The 'original' design of the LPDA [Dr. Duhamel - Collins Radio] used TWO
sections in an 'arrow' configuration .. that is .. the longer elements
were 'stacked' considerably farther apart than the shorter elements
[front of the antenna?].
This design is difficult to describe here, but has been utilized in many
VHR/UHF TV antennas over the past 35+ years.
I wonder if anyone has tried to stack a pair of F-# antennas in this
configuration!?!?!?
Don
N8DE
Chuck O'Neal wrote:
Jim Lux makes some very good points with reference to
LPDA's. I run two stacked LPDAs on 62 ft long booms, 18E
each, independently rotatable, that cover 14 - 30 MHz. They
are stacked at 55 and 110 feet, the best compromise stacking
distance for this array arrived through extensive modeling.
On 12M and 10M "side" lobes in elevation are still down
12dB. It is a great antenna, providing a minimum of 27dB
F/B, and as modeled over ground, a gain of over 16.6 dBd for
the stack fed in phase. I designed, built, and put them up
17 years ago. No problems and they still work as on day
one. Reliable, except for rotators...(another story).
Today, if I were to put up an antenna from scratch, I'd try
stacking the StepIR's. Not sure how I'd do it, yet. The
stacked LDPA array is great BUT on receive you are ramming
everything in the SW spectrum into the front end of your
receiver, so the narrow band performance of the SIR antenna
would help here. With most transceivers, I have to use a
preselector. Even with a 781 and unmodified FT-1000D's.
(Side note: You can take a power diode and a regular set of
stereo headphones during the sunspot max, or sometimes even
now!, connect them across the LPDA feedline and get near
room volume of the various SW broadcast stations coming in
when aimed at EU.) If you are an SWL as well, go with the
LPDA.
My concern is the MTBF of a stacked SIR system with all the
moving parts. Time will tell and when I take my system down
someday if mother nature doesn't do it first, (lot's of ice
up here in NE), I'll see how the SIR systems are doing.
73,
Chuck...K1KW
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "Tom Jednacz" <tjednacz@ieee.org>;
<Towertalk@contesting.com>
Sent: Tuesday, June 22, 2004 6:10 PM
Subject: Re: [TowerTalk] Re: [FCG] HF LOG-PERIODIC ANTENNAS
Comments Please
At 02:05 PM 6/22/2004 -0400, Tom Jednacz wrote:
A log periodic antenna is a compromise in order to cover
a wide frequency
range while providing an acceptable SWR in the ham bands.
Acceptable SWR over ALL frequencies would be a more
accurate statement. The
LPDA is in the class of "frequency independent" antennas,
most of which are
based on some geometric progression of spacings and
lenghts (that is, they
are self similar with multiplicative factors).
It takes many more
elements to produce the same gain as a monoband yagi.
There is always some
interaction between elements which further reduces the
performance of the
log periodic. According to Cushcraft their 8 element log
periodic is 0.4 dBi
better than a 2 element center loaded yagi. Lots of
unused aluminum. Not
much performance.
Gain is but one aspect of antenna performance, and I'd
wager probably not
the most important. Things like F/B or F/R and elevation
pattern probably
have a bigger effect on performance in a user sense than
small (<0.5 dB)
changes in forward gain. That said, I don't know if a
LPDA is going to
inherently be better or worse than some optimized narrow
band design.
Government, commercial and military customers will
replace their log
periodic antennas with the SteppIR as soon as they learn
about the
performance improvement. Covering all frequencies at
higher gain at each
frequency with four elements and low SWR instead of 20
elements is a good
deal both cost and performance.
Except that some customers need "instant" frequency
agility, as for ALE or
automatic band selection, and the SteppIR, while having
great performance
at any frequency within it's bandwidth, does not have wide
instantaneous
bandwidth, which a LPDA does.
There's also the "moving parts in the air" issue.
Military customers tend
to be pretty conservative about adopting new technologies.
They're also
not so concerned about purchase cost, but are concerned
about lifecycle
cost, or more important, "system cost", and in a HF
communications system,
I'll bet the antenna is a small part of the overall total
(the total of the
radios and the towers and the installation are probably an
order of
magnitude (or two) more than the antenna cost). If you're
in a plans
review, do you want to stand up and try to justify using
something new and
different that will only affect 1-5% of the total budget?
Especially when
there are going to be a lot of tough to answer questions:
for instance,
what's the EMP vulnerability of a SteppIR? Has it been
tested through the
full MIL environment requirements (810 and 461 are
probably both
relevant)? DoD likes to buy things that can work
anywhere (so they have
only one thing to stock in the logistics catalog), and big
old aluminum
LPDAs fit that bill pretty well. The military and
commercial folks can
also run QRO to improve the link reliability, so they're
not too worried
about eking out the last dB of gain in the antenna.
Don't get me wrong.. the SteppIR concept is wonderful,
particularly in the
ham market, which is price sensitive, willing to tolerate
potential
failures, and is fascinated with using limited power and
money to
communicate everywhere. The Fluidmotion folks will
probably also sell to
folks needing inexpensive wideband (but not instantaneous
wideband)
communications (Red Cross, Missionaries, etc.), although,
they have a lot
of the same concerns as the military: environment, no
moving parts, etc.
The C3S is a very good antenna but it is still only a 2
element yagi. The
SteppIR design has the same performance advantages as the
Force12 designs -
no traps plus it has the advantage of no extra aluminum
to detract from
performance.
I don't know that more aluminum in the antenna inherently
detracts from
performance. However, it DOES make the design and
mechanical stability more
important, and makes the design process more complex. It
also makes it more
expensive. The 1000 ft reflector at Arecibo contains a
LOT of aluminum,
and is a fairly good (!) performer at HF, inherently
broadband to boot. (I
only just learned that they do HF there:
http://www.naic.edu/techinfo/hf/hf.htm has numbers that
indicate 23 dBi
gain in a decidedly QRO operation (bring your truckload of
diesel fuel),
but that doesn't use the 1000ft dish, which was used in HF
experiments a
few decades ago.)
Again, the Fluidmotion SteppIR is a wonderful device in
the ham market, and
is one of the truly significantly different things that
has been introduced
to hamdom in general (like SSB, computers, coaxial cable,
solid state
amplifiers) because it addresses a lot of the things that
hams care about.
It's just that hams care about things that commercial
buyers don't, and
likewise, commercial buyers care about things that hams
could care less
about, and aren't willing to pay for.
Jim, W6RMK
_______________________________________________
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_______________________________________________
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_______________________________________________
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_______________________________________________
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