Filters used at W7GJ

Industrial Communication Engineers, LTD. (TEL: 800 423-2666 or 317 545-5412, P.O. Box 18495, Indianapolis, IN 46218 USA) has low pass transmitting filters with a cutoff frequency of 54 MHz. The models (and peak power handling capability) are #425 (300 watts), #426 (1500 w) and #427 (6 kw). Their model #427 claims 80 db attenuation above 56 MHz. You can get any kind of connectors on them you want, although you will probably have to provide them if you want other than N or UHF. At my installation, I use a #425 on the input of my amplifier, and a #427 on the output of the 8877 amplifier. As of 2013, I am not sure they are still in businss. More information

Additional information on low pass filters for use on 6m can be found on the N6CA 50 MHz web page.

Design and performance of a QRO low pass filter for 50 MHz is shown HERE. Using the same design, a larger version was built by W7GJ, as outlined below:

ZL1RS and YU7EF constructed a version of the above K1WHS 50 MHz Low Pass Filter but used smaller capacitors and metric wire sizes. The results are shown here.
YU7EF also built a 7 pole 50 MHz low pass filter, using the smaller 7.5 kv doorknob capacitors. A photo of the filter constructed by ZL1RS is shown to the right. Construction details and performance information on a QRO version of this filter is available HERE.
YU7EF also offered a design of a similar 7 pole low pass filter for 144 MHz:

I also constructed a filter on the back of my 2m amp, which combines two OPEN quarter wavelength sections - one for "shorting out" the even harmonics and one for the odd. It is capable of handling 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 orthagonally, 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 also would work well. I was not concerned with the impedance of the quarter wave stub sections, since they will look like a short at the harmonic frequency regardless of exact impedance. For the 150.450 MHz stub, the section should be 19.34" long, and for the 100.3 MHz stub, it should be 29" long. The longer piece was supported twice with little 1/4" thick spacer dics (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 rises up behind the amp. Maybe someday there will be photos/drawings on my web page, but in the meantime, I wanted to share this info with anyone who can use it to help get (or stay) on the Magic Band.

Additional information on well-matched and effective VHF/UHF open stub filters was prepared by G4SWX. The basic idea behind John's Coaxial Stub Filters is to keep the impedance (at the pass frequency) of the elements at the output port of the filter the same as at the input - hence you can match the whole filter by transforming the input impedance to its conjugate at the output. The simplest filter of this type uses a quarter wave open circuit @2f on the input and open circuit quarter waves @3f and 4f on the output. This is a 'magic' combination because @f the impedance of the 2f stub is the same as the parallel combination of the 3f and 4f stubs. All that is needed is to transform the impedance at one end into its conjugate at the other.

John suggests that the easiest way to do this at high power is to use a coax line rather than an inductor to do this transformation. To construct a high power filter using the above coaxial elements, John recommends a quarter wave open circuit @100MHz on the input and quarter waves open circuit @150MHz and 200MHz on the output. Join the input and output sections of the filter together with a low loss coaxial line (0.265 x velocity factor) wavelengths long. This will give you >20dB return loss ( <1.2:1 vswr) at the pass frequency. Shorted half wave stubs don't really help a lot and they tend to up the insertion loss ! A simple rule is that the more junk that you put into a filter the higher the insertion loss !

Although it takes a little bit of room to construct one of the above G4SWX filters on 6m, the coax could be coiled up, so the space required is minimal. Or, if you really want a high power and low loss version, you could construct the entire thing using square aluminum tubing welded together. If you are using rigid line sections with a single teflon spacer in the center, the lengths should be determined using a velocity factor of .985. Remember to make the section between the "input" (a single quarter wave open stub at 100 MHz) and the "output" (the pair of open quarter wavelength long stubs for 150 MHz and 200 MHz) from a 50 ohm section. To make an air line for 50 ohms, the inner conductor and square aluminum tubing should be sized as shown below:

I have an outdoor TV antenna situated very close to my transmitting antennas. When the coaxial lead from this comes into the house, I have a Radio Shack distribution amplifier to send the TV and FM signals to the appropriate receivers in the house. The gain on this amplifier had to be turned entirely down to prevent overloading. In addition, I had to install the following filters immediately ahead of the amplifier (toward the TV antenna):

1. First, I installed several Radio Shack ferrite split ring toroid filters on the RG59 coaxial cable from the TV antenna. This is to remove any "common mode" RF that is traveling on the shield of the cable.

2. Following that, I installed coaxial "T" connectors, with open quarter wavelength stubs tuned to 144.100 MHz and 50.125 MHz, to try to "trap out" any overloading signals.

3. Following that, I installed a 50 MHz notch filter (manufactured and distributed by K1UHF). This is a commercial quality filter with F connectors that has very high attenuation for signals in the 6m ham band, and is very effective.

4. Following that (just in front of the Radio Shack distribution amplifier), I installed an Ameco high pass filter (cutting off below 54 MHz) with F connectors. This was obtained from Ham Radio Outlet. This filter was grounded to a large ground wire heading directly out through the wall to an 8' ground rod.

The above filters effectively prevent unwanted frequencies from overloading the distribution amplifier. However, it was also necessary to protect the non-ham equipment (VCR, TV, telephone answering machine, and stereo amplifier/tuner) from "common mode" RF interference (RF signals that simply are passed into the equipment on the shield of the coaxial cable, or induced on the cords leading from the equipment). This was done by installing the Radio Shack split ring ferrite filters mounted as close to the equipment as physically possible, on all the speaker, power cords, and antenna leads. This combination of filters seems to very effectively permit me to receive TV and FM signals while transmitting on all bands.