[TenTec] Low Pass Filter

[JAMES HANLON]

Ed,

A properly designed filter should be terminated in its characteristic impedance at your operating frequency, probably 50 ohms, and will present a 50 ohm impedance at its input at your operating frequency as well.  So putting a filter in between the output of your transceiver and the input of your amplifier should not upset the load impedance for the transceiver as long as the amplifier input looks like 50 ohms as well.  

However, it is probably overkill to put a filter there.  One filter at the output of the amplifier will sufficiently attenuate all of your harmonics and other unwanted signals above the filter cutoff frequency that leave via the amplifier output coax lead.  Decent shielding of the transceiver and amplifier and proper filtering of the wires coming out of them, like power lines, microphone, key and headphone leads, etc, will keep any unwanted signals from getting out through open holes or on those wires.  Just about all modern equipment has adequate shielding and filtering, so you normally shouldn't have any problems there.  So one low pass filter in the coax at the amplifier output should be sufficient.  Also, I see no technical reason for keeping the coax cable between the amplifier output and the filter short, other than the possibility that the shielding on that lead is less than perfect and that would allow an unwanted signal to leak out.  As far as the impedance presented to the amplifier output, that won't be changed by the length of the cable to the filter as long as the filter works into a matched, usually 50 ohm load.  

Jim, W8KGI (PhD, EE)
  ----- Original Message ----- 
  From: Edward Sylvester<mailto:navydude1962 at yahoo.com> 
  To: Discussion of Ten-Tec Equipment<mailto:tentec at contesting.com> 
  Sent: Sunday, May 18, 2008 11:41 AM
  Subject: Re: [TenTec] Low Pass Filter


  As for LPF, should I use two; one on the output of the linear and one on the output of the radio?  Will double by RF supression or is this overkill??
     
    Ed NI6S

  JAMES HANLON <knjhanlon at msn.com<mailto:knjhanlon at msn.com>> wrote:
    There is an R. L. Drake Low Pass Filter on ebay, item 320251164898, ending at 12:58 PDT Sunday. Its current price is $8.28 with two bids. It should be a good one. 

  There is also a Nye Viking filter, item 290230177453, ending Monday at 14:16 PDT. Its current price is $1.04 with two bids. It should also be good. 

  There is another Drake filter, item 310050424821, ending at 19:00 on May 20. Its current price is $7.99 with no bids.

  There is a Drake TV-3300-LP, item 160241874048, ending at 07:53 on May 23. Its starting price is $5 with no bids.

  There is a Drake TV-3300-LP, item 2302522446865, ending at 14:20 on May 23. Its starting price is $9.99 with no bids.

  There is a Bencher YA-1, item 190222711105, ending May 25 at 18:30. Its current price is $9.99 with one bid. 

  Any of these should be a good filter for a legal limit rig.

  Jim, W8KGI

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