by Lance, W7GJ
I constructed a filter on the back of my amp, which combines two OPEN quarter wavelength sections - one for "shorting out" the even harmonics and one for the odd. Handles lots of power and I don't ever have to worry about blowing up THIS filter! :-) Sorry I don't have a scanner handy so I could provide drawings, but I will describe the construction. I used a 4' long piece of 1" square (outside dimension) aluminum tubing with 1/8" thick walls.
Off center (see center conductor lengths below to determine appropriate connector location), I mounted a pair of N connectors on opposite sides of the tubing, and drilled an access hole on a third side so I could solder a little jumper of tubing (with a small hole in the middle of it) to connect across between the two N connectors. Through the hole in the middle of the little jumper, I ran small copper tubing orthogonally, so it extended both up and down through the square aluminum tubing.
The center conductor I used for this was 3/32" diameter copper, but I think large stiff copper ground wire would work well. For the 150.450 MHz stub, it was 18-1/8" long, and for the 100.3 MHz stub, it was 28-1/8" long. The longer piece was supported twice with little 1/4" thick spacer disc (3/4" diameter teflon rod with 1/8" holes drilled in the center) and the shorter piece just had one such spacer. No fancy test equipment available here to provide figures on db attenuation at the harmonic frequencies, but I did notice my second harmonic (which was already quite weak on my stereo receiver) get even weaker.
The SWR was around 1.7:1 at 50.125 MHz, which didn't seem to cause any problems for my 8877 amplifier.
I attach it to the amp (which is sitting on top of a book shelf so the rear of it is flush to the rear of the shelf) so the long end of the aluminum tube points down and the short end up behind the amp. Maybe someday there will be photos/drawings on my web page, but in the meantime, I wanted to share this infor with anyone who can use it to help get (or stay) on the Magic Band.
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