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

[Jim Lux]

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