Tuesday, October 23, 2018

Testing my VDA on 20m

Last Monday I found some time - at last - to go out and do a proper test of the VDA I built many months ago. It is a 20m version and I only built it to test the effectiveness before deciding to build one for 40m - for which I don't have a directional antenna yet.

My 20m VDA
The only way - imo - to assess the performance of an antenna is to compare it to one or more (default) antennas at the same time. Conditions vary strongly by the minute, so you need an A/B type of setup. 

What I did was the following:

1. I set up the VDA on my 18m SpiderBeam pole with the feedpoint (at the bottom - as my VDA is end fed) at about 3m high.
2. I set up an end fed vertical for 20m with the feedpoint also approx 3m high.
Vertical at the picknick table
VDA with vertical in the background

I then performed three types of tests:

1. I ran 2 identical WSPRLite beacons for 90 minutes (0.2w output)  
2. I generated spots on FT8 with my FT857 (50w output) using an antenna switch and comparable feedlines 
3. I did some live SSB RX tests using the s-meter and one TX test (100w output) with the help of Timo OH7JHA.

One of the two WSPRLite beacons
I collected the WSPR data from the WSPR database and the FT8 reports from hamspots.net

Data collection (and some first impressions on the go)

Overall impression

Looking at all the data there is not an easy path to a final conclusion. So bear with me.
Especially the FT8 data seems to present a complex picture - where sometimes the vertical is the only one heard and then the VDA while they both have decent reports with that particular spotting station. My assumption is that the FT8 data is less reliable as the band was crowded at the time. I saw that I was often competing with other stations on the same frequency. For me that could explain why some of the patterns in the data seem to contradict / give inconclusive outcomes. 

In general I find that with 50w (@FT8) output the simple vertical generates more spots (is received more often). This makes sense as it is omnidirectional and the spotters can be found in all directions. With 0.2W (@WSPR) this is no longer the case as the omnidirectional signal becomes too weak to be picked up in many cases. In fact at one point the conditions had gone down and I received no spots at all with the vertical while still being spotted with the VDA (btw I swapped the beacons to be sure there was no influence there).

Looking at both the WSPR and FT8 data I can also see that the VDA almost always "wins" when the station is in the direction the VDA is pointing in (assuming a 60 degrees beam width).

WSPR results

Using only the WSPR data the VDA is a clear winner. The vertical only wins once and only marginally while the VDA was pointing in the other direction. There are a number of ties but when the station is in the direction the VDA is pointing in, the VDA wins 100%.
The VDA makes it across to PY (9000km), while the vertical only reaches EA8 (3000km). 

FT8 results

The FT8 data paints a less clear picture:
  • The VDA is the only antenna that reaches VK (multiple spots from VK2 and one from VK8 up to -13). 
  • When pointed towards JA the VDA creates a lot more spots than the vertical (from more spotters) but the overall SNR is only about 1dB better on average.
  • When pointed towards W the VDA is comparable to the vertical with only about 0.5dB better reports on average.
  • When pointed toward 4X the VDA clearly wins (5dB, 2 spotters) while it mostly loses when pointing the other way (as you would expect with a directional antenna).
  • In EU the VDA mostly wins when directed towards the spotter with the exception of EA (several spotters - maybe there was some obstacle in that direction?). However the VDA also loses sometimes or wins when it is pointing in another direction - which does not make sense if it is directional. It seems that in 1 skip distance the directivity is not as noticeable or other factors play a more important role (FT8 qrm, type of antenna used by the spotter, etc.).

"Live" results
The live data also gave some different outcomes. Testing with Timo - pointing the VDA to OH - gave no conclusive advantage (Timo reported an s9 on both antennas) while turning the VDA away only decreased the signal by 1 to 2 S-points. 
Listening around the band and switching between the two antennas increased my understanding of the difference in practice. When the signal was loud the difference was hard to notice.  However, when the other station was DX and weaker (FH, W) the VDA was up to 2 S-points stronger. I have made a short video of W1ZY/M station working with a mobile antenna, where - even though there is QSB - you can see the difference in RX quite well.


Getting more hard facts - so as to be able to make any claims about gain figures - would require setting up both antennas for a couple of days with the two beacons. Sadly I do not have the space to do that. 

So for now, we will have to work with what I have learned on Monday and by going through the data:

The VDA is not a miracle antenna. The theory (as I discussed here) shows that it should have around 3dB gain over the vertical I used as reference. Practice shows it has gain over the vertical that is noticeable (relevant) on multi-hop DX (eg with VK) and weaker signals but on shorter distances the effect is less clear. The data also shows that the VDA has directivity but I have not been able to assess how much that is.

All in all this to me is not enough to warrant a 40m version of this antenna. I will be taking this 20m version along on my future /P adventures as it does not take up that much extra space. If I learn more, I will share it.

If you have other experiences with this antenna or comments on the way I constructed or tested this one, let me know. I am always eager to learn more.

Tnx to Timo for his help and the many spotters (that were unaware of their support to this test).

1 comment:

  1. Hallo Lars,
    Interessant verslag van je testen met de VDA. Mijn complimenten dat je zo uitvoerigmogelijk de testen heb gedaan om een zo goed mogelijk beeld te krijgen van de VDA t.o.v. een Endfed.
    Gezien de resultaten kan ik goed begrijpen dat de VDA voor 40m nog even in de ijskast blijft staan. Toch verwacht ik dat de VDA antenne betere resultaten zal geven bij hogere frequenties van 20 tot 10m. Succes met je verdere interessante experimenten. 73 Hans, PE1BVQ.