Multi-Band Yagi vs. Log Periodic - What's the Difference?
Hey Guys, I'm a new ham with my general ticket. I would like some help in trying to understand the difference between a multi-band yagi and log periodic. What would be the advantages/disadvantages of either. I have a Yaesu FT1000 Field, AL 80B, and a Buckmaster 7 band dipole. I would like to make plans for a beam which would be mounted on a 45 feet Rohn 25 tower.
A multi band yagi covers several bands ( usually 3 or 4 ) with limited bandwidth on each one. A log periodic is a broadband array that covers a large segment of spectrum seamlessly. Log periodics are generally quite large (understatement) and heavy at frequencies below 14 Mhz. For amateur use a multi-band beam would be cheaper and easier to deal with.
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The above is a good description of the functional difference.
LPDA's usually are designed with large bandwidths to allow frequency agility - for that, they sacrifice gain and directionality - in an LPDA, typically only 3 of the elements are active at any given frequency.
The design is also a bit more interesting in that they are actually dipole array - each row is opposite polarity/phasing and the boom includes a phasing system of some sort.
There are some monster LPDA's out there, but they are rare and very expensive. I don't know of any amateur that has the really big stuff but many military installations have pretty big HF LPDA's.
I think CD makes an LPDA for something like 10 or 6 meters up to 1.2 gig - I guess if you can have just one?
Their sheer weight and wind load make them a real challenge as you can imagine.
In 37 years I have never personally (at least) known of a ham who used a log periodic array nor have I seen one in use at a ham station, just military installations.
They probably are used by only a very few hams. You won't see many of them in the ham catalogs that's for sure!
Single band or??
I would like to make plans for a beam which would be mounted on a 45 feet Rohn 25 tower.
Last edited by K7MH; 10-02-2008 at 04:27 AM.
Multi-band. I have read about numerous beams from hex to opti, trap-no trap, yagi to LP. Wow, a lot of things to think about? Good thing I'm not considering a vertical . For now, I'm leaning toward a multi-band beam to go along with the dipole. I'm sure that the xyl would appreciate a low profile something! Seems like when it comes to making a choice, budget and array profile (size) are the most important considerations?
Is it something you want to scratchbuild or homebrew yourself? It kinda sounded like that in the original post. A monoband yagi is one thing to homebrew, a tribander is another story.
Just tell her not to look up!
I'm sure that the xyl would appreciate a low profile something!
Last edited by K7MH; 10-02-2008 at 06:21 AM.
Originally Posted by kb3rrg
All the chatter about traps and everything else is largely overrated. Good antennas are good antennas, regardless if they use traps or not. As a matter of fact some systems that eliminate traps are much worse than they would be if they had traps!!!
Hygain makes some very good Yagi's like the TH-6, as has Cushcraft and Bencher. Force 12 has some good antennas, as does M-squared. Any of them, for the same boom length, will work about the same.
A quad will be no better, and some of the other things like Log Periodics will probably have less gain.
If you use a Hex Beam it will be worse, despite all the hyperbole.
That's a very sweeping statement. The Hexbeam might lose out to a full sized multiband array, but it "knocks the socks" off some of the beams of similar size. I know - I've had a Cushcraft MA5B and a Butternut HF5B alongside the Hex at different times.
Originally Posted by W8JI
Then there are the interlaced multi-band yagis. Those do not use traps but are basically several single band antennas on the same boom.
LPDAs are not rare in amateur circles, and I work stations almost every day on HF using them (generally above 14 MHz, like 14 through 30 MHz). I have one, myself, happily rotating at about 55' above ground on my tower. It's been up eight years.
Tennadyne and others make reasonably affordable and installable HF LPDAs.
They have a lot more elements per boom length than Yagis and the longest cell (element) is generally longer than the equivalent longest element (reflector) on a Yagi for the same frequency. Also LPDAs don't have as much gain as Yagis do for any given boom length. But they are relatively simple antennas to construct, don't have any traps or linear loading, or multiple insulators or other stuff that could potentially fail with weather/insects/time/exposure so once "up" they usually last a very long time.
The big LPDAs the military uses are big, heavy and expensive for most hams. I've seen some installation using towers for the booms, and having 24 or more elements on booms over fifty feet long. Not so many hams could install something like that. But the "amateur" LPDAs are smaller, lighter and less expensive than those (albeit lower performance as well).
But they are pretty popular.