[ Home ]

Conversion of a VHF Micor® Helical Resonator Front End Filter to the 2 Meter Amateur Band.

This article describes a method of converting VHF Micor® helical resonator front end filters to the 2 Meter amateur band that is easy, inexpensive, and provides performance that is equal to filters that were "factory built" for the 2 Meter band. The conversion is very quick and easy to perform without the need to remove the Micor receiver from the radio, or the removal of the filter from the receiver. The special tuning screws required can be purchased from us or you might be able to make them yourself (full do it yourself information follows below).

The VHF Micor® helical resonator front end filters that are the subject of this article were converted from the Micor® receiver range 3 ("H" split, 150.8 to 162 Mhz) or range 4 ("HH" split, 162 to 174 MHz) to cover the two meter amateur radio band. Performance of the converted filters is closely equivalent to the range 2 ("M" split, 142 to 150.8 MHz) Micor® front end filter with respect to insertion loss and frequency response, but differs somewhat in tuning ratio (MHz tuned per rotation of the tuning screw).

The conversion can be performed without the need to replace or modify the helical resonator coils that are the heart of the filter. This has become very desirable since these coils have become difficult (expensive at least) to obtain. If you prefer the more traditional conversion (i.e. changing the helical resonator coils) you can still (as of this writing) do so by contacting Kevin Custer, W3KKC.

This article includes information about the conversion methods, in case you would like to perform the modification yourself. It also includes the measurement methods used and some results of measurements I performed to document the performance of the converted filters before and after conversion. If you lack the facilities to make the special tuing screws require or to perform this conversion yourself, I (W9FIU) have an adequate machine shop, a complete radio shop and am available to assist you with the modification of your front end filter, or the entire process of repair, conversion and alignment of your Micor® receiver, exciter or complete transceiver. For more information, including modified tuning screws you can purchase and rate examples, see VHF Micor® and Syntor® "Two Meter" Conversions and other radio services.

If you plan to make modified tuning screws and/or perform this conversion yourself, be sure to read this entire article completely before you begin. The following description contains several extra steps that were done for documentation purposes. You can save a lot of work if you skip these steps.

Description of conversion method (including steps taken to document the filter performance before and after conversion):

1.) Remove the front end filter from the receiver assembly.

2.) Attach a small metal plate to the mixer end of the front end filter. This plate carries a BNC connector which can be connected to the mixer output wire from the filter. This permits the output of the filter to be connected to a spectrum analyzer for performance measurements.

3.) Connect the front end filter input connector to the output of a calibrated synthesized signal generator using a short 50 Ohm cable which adapts BNC (signal generator output connector) to RCA "phono" jack (front end filter input connector).

4.) Connect the BNC connector (see item 2 above) to a spectrum analyzer with a short 50 Ohm BNC to BNC cable.

5.) Align the filter for maximum gain roughly in the middle of the 150.8 to 162 MHz tuning range.

6.) Measure the frequency response of the filter around the alignment frequency. Measure the insertion loss of the filter at the center frequency. Measure the bandwidth of the filter at -6dB, -12dB, -20 dB, -30 dB and -40 dB relative to the peak response of the filter. Record the results. Note that measuring the filter this way (i.e. with 50 Ohm input and output terminations) does not necessarily provide an accurate determination of the filter's performance in the Micor® receiver. In the receiver the input termination is usually close to 50 Ohms, but the output termination by the receiver's mixer may not be very close to 50 Ohms. The method is thought to be adequate for comparing the performance of the filter before and after modification.

7.) Remove the bottom cover from the filter.

8.) Remove the tuning screws and the retainer (tensioner) nuts from the filter. Set the retainer nuts aside.

9.) Use a metal lathe to turn the inside end (the end opposite the screwdriver slot) of the tuning screws down to a precise 0.142" outside diameter for the last .375" of the end, then to 0.1405" outside diameter for the last one eighth of an inch from the end. This prepares the tuning screws to accept the cap described below. Note: If you are converting a "range 4" front end, you may have to make some "new" tuning screws as the normal range 4 filter tuning screws are quite short. These can be made from 10-32 stainless steel machine screws that are 1-1/4" long (or longer). The heads of these screws need to be removed and screwdriver slots cut into the end. These screws should be cut to a length of 1.10" before proceeding. NOTE: I have found that some of the normal tuning screws are made of a metal that is difficult to machine, especially to the tolerances required -- so I often make new tuning screws as described above and discard the original tuning screws.

10.) Make "caps" for the tuning screws that are .375" long from 1/4" diameter aluminum rod. Precision drill and ream the inside diameter of the "cap" to a precise inside diameter of .141" for a "press fit" over the ends of the modified tuning screws prepared in step (2) above. The cap should fit snugly over the last one-eighth inch of the tuning screw and then be pressed onto the rest of the screw until it is tight against the shoulder that is three eighths of an inch from the end. Use a small arbor press to press the screw into the cap.

11.) Press fit the caps onto the ends of the modified tuning screws. Don't worry that the length of the caps will reduce the tuning range of the filter by preventing you from screwing them "all the way out". You will be able to screw the tuning screw far enough up so that you will be able to hit the high end of the desired tuning range, about 152 MHz.

12.) Put the above assembly back into the lathe and turn down the outside diameter of the "caps" to 0.235". This step reduces the diameter of the "caps" to the proper size (determined empirically) to produce the desired 142 MHz to 150.8 MHz tuning range. It also insures that the caps are aligned (concentric) with the tuning screws. (wobbly caps are detrimental to accurate tuning and are be avoided as much as is possible).

