10 and 6 meters.

The apex is up 14 meters. The heights of the ends depends on the end and the antenna length. The tie-down points are up 6 meters (north end) and 8 meters (south-southwest end). The angles from horizontal are about 40° and 23°, respectively.

The comparison antenna is a tri-band dipole (my so-called TH1vn) up 9 meters during this test. It is oriented to favour NE and SW directions, roughly perpendicular to the inverted vee. With both antennas there are no azimuth gaps in coverage, and in many directions they have similar azimuth gain. The only significant difference is their respective heights.

All contacts were made with 10 watts from my KX3 transceiver. QRP is a good way to show up antenna under-performance. Adding +10 (or +20) db to transmitter power will cover up many antenna problems!

I'll get 6 and 10 out of the way first. I have listened on both bands but did not make any contacts. Nothing heard on 6 meters recently except for ground-wave beacons. On 10 meters there have been openings but these have been in the morning when I had little to no opportunity to operate. Most DX is at very low elevation angles on 10 meters, which should favour the inverted vee (in directions where their azimuth gains are about the same). What I found is that there was no predictable pattern for azimuth and path length: sometimes one one better, yet the other might be better on the same path the next day.

I have some ideas about why this might be happening, pertaining to the likely presence of multiple azimuth lobes in the pattern. Until I model the antenna with EZNEC this will have to remain a guess. That's my best approach since neither antenna is rotatable.

On 15 meters the inverted vee definitely shows an advantage on long paths, such as to KH, ZL and 3D2. The vee's pattern has a notch toward the south and north, and that is noticable in antenna comparisons. Therefore the dipole favours Central America and East/Southeast Asia.

There is no comparison antenna for 17 meters. Even so it works well and has garnered a number of contacts in Europe, Asia and Central and South America. I could heard the South Pacific but not well. The same is true of the Middle East and Africa. This is partly due to conditions.

Apart from azimuth pattern differences, on 20 meters there wasn't too much difference between the antenna. The greater height of the inverted vee made little discernible difference.

I was particularly interested in how it would perform on 30 meters since this is the first proper antenna I have had for this band. As with 17 meters, there is no comparison antenna for 30 meters.

I was reasonably pleased with how it did on 30 meters. The best DX was the omnipresent 3B8CF on Mauritius. Europe, the Caribbean and South America were also easy shots, despite the pattern null toward the south. Since my operating was confined to the evenings I did not test its performance toward the Pacific and Asia. Those are in any case hard nuts to crack on 30 with only 10 watts.

Some stations on 30, strong though they were, were not workable. Either they are alligators -- all mouth and no ears -- or the QRN and QRM at their end defeated my QRP signal.

Overall impressions

This multi-band inverted vee up 14 meters works just as if it were...a multi-band inverted vee up 14 meters. In other words, it works but it's still just an inverted vee. There will be no miracles.

Because the Site-B mast on which it is supported is house-bracketed it is noisier than the dipole on the Site-C tower. This is likely due to all the computers and other electronic appliances in my house and the adjacent house. When the antennas are close in performance I choose the dipole. Atmospheric noise covers up the digital noise on bands below 20 meters, so this is not a factor on 30 meters.

There were many stations on the higher bands -- 17, 15 and 10 meters -- that were barely copyable but not workable. Yet others had no difficulty. These were longer paths, including JY, 9M6, 3D2, HL, DU, KH0 among others. There is no magic sauce that can be added to an inverted vee and QRP that will convert these into QSOs. Even with no one else calling they didn't hear me or copied only a letter or two of my call.

I will have to accept these limitations due to my new status as a little gun or I must contemplate a bigger station. For the present and well into 2014 I will remain a little gun. Afterwards...we'll see.

What this means

I plan to keep this antenna for the winter season. This requires that I do a few things to weather harden it so that it survives the winter, everything from sealing joints to spreader stabilization.

The antenna fills pattern gaps in the dipole, an important consideration when one does not have rotatable antennas. It will also allow me to position the dipole perpendicular to the yet-to-be-installed 40 meters delta loop. I want them perpendicular to avoid potential interactions even though my modelling experiments indicate that interactions are low when they're parallel.

I did not bother adding 40 (or 80) meters to the multi-band inverted vee since my experience is that DX performance will be poor at this modest height. There remains some uncertainty regarding 30 meters, so I am contemplating putting a delta loop on the tower, nested within the 40 meters delta loop. This is added work and mechanical complexity that I'd like to avoid, plus my preliminary models show heavy interaction between the 30 meters loop and the tower. This does not occur to an appreciable degree on 40 meters.

I will start by loading the 40 delta loop on 30 with a tuner to determine whether there is something to be gained with this additional antenna work. Otherwise the inverted vee will be my only 30 meters antenna for the next while.

Further antenna work may be delayed until Thanksgiving (second weekend of October) due to my schedule. That's when I plan to try and turn my tri-band dipole into a quad-band dipole (addition of 17 meters), modify the antenna mount and then build the extension mast to support the 40 meters delta loop.

The weather is great for the present. It won't last much longer so I have to get cracking. Winter is coming.

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