Monday, June 20, 2011

DXpedition Antennas for Salt Water Locations

DXpedition Antennas for Salt Water Locations

A study on 20m Antennas

Which is better a 20m antenna for an island DXpedition: Yagi at 25', or a vertical or two on the beach? (Hint, You are on a web site dedicated to vertical antennas - this is a trick question!)

"Conventional Wisdom" would say that Yagi (typically a tribander or 2 element full-size Yagi) would be a great choice of antennas for a DXpedition on a beach. What makes the Yagi so great? Lets examine the reasons to use a Yagi�

The Top 10 Reasons to use a Yagi on a DXpedition, even when you are on the beach:

  1. You have used Yagis at your home station
  2. You understand how they work
  3. Yagis have directivity
  4. Everyone else uses Yagis on DXpeditions, so the must be great
  5. and�

Hmm. I can't think of any other reasons! But everyone knows that any antenna by the ocean works great, or at least "conventional wisdom" tells you so.

O.K., lets examine the use of a Yagi for a serious DXpedition to a remote island. For this discussion, let's assume you are going on a DXpedition of a lifetime to Kingman Reef (KH5K) in the middle of the Pacific Ocean. Kingman is an interesting island which is about 25' wide, and 450' long. The entire island is made up of broken shells, rocks, and rubble. There is no sand, trees, or anything else. The wind typically blows at a steady 30 mph. You are days away from any kind of medical help.

Now imagine the difficulty of assembling a typical triband Yagi, where if you drop a nut, you won't likely find it again. Then your team has to armstrong the 35-40 pound Yagi, which is on top of a 25' mast, into the upright position. I'm assuming you aren't even going to consider a rotator, which would add another 25 lbs. to the top of the mast! Remember that the wind is blowing at least 30mph, and you are standing on loose rubble. To get the Yagi installed, you will probably need 3-4 people to walk the antenna up and hold it in position while securing the base in the shells, finding stable guy points (for 2 sets of guys, or 6 ropes total) in the rubble, and adding a tag line for rotating the Yagi. Any slip-up, and one could easily get cut on the rubble, or even possibly break a bone if you fell. Any relatively small problem like that in the tropics will likely become seriously infected in short order. Based on the wind and the hazardous conditions, you will probably need 4-6 people for a few hours to safely erect the antenna. This is looking like a lot of effort for just one antenna�Ahh, but waiting hams are worth the effort aren't they?

Maybe, but your health comes first. You may wonder - is there an easier and better way?

First, lets think a little more about propagation to Kingman Reef. Kingman is in the middle of no place, more precisely, in the middle of the Pacific Ocean. Based on the distances from the 3 main target areas, most take off angles are likely to be very low, usually under 10 degrees, and often below 2 degrees! Europe is the main target, and you will likely spend half your operating time on long path (typically very low take off angles). Most typical Yagi antennas have a beamwidth of around 60 degrees, which means that you will likely have to turn the Yagi for each of the main target areas:

Directions to target areas from KH5K:

JA: ~ 305 degrees

Europe: from ~355 degrees to 20 degrees (which is the main emphasis for the expedition)

USA: from ~43 degrees to 58 degrees

Continuing with our original "Yagis are the best" scenario, you remain convinced that the Yagi is still the best antenna. What if I told you that a simple 1/4 wave vertical on the ocean, or better yet a 2 element vertical dipole array, would be a far better performer! I can hear the pundits saying: "Verticals are for kids!" or "Real men use Yagis!" You believe there is no way that a Yagi could be inferior to a vertical!

So why do you think that verticals work so poorly? Because "conventional wisdom" says so? Unfortunately "Conventional Wisdom" on the performance of vertical antennas usually comes from comparing a Yagi at home to the trapped vertical in the back yard. At home the Yagi will be better, but not on the ocean! On the ocean, you will see that verticals are the clear choice for high-performance antennas, that just happen to be easier to install too!

First off, let me dispel one long-standing myth: Horizontal antennas over salt water do not get any enhancement from the salt water (from increased ground conductivity). Well, to be exact, almost all useful angles for HF propagation get little or no useful enhancement. The horizontal antenna (and so does the vertical) receives a benefit from the ocean, because the ocean presents an undisturbed foreground for the incoming and out-going energy. Salt water also has less loss than typical ground for every reflection. To show you the impact of ground conductivity, the following two Figures were created: Figure 1 (below) shows that the take off angles in the 30-90 degree range (straight up) do get some limited enhancement from the salt water. Figure 2 (below) compares a 2-element vertical array over land and salt water. The only antenna that getssignificant signal enhancement by being next to, or over, salt water is a vertical. In fact, verticals get about a 6 dB increase of gain when placed over salt water, and the radiation in the pseudo-Brewster angle is filled out (which is radiation under ~12 degrees). So the salt water is enhancing signals right were most DX signals are arriving - in the pseudo-Brewster angle!!

Figure 1: 2-element Yagi at 25' over land and salt water. Peak gain is 10.40 dBi (over salt water) at 38 degrees take off angle in this design.

Figure 2: 2-element vertical dipole array (parasitic) over average land and salt water. Peak gain is 10.01 dBi (over salt water) at 8 degrees take off angle in this design.

OK. Let's get back to Kingman.

