Unfortunately for us all, many linear amplifiers have some gain at VHF
frequencies. In fact, much has been written about the idea that gain at VHF
frequencies can cause runaway "glitches" in linears, in some cases
destroying them.
There are always sources of VHF energy at the input of a linear, even if
only occasional switching transients from keying the rig on and off. One
could easily imagine these being amplified unintentionally.
Thus was my reasoning for having a filter after the linear.
Now, I chose to put it after the tuner. In my case that leads to no issues,
as my antennas are resonant, 50 ohm nominal loads. But if someone else's
antennas really needed a tuner, then obviously one could place the filter
after the amp but before the tuner.
Regardless of all that, I still think one is probably enough, and my
practice is likely overkill.
73 de Gary, AA2IZ
----- Original Message -----
From: "Rick Denney" <rick@rickdenney.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Monday, May 19, 2008 3:50 PM
Subject: Re: [TenTec] Low Pass Filter
> Bob Close writes...
>
> > IF the transceiver had harmonics and those were transferred to the
linear,
> > would they not be amplified by a factor of ten?
>
> Seems to me a low-pass filter is not a band-pass filter, and isn't
> there for normal attenuation of close-in spurs. It seems to me there
> for attenuating VHF spurs above 30 MHz so that they don't get into
> nearby TV's or FM radios (or cordless phones, stereo speakers, etc.)
> Those spurs should be pretty small to begin with for well-designed
> stuff that isn't overdriven, and a single low-pass filter on the
> output is enough to clean up any little bit that is there.
>
> The only low-pass filter I have is the low-pass effect of my Ten Tec
> 229 tuner with its L network. We see no RFI on our TV. I doubt that
> any more low-pass filtering would be of much use.
>
> Rick "lucky to live in a rural area" Denney
>
>
>
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>
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