If some are seeing an SWR much closer to 1:1 ... then there WILL be
interaction guaranteed.
Is it hurting anything? Was that rhetorical? I ask because although I've
done quite well in range testing these antennas I don't have a means to do
BOTH the 40 and the 15. So - I don't know! Thus I'd LOVE to have a model
that seems real that can help - but that hasn't been achievable, yet.
actually - I had recently created a new model for the LLDD ..... it will
work sadly sans the sleeve only ... so I just created an ASCII file of it
and imported it to the 6el15 model that I have. Put the 40 at 124'.... the
15 is at 90' (where my top one will be) ... this is just a dipole on 40, LL
as the M2 is ... aligned it over D1 of the yagi ... before I did that the 15
was something like 11.5db gain and 25 db FB ... after I did this the gain
was 5.8 db and the FB was negative!! It was pointing in the opposite
direction so a 30 db change in the FB! The SWR was 5+. Now we all know that
is not the real world results people get. The SWR generally isn't impacted
much (and in most cases its unknown how much as often both antennas go up
together and are not tested separately at height) - and who knows what the
gain and FB impact is?
Anecdotally, which I really don't like, anecdotal information that is,
people will say "It works GREAT!" ... but for those that have had a dipole
up high in the air - it works extremely well as well! HEIGHT being the key.
(or: and I don't say this to make anyone mad, Mosley beam owners will swear
by their antennas!)
The 40 as its own model seems to be good - the 15 is most definitely good...
the ASCII import is from EZNEC (wish they had a way to create this
compatible import file but oh well...) ... thus my quandary ... experts
please!
g.
-----Original Message-----
From: john@kk9a.com
Sent: Thursday, June 2, 2016 11:17 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Modeling question - for the experts!
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@contesting.com>
Subject: [TowerTalk] Modeling question - for the experts!
From: "StellarCAT" <rxdesign@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.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|