[Boatanchors] Low Pass Filter Issues

[Glenn Little WB4UIV]

A low pass filter is a low pass filter. A low pass filter passes all 
frequencies below its cutoff frequency. If properly designed and built the 
filter will be more or less flat with minimum attenuation below the cutoff 
frequency. If the filter is designed as a low pass filter for VHF, it will 
probably pass all frequencies below about 200 MHz with minimum loss. This 
includes HF. On the other hand, if the filter is designed as a low pass 
filter for HF, it probably passes frequencies below about 50 MHz. This will 
not work for VHF.

I hope that this answers your question. The VHF filter will work on the HF 
rig, but not well as it will allow TV channels to to antenna. The HF filter 
will not work on VHF.

73
Glenn
WB4UIV

At 09:36 PM 11/04/04, Duane Fischer, W8DBF wrote:

>
>Hello All,
>
>Today I listened as several Hams in a local ham supply store argued about low
>pass filters. It was for a two meter VHF transceiver. One insisted the filter
>had to be for VHF use only and would severely inhibit a HF rig from
>transmitting. Or the converse, that a low pass filter for a HF rig would
>severely inhibit a VHF rig from transmitting. Another said the same low pass
>filter would work on either a HF or VHF rig. What exactly is the truth 
>here?
>
>Nowadays many new HF transceivers come with a low pass filter included in the
>design. How effective are they as compared to the external ones?
>
>Do any VHF rigs include them nowadays?
>
>Thank you for your wisdom.
>
>Duane W8DBF
>
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