>What happens if we take the N3OX approach, but put two, or possibly three
>wires going up the outside of the fiberglass,
If you work it right, you should be able to get a good parallel vertical
going that way.
I've seen it done a few times with fairly long standoffs to the sides. It
would be tough to do with wires wrapped tight around the pole ;-)
You might need to detune/ground/lift some wires for various band
combinations to avoid high angle radiation, like if you had a 15m element
and a 40m element, you'd have to detune the 40m one to avoid it radiating
like a 3/4 wave vertical.
But if you *did* have a 66 foot vertical that you wanted to use on 15m, you
could drive the 66 foot element and the 1/4 wave wire *differentially,* like
a J-pole.
I chose the simplest approach for me: base matching on 160m through 30m,
because I didn't care about 20m and up on my 60 foot antenna (because of
radiation angle problems and the fact that I had other antennas) When I
built the 40 foot antenna originally, I never actually had the 20m matching
network installed, but figured I'd run the numbers and toss it in there
because the antenna works well on 20.
But I've got the tools to design networks, it' s just EZNEC to model the
impedances, one of any number of tools to calculate the right L network,
some junkbox capacitors and home-wound coils. I use some of the late
G4FGQ's programs (archived at zerobeat.net) to calculate the right values of
inductance and capacitance, and also to calculate the physical dimensions of
air wound coils from ordinary house wire with the insulation stripped. It
helps that I have a lathe to make coil forms on ;-)
Others who don't want to go through that stuff can certainly go the parallel
vertical route, trading element mechanical complexity for electrical
simplicity.
- - -- - -
Or you can just accept some extra loss and use a tuner in the shack for the
ultimate in simplicity, and that's what's causing these things to sell like
hotcakes. There are a lot of bands where a 43' vertical with the right
balun and a short run of low intrinsic loss coax to a good tuner isn't going
to be so bad, and others where it will be terrible.
With the right transformer in the "typical" installation of maybe a few
dozen feet of RG-213 to a tuner, the losses on the bands 40m and up tend to
stay under 3 or 4dB, maybe 5dB if your coax run is long.
On some bands in pretty typical installations, you'll get within 1dB of what
would happen if you went to the trouble of base matching the thing!
But the eHam thread shows the danger of not understanding the antenna
system. It seems like a popular 43 footer and feedpoint transformer
combination actually drives the coax shield quite high in voltage, and
depending on the completion of the coax shield common mode circuit could
cause HUGE problems in performance.
For other users it will cause very few problems.
" Is 43 feet some magic
length?"
It's not close to half-wave resonant on any band. Probably gets closest on
30m, but it's still three feet off there. So the feedpoint voltage is
moderate across the bands, and the SWR is not horrific until you get below
40m. SWR runs <6:1 w.r.t. 200 ohms across all of HF until you get down to
80m.
Electrically, maybe it's an exercise in calculated mediocrity ;-)
But I think I have a theory why people love these things so much.
They're very simple to install, very simple to maintain, they have no
adjustments at all, score one.
The original, at least, is gorgeously made and quite impressive looking.
People bust pileups better when they feel good about their antenna. Score
two.
They tend to be sold as massive flamethrowers of an antenna that will give
you a big signal, and they're expensive for what they are. I think this
convinces people thoroughly that they need to not screw up this awesome
antenna by not laying down enough radial field and by using cheap coax.
So people buy their big gun 43 foot vertical and then they back it up with
8200 feet of radial wire and spring for LMR-600 coax for the feeder, and the
$180 balun with a core sized for a shortwave broadcast station. This
replaces their Butternut on a ground rod fed with ratty RG-58. That's the
hat trick.
Of course they're happy ;-) The fancy 43 foot antenna inspires people,
finally, to crawl around on the yard for a day stapling down miles of copper
and to replace their 20 year old coax with hardline.
Whether or not you can pick up 3dB on the average band (much more on 80/160)
if you stamp out the line loss hardly matters. I think people like these
things because they find them to be WAY BETTER than the thing they
replaced.
A lot of that probably has to do with the rest of the installation, but
there you go.
73
Dan
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|