Tuesday, November 21, 2017

Looking at a VDA as a portable antenna (theory)

Always looking for more effective antennas for working DX while /P, I have been investigating the two element VDA (vertical directional array).

VDA design

Stories about the VDA
On the web there are some different stories about this antenna. Everyone seems to agree that this antenna is a great antenna for the expedition to a coastline where the antenna can be put very near to salt water. As with all antennas, the salt water with its high conductivity improves the antenna radiation as the ground losses decrease.

Only so much of our world is coastline, so what happens when you use the VDA over "normal ground"?

Some say it becomes useless. This is however only in comparison to the performance over salt water. In this comparison a lot of antennas are useless over normal ground. We know ground conductivity impacts the antenna radiation pattern, but it is quite another thing to say that you should forget about specific antennas because they perform better over salt water.

The backdrop - what is my reference?
My reference antenna and the one I have used most often, is the vertical end fed half wave dipole. It is very easy to set up (down to 40m - when you go lower other factors come into consideration) and its performance is quite good. Any antenna has to beat this setup to be worth my attention.

I have also used inverted V's and sloping wires. These antennas are definitely better on shorter skip (high angle radiation) but do not increase DX performance. My portable hexbeam beats all of the other antennas but is considerably more work to set up, does not fit everywhere (exp. in a forest), and is only a solution down to 20m. 

My default mast is the 18m high Spiderbeam HD fiberglass pole. I have a higher one (26m) but I consider taking that one as extra effort. I also have a aluminum mast for my portable hexbeam but that is also quite some work to setup. Note that for the end fed vertical a much more compact 12m pole suffices down to 20m.

Looking at it from a portable operator, there are a couple of constraints you have to deal with that limit the antenna performance you can achieve. Winning a couple of dB on low angle radiation is quite a challenge without adding a lot extra complexity.

Over to the VDA model
Okay, so we are looking for a portable antenna with DX specs that are better than those of an end fed half wave set up vertically, without adding too much complexity.

Would the VDA be an option?

Before building anything, I usually model the antenna to see what it will theoretically do. Using 4nec2 the performance of the VDA varies a bit depending on the dimensions you choose for the elements and their distance. I modeled four versions I found documented on the web over ground conditions found in the area I live in (no salt water I'm afraid). The versions are by: PA3FYM (blue pattern below), OH1TV (red pattern), OZ1CX (green pattern) and F4BKV (pink pattern). You can see some design choices with OH1TV opting for higher gain and a lower F/B ratio. They all achieve maximum gain around 20 degrees radiation angle - a good angle for DX.

Four different versions of one antenna

Now how does this compare to my tried and trusted end fed vertical?
The 20m end fed vertical can be set up using a 12m pole, but I would most often use my 18m pole.  So I compared the OH1TV version of the VDA (highest gain) to an half wave end fed with its top at 12m and one at 18m.

The angle of maximum gain is approximately the same - and all in the low range of 15-20 degrees (good for DX-ing). The VDA at approximately the same height as the end fed (12m high), gives a considerable gain of 4dB. All things equal this would mean you would need 2.5x the amount of power to get the same signal at the DX station.
Now when the end fed vertical goes up, the gain increases. The difference is now down to 1.4 dB - corresponding to a power increase of 1.4x 

There is a bit more to say than just these maximum gain figures. With the VDA you lose the omni-directionality that is very convenient when you are not working someone in particular - e.g. when activating a nature reserve. But another factor (for me) is that I am testing this on 20m with the intention to make one for 40m later. In that case the reference end fed antenna is not as high (relatively) as I can get it on 20m, so the difference will be bigger in favour of the VDA. 

All in all this does trigger my curiosity sufficiently to build one. I am planning to make one for 20m first and test the real life performance differences using my WSPRLite beacons. To be continued...

Monday, November 20, 2017

Portable at PAFF-0117

Last week I had a rare opportunity to go out again /P for a longer stretch of time. I decided to head to the south of the country to activate a nature reserve I had not visited yet: Weerter- en Budelerbergen & Ringselven.

Because I had some time on my hands and I had not been on the radio for weeks, I went for the "full monty" - taking a hexbeam for the higher bands and a delta loop for 40m. I was not going to let the conditions ruin my day.

When I arrived at a lake in the nature reserve I was suprised to find a nice sturdy gate - placed by a local fishing organisation to corner off a piece of the lake. This made setting up the hexbeam a bit easier.

Alu mast nicely supported by a fence

There was some old metal pole a bit further away that was helpful for keeping my Spiderbeam 18m mast up. This mast was holding the full size delta loop for 40m. I had not used that in the field apart from some initial testing.

