Performance Measurements of a Motorola VHF Micor
Filter Modified for 220MHz
All photos clickable for a larger view.
The VHF high-split Micor 5-Pole
Helical Filter Modified for 220 MHz.
This entire effort was brought about in an attempt to assist my friend
Gary, KH6JTM resolve
some interference issues with his 224.580 MHz repeater in NE
Albuquerque. Gary built his
220MHz repeater by modifying a high split VHF Micor using instructions
he found at the Repeater
Information website. These modifications include steps to convert
the 5-pole helical filter from 160MHz to
220MHz. It "peaks up" at 220MHz, but just how well was it working
compared to the stock
filter, and would there be any benefit to adding a second filter ahead
of the receiver? I have access to a Hewlett-Packard 8753C 6GHz network
analyzer at work, so I offered to sweep the filter and test Gary's
modified 220MHz preamp at the same time.
Here is the HP8753C Network Analyzer used
for these test measurements.
Gary performed the initial modification per the documentation to the
helical filter I tested. I modified it a bit further by replacing the
phono connector at the input, and
plain wire output with chassis mount SMA connectors. I also changed the
output tap from
about 2-1/2 turns from the bottom to 1/2 turn from the bottom, matching
the tap point of
the input coil. I was hoping to bring it closer to 50 ohms since this
would be used in
a 50ohm in/50 ohm out application. It ended up being around 100 ohms
but I did not consider it worthwhile to try to get the match any better
(plus it was mechanically difficult). That being done, the filter was
connected to the analyzer and the following plots were created.
Photo showing the filter wth added SMA Connectors
Photo showing output resonator tap details.
This plot shows the 3db bandwidth to be 3.5MHz, with
5.1dB insertion loss. The ripple is indicative of overcoupling between
the poles, likely intentional on the part of Circle-M to widen the
passband of the filter to operate over the stock Micor's intended
Here's a wideband response plot. Spurious responses are
far enough away and low enough not to worry about
This is Gary's modified preamp. This plot shows the gain
bandwidth to be about 70 MHz, with the peak gain about 14dB at -40dBm
Out of band response is lower than I expected.
Just for giggles, since the analyzer was already set up
and cal'd, I hooked up an old Motrac helical filter. This one doesn't
appear to be as overcoupled as the Micor. Insertion loss is about the
same at 5dB.
Here is the modified helical filter and preamp working
together, with -40dBm of drive. Between the selectivity of the amp and
filter combined, this setup might be of some worth. On the air testing
will tell all.
The bottom line is that a modified Micor helical works just fine on
though it is a little broad and lossy to be considered as a high-Q
Putting an additional one inline, while improving out of band
incur yet another 5db or so of loss in addition to the 5dB of loss
the helical already in the radio. 10dB of loss before you even get to
radio seems a bit much, and adding a preamp to that brings with it a
new set of problems. If greater rejection than what is provided by a
5-pole helical filter is required, a much better solution would be a
Many thanks to Gary, KH6JTM for allowing me to keep his filter and
preamp for a few weeks while I got around to measuring them.
Pane Relief Computer Services