VHF Filters for Experimenters

Since 2004, we have been developing roofing filter mods for some of the more popular radios. The listing of what we have available is on the Performance Enhancements page on our website. Some radios are easy to modify because they have a coax cable between the output of the first mixer and the input to the second mixer. Other radios have this circuitry all on one pc board, and they require some pc surgery to add the modification. We have chosen to provide mod kits for radios for which installation is easy. However, many people have been asking about other radios for which we don't provide mods. Now we are providing filters for people who want to create their own modifications or who want to experiment.

The table below lists the first IF for many of the better radios. We have some of them in stock and some on order.

Frequency MHz Radio Main Rx Sub Rx

69.4500 FT-2000 X

48.6400 FT-1000D X

73.6200 FT-1000D X

47.2100 FT-1000MP, MkV X

70.4550 FT-1000MP, MkV X

45.7050 FT-847 X

68.9850 FT-920 X

47.0550 FT-980 X

64.4550 IC-746, IC-756PRO I, II, III X

69.0115 IC-756, 765, 775 X

70.4515 IC-761 X

46.5115 IC-781 X

45.0000 Orion, Orion II X

73.0500 TS-450, 690, 850, 870, 950SDX X

44.9300 TS-930 X

45.0500 TS-940 X

40.0550 TS-950SDX X

These filters are the same type of filters used in our roofing filter mods.

Frequency MHz	Radio	Main Rx	Sub Rx
69.4500	FT-2000	X
48.6400	FT-1000D		X
73.6200	FT-1000D	X
47.2100	FT-1000MP, MkV		X
70.4550	FT-1000MP, MkV	X
45.7050	FT-847	X
68.9850	FT-920	X
47.0550	FT-980	X
64.4550	IC-746, IC-756PRO I, II, III	X
69.0115	IC-756, 765, 775	X
70.4515	IC-761	X
46.5115	IC-781	X
45.0000	Orion, Orion II		X
73.0500	TS-450, 690, 850, 870, 950SDX	X
44.9300	TS-930	X
45.0500	TS-940	X
40.0550	TS-950SDX		X

Wiring at VHF needs to be done with very short leads. In particular, the grounding of the filter case must have very low inductance to prevent “blowby”, or poor isolation. At 73 MHz an inductance of 2 nano-Henries will insert 1 ohm in series with the filter. A wire less than ½ inch long has this much inductance. This can result in blowby which is only 34 dB down. Care should be taken that all of the signal and return current passes through the filter and its case. If a common ground is used between input and output, the filter isolation will be compromised when some of the signal return current flows through the ground plane. For example: a copper ground plane should be split to separate input and output signals. All of the Inrad roofing filter assemblies are designed this way.

If you are designing an experimental up conversion radio using one of these filters, it would be wise to select the second IF using the same frequency as the original radio. That is, select 9.0115 MHz for the second IF to go with 69.0115 MHz in the first IF, as in several of the Icom radios. This should minimize the spurious signals generated in the mixing process. Most up conversion radios use 455 kHz as the third IF.

FILTER SPECIFICATIONS

Bandwidth: 5 kHz typical

Insertion loss: 5 dB typical

Case: Shown below

Temperature range: 0 to 50 degrees C.

Input/Output match: 50 Ohms

Operating level: 0 dBm maximum

Isolation: -60 dB typical

Price: $125

Order these filters by frequency only.

TYPICAL FILTER CHARACTERISTIC

FILTER CASE DIMENSIONS (mm)

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Bandwidth:	5 kHz typical
Insertion loss:	5 dB typical
Case:	Shown below
Temperature range:	0 to 50 degrees C.
Input/Output match:	50 Ohms
Operating level:	0 dBm maximum
Isolation:	-60 dB typical
Price:	$125