[TowerTalk] B&W terminated folded dipole

[Jim Lux]

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