An FYI for those interested.
Note this is ONLY about an M2 40M4LLDD. There are other 40 antennas that
are/were intentionally designed not to have an interaction issue with 15
meters. JK, Optibeam, (some) F12 amongst them. Moxon’s also inherent to their
own design.
In my situation I have 2 M2 6 element 15’s. What I call 15M6DX-100. They’ve
been range tested and are very good performers re match and FB. The plan is for
a rotating tower with 2 of these – one at 45’, one at 90’.
I will also have, on that same tower and initially planned in the same plane an
M2 40M4LLDD. It too has been tested although unlike the 15’s above, which were
range tested here, its FB testing has been anecdotal using signals heard on the
band as well as the RBN. It seems to be similar to the M2 published numbers. So
I can personally say that the worries of Linear Loading, at least in my mind,
or no longer there. It WORKS. The SWR is actually slightly better than the
manuals numbers (cut CW/LO PHONE). It will be mounted at 124’.
My original plan was to model/design a sleeve to force this antenna not to
interact with a 15 meter yagi mounted vertically above or below it. I had asked
for input from owners of the 40M4LLDD – its 15 meter “SWR” ... I got 2
replies... one said “low” and the other was a value of 1.3. So it was my worry
that with that low of an SWR and with the antenna anywhere near the 15 yagi it
WILL interact. How much is anyone’s guess. No longer a guess for me though –
read on.
Efforts to model the 40M4LLDD have succeeded to a limited extent - I created
one that worked but wouldn’t allow SWR checks above about 20 Mhz (kept getting
“negative power” errors). Grant, KZ1W, asked for my model and kindly worked on
it and got it to run. Now unfortunately I can’t get my ‘sleeve’ concept to work
in the model so once again I’m stuck. Note the ‘sleeve’ was tried on an actual
element of this beam. Used as a dipole – it did resonate somewhat on 15 as
built – and the sleeve improved it (moved the SWR up to about 6:1) so it
appears the concept worked. But in discussing this further with Greg, W8WWV, we
both agreed that we don’t know if any knock-on effects to the 40 itself will
occur. SO although it might solve the issue with 15 it might just hurt the 40
in the process (or cause issues to the 20 that will be even closer to the 40).
A situation I’m unable to have a range test for or a set up for.
So I decided to instead just turn the 40 ninety degrees to the tower. I already
plan to have an 80/75 dipole at the top turned 90 deg to the tower – so now
these 2 bands will be turned and I believe the GH control can reflect that
offset.
But before I do this I decided to test for interaction to a 15. Both inline –
and at 90 degrees.
These are the results:
-40 mounted at 37’ and rotatable. It has been used for a few weeks now – works
great.
-15 mounted at 12’ (just over 1/4 wave) directly below and as it would be
“inline” with where it would be in relationship to the 40 – i.e. not shifted in
any direction from the tower.
SWR measured with the 40 turned 90 degrees to the 15. The results were about
what I had gotten months ago when the 15 was tested by itself. Very good.
SWR measured with the 40 inline to the 15. The SWR went up slightly both in
value and frequency. Note if someone was using both of these antennas in this
situation and were, due to the issues of installing antennas, not able to check
the 15 without and then in the presence of the 40, they’d think the results
were very good and be perfectly happy with them. They could honestly say “the
15 works great”! But – it DID change.
In all my modeling – still very green at it and far far from the ‘experts’ – I
did notice that FB was the most sensitive variable/parameter of a yagi. It will
show the effects of changes and interaction before you’ll see changes in the
SWR. SWR is second most and a ways back in sensitivity, and a good distance
back is the gain – you almost have to work to reduce gain.
FB tests:
-with the 40 turned 90 degrees to the 15 I range tested the 15 (XG3 with 10 db
pad (20db RL) connected to a 10’ dipole mounted about 25’ up and about 550’
from the test antenna – flat SWR so expect (and get when testing) flat results
across the band). The FB was about what I had previously tested with the caveat
that I hadn’t checked it at 12’! It was centered, the FB peak, pretty close to
where I had previously measured it and the values indicated FB was there
changing about 20 db from the band edge, to the peak and back to the band edge.
-with the 40 turned inline to the 15 the FB disappeared! It was gone! The
signal was 20 db stronger and basically flat across 15. Note that is about as
much as what would be ‘normal’, at the band edges and off the FRONT of the
antenna. SO there is severe interaction – and this is with 25’ separation.
Now some will say the 15 was too low. Indeed I’d agree that because of that the
results will vary probably with measureable, if not impaired FB found when the
antennas are much higher – but I’d also say that there is interaction and it
will be there wherever the antennas are assuming the spacing is this or less.
I’d guess and those modelers out there that know might agree that because we’re
now in to phase issues that the FB impact will change as the spacing changes
getting better or possibly even worse – but I don’t know that you’d ever get
back to the expected results of the 15 alone.
Please write direct if you have any other questions. I’m done – antennas built
– tested and now on to the tower – FINALLY. Thanks for the bandwidth guys –
sorry about all my posts... thanks to all that have contributed.
Gary Myers
K9RX
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