Research of the MICOR high-band helical resonator casting

By Kevin K. Custer  W3KKC and M. Scott Zimmerman N3XCC

October 13 2002
Re-written  due to OEM coil availability - or rather the lack thereof...

Some Background:
The MICOR 150.8 to 162 MHz. high-band receiver is the most common unit you will find in the used/surplus market.  These receivers were built in the day when most radio systems had only one or two channels of operation.  They were designed to be highly selective from the standpoint of both the front-end and I-F stages, and were built to operate on a narrow portion of the spectrum.  Parts for this type of equipment were specified to purposely limit the tuning range.  This allowed for more precise tuning and better stability from both a mechanical and electrical standpoint.  Channels were usually paired close together so the receiver would operate without reduction in receiver sensitivity as it was greatly reduced when trying to receive more than a few hundred kilohertz from where the receiver was tuned.  Referring to the manual and parts list shows the intended tuning ranges and the parts associated with them.

The cost of the factory 132 - 150.8 MHz. coils increased drastically over the last several years and replacement by this method is expensive to say the least.  In 2002 Motorola reported there are no more OEM (factory) 132 - 150.8 MHz helical resonator coils available for the MICOR. This prompted us to do the following research and seek other methods of modification that would give the same results (exactly the same) as the factory low split coils.

The factory helical resonator coils for the MICOR high-band receiver come in two frequency splits even though the receiver RF board is built in four splits.  The low and high splits for the helical resonator coils are 132 - 150.8 and 150.8 - 174 MHz.  It *seems* that Motorola had "sub-splits" within the low and high splits.  The difference in sub-splits is in the length of the tuning screws. We found that Motorola used two different length screws depending on sub-spilt and actual frequency the radio set was originally built for.  I always thought the screws were the same length, no matter the frequency, because there is no breakdown of the differing lengths in the parts list of the manual.  What we discovered is that if a front-end casting was to tune from 150.8 to 162 MHz, Motorola placed 150.8 to 174 MHz coils in the casting and used tuning screws that are 1.25" long.  If the casting was to be used above 162 MHz, the screws were either 3/4" or 1" in length depending on exact frequency.  In NO instance is the factory screw length longer than 1.25".  Why?  At the point where the 1.25" screw is fully inserted into the casting (just before the tensioning nut pops off) maximum capacitance on the end of the resonator occurs.  This means longer screws are of no benefit in making the resonator tune lower.  In other words the 1.25" screw tunes the resonator to its lowest practical frequency.  Placing longer tuning screws, larger tuning screws, or sleeved screws into the casting creates two problems.  First, the width of the bandpass is greater due to "pulling" the coil to resonance.  Second, Insertion Loss (IL) is greater than if the correct coils were placed in the casting.   These two things are unacceptable in duplex repeater service, especially on a 600 kHz split.

  I have a HP VNA that allows us to do very accurate filter testing.  We swept some factory original high-band, high and low split helical resonator castings to get a benchmark on how far they'd actually tune and determine their bandwidth and insertion loss at several points across their intended tuning range.  We agreed to use the mixer FET on the RF board in our testing because the tap point on the output of the resonator casting is not at the 50 ohm point, so connecting 50 ohm equipment to it won't show the filters exact response.  What we did was bolt the helical casting to an old 150.8 to 162 MHz. RF - I-F board and power it so the mixer FET was operational.  This allows the match on the output of the resonator to be exactly the same as what it is in actual operation.  We simply capacitor coupled RF from the output of the FET to the test equipment.  Using this method we were able to make several response and loss measurements with the factory original high and low split helical resonator castings to establish the benchmark.  The results were documented and used in comparison with several coil/screw modifications to be absolutely certain of measurement accuracy.

  After extensive evaluation of all of the mentioned/popular methods to tune the 150.8 - 174 MHz. coils into the 2 meter ham band (bigger screws, longer screws, screws with sleeves, etc.) we concluded the only acceptable approach is to either use the original 132 - 150.8 MHz. helical coils (which now are no longer available) or fabricate our own coils that would properly tune this band.

Scott and I spent numerous hours coming up with a method of fabricating coils for the 2 meter band - a method that would allow any factory length screw to be used to tune the 2M band.  We now have a method that works very well, is repeatable, and tunes exactly like the OEM low split coils.  How?  We turned out a coil form that allows us to wind a replica of the low split coil.  This conversion method is perfected and has been used on hundreds of receivers over the last several years. Our new coils perform identically to the factory original 132 - 150.8 MHz. coils - with sensitivity and selectivity identical to that of a receiver using a factory original low split resonator casting.

Don't be fooled by methods claiming to allow 150.8 to 174 MHz. coils to be tuned into the 2 meter ham band.  While you can pull them that low, they won't perform like the real thing  - I guarantee it.  Besides, our method is as affordable as any other offering, and you get the real thing.

If you are interested in having your MICOR high-band preselector work properly in the 132 - 150.8 MHz range,  Click here.

Copyright 10-13-2002  Kevin K. Custer  W3KKC
All Rights Reserved.

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