Tuesday, May 9, 2023

A 6m beam for portable operations

There is always a need for another antenna. If this does not sound like a self evident truth you are probably reading the wrong blog.

For a while I had been contemplating building a new 6m beam. I have the DK7ZB multiband beam for 2-4-6 meters. When I made it, I thought it would be a nice addition covering these three bands. In practice however I found I don't really use the 2m and 4m bands and therefore have an overly complex antenna with 7 elements while only using it as a 2 element 6m beam. To make it transportable I have to remove (parts of) the 4m and 6m elements. With the 2m elements the antenna is rather bulky to transport.

I was looking for an antenna for 6m with more gain but less complexity in assembly and that would be easy to transport. One of the things I wanted to avoid was having to use bolts and (wing)nuts to attach the elements - as these small parts have a tendency to go lost when you drop them while out in the field.

My ideal "high gain but still transportable" option came down to a 4 element antenna with a boom of 4m. Split in two the boom is transportable. The same goes for the elements. Now all I needed was a way to attach the elements without using any loose parts.

I remembered an old 11m vertical that had radials you could screw into the base. What if I could make some construction to be able to screw the elements in position?

Keeping the antenna light (it is for portable operation) I chose 8mm diameter tubes for the elements. The inner diameter of this tube fits 6mm metric threaded rod. Glueing these in I had my threaded elements. 

Now for the other side I looked at a threaded rod extension (coupler). There is a round version and with the 6mm threads it snugly fits in a 12mm diameter aluminum tube. 


With this concept working I continued developing the antenna by spacing the elements along the boom (using plastic element holders to attach the elements to the top of the boom) and making them to size.

For the dipole I used a plastic pipe to created a mechanical non conducting connection. I inserted the same threaded rod extensions I used for the other elements. In this case I soldered a piece of wire to the extensions - creating a connection point for the coax.




I then created the quarter wave impedance transformer as specified by DK7ZB and put all the pieces in a small junction box. Note that I cut out an opening in the bottom of the junction box to be able to an element holder for the dipole. That way I know all the elements are nicely in parallel.

In the end there is one spot left where this antenna has bolts and wing nuts: at the point the two boom parts meet. Perhaps I can design some other solution there in the next version.

Time will tell if the threaded connection is an improvement or that it will drive me crazy when dirt gets on the threads (quite imaginable in the field). For now I like the compact package the antenna constitutes while disassembled and I like the way I can quickly put it together.

After completing construction work last Sunday I took the antenna out for a test. For some reason my VNA showed it 2Mhz off with a very narrow bandwidth but the radio was quite pleased with SWR below 1:1.5 in the relevant section of the 6m band. Of course there were no conditions when you need them but PE5TT was available for a simple test. At 25km distance with a comparable antenna we had the same signals both ways. A short test gave an impression about the profile: front/side looked like >20dB while front/back seemed more like 12dB. 

A more thorough test would see me going out with both the 2/4/6 multiband beam and this new beam and compare them side by side. Probably something to undertake in the summer sometime. For now the beam will accompany me on the upcoming trip to HB0 with YNOMY DX Group this weekend.

More info about this antenna will certainly follow. Watch this space...

Friday, January 6, 2023

Delta Loop vs End Fed Half Wave on 20m

Recently I came across a post by a German OM DL3TU who was quite pleased with the results of a delta loop on 20m.

I have a delta loop for 40m but rarely use it as it is quite big. For 20m a delta loop becomes very manageable. 

As my best DX antenna - the portable hexbeam - is not always an option from a time, effort or space perspective, I am always curious if there is another design that could beat my default go to option: the vertical end fed half wave antenna.

So I read through the notes of DL4AAE (who designed the antenna DL3TU wrote about) and built the same antenna. The nice thing about the version he designed is that it has an impedance of around 50 ohm. The design I used for 40m has 75 ohm and therefore needs some matching solution (like a quarter wave piece of rg59).

The new delta loop
Two antennas in the field
One of the beacons

This afternoon I went out to test the effectiveness of the antenna as compared to the 20m EFHW vertical. As always I did this by running two identical WSPR beacons for a while. I didn't have a lot of time before sundown so the dataset was put together over 30 minutes in which both beacons transmitted 9 times.

After examining the WSPR signal reports I found an interesting outcome.

Blue points are the averaged differences between the signal reports of the same spotter for the Delta Loop over the EFHW. The spotters are plotted by their distance from me.

The plot shows that in EU to about 3000km the delta loop beats the EFHW at most spotters, although four spotters only heard the EFHW (orange cross) and the EFHW got more reports in total. So it would probably be hard to tell the difference between the two within EU during a field activity.

Outside of EU the story is a different one. There were three DX stations picking up the WSPR signals. W7, VE6 (6000km) and VK5 (15000km). The VE6 copied both beacons a couple of times but the EFHW was clearly stronger (by average 8dB). The W7 only copied the EFHW but only barely and once - so we will skip that one. The VK5 copied the EFHW 4 times without picking up the delta loop beacon. 

The delta loop deserves a bit more testing but at first sight I would choose the EFHW for both its performance and practicality.