When the antennas were up I first collected J5 on two bands (atno for me) and TO2 on a new band. Then it was time to wake up the WWFF chasers.

Hexbeam at about 10m/30ft

I started on 40m as the higher bands were still rather quiet. QRM was manageable and the flow of chasers was quite constant - logging 116 calls in the first hour.

Then it was time to try 20m with the hexbeam pointing east. The first contact was a nice surprise: a VU2 who commented on my signal level - being 9+10dB at his end. I worked VU only a couple of times before so I was thrilled that he came to say hello and even more so that signals were that good. He was followed by a number of EU chasers. As the earth kept turning North America woke up. K1RO was the first one to make it across. It was time to turn the hex that way. Signals were never very strong but QRM was low on 20m, allowing me to work 23 stations from 15 US states and 4 VE provinces. Not bad at all.

In the meantime I was visited by a member of the local fishing club who wanted to know what I was doing. He made a minor point about me using their gate as a support but was satisfied after a short explanation of my intentions.

After a while I did not get any response on my CQs anymore and dusk was setting in. So I decided to take down the hex while I could still see what I was doing and continue on 40m until I had to leave. Before doing that, I called TI2 and logged him on a new band (17m).

Taking down the hex at dusk

Sadly 40m had turned into a very noisy band. It was hard to copy anyone at signal levels under s9. I did seem to put out a nice signal though as I saw a spot on the cluster from W3 and an OM from A7 (Qatar) came by to ask me what the heck my setup was, as I was that strong. The delta loop has made it into my favourite set of antennas. I logged another 60 chasers through the QRM.

In just under 4h radio time I logged 254 calls from 44 DXCC, 15 US states and 4 VE provinces. ODX was TI2 at 9100km.

Tnx to all the chasers that came by.

Friday, October 27, 2017

Test: End fed wire versus AlexLoop on 20m

Recently I did some tests with my two WSPRLite beacons. I started testing my AlexLoop against a 20m long end fed wire on 40m. The wire beat the loop quite dramatically.

One of the comments I got was that the loop should do relatively better on higher bands. That is why I ran a test on 20m as well. I collected the data about 2 weeks ago but due to other commitments I only got around to analysing it today. 

So what was the set-up? As in the previous article, I set up the AlexLoop in the garden with the center of the loop at about 2 meters high. The end fed wire this time was 10 meters long and ran from the back of my house into the garden sloping from about 7m high to about 2,5m high. I ran the beacons on 200mW for 2 consecutive days. 

The WSPR data shows:

  • 514 spots from 49 spotters were received for the beacon on the wire 
  • 436 spots from 29 spotters were received for the beacon on the loop 

This already gives you some impression about the difference. More stations were able to pick up the signal from the wire. 

Looking at the spot map below (distance in km vs SNR) you can see the orange profile of the wire is slightly shifted to the right (higher SNR) as compared to the blue profile of the loop. It is not as big a difference as on 40m though. 
More prominently you can see that the odx is again much higher for the wire as the signal was picked up across the pond in East Coast US (orange spots on the top).

This time the overlap is bigger, which makes it interesting to zoom in on a single spotter to see the actual signal strength over time. I selected one spotter that generated the most spots (EA8) but did check if the profile was any different at other spotters. This seems like a good sample for those stations that heard both. The orange line shows the SNR of the end fed wire, while the blue one shows the loop. On average (also looking at other spotters) the loop is 3 dB down across EU.

There are not enough spotters to make further assumptions about performance differences at specific distances or directions. What you can see is that for DX the signal from the wire is received within a 6dB range of SNR. The loop is not reaching the lowest end of that SNR. So it seems the wire is beating the loop everywhere but more so on DX.

Sunday, October 15, 2017

One man set-up for the 26m Spiderbeam pole

In January of this year I received the Spiderbeam 26m HD fiberglas pole. It is the tallest they have and afaik the tallest you can get. Cool for low band antennas with little compromise.
I used it only a few times as I can normally get all the antennas I have on my 18m pole (or on my aluminum mast) and I simply did not have the time to build and test low band antennas.

When I did bring the mast out I was reminded of its size and weight - 2m when collapsed and an interesting 18kg. If you have a good support - like the one I build that I attach to my car, or some strong fixed pole you can tie the mast to - it is possible to set this up on your own. Otherwise you need more people. When we used it in the open field during the PACC, we need 3 people.

A few months ago I build a simple support system for my 18m pole that allowed me to set this mast up in the open field without any hassle. I have been using this more often and started to think if I could make something for the 26m mast as well. I wanted to use a bigger surface this time, but use a different structure because it would become too bulky if I just enlarged the 18m version.

