[TowerTalk] Modeling question - for the experts!

[StellarCAT]

Ok... so here is my situation. 

I will have 6/6 on 15 with the top at 90’. I will have the 40M4LLDD at 124’ so just 34’ above it (a good distance for stacking on 15!)

I had asked for others with this antenna to test it for SWR on 15 and two replied. One said it was low, the other said it was low and provided a value of 1.26:1. That surely seems low enough that it would indeed couple in energy and re-radiate probably seriously impacting the FB/FS. I tried my ‘sleeve’ idea on this dipole and sure enough it moves the lowest point down to 18Mhz and the SWR on 15 is now about 6:1. 

I had posted regarding this situation a number of times – possibly ad nauseum... for that I apologize. 

I then built the full antenna and have been using/testing it for a couple weeks now on my test tower at 37’. On 15 it measures, unfortunately, neither too high nor too low. It is 2.7:1 at its lowest and using the SARK110 I can see that it goes lower (2.3)  than this lower in frequency so if it moves upward as I go to the final height it might very well be relatively low on 15 ... I say might be because who knows what will happen – however it is appropriate to reiterate that 2 others found it to be low – lower than this. 

So I don’t know if I want to do the sleeve on all the elements. This is why:
-I don’t know what impact it will have on 40 itself. 
-I don’t know, if I add the sleeves, without getting it to height if it will “do its (wanted) thing” 
-its obviously adding additional complexity to the antenna and increases the possibly of mechanical issues if one or more of the wire sleeves fails. 

SO – as I’ve mentioned I find that modeling is not as reliable as others suggest. It becomes a guessing game of sorts trying to figure out why it isn’t behaving as say a real world test suggests it should (and one has to be informed enough to even know that it is/isn’t) or worse yet if you have no real world results yet – can you trust it? My limited experience on the 15/20/40 modeling/tests I’ve done says no – you can’t. [my models have been checked by a very experienced modeler who has written many an article on the subject – nothing obvious found wrong with them]

So here are the questions:

1) WHY when I model a full size 2 element 40 does it not have a low SWR on 15 when I know in the real world people have said it should/does? I just tried it again using a wire antenna – SWR on 40 drops to about 1.1:1 ... the lowest it is anywhere near 15 is 5:1 at 22 Mhz. This is real average soil and at a height of about 60’. 

2) The LLDD isn’t something that models really easily. Since there are these LL wires at an angle (I have a model if anyone wants to write direct) ... so the issue isn’t getting it to work on 40 – it seems to model ok there – rather it is to get it to test (check the match) on 15. Because of the requirement to align segments this gets to be a bit difficult to do although I managed to get it to work on the basic element. But on adding in the sleeve I’ve found it impossible to do. One gets a negative power error because of it and hours spent trying to align segments hasn’t resolved that error. 

3) I’ve thought about just modeling a 4 element full size and adjusting it such that it shows a low(er) SWR on 15 ... THEN use it to see how it effects the pattern of the 15 spaced at the 34’ distance it will be at... but once again I don’t know if that is a close enough approximation and of course I’d be manipulating the results to get what I think it should be... 

Gary 

ps: if the reader is of the mind “why bother” or “if it were me I’d just put it up and not worry about it” ... I can understand that thinking – but with all the expense and time of trying to optimize and check things for me at least I’d rather not be ignorant of the (possible) results – I’d rather understand the implications, if I can, so I can then decide what to do... which might very well be just put it up.