wf-design

IV3PRK Pierluigi “Luis” Mansutti

160 Meters: DXing on the Edge

The Waller Flag receiving antenna.

Searching for the best design

The new WF will be built on an 8 m. long aluminum (50 mm.) boom at 13 m. height, attached to a fiberglass mast on top of an insulated 12 meter small tower. Vertical wires are supported by 3 m. long fiberglass tubes (the first three sections of cheap 5 m. fishing rods - diameter from 29 to 16 mm.).

Not yet decided the wire type, between

#13 gauge “Silky” wire by “The Wireman”,

as suggested, or normal #14 house wire.

These are the wires lenghts in the EZNEC

modeling design:

wires 1, 3, 5 ,7 = 6.00 meters;

wires 2, 4, 6, 8 = 2.90 meters.

Varying the loads with AutoEz.

Load 1 is on wire 3 and

Load 2 is on wire 7.

The best F/B (Front to Back) results are shown at load 1 = 580 and load 2 = 605 Ohms,

with an impressive 45 dB peak at 22 degrees elevation.

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The RDF (Receiving Directivy Factor) variation is less pronounced but confirms to be better in the same region.

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Feed system.

In the “Insr.Objects” sheet of AutoEz we substitute the sources on wires 1 and 5 with the transformers and phasing lines connecting them to a central feed point (the new source).

Being the loads in the 600 Ohms area, it is better to use a 6:1 rather than a 9:1 transformer. The SWR now is well below 1.5:1 as recommended by N4IS. ===>

Phasing lines length.

Now, with dimensions and loads fixed, let’s see what happens by varying the phasing lines lengths. Line 1 (to the front loop) is kept fixed at 5.00 m, while line 2 (to the back loop) is changed from 4.00 to 6.00 meters. Phasing lines are 100 ohms impedance, made by paralleling two RG58 cables. Here it is not easy to take a decision. ===>

Keeping fixed the elevation angle at 22 degrees, if we look at the Front to Back (red line), we see the best results (F/B of 45 dB) occurring with the phasing line to the back loop shorter than that to the front loop. But if it is more important the Front to Rear ratio (blue line), than our phasing line to the back loop should be longer (5.7 m.) than the front one.

As a matter of fact, if we examine the elevation lobe, we see that the blue pattern should be better, with a lower high angle radiation on the back and a 3 dB gain in the front direction.

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Thus, as far as the Front to Rear raport is concerned, a phasing line lenght of 5,90 m. to the back loop should be preferred.

Also on the azimuth plot we see that the blue lobe looks better. The highest 45 dB null of the red pattern is very sharp, only at 0°, but without any practical effect. ===>

RDF-Receiving Directivity Factor

This is a new parameter introduced by Tom, W8JI, for eveluating receiving antennas (see Low Band DXing by ON4UN, 5th Ed. pag. 7.10), and according to it, the line to the back loop should be half meter shorter than that to the front one - 4,50 meters to the back vs. 5.00 meters to the front - and that agrees also with the best Front to Back. ===>

In this case we achieve a broad RDF above 11.7 dB, which is comparable to a four square array or long Beverage, with the plus of beeing rotatable!

Loop dimensions.

This graph shows the RDF by varying the horizontal wires length on three different boom lengths. The curves are pretty flat, with just a small dB fraction improvement by making the loops wider.

Not critical at all and not worth of making them too big ===>

Quite a different graph we get from the same AutoEz run if we look at the F/B keeping the elevation angle of 22 degrees! ===>

There is a sharp peak for the loops 2.9 m. wide with 2.2 m. separation on 8 m. boom (second with 2.7 m. wide loops separated 1.6 m. on 7 m. boom).

So, of course, bigger is better and this is confirmed also by the same AutoEz run, looking at tha gain.

Max Gain increases with loop dimensions… but in the real world it has no meaning beeing actually a small reduction of the negative gain ===>

(from -55 dB to -53 dB !)

Nevertheless, we are dealing with these numbers and, as usual, also a decibel fraction makes the difference!

Now, we are investigating on the RDF variations by lowering the antenna to 4 m. o rising it to 30 meters.

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Still looking at RDF, this is a simulation varying the half vertical wires of the loops.

As happened for horizontal wires, there is not a great difference in the RDF by making greater the loops ===>

On the contrary, the same variations have a pronounced effect on F/B. This graph shows that the best vertical dimension, for a 2.9 m. wide loop, is 6 meters (3 + 3 vert. wires) ===>

Checking the load values.

So, before choosing the final Waller Flag design, let’s check again for the best loads. These are the dimensions so far definite:

Boom 8 m. – 12 m. high – Vertical wires 2 x 3.00 m. – Horizontal Wires 2,9 meters. Phasing lines: 5 meters length to the front loop and 4,60 m. to the back loop.

At the beginning of modeling we put 580 Ohms on Load 1 (the front loop) and 605 Ohms on Load 2 (the back lobe) and that's confirmed to be the best (red line) with a F/B raport of 45 dB ===>

Keeping the same parameters (i. e. varying the difference between L1 and L2 from 20 to 40 Ohms) this is a run aimed at RDF and we see that L2 is a bit better at 600 Ohms (+20), but, again, nothing is not critical and don't worry too much for precise resistor values! ===>

But, again, is Front to Back at DX low elevation angles the most important parameter, or F/B at continental high elevation angles to limit background EU QRM?

Or, is it better to take the Front to Rear parameter mostly into consideration? Everyone has is personal choice.

This graph confirms that also F/R is better in the 575/600 region at 22 degr. elevation, while higher load values improve higher angles radiation lobes (45 degr.)

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Anyway, the RDF graph indicates that the loads should be 575 Ohms (V1) in the front loop and 600 Ohms (V5) in the back one, with definitely 25 Ohms of difference between each other.

This is the final EZNEC model showing the complete set-up; X1 and X6 are the transformers connected to a central source. ===>

Transformers.

For this last definitive antenna design, let’s check again the transformers windings.

They have been calculated with my simple Excel spreadsheet.

With 600 ohms load as primary impedance and 100 ohms (phasing lines) as secondary, the rounded number of turns on a typical BN73-202 toroid results to be a 5 vs. 2 turns winding.

Entering those 6:1 Xfmrs in the EZNEC model to check SWR, we find it good below 1.5 line in the 600 ohms load region, quite enough for receiving purpose ===>

Luis IV3PRK September 17, 2016