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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