Walking around in a DIY store I stumbled upon a flat roof vent. The inner diameter is just over 110mm allowing the 26m pole to slip through with the end cap on.

Now picture this thing pressed between 2 layers of wood with holes on each corner for stakes / pegs. I could just see this working. So I took this home and started working on the two layers of wood.

I chose plywood and cut two pieces approx 60 x 60 cm. I chose the bottom layer a bit thicker so I could sink the bolts I was going to use into the wood (and that way keep an even surface). In the picture you can see that I already cut a hole in the middle with the largest hole saw I have. That saw is only 83mm so I next I enlarged the hole.

Enlarger the center hole
The intermediate result - starts looking like what I had in mind

I added 4 bolts though both plywood layers and the metal, made 4 large holes (25mm diameter) near the corners and glued the two layers of wood together. Another thing I did (like with the 18m pole) is to add a fixed ring on the top of the lower segment of the 26m pole and add three guy rings to it.

(sunk) bolts through the wood and metal

Fixed guy rings

I finished the work yesterday, so today I could make good use of the brilliant weather we have at the moment to go out and see if my construction works.

Just for a comparison of size I put the 26m and 18m poles side by side with the two mast supports.

I then started setting up the 26m pole. I four stakes in the corners of the plate I constructed. They were of different size and mostly to long as you can see. However, they still do the job.

As I hoped, the collapsed pole slipped in nicely and stayed upright - even before I guyed it. That is very convenient as it allows you to add the guy lines one by one and adjust the tension of each one until the pole is fixated and completely vertical. 

I then pushed the segments out with my 20m long end fed wire attached to the upper segment (while I was there I might as well make some contacts). I was lazy and only wanted to assess if my idea worked so I did not guy the pole on any other level. 

As you can see the pole was not completely vertical. This was due to a lack of tension in the guy lines (will put on more tension next time) and of course due to the lack of guy lines higher up. It stayed up though and I was able to make a couple of WWFF contacts in the half hour I was there.

It is easy to set up and break down the way it works now, so I am pleased. The one thing I do need to add is a small step or something else that allows me to stand a bit taller. I am 2m tall but still the pushing out of the last segments is heavy work.

Saturday, October 14, 2017

Test: End fed wire versus AlexLoop on 40m

Recently I acquired two WSPRLite beacons. I want to use them to test various antennas to find out which ones are most effective /P. I have built a number of different antennas and I still have a few projects planned. Modelling antennas gives you an impression what to expect but you only find out in the field what the real performance is. Actually, you need (more or less) parallel reports to account for changing conditions and WSPR is a nice system to get these reports accurately. 

Two identical 200mW WSPR beacons
I started using the beacons at home testing the performance of my AlexLoop against my default antenna - an end fed wire. At home I use 20m of wire that is resonant on 10, 20 and 40m. That wire runs from the back of my house at approx 8m high to a corner of my garden and then still a couple of meters along the far end of the garden - sort of an L-shape. It ends at approx. 1.5m high. Nothing special - not your DX killer.

Although this is a convenient test for me to do at home, the comparison is not that exotic as both a low hanging and sloping end fed wire and a portable loop are realistic /P antenna options.

The first tests I used the AlexLoop in the attic on 20m, 30m and 40m. It turns out that although the loop is higher there, the attic itself attenuates the signal too much. It is just not picked up while I do get spots from the end fed wire. So I moved the loop to the garden at a height you would put it at when going /P - the center at about 2m high.

AlexLoop in the garden
I tuned the loop to 40m. Now spots were coming in for the AlexLoop. Running both beacons for 24h on 40m I can get a good picture of the difference between the AlexLoop and the (low) sloping end fed wire. 

Comparing the spots of the two antennas from the same RX stations, I find that the loop is mostly 10dB or more down from the end fed wire. That is an enormous difference. There is no advantage to be found on any distance for the loop.

Odx for the AlexLoop was EA8 with 3000km (2 spots from EA8) with 39 different spotters over the measurement period.
Odx for the end fed wire was N8 with 6500km (14 spots from W) with 110 different spotters over the measurement period.

The graph above shows the spots for the end fed wire (blue) and the AlexLoop (orange) on 40m over the same measurement period. The distance to the receiving station (in km) is on the vertical axis and the signal strength (in dB) reported by the receiving station on the horizontal axis. 

Apart from the two orange dots at 3000km (EA8) the AlexLoop does not reach much further than 1000km. You can also see that the end fed wire gets better signal reports - with significantly more dots to the right. 