13.) Replace the modified tuning screws into the filter. It is easy to do this from the bottom of the filter since the bottom cover was removed. Once the tuning screws have been replaced, replace the retainer nuts also. Tighten the retainer nuts only enough to make the tuning screws have some friction against turning.

14.) Replace the filter into the test setup and re-align the filter, this time to a frequency within the two meter ham band. I used 146 Mhz for my measurements on the converted filter.

15.) Repeat the measurements in step 6 again, this time around the new alignment frequency. Record the results and compare them with the results found in step 6.

NOTE: it is normally not necessary to document the performance of the converted filter, etc. If this is not desired, the conversion process is much simpler. The front end filter does not have to be removed from the receiver. Simply remove the bottom cover of the filter, remove the tuning screws and the retainer nuts. Modify the tuning screws as described above. Then replace the tuning screws, retainer nuts and the bottom cover of the filter. Align the filter normally and you are finished.

Measurement results:

I found no significant difference between the insertion loss of the filter before and after conversion. In both instances I measured an insertion loss of about 6 dB.

I found no significant difference between the filter response measured in step 6 ("normal", before conversion) and those measured in step 15 ("converted", after conversion). The measured response for the converted filter is shown below. The bandwidth indicated is the frequency difference between the "dB down" point on the low side of the response curve and the similar "dB down" point on the high side of the response curve.

dB down	Normal Bandwidth	Converted Bandwidth
-6 dB *	1.2 MHz	1.75 MHz
-12 dB	2.7 MHz	2.4 MHz
-20 dB	3.5 MHz	3.1 MHz
-30 dB	4.6 MHz	4.1 MHz
-40 dB	5.8 MHz	5.3 MHz
-50 dB	7.6 MHz	7.2 MHz
-60 dB	>10 MHz	9.8 MHz

The "-6 dB" bandwidth is a little difficult to assign, especially in the case of the measurements made before conversion of the filter. This is because the passband ripple is quite large, almost 6 dB in the case of the normal filter. This could probably be improved by sweep alignment of the filter, but this is not the alignment procedure that is normally used.

The converted filter actually shows apparent better performance than the normal (pre-converted) filter, but this is not really significant because the center frequency has also been lowered. In terms of "percent" bandwidth (the measured bandwidths expressed in terms of percent of center frequency) the performance difference becomes insignicant.

I have also compared the performance of this converted front end filter with that of a front end filter that was "factory built" for the 140 MHz to 150.8 MHz range. No significant difference was observed in insertion loss or frequency response between these filters.

The table below show the performace of a converted "range 3" front end filter and also a converted "range 4" front end filter. These were measured after conversion to the two meter band.

I made some photographs of the response curves of the converted (range 3) filter, which are shown below.

The above photo shows the "swept response" of the converted filter around the center frequency of 146 MHz. The signal generator output was set to -30 dBm, the top of the screen of the spectrum analyzer. The insertion loss of the converted filter is seen to be about 6 dB. The vertical scale is 10 dB per division. The horizontal scale is 1 MHz per division. The peak response is at 146 MHz. The response shows some passband ripple, which is normal for this type of multi-resonator over-coupled filter. The response is somewhat asymetric, which also normal for this type of filter, especially if not "swept aligned" (as is the case here).

The above photo shows a closer look at the central portion of the response curve. For this measurement, the signal generator output was increased to -24 dBm, the vertical scale was set to 2 dB per division and the horizontal scale set to 500 kHz per division (note: screen says 50 kHz per division, but the correct scale is 500 kHz per division).

Summary:

This article describes a fairly simple and economical method of modification of the Micor® receiver front end filter from the 150.8 to 160 Mhz range to the 140 to 152 Mhz range. It also describes test results that show that this conversion can be performed using this method with no noticable degradation of filter performance. A Micor® receiver can be converted using this method with no loss of sensitivity or front end selectivity. The tuning rate (MHz tuned per turn of the tuning screw) of the modified filter is slightly faster than the original filter, but not excessivly so.

The main difficulty with this conversion method is that fairly precise machine work is required to produce the press fit between the modified tuning slug and its "cap", and also the necessary concentricity between the tuning screw and the cap. This is not that great a difficulty, however given access to even a small metal working lathe. This is a superior approach to using glue, etc. due to the good electrical contact produced between the tuning screw and the cap, and the permanent nature of the press fit joint. The converted front end filter is expected to be as stable and rugged as the original in addition to producing normal performance. This approach has been shown to be useful in the conversion of "H" and "HH" split Micor® radio receiver front end filters to the two meter amateur band. The "M" split (142 to 150.8 MHz) receivers don't require conversion as they were factory built to cover the two meter amateur band. The "L" split (132 to 142 MHz) receiver front end filters also don't require front end filter conversion as they will tune up to the two meter band without difficulty (though the local oscillator chain of the "L" split receivers does require conversion before it will tune up on the two meter band).

One important caution. Don't try to screw the tuning screws out through the top of the filter after this modification has been made. When the "cap" reaches the top of the filter, you should not "force" the tuning screws any further. This will damage the modified tuning screw, possibly even force the cap off.

If you have any questions or comments, or would like to have me perform this modification on your Micor® receiver(s), you may contact me via eMail by going to my Home Page (use the link just below) and choosing the "About Ries Labs" link.