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Author Topic: Low Pass Filter installation  (Read 522 times)
N7GCO
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Posts: 146




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« on: January 01, 2008, 08:23:38 PM »

I have a BENCHER YA-1 Low Pass Filter I would like to install in my shack, but am not sure the proper location sequence. I am using in sequence: a Icom 746 transceiver,  Icom PW1 Amp, RadioCraft 3000 Pro SWR/Power meter, to a yagi antenna.

Does the low pass filter go:
Between the transceiver and the amp?
Between the amp and the swr/power meter?
Between the swr/power meter and the antenna?
or other?

I did a web search and did not find the answer.

Thanks
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N4JOH
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Posts: 193




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« Reply #1 on: January 01, 2008, 08:38:35 PM »

I have mine coming out of the back of my Amplifier,,,, John
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KZ1X
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Posts: 3228




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« Reply #2 on: January 01, 2008, 08:38:49 PM »

I doubt you need it, but put it at the output of the amplifier.
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AC5E
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Posts: 3585




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« Reply #3 on: January 02, 2008, 04:28:15 AM »

Since the amplifier is also a harmonic generating device it should be placed after the amplifier.

Since high SWR will put high RF currents and voltages on the capacitors in the low pass filter it should be placed before the tuner.  


73  Pete Allen  AC5E
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WA3SKN
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Posts: 5493




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« Reply #4 on: January 02, 2008, 04:38:24 AM »

I normally recommend the "last 50 ohm point in the system", but the amp output would be a good place.

-Mike.
 
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AD5X
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Posts: 1432




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« Reply #5 on: January 02, 2008, 05:11:36 AM »

I agree with KZ1X - i.e. do you really need a LPF?  This tended to be more of a necessity before CATV when your neighbor's TVs were picking up programs off-the-air.  Even though the amp's harmonic output levels met FCC requirements, sometimes there was still enough energy to cause TVI.  Remember that the LPF will only help with harmonic RFI above the filter's passband.

Phil - AD5X
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WB2WIK
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Posts: 20611




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« Reply #6 on: January 02, 2008, 09:51:32 AM »

After the amp; although the PW-1 has internal automatic bandswitching low pass filters in it and the extra "outboard" filtering probably won't do anything useful.

The Bencher filter cuts off just above 30 MHz, so it is equivalent to the automatically switched LPF the PW-1 selects for 10m/12m.   But the PW-1 has far better filtering on 160-->15m, as those are bandswitched and selected, unlike the Bencher which has one cutoff frequency, and that's it.

WB2WIK/6

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AG4RQ
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Posts: 300


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« Reply #7 on: January 03, 2008, 01:03:35 PM »

I don't have an amp, but I have my low-pass filter between the radio and the tuner. The tuner is the last place I run any coax. I use 450-ohm ladder line from the tuner to the antenna.

If I was using an amp, I would place the amp between the radio and the low-pass filter. In other words, radio, amp, low-pass filter, tuner, antenna.

I forgot to mention that my tuner has the Watt/SWR meter built in.

73,
AG4RQ
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AG4RQ
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« Reply #8 on: January 03, 2008, 01:07:46 PM »

If using a separate Watt/SWR meter, it should go after the tuner and it should the last thing before the antenna. That way, you get a proper reading of power and SWR after all else inline.

AG4RQ
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AG4RQ
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« Reply #9 on: January 03, 2008, 01:12:53 PM »

"do you really need a LPF?"

Yes. I don't want harmonics from my HF rig messing with anything above 10m. TVI isn't the only interference you have to worry about. There's 6m amateur, 6m business band and everything above that. Maybe it's a waste, but why take chances?

73,
AG4RQ
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