So, my first test of the beacons worked out well and I have my first bit of data on two antennas. My idea is to create a separate section on this website dedicated to antenna performance based on future experiments with the beacons.

Wednesday, September 27, 2017

Projects - 4m slim jim antenna

As I wrote before I now have a working 4m set-up using the 4m transverter kit from DF2FQ (XV4-40). I also have a compact 2/4/6 meter beam that I will describe in more detail in a future post.

Although the beam is compact, I cannot use it at home as it is too large. How can that be? Well, I don't have a fixed mast - all my antennas start from my attic. I use a 20 meters long sloping end fed wire from my attic into the garden in an L shape for 80m-10m (resonant on 40/20/10m - works OK for 40-30-20). For 6m I use a vertical end fed wire that I put on a fiberglass pole in the attic and then push up through an attic window (I sometimes use this construction for lower bands as well). For 2m I have a compact 4 element beam that I can just push through the same attic window. I attach that one to an aluminium push up mast.

The 2/4/6m beam is just too large to fit through the attic window. So for working Es - one of my favourite sub hobbies - I needed a solution for 4m (having 6m and 2m covered). With Es you can get good results even with a "simple" vertical - as I have already experienced using my end fed wire for 6m. So, it was time to create a 4m vertical. Using a bit of ladder line PG8M had lying around and a nice online calculator by M0UKD (with construction tips), I created a slim jim for 4m.

Ladder line as the basis for the Slim Jim antenna

The online calculator and all the other info on John's page give you a good starting point but then you are tasked with finding the exact feed point position. As John M0UKD notes, it is important to perform the final tuning in a set-up that resembles your target situation. I stuck the ladder line to my fiberglass pole in the garden and pushed that up a couple of meters to make sure the antenna was away from obstacles (as I will be using it when I push it out from my attic). I tried different feed point positions, each time making measurements with my VNA.

Locating the feed point
Finding the feed point position is a trial and error process. This is slightly complicated by the wires being insulated. I didn't like the idea of having to remove the insulation on a large stretch to find the right spot, so I used the set-up in the picture above: two needles connected to my VNA. That way I could try different points and check the effect on the antenna SWR curve.

Finally I added a W2DU style current choke on approx 1m of coax (soldered to the feed point) that conveniently terminates with a SO-239 connector: ready to go.

The end result gives me an antenna with an SWR of 1:1.6 on 70.200. I might improve that a bit more by tweaking the stub length in the future but as it is I am pleased to know I am ready for next year's Es season.

Saturday, June 24, 2017

First 4m QSO - getting a transverter kit working

Almost a year ago I bought a 4m transverter kit from DF2FQ (XV4-40). I have basic understanding of electronics, acquired in part through the hobby but building a project like this was new to me. With my mechanical engineering background I am more confident in building robust stuff like antennas, masts, supports, baluns, etc. Soldering loads of tiny components on a board seemed challenging. 

However the level of documentation provided by DF2FQ and some motivating words by PG8M - who was already using the transverter - pushed me to the point that I ordered the kit. When I received the package I must admit it looked daunting. 

Lots of parts..
..and rather tiny

I started building the kit sometime in August last year. I tried to work as meticulous as I could (a challenge when you are as impatient as I am). When I had completed the kit and ran the first tests, I found that I received 70 Mhz signals with the radio tuned to 29Mhz. Hurray!

However, when I tried to transmit, the transverter appeared to be dead as a dodo. No LED, no signal. A very disappointing result. I did some tests (as far my knowledge goes) and could not find the cause of the failure. So the kit went back in the box it was shipped in. Time for other projects.

The Es season is rather good this year and I have enjoyed the effects on 6m but 4m was open a lot of times as well. I really had to get back to the abandoned transverter project. PG8M had already kindly offered to help me months ago and today I took him up on his offer. We went through the error checking process step by step, finding and fixing four major issues. It turned out I had made two soldering errors and two of the smd resistors were faulty. Whatever happened to these resistors I really don't know.

PG8M trying to locate the next error

The good news is that we ended up with a transverter that delivered up to 35W on 4m with 5W drive on 10m. 

There was no one around to do an on air test so back home I hooked up the transverter and put my tri-band yagi on a pole @2m high in the garden. PG8M was QRV 30km away with his 4 ele beam pointing in my direction. That is when I made my first 4m QSO.

DK7ZB tri-band beam in action twice this week

It is a bit late in the Es season but I am sure I will get back to this band in the future. The next thing I need is an antenna I can set up quickly at home in case of any high MUF situations. For 6m I use an end fed wire that I can attach to a pole that I push through the attic window. For 4m I am going to try a slim jim vertical. More on that later.