Hi Grant - my numbers were as found on a recently calibrated (by Larry)
LP100 as well as the rig as well as the SPE 2K. All agree quite closely.
I've not seen any irregularities here using the SARK so I don't think I have
any local stations to screw with it.
Gary
-----Original Message-----
From: Grant Saviers
Sent: Thursday, June 2, 2016 11:50 AM
To: StellarCAT ; tower
Subject: Re: [Bulk] [TowerTalk] Modeling question - for the experts!
I suggest you retest the 40m beam with a transceiver with a decent SWR
meter. I've found that high larger antennas (80/40 dipoles and beams)
do not measure well on the SARK or AIM4170. There is something at my QTH
which influences the results to higher SWR's than when measured with a
rig at 5w and with a classic SWR meter. Two weeks ago we were tuning two
Field Day 80/40 fan dipoles at 65', one for ssb and one for cw and had
difficulty with both the SARK and AIM. We finally used my K3 and
another quality swr meter in line (they agreed). We also had this
problem with a different AIM4170 when measuring the highest antennas on
my 140' tower. Paul, who was putting up the beams said he frequently
has this problem with his AIM.
The SPE 2K antenna side swr meter agrees as well with the K3 and Pro3
measurements on those beams. The P3 is measuring through a 5B4AGN
bandpass filter set, the K3 and SPE are direct. The error was about the
order of magnitude you are experiencing 2.3:1 vs 1.2:1.
I don't have any BCB stuff within 5 miles, but do have a HV transmission
system about 400 yds away. It is ok quiet 160 and up but perhaps there
is something else being coupled into the analyzers. I should do some
more homework with bandpass filters on the analyzers etc. That's on the
to do someday list.
I had a MFJ269 but that was hopeless at another QTH due to coupling of BCB.
Sound like you will have a great antenna system. If I get a chance
after Seaside, I'll check my 40m W6NL Moxon and 3L full size 40m for 15m
swr. I suspect your LL changes the "rules" substantially.
Grant KZ1W
On 6/2/2016 5:38 AM, StellarCAT wrote:
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.
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