Two elements YAGI for field day operation

With the success on the invisible attic wire beam, a lot of club members showed their interest on this trial and discussed possibility of similar application. It was finally decided in a morning net that we should turn this into a club project - make a beam antenna for the club's annual field day activity.

A special work group was formed in Apr-98 to host this build program. Further meeting within the work group defined the following criteria:

Model simulation of the plan

Detail of the technical parameter of this antenna can be found at http://www.qsl.net/ve3rgw/antfarm/antfarm.html .

Data of this beam had been fed into an antenna modeling software to find the best configuration.

 

 

Above figure shows general structure of this dream beam. It composed of two inverted V dipoles placed 10 ft apart. This distance is chosen to resemble 0.15 wavelength element spacing at 14MHz. We tried various combination of ground clearance, tilt angle of inverted V, different length of the wires. Final result from the simulation indicates that the antenna must have at least 15-ft average clearance from ground surface. Reducing this ground clearance will raise the take off angle of the beam to a situation that it will no longer be suitable for DX operation.

Projection on horizontal plane indicated a pattern like a dipole but with a 7db difference between front and back direction. Very strong attenuation is seen on the left and right side (over 18db drop as compared to front).

 

Vertical plane projection (realistic ground reflect) indicated a 40 degree take off angle with a similar F/B ratio. The higher the antenna from ground level, the lower the take off angle. Above result is based on condition that lowest point of the antenna will be 10 ft from ground.

The supporting mast is finally decided to be made of two 10-ft fencing pipes (stacked on top of each other). This 20-ft. pole is then secured in standing position by 3-nylon guy wire. The guy wires are then pinned down at 20 ft. from the main mast and 120 degree apart from each other. In order to allow the boom to rotate freely in future, the boom is loosely clamped to the mast with an oversized U-bolt. The whole boom is then hanged from the tip of the mast with two nylon ropes (like a suspending bridge structure).

The wire elements of the antenna (extending from the two ends of the boom) are then terminated on insulators. These insulators are then pinned to ground via nylon ropes. In brief, the foot print of the whole array turned out to be close to a square patch 65 ft x 20 ft.

In order to work on 40m, the ropes used to secure the driving element are replaced with two runs of wires (15 ft. in length). By connecting these extra wires with the driving element (over the insulator), the antenna becomes a 40m dipole. That's why there are four insulators on the front but only two at the back. These four legs of wires and ropes are then pinned down to ground at a distance of about 35-ft. from the center mast. A 10-ft. section of rope is placed on each side between the insulator of both 20m sections (main driving element and the reflector). This is to ensure a constant spacing between the 20m elements. There are two kinds of insulators in this antenna, 4 of them have two holes (used for termination of the reflector elements and the two 40m elements to ground. The other two are triangular in shape and have 3 holes. They are for terminating the 20m directors, the 40m add on elements and the nylon rope that controls the spacing between director and reflector.

 

Marking anchor pin location with fixed length rope from center.

Boom, main mast, suspending ropes, U-bolt installed. No wire elements yet.

 By using a vertical drop line, the standing angle of the main mast is verified.

(click for magnified view)

Completed wire beam and the tension ropes. Rotation is possible by releasing the 4 tension guy wires from their anchor and steering the whole system along to the newly desired azimuth by four people.

Close up view of the boom and the antenna, the R.F. choke can be seen slightly behind the feed points of the driving element. No BALUN is used in this case. Matching is achieved by cutting length of the driving element for this specific coaxial run application.

Tune up and directive effect verification

We have precut the 20m elements and the 40m extension elements with 196" length. Once the mainmast is up, we start trimming on the 20m director. This has to be done before setting up of the 20m reflector. Our computer simulation is based on bare wires. Since we used insulated wires for our antenna elements, the actual length is shorter. It turned out that we need only 193" to achieve the lowest SWR for the director dipole alone (close to 1:1.2 at 14.2MHz). Setting up of the reflector element lowered the frequency to 14.15MHz but raised the SWR slightly. We don't have time to work on the 40m that day so it is still on its default extend length 195".

 

 

Normal procedure to test directivity of an antenna system usually involves high precision rotators to control azimuth and tilt angle of the antenna. A field strength meter will be placed at a fixed distance from the antenna. A signal is then transmitted by the antenna while reading is recorded by gradual change of both azimuth and tilt angle.

We can only perform the directivity test with a simplified but reversed profile. Instead of turning the antenna, we turn the measuring spots at fixed distance but different angle from the antenna array. This method is not accurate due to influence from the surrounding reflection and is limited to test for the H-plane on zero degree elevation. Yet result still indicates a close match with predicted outcome based on our early model simulation.

We finally came to the moment for a real test. The initial bearing of the antenna was set for 270 (true north) since our targets for that day were stations from west and central Canada. We picked up VE7 (Vancouver) at S10+9 with ease but when it came to the moment to contact VE6 (Calgary); the signal turned out to be only S1. By turning the azimuth to 300 (true north), the signal pops up to S3-5. This effect really surprised most of us since we never can imagine a 2 elements beam can be so sharp in beam angle. But it could be a mismatch effect between the antenna's take off angle with the angle of incidence of the incoming signal. This concluded that it might be necessary to include antenna tilt control in our next attempt to use this antenna in future or we have to find ways to raise the antenna further.

In a whole, we are very satisfied with the experience and outcome of this project. The construction team already planned to try for a diamond quad next time. Stay tuned.

 

Last updated by VE3RGW 25/July/98.

 

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