[TowerTalk] Modeling question - for the experts!

[john at kk9a.com]

A full sized 40m dipole will have a high impedance on 15m and the SWR will
be over 2:1 so the 2.7:1 SWR on your beam indicates that it is resonant.
Is it hurting anything?  Your model should show the interaction using the
stock 40M4LLDD antenna and spacing to other Yagis.

John KK9A


To:	"tower" <towertalk at contesting.com>
Subject:	[TowerTalk] Modeling question - for the experts!
From:	"StellarCAT" <rxdesign at ssvecnet.com>
Date:	Thu, 2 Jun 2016 08:38:35 -0400


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.