Sure the zepp is short for zeppelin. In its original form the object was to
keep high RF voltages away from the explosive hydrogen.
If I had computer skills I could draw you a pic, but you can imagine a
quarter wave length of open-wire transmission line with a half wave attached
to one leg and the whole thing hanging vertically down ; with a suitable
weight it would not slope backwards too much when under way at less than 100
knots. feed would be at low voltage/low impedance end.
then someone put it up on land upside down but with the two parts at right
angles and the half wave at the top. Still called a zeppelin but shortened
to "half-wave Zepp."
Then someone (probably Dud Charman ) declared it unbalanced, which indeed it
was , and fed it at the centre instead. Hams used it on multi-wavelengths
which means it is just a center fed doublet , but the name Zepp. stuck.
Then someone used it for two metres vertical polarisation by straightening
it out again and called it a J-pole [because with a wide spacing of the
feeder compared with the element length it looks like a letter J. ] Grand
circle completed but when its bent and horizontal its a "end-fed zepp" but
when it's straight and vertical it's a "J-pole." In what I presume was an
effort to balance the feed again, the half wave was made folded and brought
close to the open end of the feeder (where it is out of phase ) and close
enough for a capacitive coupling there.
I presume Jim comes from J . Why it is Slim I know not and Mr. Judd is a
silent key now.
What goes around, comes around or some such sentiment.
You young G4 kids don't know nuffink [ :-)) ]
Bryan
Bryan
----- Original Message -----
From: Andy <[email protected]>
To: <[email protected]>
Sent: Wednesday, February 23, 2005 4:51 PM
Subject: LF: RE: Re: Off Topic Ft101zd
Off the shelf A/D converters (mostly single chip) have come on in leaps
and
bounds in recent years, thanks to (dictated by ?) the mobile phone
industry.
I think most cell phone base stations just downconvert and digitise the
whole
band in one go. So, with forward power control implemented, 14 bit (16
bit
now) resolution is adequate. The GSM band is 25MHz wide so 65MHz
sampling is
adequate. This is just about suitable for entire HF band digitisation.
At
least, on a good day provided there are no mega-strong signals present
such as
on-board transmitters - there often will be!
A/D converters got to this point over a very short period, a few years
ago. In
1994, such 14 bit devices cost 10000 pounds each (from Analog Devices),
now
they cost a few tens of pounds.
Now, I have heard of 100MHz devices - which are needed for 3G phones -
but
things seem to be moving more slowly now, so it looks as if technology is
the
limiting factor, or more likely the money that the mob. phone industry is
prepared to throw at development of better A/D converters.
However, the product I mentioned, was produced, by Hewlett Packard I
believe,
some time ago specifically for intercept equipment for communications and
radar. It was in the HP catalogue at one time, but going back a few
years.
Don't have the exact hardware details to hand, but vaguely remember it
was
made up of discrete components, and was almost certainly a sigma delta
type of
A/D ( one or two bit sampling very, very fast, ie. several GHz, followed
by
processing, Same idea as in Codecs on soundcards) and cost an awful lot
of
money. I would have cost tens or hundreds of thousands of dollars for the
complete system.
While it may have been enhanced a bit by now , more likely its been
forgotton.
Contrasting the massive leap forward in technology in the commercial, and
particularly mobile phone industries, with the slow down / stagnating of
purely
military research since the end of the Cold War, I guess much comint (and
software defined radio) hardware just uses off the shelf devices now.
On a totally different matter :
Have just read that the Zepp antenna is so named as it was once used as
the
antenna towed behind Zeppelin airships. Does anyone know if this is so?
I've
certainly never come across any other derivation of its name.
Andy G4JNT
for [the modern day equivalent equivalent of WW2 monitoring
> >stations] that digitises the entire HF spectrum in one go.
> >
> Do really exist A/D converters capable of 24-bit resolution at 30 MHz
> bandwidth ?
> If the answer is yes, then recently I haven't paid too much attention to
> what was
> happening in the technology field...
>
> 73 Alberto I2PHD
>
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