VDA - the antenna of choice?

[DL6GCA]

Of course I love DX … and best of all as S2S … and what is the best way to do that? … with a beam, of course.

But which beam should you drag to the summit?
The requirements are clear: you have to be able to set it up easily and quickly on your own. It must weigh almost nothing, must not be bulky and must be able to hang on the mini mast from dx-wire, which I always have with me.

That doesn’t leave much choice.

If you browse the www, you’ll eventually come across reports about VDA.

Classically, the radiator of a VDA is fed in as a dipole. Since I want to keep the mechanical effort as low as possible for SOTA, I thought it must also work with endfed. When I searched the www, my suspicions were confirmed. So I built another 1:49 balun and cut the wire to length for a 20m radiator.
The length of the reflector and all other dimensions were quickly found.

It was clear that I didn’t want to have a horizontal bar to spread the wires. That’s why I pulled them apart with strings that I fastened to the ground with tent pegs. (I would have fixed the distance with a string in the middle of the element).
So banana plug - radiator - cut string for the distance over the top of the mast to the reflector and then a string to hold the reflector. Then 2 tension lines.
This way the whole antenna consists of 3 bundles + balun and coax.

After I had prepared everything, today was the day of truth. I quickly realised that the mini mast from DX-Wire was too small. I couldn’t attach anything to the top, then everything bent too much and the wires of the elements lay on the ground. It was too short for deeper mounting.
Fortunately, I still have a mast with 12.5m. It also bent, but it was high enough.

.

In the background the mini mast with the endfed (for 40, 20, 15 + 10m) which I have set up as a comparison antenna.

.

Super SWR

Now it was time to compare the antenna. If you build something like that, it has to be worth it.
As a comparison antenna, I set up my Endfed, which is resonant on 40, 20, 15 and 10m. Its wire was fed just above the ground and then goes up the mast and in about 8m diagonally down to about 1m above the ground.

In general, I am less interested in theoretical values than in practical results. I want to hear it.

I stood between the antennas with the KX2 and kept changing the BNC connector.
I quickly noticed that the VDA was clearly directional. Stations that did not come from the main direction were more audible in the endfed. This was clearly noticeable!

The VDA was oriented towards the USA. There are always loud stations there in the afternoon. I was also lucky and was able to listen to a morning round, which must have been from different US states. Unfortunately they never said their call signs…
Here I could hear a very slight tendency towards the advantage of VDA over Endfed.

And now? Is it worth it?

In my opinion, no. Of course, at an VK or NA-EU-S2S party you can set up the antenna in the appropriate direction. You could even turn it - you just have to move the tent pegs. But I don’t think it’s worth the effort. The gain is too weak for that. Besides, you get the disadvantage of receiving much weaker signals in other directions!

73 Armin

[EA2BD]

Hi Armin,
it is always interesting to read your stories, moreover when they are antenna testing!
Thanks for sharing your results. I believe a VDA is a real performer near the sea, where the salted water makes the best out of it. If I’m not wrong the reduced height over ground in a regular terrain, as in your situation far from the sea, makes this an average vertical dipole with certain enhancement towards the beam, but not much as you could expect.

Anyway I’m sure you enjoyed cutting the wire and puttin it up; it is lovely to see them on the air.

73 es CU Ignacio

[MM0FMF]

@DL6GCA

And now? Is it worth it?

Where’s the salt water ground plane in your setup?

[DL6GCA]

@MM0FMF

Where’s the salt water ground plane in your setup?

…now i see my mistake… it was all wet though, because it had rained for a long time… but I had forgotten the salt shaker with my breakfast egg.

[MM0FMF]

They don’t work very well unless you are right on the coast, on the beach where the tide can reach.

[M1BUU]

I built a vertical array with two 17m quarter wave verticals to attempt to work VK0EK some years ago. Unfortunately there was a solar storm during the weekend I had allocated, so I never did work Heard Island. The vertical array seemed to work extremely well though. The QRM cancellation from the back was astounding!

I used the Christman phasing method to feed the verticals and 4 radials on each antenna. I used string to set the distance between each vertical antenna. The vertical antennas consisted of 7m fishing pole with wire elements made from 7/02.

I have used the antenna for SOTA, but for the EU/VK event and it was difficult to get my head around long path and short path. It would be much easier for the Transatlantic event where you could just rely on one beam heading.

73, Colin

[MM0FMF]

@M1BUU

a vertical array with two 17m quarter

That’s a quite a different beast to a VDA though Colin.

[VK1DA]

I was also puzzled by LP and SP until I read an article explaining that because on a globe, the shortest distance between two points is a straight line, the direction can be found using a piece of string (pulled taut) on a globe passing through the two points involved eg England and Australia. When the string is as short as possible, that is the short path. The long path is 180 degrees from the short path. No other direction will reach that same destination. So the rest of the string can be extended in a straight line all the way around the globe, forming a perfect circle. In fact a great circle.

Now when I hear someone calling “CQ dx long path”, I wonder long path to where? Every direction is both a short path and a long path to a lot of locations, all somewhere along the great circle formed by the string.

Sometimes the polar anomalies caused by the curving of the magnetic field near the globes make paths a little different from the directions taken from pure “global string theory” (!) but those cases are rare. And with antennas that have a 60 degree beamwidth you can never tell.

The simplest directional antennas are bidirectional. So positioning for the short path direction also satisfies the LP.

On the dynamic beams like steppir and ultrabeam, the controllers have a SP/LP switch. This merely swaps the lengths and roles of the director and reflector (on a 3 el yagi) allowing you to check the opposite path in a few seconds, rather than waiting for a rotator to turn a beam around. Good demo of the 180 degree relationship.

