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
>
> _______________________________________________
>
> 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.
>
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_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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