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SteveCG · 31st Dec 2020, 1:15 pm

Well, I've done some more modelling.

Since I do not have an aerial for which I've independent experimental results to test these modelling tools I decided that the NBS designs could act as a substitute. So I modelled, using 4nec2, the 5 element design (0.8 lambda length) and the 6 element design (1.2 lambda length) for a frequency of 99 Mc/s. I used the default element wire diameter of 0.0085 lambda and a boom diameter of 1 inch (I use Imperial units). The results from the modelling were in good agreement with the NBS designs. I noticed that the "bandwidth" of the NBS designs was about 6 Mc/s, which rather fitted a single channel for the old analogue System M, 525 line, TV standard.

If you arm-wavingly widened the bandwidth to about 10 Mc/s then the forward gain was consistent with the gains that I was getting from modelling the dimensions of the real commercial FM aerials. By the way, these gains were about 2 dB less than the manufacturers has claimed.

Nevertheless, the 2% frequency shift I started off questioning was present in the commercial designs but NOT in the NBS design.

More puzzling to be done ...

31st Dec 2020, 1:15 pm	#7
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Re: Modelling TV/FM Yagi Aerials Well, I've done some more modelling. Since I do not have an aerial for which I've independent experimental results to test these modelling tools I decided that the NBS designs could act as a substitute. So I modelled, using 4nec2, the 5 element design (0.8 lambda length) and the 6 element design (1.2 lambda length) for a frequency of 99 Mc/s. I used the default element wire diameter of 0.0085 lambda and a boom diameter of 1 inch (I use Imperial units). The results from the modelling were in good agreement with the NBS designs. I noticed that the "bandwidth" of the NBS designs was about 6 Mc/s, which rather fitted a single channel for the old analogue System M, 525 line, TV standard. If you arm-wavingly widened the bandwidth to about 10 Mc/s then the forward gain was consistent with the gains that I was getting from modelling the dimensions of the real commercial FM aerials. By the way, these gains were about 2 dB less than the manufacturers has claimed. Nevertheless, the 2% frequency shift I started off questioning was present in the commercial designs but NOT in the NBS design. More puzzling to be done ...