Birds of a Feather
Although over the past year I've provided details on the LPDA (Tri-Band Log-Periodic) that I built in my workshop covering the bands of 2 Meters, 1 1/4 Meters, and 3/4 Meters as seen in the above photograph, I have been meaning to update some of the information on it, as over the last year while operating with the LPDA, I had found a few places where the design as built needed some tweaking.
The original ball mount in particular proved to be too light as built, so I discarded it, and replaced it with the much more robust ball-mount (built from black Delrin) as seen here in place on the antenna, that allows for quickly positioning the antenna in either a horizontal or vertical position on the push-up mast when mounted on my mobile.
If I were building the LPDA as built over again, I would select 3/4"OD tubing in place of the 1/2"OD tubing used with this antenna for the booms that also act as the transmission line, as the 1/2"OD tubing with the various over-sized composite fasteners that pass through the booms on this antenna, do not allow enough room to route the feed coax through the shield attachment boom, an oversight on my part when I drew up the drawings for the LPDA as designed and built.
Now the antenna performs fine with the feed coax routed along the outside of the shield attachment boom, but doing it over again, I would strip the outer housing off of the coax before routing the coax through the shield attachment boom.
Still, the tradeoff is that the antenna as built, is lighter in weight than the antenna would be if built with 3/4" OD booms, there's always a tradeoff to everything isn't there.
Just so you know, LPDA's are quite particular as to how the feed coax is routed to the antenna, and that's why you see the coax drooping in its approach from the mast to the rear of the LPDA. This method of routing the coax is common practice with commercial LPDA's as well.
The antenna as built initially also needed some slight modifications to be made to tame the SWR on the 220MHz band, although as you can see by the 3 individual analyzer readings for each band, that this has been resolved, with the antenna proving to be a terrific performer as well as being resonant everywhere across all 3 bands.
Expand the photo for a closer look.....
So what your seeing here is my recently completed Log Periodic antenna. covering the 144, 222, and 432 MHz bands.
There are various types of log periodics that one can build, however this one is the most common form of log-periodic antenna and is know as the log-periodic dipole array or LPDA,
My LPDA consists of 16 half-wave dipole driven elements of gradually increasing length, each consisting of a pair of 6061 aluminum alloy rods mounted on two 6063 aluminum alloy booms.
The elements are mounted close together in a line, connected in parallel to the feedline with alternating phase.
Electrically, it simulates a series of three-element Yagi antennas connected together, each set tuned to a different frequency, and in this case for the complete bands of.....2 meters.....1. 1/4 meters.....and 3/4 meters.
My LPDA looks somewhat similar to a Yagi antenna, in that they both consist of elements mounted in a line along a support boom, however they work in very different ways.
Adding elements to a Yagi increases its directionality, or gain, where as adding elements to a LPDA increases its frequency response, or bandwidth, although the log-periodic still has more gain on all the bands it covers than a 3 element yagi designed for the same bands.
Every element in my LPDA is active, that is, connected electrically to the feedline along with the other elements, although at any one frequency most of the elements draw little current from it.
Each successive element is connected in opposite phase to the active connection and in this case each boom
.The two booms are being feed with the coax connected at the front of the antenna, the center conductor is connected to the one boom, and the shield is connected to the second boom, therefore the two booms act as the transmission line.
It goes without saying that the two booms with their respective elements are insulated away from one another.
Click on the photo for a close look.....
Log Periodic loaded in my mobile.....
Log Periodic mounted on the mast of my mobile.....