"David" wrote in message
...
Dave,
... The article [Sperrtopf ] mentions the top part as 1/2 wave ...
If this is the case, how come the radiating element is 1480
long ?

Dave,



If you read his shopping list you see that his drawing is wrong. The
_TOTAL_ length of the small pipe is 1480. Still a vertical half wave
radiator.




I see no one took this part so I will address the question of what these
antennas are, but second. I also read your later post describing your
application very well and make this comment first.



If you want to simply 'build' the antenna from physical dimensions
derived from another frequency or antenna or just from "scratch", I see two
significant problems.

One is that if you have no measuring equipment for 915 MHz, you run a real
risk of not getting the size close enough regardless which type of antenna
you choose. This could wipe out all your desire to get "reasonable gain".
While RF is commonly called "Black magic" there are strays that can be
understood only to some extent and therefore they can easily get you off
your target. The other is that the plastic "Radome" *will* have an effect
on the resonant frequency (lowering the frequency), so comparing it to an
"open air" version moves you off your target as well. A common issue
relating to both of these is that as you make an antenna with more and more
gain, the dimensions become more and more critical to get the best
performance.

As a conclusion to this I would recommend setting up the system with a
simple, good old quarter wave ground plane and see if you really need
"reasonable gain".



Now, on to the "what-is-this-antenna" question you asked earlier. A
"J-Pole" (or as I consider, more correctly) a "J" antenna is a vertical half
wave antenna (radiating element). It is fed at the bottom (end fed) with
an approximately quarter wave long, shorted line, or matching section, or
matching stub - call it what you wish. This matching section is constructed
of open, parallel wire transmission line. The match to 50 ohms is obtained
by connecting the feed line to a point on the matching section closest to 50
ohms. As a simplification you can look at the classical quarter wave
shorted stub. Where the impedance goes from zero at the shorted end to a
very high value at the open end. Connecting the high impedance (end of the)
half wave radiator to the high impedance end of the stub leads us to
conclude that we ought to be able to find any impedance we want (between
"high" and zero) somewhere along the length of the stub - and we basically
do.

.. The length of the matching section and the point where the t-line
connects both can be adjusted to get to the best match - though this may not
be easy given the construction method used. It is called "J" because when
you put the matching section in line with the radiator (and there is nothing
requiring this) it has the physical shape of a "J". The matching section
could be at right angles to the radiator destroying the "J" shape and the
concepts described here are the same (there are antennas done this way). I
described it as I did so I can replace the open wire matching section with a
coax matching section of the same properties and get the Sperrtopf antenna
exactly. So the Sperrtopf can be called a "Sleeve J-Pole" if you like. Or,
for those enamored with the "pole", perhaps a "Sleeve-Pole".



The "sleeve" antenna, or "sleeve dipole" I also a vertical half wave
antenna, but it is center fed . Start with an ordinary half wave, center
fed dipole. Turn it vertically. Feed it with coax (yes, I know there is a
balanced to un-balanced issue, but stay with me). Now, convert one half of
the dipole (one quarter wave length side) from a wire to a pipe. Make this
the half which has the coax shield connected to it. Then, take the coax and
rather than running it away at 90 degrees, stuff it down the center of the
pipe, out the end and to the transmitter. This is the sleeve dipole. Some
may consider the sleeve simply to be a "choke" keeping current off the
t-line and there are usually several models which can be used to explain one
configuration.



The referenced Microwaves article shows one type of these, the ordinary
sleeve dipole. Looking at Fig 3 and the Appendix B picture, Antenna 3 is
clearly the classical vertical sleeve dipole.

If I read this correctly, the sleeve dipole closely mimics the ordinary
dipole as one would expect. The "spaced gap" antenna (#2) appears close
with possibly a little less gain due to the flattened pattern (more
radiation at 30 & 150 degrees). Either the #2 or #3 look like they are easy
to construct, with the cautions I mentioned above.



Finally, the antenna you described taking apart, looks to me like the #1
antenna (Extended Inner" with a choke 1/4 wave back from the shield end.
Too bad the Microwave paper didn't try this.



73, Steve, K;9.D,C'I