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

To: "Chuck O'Neal" <>
Subject: Re: [TowerTalk] Re: [FCG] HF LOG-PERIODIC ANTENNAS Comments Please
From: Don Havlicek <>
Date: Tue, 22 Jun 2004 19:49:13 -0500
List-post: <>
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!?!?!?

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

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.


----- Original Message ----- From: "Jim Lux" <>
To: "Tom Jednacz" <>;
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


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


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


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


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: 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: 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.

TowerTalk mailing list


See: 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.

TowerTalk mailing list


See: 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.

TowerTalk mailing list

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