Jim,
Good question (why doesn’t M2 just redesign it).... note on the F12 the
distance from the boom to the LL is considerably less than 1/4 wave on 15....
once the LL is reached I believe (and another antenna manufacturer whom I
talked to agrees) it acts as a trap to isolate the antenna up to that point...
and since it is short – the resonance on 15 isn’t there (but would be there on
12!). The 40M4LLDD has almost a 1/4 wave on 15 up to that point which would
mean a rather large change to the antenna to (possibly) eliminate the
resonance. Also most say “so what” .... not knowing how they’re being impacted
and thus there is no pressure to make the change. I believe now with 3
manufacturers making antennas (40M) designed not to interact at least on 15
there might be some amount of pressure.
Also note this is not at all to impugn M2... I believe they make an excellent
line of antennas for a quite reasonable price. When done I’ll have with one
exception an all M2 antenna farm from 40 through 2 meters – 9 yagis in all. The
one exception is the F12 Magnum 80.75 rotary dipole which I love and which
worked extremely well in Arizona. It will be at the top at 144’.
I tried using the Optibeam model shorted at the boom end ... that didn’t work.
So Instead I tried using what VE4XT had suggested to me which was to short it
at the far end... and that seems to work well. You can indeed adjust the length
to move the resonance point – and if not careful it can end up instead at 17
meters... the final model I have puts it above 17M and has 30:1 SWR above 20
meters through 30 Mhz.
NOTE: this whole discussion – this possibility of a test is thrown out for
those in a position like myself to test it if they want to... I have the
antenna, new in the box. I will assemble it and try it. We’ll see. I’ll report
the results.
Gary
K9RX
Date: Mon, 15 Feb 2016 08:56:36 -0500From: "StellarCAT" <rxdesign at
ssvecnet.com>To: "tower" <towertalk at contesting.com>Subject: [TowerTalk]
Changing interaction of M2 40M4LLDD on 15Hello to those that have the
40M4LLDD.... I believe I have found a way to eliminate the interaction of the
40 to a 15. I?ve done a good bit of modeling and came up with a possible
solution. I?m happy to pass this along to anyone that wants to try it. I?d love
to know if it works as modeled. Write to me at my QRZ email address for
details. I hope to build the antenna, test it at 36? (test tower) and then add
this to see if indeed it does the trick. Note although I (believe) I know what
needs to be done I don?t yet know how it would be done mechanically ? i.e. the
specifics. It isn?t really complicated at all, pretty basic change in principle
so shouldn?t be difficult.Gary MyersK9RX
## so why doesn’t M2, just build the damned thing right the 1st time ???
Optibeam uses decoupling stubs on their version of their 4 el 40m yagi. One on
each side of boom,so 8 x stubs in all. The stubes on the optibeam appear to be
small gauge al, coming out from the 40m els..at at right angle for a couple of
inches, then parallel to the ele for a couple of feet. Far end of stub is held
in place with a small insulator back to the ele. You can see the concept on
their install manual. Done right, the 40m yagi can be very close to a 15m yagi.
These ants have to resonate some where, so the resonance will no doubt be
shifted higher up in freq.
## F12s N series of 40m eles, on their 1-2-3 ele 40m ants.... don’t resonate on
15m. However they will resonate on aprx 24 mhz...which will screw up a 12m
yagi. f12 claims their N series of 40marrays can be within just 3 feet of
another F12 monobander or tribander ( 20-15-10m). F12 didn’t use stubs. They
used longer eles, ranging from aprx 59 feet for the ref to 51 feet for the dir.
Longer eles + LL rods.
## This 40m yagi and interaction to a 15m yagi has been an issue for decades
now.....and is easily resolved. Dunno why the folks at M2 have not figured it
out...yet. Jim VE7RF
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