Ever wonder what causes a solid-state RF amplifier to fail? One answer is heat; the other major contributor is transient voltages. Most of us know to keep equipment cool, but few understand that voltage spike-induced transistor failures can be prevented with pure science.
Tube arc-overs create voltage spikes in transmitters. Because of that, I highly recommend a shorted quarter-wave stub after any solid-state amplifier feeding a tube input. They normally go at a 50-ohm point in the system and provide a DC short to voltage spikes, while passing RF with little or no loss.
Continental Electronics has long suggested these for its FM exciters. I build them for clients and you can, too. (Just contact me via my website if you have questions.)
RF theory and practice tell us that a quarter-wavelength of transmission line will ignore the frequency of interest if it is on a T adapter in the RF path and is shorted at the far end.
That’s right — you short the center conductor to the outer conductor at the end of the cable. The short will be what DC or transient voltage spikes will see. The trap will short everything but the frequency of interest.
In the case of an 88 to 108 MHz FM system, a 50 ohm RG-58, RG-8, RG-213 or similar coaxial cable is just fine for the use. The important part is that the cable needs to be sized in length for the frequency you want to pass.
As you know, each coaxial cable type has a VF (velocity factor). So a quarter wavelength of line would not be the same physical length as a quarter wavelength in free space. Let’s say you are operating at 98.1 MHz. A wavelength at that frequency is 120.4 inches (about 10 feet). A quarter of that is 30.1 inches.
RF travels much slower when going through a transmission line. A typical example is Belden 8259 RG-58A/U. Its velocity factor is 66 percent. To achieve an electrical quarter wavelength at 98.1 MHz, that line needs to be 66 percent shorter, which turns out to be 19.87 inches.
Making one of these will require a connector on one of the cables. I wrote an article about that subject in Radio World last year (“Installing the Connectors the Right Way,” Oct. 10).
Hopefully that will help. You can do fairly well by calculating the line length for a particular frequency, but there is the length of the T adapter to contend with. I use a return loss bridge to trim the cable length experimentally so that it is exactly on frequency.
THE SETUP
Put a 50 ohm dummy load on a return loss bridge connected to a spectrum analyzer with tracking generator. Then sweep the frequency of interest. Your test arrangement should show 30 dB or more of return loss when the dummy load is attached.
Install a T adapter in series with the dummy load and the result should be the same. Connect a piece of coaxial cable to the open port on the T, and your return loss will become terrible. Use wire cutters to cut into (but not through) the line, at a length longer than you calculated or think it will take to make the trap.
Cut through just enough to short the center and outer conductors together. The spectrum analyzer should show a return loss dip at a frequency lower than the desired one. Then cut again making the line shorter by maybe 1/8 inch. The frequency should go up.
When you get close to the desired frequency, cut through the cable entirely. Then carefully bare the center conductor near the cable end and twist the outer braid onto it.
Experimenting, keep trimming just a bit at a time until you are on frequency. Then whip out your soldering iron and solder the end of the center conductor to the outer conductor at that point. Verify that the best return loss happens at the desired frequency. If the frequency is too high, throw the cable away and start over. If it is too low, just keep trimming the cable shorter.
When you are done, you can confidently install the shorted quarter wave stub trap with T adapter in a transmitter or on the back of an exciter. There should be no change in VSWR.
When I build these traps, I usually use RG-58 or RG-8 type cable. You will more or less be pushed into one of those two cable sizes because the equipment will have a BNC connector for up to 250 watts or an N connector for up to 1000 watts.
I even built one for a 10 kW FM transmitter, using 1-5/8 inch rigid transmission line. In that case, the shorted quarter-wave stub trap was used for attenuating the first harmonic (twice the carrier frequency), which is sometimes misnamed as the second harmonic.
Another was built for 950 MHz, but it was less than two inches long. The goal was to protect an STL receiver from lightning surges.
In conclusion, I’d like to say that the more we know about basic theory, the more we can use it to make stations better.
Mark Persons, WØMH, is a professional broadcast engineer, certified by the Society of Broadcast Engineers. He has more than 30 years’ experience and has written numerous articles for industry publications over the years. His website is www.mwpersons.com.
