At 11:04 AM 9/4/2005, Jim Jarvis wrote:
>It's not quite a dummy load, folks. But it's not very
>efficient. My only experience with it was in the marine
>SSB bands. The Sea Education Association base used one to
>communicate with its sailing research vessels, when my
>daughter took a semester at SEA.
>
>
>The B&W topic was raised in two contexts. One, Jim Lux's modelling
>efforts.
For which I've been preempted by L.B.Cebik's fine work, saving me the few
hours. Now I can concentrate on "the other antenna being modeled" for
comparison (about which more later).
> The other by a chap who is looking for an ALE antenna.
Which oddly, is my application as well.
>Automatic Link applications spread over many bands, as we go through
>diurnal ionospheric shifts. Is a -3dB or -6dB gain antenna useful,
>to enable fast changes? Perhaps....and perhaps -10dB is not.
>
>If I were contemplating ALE operation, I think I'd look seriously at
>off center fed dipoles...and a modern implementation of same called
>the Carolina Windom. That, with an auto-tuner, should be able to
>handle the wideband requirements of ALE.
I'm not sure the OCF dipole has all that smooth an impedance. When last I
looked at it, you could find a length that worked moderately well on all
ham bands (in that the feedpoint impedance had a magnitude of around
200-300 ohms or so, so a 4:1 transformer matched it reasonably well), but
it had some nasty bumps.
It's one of the things I'm looking at, along with a "multi wire" type dipole.
Autotuners are nice, BUT, if the tuner's at the transmitter, and the (coax)
feedline is long and lossy, you may not wind up a whole lot
better. (another thing going into my analysis)
There's also the reliability issue (you've added a box with lots of
electromechanical parts in it).
A decent antenna tuner AT THE ANTENNA would probably work nicely, but, so
far, nobody is making an intelligent and reliable one. (by this I mean one
which
a) can handle a kilowatt (for real.. not ICAS, I mean, a full kilowatt
average power (not PEP), 100% duty cycle running digital mode type traffic)
(actually, 250W continuous would probably be ok, since you're NOT getting
the 3-6dB hit of a B&W type)
b) doesn't require any separate wires (i.e. power goes up the coax, along
with control signals and status back as required)
c) is weatherproof (salt spray, rain, decent temp range, etc.)
)
There are some tuners that are "almost there", but miss on one or another
of the above list.
Also, if you're changing frequencies "really fast" (i.e. a frequency hopper
or a direct sequence) then a automatic matching box is probably not going
to hack it. I note that there are HF spread spectrum radios out
there. They need fairly wide instantaneous bandwidths (somewhere more than
10kHz and less than 1 MHz) once they've done their channel sounding to find
the right band. These kinds of radios are fielded by government agencies,
and not so oddly, such folks are a big customer of the passive broadband
B&W, which would work quite nicely with them.
Jim, W6RMK
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|