I suggested that a simple vertical would be better for most useful take off angles� let me show you why. Figure 3 (below) compares a 2 ele Yagi at 25' height over salt water compared to a 1/4 wave vertical over salt water. The antenna with lots of gain at 40-degree take off angle is the 2-element Yagi at 25'. The other antenna is the 1/4 wave vertical. Notice, at take off angles below 10 degrees, the vertical is the hands down winner! You may point out that the Yagi has more "gain". This is true, but the extra dB or two gain is at take off angles that don�t matter! A simple vertical by the ocean can and will usually outperform a Yagi on most typical DXpedition paths, because the energy from the vertical has fewer hops to the target! It's all a matter of the angles of the arriving signals.

Figure 3: Yagi at 25' compared to a 1/4 wave vertical over salt water

As you saw in Figure 3, the 1/4 wave vertical is a good performer. Even better for our Kingman Reef expedition is that a single vertical is really light weight (maybe 3-4 lbs for the 16' vertical), and can be assembled and installed in just a few minutes. Compared to the Yagi, the vertical only needs some rudimentary guying, and you don�t have to worry about turning the antenna! And since you are going there to operate, you can be on the air in 30 minutes if you wanted! Try that with the Yagi.

I bet that some of you aren't convinced yet that a single 1/4 wave vertical is a good enough choice. OK, lets up the "vertical anti" a bit, and rather than a 1/4 wave vertical, lets examine a 2-element parasitic vertical array using 1/2 wavelength vertical dipoles. To better understand what this array looks like, picture a 2 element Yagi standing vertically, but minus the mast and boom to support it (much less overall weight). (see picture to right)

There are a few advantages of using vertical dipoles (vs. 1/4 wavelength elements): they eliminate the need for radials, and by raising the feedpoint, you achieve an additional 1.5-2.0 dB of gain at low take off angles. A parasitic vertical array (vs. a phased array) is an easy way to increase gain and directivity, yet is still very easy to install and tune up. The parasitic array needs less parts than phased array, which is an important aspect on DXpeditions. Figure 4 (below) compares the 2-element Yagi at 25' to the 2-element vertical array using vertical dipoles. You can see that the vertical array now has nearly the same peak gain as the Yagi, but the gain is focused right where most of the arriving signals are coming from: 0-10 degrees! And the verticals are now the clear winners from 0-20 degree take off angles - who could ask for more? Since many of the signals are arriving at around 1 degree take off angle, often the verticals can be 20 dB stronger than that Yagi! Talk about a band-opening antenna!

Figure 4: Yagi @ 25' compared to a 2-element vertical array using vertical dipoles

But I'm sure that the Yagi-lovers are saying: "I bet if you were able to get the Yagi up to a good height like 50', that it would surely beat the vertical array. Think again dude! First off, installing a Yagi at 50' on a DXpedition is difficult, if not impossible. Secondly, it still doesn�t compare to the vertical array! Figure 5 (below) compares the 2-element 20m Yagi at 50' to the vertical array. Notice that the Yagi now has a large amount of energy being radiated straight up. Sure the main lobe is lowered, but the 2-element vertical will still be better on most DX signals.

Remember, that a typical tribander weighs at least 35-40 lbs., and add another 20 pounds or so for the mast, for a total of at least 55-60 lbs. A two element full-size vertical dipole array for 20m weighs no more than 18 lbs. if properly built. A vertical array is much more efficient if you consider a key metric for DXpeditions: dB per Pound of Antenna.

Figure 5: Yagi @ 50' compared to 2-element vertical array using vertical dipoles

And you still need more convincing? OK, Figure 6 (below) is the energy at 5-degree take off angle. The antenna with all the signal (the big one) is the 2-element vertical array which has 9.9 dBi at 5 degree take off angles. The antenna that is down 12-20 dB in all directions is the good ole Yagi at 25'� Another key point is that a 2 element vertical array has a very wide pattern, which is on the order of 120 degrees for the -3dB points. Thus the 2-element vertical array can cover all 3 main target areas without turning the antenna!. In addition, if there are callers from other directions (assumes there is a water path in those directions), the vertical has more useful gain than the Yagi does on azimuths that are off the side or back of the antenna.

Figure 6: Energy at 5 degree take off angle of a Yagi @ 25' compared to a 2-element vertical array using vertical dipoles

Some of the pundits might still say this was a "Made up" example�Sorry to disappoint you, but the Kingman Reef example was real, and was based on the planning discussions for the upcoming Kingman Reef DXpedition. For many island DXpeditions, vertical arrays should be given more consideration on the high bands than they currently are receiving.

The computer models have been verified by empirical testing we did from salt flats while shooting over salt water. The measured data follows the model. Verticals were selected for the "Team Vertical" locations in the Caribbean and comparisons to full-size Yagis were made, confirming the models, as well. The comparisons were done over many continuous hours of switching back and forth, with differences of up to 9 S-units (on an 'MP meter), averaging 2-4 S-units, in favor of the verticals. The Team Vertical Yagis were occasionally used during the contests, but only to cover directions where the verticals has nulls. This might be one of those situations where if you haven't tried it, you don't realize what is happening. How good is a larger vertical array using vertical dipoles? Tom, N6BT, has commented that they are truly on par with the large commercial curtain arrays he used in Saipan. Salt water is the key to verticals. If you can get close to salt water, or literally sit right in it, such as at Kingman Reef, verticals are the answer to high performance antennas - besides being significantly easier to transport and set-up.

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