73 Andrew VK1DA/VK2DA

[DL6GCA]

@VK1DA

Good demo of the 180 degree relationship.

…so I think that for an intercontinental S2S party a Quad or Delta Loop would be a good idea… and on the summit certainly more effective than the VDA

73 Armin

[VK3AFW]

Hi Armin,

Thanks for the report.

I have some comments.

Well you know that if you achieve gain it will be at the expense of signal in the non preferred directions. For two elements this means suppression of the back of the beam signal.

Long Path or Short Path. Pick one.

Now two close spaced elements will give you about 3 - 4 dB gain. Useful but not meter bending.

For DX we can do better with a vertical over 4 radials. The power will be sent at a lower angle than from a beam near the ground No salt water required.

VK1AD and VK3ZPF are exponents of verticals with elevated radials.

If it is 5/8 wave long then we will see some impressive results if installed on a ridge.

A half wave end fed dipole even only at 7 m apex height will give good results on 40/20/15 m gand on a ridge the DX can be amazing. I think your log shows this.

A conventional two element horizontal beam 9 m above ground on a ridge will be as good as it gets for a one man carry in station but it’s getting to be a bit hard for any but short hikes.

I favour the simple one element wire tuned doublet or halfwave because they are compact and easy to setup and use.

I would have kept away from the wire fence btw.

Keep up the reports.

73
Ron
VK3AFW

[HB9DIZ]

@VK1DA

Now when I hear someone calling “CQ dx long path”, I wonder long path to where?

Life is too short to miss opportunities, so bag the QSO.

[HB9EAJ]

Hi Armin,

@DL6GCA

Is it worth it?

Thanks for your conclusions!
Some time ago I also looked at this design for more directional gain but forgot about it, because my telescopic mast is not as long as yours. Now you saved me some work

73 Stephan

[DH8WN]

@MM0FMF

Where’s the salt water ground plane in your setup?

But its not a ground plane antenna. A high conductive ground is important if the feeding point on the lower end has a low impedance. Here the radiator is Lamda/2 long with a high impedance at the bottom. A high conductive ground is not important.

73, Ludwig

[MM0FMF]

Sorry poor terminology from me Ludwig.

If you look at where these antennas are used effectively it is always on a beach. If you read the linked article you see it talks about 0.64λ verticals and their gain. Or these classic VDA designs where it says

“8dB gain relative to a low horizontal beam reported
when used over sea-water wetted sand, or when used
elevated over normal ground”

If you don’t use over such ground or raise it well clear of the ground then the “magic” is missing. i.e. it’s not a brilliant DX antenna. In this case it was neither elevated nor over “sea-water wetted sand” which can be inaccurately expressed as salt-water ground plane.

Some friends who do HF contesting made some VDAs for 20 and also 17m. They tried on a beach and they worked well. Then they moved inland a few hundred meters and they were not very good. The whole purpose of the VDA is it doesn’t need a big mast unlike a Yagi and so is ideal for DXpeditions going to remote islands etc. You could just use classic verticals for your DXpedition but the VDA in the right circumstances can be outstanding.

[DH8WN]

Hm, two or three (?) years ago there was an article in our HAM magazine FUNKAMATEUR. A local HAM made tests with a VDA in his garden, far away from a see or salty water. He got nice results working DX in the main direction, also by comparison with a different good antenna. (After he optimiced his antenna by tests he asked me to simulate his antenna. My results were fitting his.)

Maybe the results would be better over a high conductive ground, but why?

@MM0FMF

0.64λ

This is close to 5/8 Lambda with a lower impedance feeding.

73 Ludwig

[M0TTQ]

I have done some tests next to the sea on 20m with a straight end-fed vertical radiator and reflector (so 2x 12m poles - not a SOTA-friendly load). Without measuring precisely - I did some tests into a webSDR about 3 miles away which was in line with the fence I was tying the masts to - I found a few dB forward gain and a few more than that front-to-back. If there is a fixed direction of interest you work regularly, you could optimise the spacing and element length but for general SOTA use I think there are probably other things you could do that are a more efficient use of resource.

I think for the higher bands maybe there is more in this kind of thing, there are designs for vertical end-fed Moxons which could work well on 10 or 12 using fairly light fibreglass spreaders. I have a 10m one I need to finish optimising.

[VK3AFW]

All antennas are affected by the ground. The ground provides a mirror image of the antenna. If the ground is not highly conductive then energy is lost in heating the soil. Hence moving from being in the tidal zone to being in the sand dunes produces a lower performance.

Elevation of the antenna and adding a radial system reduces the ground losses.

73
Ron
VK3AFW

[DD5LP]

Hi Stuart,

Talking of vertical Moxons, I have two built to my own design (actually adaptation to the standard design) for 10 & 15m that I have tested in the garden but need to do more tests really “in the field”. I have posted about them here on this reflector before.
The problem is that with two masts and guy ropes in addition to the antenna - as you said, these are not really “SOTA friendly” antennas.
Now, the VP2E, IS more portable but I’m not convinced that they work that well under 80m and mine are for 40 & 20m. They don’t seem to have the directivity that is claimed.

Yes, winter is coming in the Northern Hemisphere - EXACTLY the right time to go out and test antennas! HI

73 Ed DD5LP.

[W6PNG]

Here’s another source of design instructions etc for a VDA by a successful group of DXPeditioners.

I agree with Andy that they are typically deployed in beach/salt water area.

http://lagunaria-dx-group.org/projects/4ElementVerticalDipoleArray_EN.pdf

Paul

[DL6GCA]

Great report… thank you for sharing!

… and yes - the VDA seems to be nothing for a SOTA aktivation… unfortunately.

73 Armin