Sunday, April 9, 2017

Project - Changing a CB beam into a 10m beam

(originally posted in June 2012)

As blogged before I got myself a cheap 11 meter 4 element yagi (Sirio SY27-4). I am now in the process of making a manageable yet strong and high enough mast to use in a portable set-up as well as optimising the antenna itself.

Today I started using a modelling program called 4nec2. It took me some time to figure out how to use it but in the meantime I have modelled a few different configurations of the Sirio.

This is what the antenna radiation pattern looks like when used in its original configuration on 27.2Mhz

Horizontal pattern (original) 3d pattern (original)
The main figures for this original configuration @ 27.2Mhz: Gain: 8.39dB, F/B: 18.5dB and beamwidth: 60 degrees.

Paul MM0ZBH advised me to shorten all elements by moving all sections in by fixing them with only one screw (they are normally held by 2 screws per section). This means a reduction of the elements lengths by 8cm. This slightly changes the relative size of the elements (relative to eachother). So I was wondering what would happen to the radiation pattern. It looks like this:
Horizontal pattern (one hole shorter) 3d pattern (one hole shorter)
The main figures for this shortened configuration @ 28.45Mhz: Gain: 8.59dB, F/B: 13.9dB and beam-width: 60 degrees.

Gain is up a bit but F/B ratio suffered from this change. F/B ratio is however not a main concern for me in the portable set-up. To be sure that this configuration would be a good one I also compared this to a completely scaled down version of the antenna - changing the element lengths and boom position by x% based on the higher target frequency. The outcome of this exercise:
Horizontal pattern (scaled down) 3d pattern (scaled down)

The main figures for this scaled down configuration @ 28.45Mhz: Gain: 8.41dB, F/B: 18dB and beam-width: 60 degrees.

The F/B ratio is back up to 18dB. Gain is roughly the same. To me this does not seem like a worthwhile change as it involves adding new holes to the elements. It might be a good step when F/B ratio is relevant.

While researching antenna models I also came across a model designed by DF9CY. Christophe apparently also changed an 11m beam into a 10m one. The radiation pattern of his antenna also looks very good. The position of the elements along the boom is a bit different: the dipole is much closer to the reflector.

The radiation pattern of the DF9CY yagi:
Horizontal pattern (DF9CY) 3d pattern (DF9CY)
Christophe's version performance @ 28.45Mhz: Gain: 8.64dB, F/B: 16.3dB and beam width 60 degrees.
The performance looks a lot like the scaled down version of the Sirio. Christophe achieved a bit more gain while opting for a bit lower F/B ratio.

All in all this tells me that taking the Sirio and changing it by just moving all sections in by one screw hole leads to a very acceptable 10m beam that is hard to beat by any other configuration (if you take the 4m length of the boom as a given). The sigma match for this configuration needs to be 22cm extended for the SWR to be flat around 28.4Mhz.

Another advantage of taking the "one screw hole"-approach is that you can use it the other way around and create a rather decent 12m antenna. If you keep the boom positions as they are and move all sections of all elements out by one screw hole (from the original two screw connection) you get a 12m antenna that works as shown below.
Horizontal pattern (one hole larger, 12m) 3d pattern (one hole larger, 12m)
The performance @ 24.65Mhz: Gain: 7.61dB, F/B: 9.02dB and beam width 60 degrees.
This is not brilliant - especially the F/B ratio is low - but compared to my vertical wire this should deliver a better performance. The question is whether I will be able to tune this antenna using the standard gamma match. Will have to find that out later.

Note that all models above are based on an antenna in free space. If you take the chosen configuration for 10m ("one screw hole shorter") and put it on 6m height (what I am aiming for). You get the following performance:

Horizontal pattern Vertical pattern
The performance @ 28.45Mhz: Gain: 12.58dB (at 25 degrees), F/B: 15.4dB
At 15 degrees elevation (more relevant for DX) the gain is still 12dB. This should help me to get in touch with some DX.

Update November 2012: In the meantime I built the portable alu mast to hold this antenna. The mast goes up to approx. 13m (43ft) without the need for guying (even with the beam). I used this combination during CQWW SSB 2012. The beam was at that time around 11m (37ft) high. It then has a theoretical gain of 13.6dB at 15 degrees. Perfect for DX-ing on paper but also in practise. Check out my experiences here.

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