Dave.... please send me that file if you don’t mind (and if it is ok with
Optibeam).
I asked Richard, N5ZC to test his 40M4LLDD recently .... I asked him to check
to see what the SWR was on 15 (and 20) meters. I have plans on having stacked
6/6 on 15 with the top antenna 30’ from this 40 meter beam...
he got back to me ... the 20 seems fine ... but on the 15 check: sure enough –
1.3:1 at 21.2! OUCH! That concerns me a bit.
I modeled it – but I didn’t want any issues related to modeling the linear
loaded 40 and instead modeled a 2 element full size... with that resonating
properly on 40 I tested it on 15 and found it was not resonate on 15 but rather
higher up – maybe around 22 Mhz. And likewise had no effect on SWR/FB/gain....
I’m not sure if this is the case and the LLDD because of its altered design is
indeed resonate lower .... or what.
I know a good deal use the antenna, the 4LLDD, alone – I can’t afford to have
yet another tower just for it...
Gary
K9RX
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
My OB2-40 isn't connected up right now so I can't scan it outside the band, but
I do have a pretty good EZNEC+ model of the antenna that I worked up based upon
actual physical measurements I took of everything ... elements, coils, and the
decoupling stubs. That EZNEC+ model gives SWR and gain plots virtually
identical to those supplied by DF2BO, as well as identical to actual SWR and
pattern plots I took once the antenna was on the tower ... which surprised me a
bit since NEC2 isn't known for modeling closed spaced elements all that well.
Anyway, my model shows no resonances above 40m until a small dip at 29.5 with
about a 12:1 SWR at that point. There is a rather strong resonance at 47.8 MHz
(about 1.2:1 SWR) which matches my on-the-air experience that I can get a
decent SWR and slight directivity at the low end of 6m with the OB2-40.
I'm not actually sure the antenna "has to resonate somewhere" near it's third
harmonic. It seems to me that the decoupling stubs could conceivably kill the
resonance there without shifting it.
For what it may be worth ...
73,
Dave AB7E
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|