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Radio Broadcast Technical Consulting and Sales
 
10032 Island Drive, Brainerd, MN  56401

  FM Quarter Wave Stub

Built by M. W. Persons and Associates, Inc.

Made in Minnesota



Shorted Quarter Wave Stub Trap with N Connector
 

These are RF band-pass filters/shorted quarter wave shorted stub traps.  They are a DC short that will pass an FM radio frequency with little or no attenuation.  The left photo shows one with an N connector near a 12-inch ruler for size perspective.  The one below is built with a BNC connector.  Each one has a T connector so a normal FM transmission line can be interrupted for filter installation. 


Shorted Quarter Wave Stub Trap with a BNC Connector

 

Above is the test setup required to calibrate the trap to the correct frequency.  A return loss bridge is used in conjunction with an RF spectrum analyzer and 50-ohm dummy load.  The analyzer display is a VSWR picture of the performance of the trap at the frequency of interest.

These filters are normally used at the output of an FM (88 - 108 MHz) solid state broadcast exciter as a voltage spike protector.  It is, in essence, a shorted quarter wave stub made of 50 ohm transmission line.  They are needed on FM transmitters where the next stage of amplification is a tube.  An arc in that tube could send a spike of hundreds or thousands of volts back to the exciter causing failure of the exciter's output stage. 

Since the filter is DC shorted at the end, it presents a direct short to any DC voltages.  It passes the FM frequency of interest because the cable is cut to an exact length.  Each filter is cut and tested using a spectrum analyzer with tracking generator and return loss bridge.  The return loss at the specified frequency is typically 45 dB, which a VSWR of 1.01:1.

Sometimes they are used at the output of a low power FM station or an FM translator to give lightning protection to the final amplifier.   They will also help reject second harmonic radiation as well as offer some attenuation to carriers that are not on the frequency they are cut for.  

FM BNC Stub Trap:  Filters for 88 to 108 MHz using high quality 50 ohm Belden 8259/RG-58A/U or equivalent cable, with a BNC connector and T adapter, are $68.00.  They are rated at 250 watts.  

FM N Stub Trap:  Filters for 88 to 108 MHz using Belden 9251/RG-8A/U or equivalent high-quality 50 ohm cable with an N connector and T adapter are $98.00.  They are rated at 1000 watts.  

Specify frequency at time of order.  Cables for other frequencies are available.  Pricing upon request.  

Here is a stub trap made of 7/8" rigid line with 7/8" EIA connectors.  This one will help protect a 2 KW FM transmitter from lightning problems. 
Material costs were high so the price was $1200 in September 2013.

 
Here is Mark in the shop demonstrating that a shorted quarter wave stub makes a good second harmonic trap for an FM broadcast transmitter.  Attenuation at twice the station's assigned frequency exceeds 30 DB. 

The trap is a 1-5/8" coaxial cable T with an adjustable quarter wave shorted section attached.  Input to the T is the station's transmitter.  The T output goes to the transmission line leading to the broadcast antenna.  A dummy load, to the right of the T, is shown in this photo.  It was used for testing purposes.

On the bench is an IFR A-7550 Spectrum Analyzer with tracking generator. The IFR A7550 works from 9 KHz to 1 GHz.  It is excellent for looking at RF emissions and RF circuits.  Analysis is done in minutes rather than hours by previous methods.

 

The left plot is a return loss measurement of the filter. The operating frequency of the radio station is in the middle where the match is the best. The right plot shows return loss at both the station's frequency and at the second harmonic. The filter absorbs second harmonic transmitter output while passing the station's main signal without attenuation.

Questions?   

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page last edited 01/12/2014