[TowerTalk] [Bulk] Change in Frequency As Antenna Height Rises

Mon May 30 22:53:15 EDT 2016

Re the element to boom plates/clamps.  I've found they make a noticeable 
difference with NEC4.  However, they are hard to model since the Lawson 
substitute element diameters are so short to cause NEC 2 or 4 to not 
like the segment length. They also have a big diameter step which NEC2 
doesn't like.  Lawson states his taper correction formula is based on 
small diameter steps.   I conclude that those who swear by insulated 
elements do so to not have to swear at plates in the modeling.

Lawson (page 7-12) shows a 0.5% increase in resonant F for a 6x4" 
element to boom plate on a 1" diameter 46Mhz dipole.  Your measured 
21375KHz measured vs 21140KHz modeled with no plates which is a 1.1% 
change in freq with perhaps a larger plate for a 3" boom, but at half 
the frequency.  Direction ok, magnitude seems high.  Does the plate 
effect add up with 5 elements? Perhaps.

So if "relative results" (does that mean range vs model?) are better by 
leaving them out using NEC2, it may be some combination of these 
modeling problems.

I'm not as sure as you are that 2 out of the 3 variables (swr, gain, 
f/b) fully define the performance of antennas over a band.  Sure, f/b 
and gain correlate, but the shape of all three curves vs frequency is 
another matter, with many possibilities. Especially after seeing dozens 
of end point "triple curve plots" that the AutoEZ optimizer has 
generated after many thousands of trial designs.

Congrats on having set up a range - a significant accomplishment.

73,

Grant KZ1W

On 5/30/2016 11:10 AM, StellarCAT wrote:
> agreed - but I have modeled my 6 element 15 and 5 element 20's. They 
> are conversions using a 15M6-125 to create a high strength 15M6DX (as 
> it models much better, especially in a stack) which I call 15M6DX-125 
> ... and using a 20M5 to make a 20M5LGS-100 ... again better 
> performance version ... and I have range tested these. I don't have a 
> way to measure gain but using both SWR AND measureable peak FB points 
> I can see how and if it agrees with the model (and assume, I believe 
> correctly, that having these 2 variables means the gain will follow as 
> well).
>
> So does it agree with the model? Generally it does not - or at least 
> not absolutely - it does relatively ... so the model is off in 
> frequency ... or stated another way using the dimensions from the 
> manufacturer produced results that were frequency-wise different from 
> the model - but the patterns are the same. So the only change is a 
> shift in where it all is. I'm sure the gain is also appropriately 
> placed when I've done this shift.
>
> Also I'll note, and leave it up to others to debate, I built the 15 
> exactly (no worse than 1/8") to the M2 manual - the only differences 
> are the boom size going from the lighter weight 2" - 2 1/2" used on 
> the standard 15M6DX to the -125 boom (3") ... and the clamps are 
> different.
>
> Now I've found that clamps seriously mess up the models - I find it is 
> more accurate in results (again relative results) to leave them out - 
> as well as the swaging (my first models included it) and I couldn't 
> find a clean way to model the boom with NEC2 but using Lawsons boom 
> compensation values it should be trivial.
>
> The results? The 15M6DX showed a FB peak at 21375Khz! I had to 
> lengthen all elements by 3" each to get it to I believe 21140 where I 
> wanted it. The peak FB was ~48db b4 the change and about 44 db after. 
> So - are their dimensions actually producing high results? Or are the 
> clamps and/or boom causing THAT much of a change (I don't believe that 
> at all).
>
> Finally the 20M5LGS made form a 20M5. Here the boom is the same, 3", 
> the clamps are different but because its 20 meters (lower frequency, 
> less percentage difference) I suspected they would have little effect 
> overall. I built exactly to the manufacturers dimensions ... the range 
> testing showed very close results re FB peak frequency and 
> distribution as well as an SWR curve that was nearly identical to the 
> model ---- with again the caveat that the model was at a different 
> frequency! [I was able to make one single change to the manufacturers 
> dimensions - I changed the hairpin moving it by 1" and that improved 
> the already very nice SWR curve making it no more than 1.30 across the 
> band).
>
> So my take is that these tools are not useable as an absolute tool - 
> they are accurate in their results but the relationship of the results 
> to the actual frequency is relative and needs to be compensated for.
>
> oh - and I'm told - if the elements were isolated and I was using NEC4 
> that the results would be accurate both in a curve by curve comparison 
> (model to actual) as well as absolute frequency placement... of course 
> starting with all M2 antenna I wasn't about to isolate them - and NEC4 
> is a pretty penny additional.
>
> Gary
> K9RX
>
>
>
>
> -----Original Message----- From: Grant Saviers
> Sent: Monday, May 30, 2016 1:45 PM
> To: Jim Thomson ; towertalk at contesting.com
> Subject: Re: [TowerTalk] [Bulk] Change in Frequency As Antenna Height 
> Rises
>
> Ok, another cut at this with a much more complex model - 6 element 17m
> beam on a 48' boom.  (I happen to have that model in EZNEC Pro4 and
> AutoEZ).   Finding "resonance" isn't of much interest.  What is of
> interest to me is the change in swr, gain, and f/b over changes in
> height for a design I've been optimizing.
>
> Here is a verbal description of results:
>
> 1. Gain changes smoothly upward from 0dbi at 5' to 16.7dbi at the
> optimized height of 100'.
>
> 2. SWR decreases from 2.6:1 at 5' to 1.3:1 at 15' and  then decreases to
> design height value 1.16:1 by 30' up.
>
> 3. Minimum f/b is 28db vs the optimized value 30db  also at 15 ft.
> However, maximum f/b increases significantly at selected frequencies as
> the antenna is elevated.
>
> A quarter wavelength on 17m is about 14ft, so this modeling of this beam
> would indicate that a bit more than 1/4wl height is sufficient to
> predict results at much higher elevations.
>
> My modeling (as G3TXQ comments) shows that max gain, min swr, and max
> f/b all happen at different frequencies.  So, "tuning" a beam with swr
> seems like a trip to Vegas re what the performance will be.  I think it
> is at best a quick check that there are no gross assembly errors. I've
> had the AutoEZ optimizer generate beam models with 1:1 swr, but with
> reverse patterns, straight up patterns, no gain, etc.
>
> So for beams, I think good models/modeling provide the correct
> dimensions.  The above results were from EZNEC Pro/4, dbl precision,
> driven with AutoEZ to generate 250 test cases, frequency x elevation.  I
> wouldn't try analyzing this without AutoEZ.
>
> Grant KZ1W
>
>
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