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Old 29th Dec 2020, 11:05 am   #1
SteveCG
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Default Modelling TV/FM Yagi Aerials

Are any other forum members familiar with the MiniNec and Nec family of computer modelling tools?

See for example: https://www.qsl.net/4nec2/

I have a question for them arising from my experience of trying out the measured dimensions of a few Yagi designs that were on the UK skyline in the 1950s, 60s, 70s and 80s.

I am finding that the aerials seem to be working as if they were designed for about a 2% lower frequency than their intended use. Eg a FM aerial produced for the 88-100 Mc/s band seems to cover the 86-98 Mc/s range better.

My question is this: Have others using these tools found this effect?
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Old 29th Dec 2020, 11:54 am   #2
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Default Re: Modelling TV/FM Yagi Aerials

Can’t help with MiniNec but during the 50’s and 60’s the transmissions centred around 92-93Mhz so 86-98Mhz seems reasonable.
Holme Moss was 89.3, 91.5 and 93.7Mhz, admit I have not looked at other areas.
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Old 29th Dec 2020, 1:43 pm   #3
Julesomega
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Default Re: Modelling TV/FM Yagi Aerials

I have some files left from when I was into Band II dx, and they give similar results. I've said before that the main UK manufacturer of FM aerials continued production of the same design until only a few years ago, despite the gain dropping to zero for the Classic FM frequencies, and the direction reversing above that
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Old 29th Dec 2020, 2:38 pm   #4
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Default Re: Modelling TV/FM Yagi Aerials

Have you done any 'correction' in your modelling to account for the thickness of the elements? I remember a few decades back when the US National Bureau of Standards released Peter Viezbicke 's maths for their Yagi designs that there was a correction-factor for different element-diameters.

"The larger diameter elements yielded maximum gain at shorter lengths while the smaller diameter elements yielded maximum gain at correspondingly greater lengths."

Google for "NBS Technical Note 688".

My Fuba UKA8 was designed for 'european' full 88-108MHz coverage and seemed to work rather well.
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Old 29th Dec 2020, 4:37 pm   #5
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Default Re: Modelling TV/FM Yagi Aerials

With 4Nec2 I have been been to model the diameters of the elements and the boom, as well as the material they are made out of. Indeed I have read the NBS 688 note.

Since the Nec computer programme(s) originated from a "Serious" player I find it hard to explain the discrepancy... Yet my concerns are there for aerials from different manufacturers in all of the three VHF Bands.
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Old 30th Dec 2020, 12:56 am   #6
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Default Re: Modelling TV/FM Yagi Aerials

I see that a new forum has just been set up for MMANA-GAL & GAL-ANA
users: http://gal-ana.de/forum/
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Old 31st Dec 2020, 12:15 pm   #7
SteveCG
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Default 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 ...
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Old 2nd Jan 2021, 9:43 pm   #8
Ted Kendall
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Default Re: Modelling TV/FM Yagi Aerials

Quote:
Originally Posted by G6Tanuki View Post
Have you done any 'correction' in your modelling to account for the thickness of the elements? I remember a few decades back when the US National Bureau of Standards released Peter Viezbicke 's maths for their Yagi designs that there was a correction-factor for different element-diameters.

"The larger diameter elements yielded maximum gain at shorter lengths while the smaller diameter elements yielded maximum gain at correspondingly greater lengths."

Google for "NBS Technical Note 688".

My Fuba UKA8 was designed for 'european' full 88-108MHz coverage and seemed to work rather well.
Scroggie mentions a correction factor of "a few percent" on element lengths in an aerial array but doesn't go into details.
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Old 3rd Jan 2021, 9:42 am   #9
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Default Re: Modelling TV/FM Yagi Aerials

An update:

I rechecked my modelling of the commercial aerials and compared it with what had been claimed over the years. What I found was that later claims were the same figures as earlier ones but now listed as 'peak' gains. These peak figures fitted closer, but still below, to what I found modelling the aerials with their peaks.

... work in progress ...
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Old 4th Jan 2021, 9:37 pm   #10
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Default Re: Modelling TV/FM Yagi Aerials

Were they perhaps quoting dBi rather than dBd ?
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Old 5th Jan 2021, 10:20 am   #11
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Default Re: Modelling TV/FM Yagi Aerials

John,

The units the manufacturers have used over the years have 'evolved' shall we say. Indeed for some, the descriptions of their differing aerials was more on the lines of "for strong areas, medium range, medium-long range, and fringe". What I mentioned above in the posting was for when dBd was explicitly stated.

Last edited by SteveCG; 5th Jan 2021 at 10:25 am. Reason: added clarity
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Old 14th Jan 2021, 11:28 am   #12
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Default Re: Modelling TV/FM Yagi Aerials

A further update:

1) I have found that 4nec2 does not model the presence of a metallic boom.

2) I have entered the dimensions of more Band I, Band II and Band III commercial aerials into 4nec2. Again we have a frequency discrepancy. 4nec2 produces results that are about 2% low. For example an aerial that was sold as working at 188 Mc/s (channel B8) seems fitted for working at 184 Mc/s. Likewise a 100 Mc/s aerial fits 98 Mc/s and a 50 Mc/s aerial fits 49 Mc/s.

3) The aerial gains I noted earlier are all for 'free space' conditions - for example an aerial pointing up into the sky (Radio Astronomy) nearly suits this condition. If though, you point the aerial to the horizon then you need to include the presence of the terrain and this alters the polar diagram appreciably and the notional gain. Indeed the gain to the horizon tends to zero - clearly unrealistic for practical TV/FM operation.

Trying to make consistent sense of all this is work in progress ...
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Old 14th Jan 2021, 12:37 pm   #13
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Default Re: Modelling TV/FM Yagi Aerials

Mulling over point 2) I think I've found a form of consistency. Since the commercial designs all have metallic booms then to model them using 4nec2 - which it appears cannot handle them at all - I need to reduce the lengths of the elements to the boom-less lengths appropriate for say an insulated boom. This would raise their apparent working frequency (as listed by 4nec2) probably to the intended frequency of the commercial designs.

Thoughts please, as sometimes I feel I'm chasing my tail over this!
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Old 14th Jan 2021, 2:05 pm   #14
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Default Re: Modelling TV/FM Yagi Aerials

Any correction for boom structure will be less than you can measure; in any case it will pale into insignificance compared with the corrections needed for any mast it is to be mounted on + the feeder needed to get any signal out of it + the surrounding rooftops + the bird sitting on a nearby tree etc etc
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