[TowerTalk] Re: [FCG] HF LOG-PERIODIC ANTENNAS Comments Please

Tue Jun 22 20:49:13 EDT 2004

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 at earthlink.net>
> To: "Tom Jednacz" <tjednacz at ieee.org>;
> <Towertalk at 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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> Toll Free, 1-800-333-9041 with any questions and ask for
> Sherman, W2FLA.
> 
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> 
> 
> _______________________________________________
> 
> 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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