Log-Periodic Antenna Design

Hi Rich,

Someone else has already provided the translation, so that will take care of some of the wholes in data entry and reading.

Tau is related to the shape of what you call the pyramid, or more actually to the angle of the taper. This defines the smoothness of matching across the span of frequencies you entered into the program. As you can see, it also relates to the available gain. Another correlative is it also relates to its length. All of these things are trade-offs that lend to the rule of choosing what you want most and giving up on the rest.

I note elsewhere that you wanted something about 1 foot long. There is a Tau that will give you this boom length, and you got it on the first guess.

You should start building what your screen shot gave you IF that is the span of frequency you need (already questioned by other correspondents).

73's Richard Clark, KB7QHC

Why don't you sell your house and move to a place next to the TV transmitter station. You will only need a piece of wire then. Or the bare finger. Must be wet, of course.

w.

Here in the Tyler, TX area, the TV stations are continuing their analog broadcasts on their previous channels. Their digital signals are on different channels. When the switch- over comes, they plan to switch their digital channels over to their previous analog channels.

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73, Cecil  http://www.w5dxp.com

Some of the converter boxes have a signal strength indication. My "Insignia" box does. It allowed me to find a sweet spot for channel 10 DTV reception using RS rabbit ears.

--
73, Cecil  http://www.w5dxp.com

I bought one, a Phillips MANT310/MNT310F - didn't work as well as the non-amplified RS one I already had.

--
73, Cecil  http://www.w5dxp.com

Log periodics are not necessary for sub-octave operation.

I did the same thing and I was surprised how many digital chanels I got. I was at a dollar store and saw the RCA UHF/VHF loop/ears for $5 and that gave me considerable better reception.

greg

Ever hear of the BOW TIE. Its a sort of Fractal antenna. Having a wide band. It could be improved by making it a full fractal.

greg

Actually, I had to construct my own with a broadband MMIC device, to get decent functionality ... however, my mother has an el-cheapo unit she purchased from some yard sale, it/they work surprisingly well!

Location, reflections, etc. can vary ones experience greatly--but hey, they usually beat a wire coat hanger! :-)

Regards, JS

snap:

I have used the procedure in the following paper:

I slightly modified the design (e.g. I used several different diameters for the elements), but basically I used the same formulae. I did not use their formula for deciding the right spacing between the support tubes to get the right characteristic impedance, because I suspect that maybe it is only valid over a limited range of spacing. Instead, I measured the characteristic impedance of just the tubes with no elements attached on a VNA that could plot impedance vs distance in a sort of TDR mode, and I adjusted the spacing until it was right. I epoxied some very small (roughly 1mm cube) spacers between the support tubes to keep the distance right, once it was adjusted properly. It is important that the space between the support tubes is mostly filled with air, not epoxy or any other dielectric, because the wave needs to propagate at the right speed between the tubes so that the elements are fed in the right relative phase, and this won't happen if there is something with a higher dielectric constant in there. I used those brass tubes that you can get in hobby shops, but unfortunately brass is fairly resistive and I did not silver plate it. I would have used aluminium if it were easier to solder in a reliable way.

The finished antenna had a good return loss over the desired frequency range (sorry, can't remember the numbers) and it did the job for which it was intended although I don't have a measured gain value etc. because I don't have an antenna range.

I have a spreadsheet for calculating the element lengths (in Openoffice.org format) if you are interested.

Chris

...

Cookbook, if at all possible. Like, element lengths and spacing, and the angle between the booms - which brings up a question - what effect does it have when you take a flat L-P and "open it up", like to a pyramid shape - I was doing a thought experiment with this and when it's completely "flattened out", (180 degrees between the booms), it looks like a Bow Tie!

Thanks, Rich

Hey, "John Smith", why don't you go outside and play hide-and-go-f*ck-yourself?

Cheers! Rich

"Best Regardses"? ;-)

Anyway, thanks for this - I'm feeling a lot better about this project now thanks to your help and that guy that translated it for me (forgot the name, but he knows who he is - Thanks Again!);

I might even start cutting wire[1] and soldering this weekend! ;-)

Thanks! Rich

[1] GTAW (Gas Tungsten Arc Welding) filler rod, with some coppery- colored coating that solders even better than copper, about .030 dia., and as stiff as piano wire.

Apparently I'll only need up to ch. 52; have to look up the carrier freq. on that.

Ch. 14 = 470-476 MHz Ch. 83 = 884-890 MHz I think.

Now all I need to do is the algebra. ;-)

52 - 14 = 38 38 * 6 = 228 228 + 470 = 698 698 + 6 = 704

Cheers! Rich

Dude! Awesome! And this one doesn't even have a reflector!. I wonder how far I should hang it in front of the venetian blind. ;-)

Thanks! Rich

Your clip lead is too long. Try 1/4 wave.

I got good reception using a 4 1/2 inch length of 24 gauge solid poked into the F connector for a (line of sight) 45 kW station, 20 miles away. (Most of the local stations run in the 500-1000 kW range).

Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)

Yes - somebody just pointed to a youtube of somebody making one out of coathangers.

I think I'm gonna do that.

Cheers! Rich

Orfanidis's book on electromagnetic waves and antennas is online, and covers LPDAs.. Kraus is better, but you'll have to fork out at least $20+shipping for a used copy. Kraus *is* my recommendation if you have to have a single antenna book, though.

I think a bow tie's design is actually motivated more by one of the "standard" ultra-wideband antenna designs, that of a pair of opposing cones touching each other at their narrow ends, being translated down into 2D.