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

To: Tom Jednacz <>, <>
Subject: Re: [TowerTalk] Re: [FCG] HF LOG-PERIODIC ANTENNAS Comments Please
From: Jim Lux <>
Date: Tue, 22 Jun 2004 15:10:08 -0700
List-post: <>
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

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


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