RCARES Dualband Logperiodic
From Megawiki
These antennas can be made resonant on harmonic frequencies with lengths that are fractions of the target wavelength. Many antenna designs have multiple resonant frequencies, and some are relatively effective over a very broad range of frequencies.
Although log periodic antennas have slightly less gain at any particular frequency when compared to other directional antennas such as the Yagi-Uda, the difference is made up in the extremely wide bandwidth they can achieve. Commonly the antenna is driven by alternating elements with a circa 180o (also known as a π radian) phase shift from the previous element. This is normally done by wiring the elements alternating the two wires by a balanced transmission line.
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Reasons to build a log periodic
- You need to get in on voice or digital, and an omni won't cut it you need a directional antenna
- We experienced this in several events this year.
- Yagis need matching, and can get large.
- Yagis or multiple mono banders, would need duplexer with dual band.
- Log Periodic type, Directional, and has pretty good gain.
- Gain estimated is about 6.5 Db on Two meter and 7 Db on 70 cm.
- The participants expressed an interest in building log periodic antennas
So over the next few weeks the RCARES digital and EMCOMM groups will be building some directional log periodic antennas. The best help on assembly will be in person at the scheduled meetings.
These can be used where an Omni antenna doesn't cut it , Yagi is too bulky.
Building a log periodic
The boom
For the first part of the project, we will be marking and drilling the aluminum tubes on the drill press. Take 1⁄16" aluminum 1⁄2" X 36" tubes and mark the points where you are drilling with a drill press. The following measurements were calculated to be center points of the log periodic boom.
There are a total of two booms and 10 radials of the sizes below From the top of the boom:
- Radial #1 1⁄4"
- Radial #2 51⁄2"
- Radial #3 111⁄8"
- Radial #4 171⁄4"
- Radial #5 23 1⁄2"
The holes are the same on lower and upper booms, but screw directions alternate sides to operate the log perodic.
Note i usually use a drill bit the size of the screw. I also utilize a board as a jig, I Drilled one hole and put a screw in to hold the boom to the board. This helps stablizes the boom so it is easy to work on. Make sure to drill thru the boom all the way. Use a metal drill bit and slow setting an a drill press so you don't melt the alluminum.
You can drive the screws into the boom using a power drill, this will actually create the threads as you put them in. this helps hold the screws into the boom.
Radials
The radials taper out in a logarithmic fashion. After acquiring the inserts and aluminum arrow shafts you first need to cut them to length. Using a small pipe cutter cut two arrows shafts to each of the lengths.
- Radial #1 161⁄8"
- Radial #2 161⁄2"
- Radial #3 173⁄8"
- Radial #4 177⁄8"
- Radial #5 181⁄2"
After that is done you will need to scrape the inside of the tubes with a Dremel tool to ruff up the inside surface and take off any anodizing or coating to make sure good contact is made. This is very important
Next to adhere the inserts to the arrow you use contact cement. ONLY put a LITTLE strip of glue on one side of the insert. Tap it into place gently with rubber mallet, and wipe off any excess glue.
Take a DVM or continuity meter to verify continutity on the shaft to the insert on the all radials.
Complete project parts list
* 2 x 1⁄16" aluminum 1⁄2" x 36" tubes. * 1 x so-239 Bulkhead mount (4 hole) like on the back of your radio * 10-1816 to 1819 type cheap practice arrow shafts aluminum arrow shafts with screw type inserts * Toilet seat bolt kit 5⁄8 " diameter or smaller, and 6 washers to join main booms. * 4 1⁄2" self tapping screws * 6" of 18 gauge wire * silicone * PVC pipe for mounting.
- Total 16 stainless machine screws #8-32 x 11⁄2" long make sure these fit with the following:
Contact Dave Voss if you don't have arrows yet. - we are looking for other suppliers. The local one we found doubled thier prices. - little do they know there are lots out there.--KB9MMA 09:02, 1 December 2006 (CST)
We will discuss mounting arrangements, some PVC elbows etc will be needed later. -Those are fairly cheap. Total cost is around $38 estimated, and we will be purchasing them over weeks, so no need to strain the wallet right away.
