OARC Transmission Line Project 2013 Session 3 Home TCA Index TCA Files Antennas Presentations Oarcproject  2013 Oarcproject 2015 Oarcproject 2017 Gallery Contact
In this session,  we will design, build and test a one/two transmission line stub filter that is commonly used to isolate two radio stations during Field Day where the radios are located close to each other and working on different HF bands such as 40 and 20 metres. Stubs like these are commonly used from LF through microwaves. Worth learning about. Here is the problem: Consider a 40 metre radio transmitting 100 Watts.  All transmitters will generate a small amount of broadband noise and harmonics. In this case, a significant amount of noise can interfere with a nearby 20 metre receiver. A 20 metre notch filter placed at the 40 metre transceiver will reduce the noise to a reasonable level if the transmitter oscillators are well designed and the harmonics are well controlled. The 40 metre signal can still overload the 20 metre receiver at its front end. To overcome this problem, a 40 metre notch filter is placed at the 20 metre transceiver The stub filter shown below solves this problem using two principles: 1) HF radio bands are harmonically related: 3.5, 7.0, 14.0 , 21  and 28 MHz 2) Transmission line stubs also repeat their impedance in the same fashion. WARC bands are not harmonically related and are also not used in Field Day. FIlters for these bands must be constructed from conventional band-pass filters using inductors and capacitors. The Filter The photo below shows two stubs connected across a 50 Ohm system which is terminated in 50 Ohms.  One stub is shorted at one end while the other stub is left open.  The filter will produce a short circuit across the line at frequencies where the line lengths are multiples of one quarter of a wavelength.  For example if the open line is one quarter of a wavelength at 7 MHz, a short circuit occurs at 7 and 21 MHz producing a notch filter at those frequencies. If the short circuited line is also a quarter of a wavelength long at 7 MHz, a short occurs at 14 and 28 MHz. So, in this case , the bands that will be nulled out are 40, 20, 15 and 10 metres.  The filter passes the signals in the 80 metre band. The filter can be used in three modes: 1) Use only a shorted stub (quarter wave at 7 MHz) : place at the output of a 40 metre transceiver. 2) Use only an open stub (quarter wave at 7 MHz)   : place at the output of an 20 metre transceiver. 3) Use both stubs for an 80 metre transceiver.
The Experiment
We will conduct the experiment at higher frequencies using the 3.2 metre transmission line that you already have. 1) Cut the line into two equal parts. 2) Take one piece and expose approximately 1/4 inches of the center conductor and solder it to the the outer braid of the cable. 3) Now trim the open circuit piece to the same length as the shorted section. 4) Measure the lengths of the lines (approximately 1.6 metres) You should now have the two stubs (see above photograph) with connectors. These stubs will then be connected to the AIM impedance meter as shown above: first the open circuited line, then the shorted line and finally both. You can calculate the expected notch frequencies using TLDetails and its Freq -Vf -Len-WL conversion utility given the cable type and its length. Now measure the filter (three modes as above) using the AIM 4170 VNA to check out the design and see if it must be tuned.