Author Topic: High-Pass Filters  (Read 279 times)

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Offline W5HRO

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High-Pass Filters
« on: April 08, 2014, 11:25:51 AM »
One thing about many of the old vacuum tube and most solid state receivers is they have dynamic RF amplifier front ends. This was a design trade-off made back in the earlier days of radio to quickly get multiple band coverage without the need for plug-in coils for each band as with the National HRO receiver line for example. Most receivers rely solely on IF filtering for selectivity. This design trade-off resulted in very poor RF amplifier selectivity with hardly any rejection to strong broadcast station interference or other interference down lower starting at DC on up. This can often lead to many receivers being unusable at certain locations and even intermod products getting through on the higher bands.

I recently purchased the premium Kiwa Electronics filter attached below and I am currently using it on my NC-303 receiver. I need to modify the receiver's input to a 50-ohm termination though to get the best performance. I'm thinking of running coax from the SO-239 connector on the back of the receiver to a 51-ohm resistor to ground to the 100pF coupling cap then to the 1M grid-leak resistor.

Offline W5HRO

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Re: High-Pass Filters
« Reply #1 on: April 08, 2014, 11:26:10 AM »
Attached below is my idea to modify the NC-303's antenna input circuit for use with the external high-pass filter. There is no telling if the receivers input circuit is actually 50-ohms or not. It could be anywhere from 5-ohms to 105-ohms. Something like the below should work on most old tube receivers to get a 50-ohm termination at the input.

Offline KM1H

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Re: High-Pass Filters
« Reply #2 on: April 14, 2014, 06:40:51 PM »
How much BCB RF do you have there? I have 7  5-10KW stations within 5-10 miles and they dont bother my NC-303 even on 160/80 with inverted V's at 180' on top of a hill and LOS to the AM towers. Coax fed antennas and using the radios antenna trimmer do the job IMO.

OTOH I had to build filters for the 500-750' Beverages for use with a TS-940 on 160, the TS-950SD seems to be immune.

The sliding coil carriages of the NC-100 thru NC-240D and variants use very similar LC assemblies as the HRO and are also bullet proof

Offline W5HRO

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Re: High-Pass Filters
« Reply #3 on: April 15, 2014, 07:32:29 AM »
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How much BCB RF do you have there? I have 7  5-10KW stations within 5-10 miles and they dont bother my NC-303 even on 160/80 with inverted V's at 180' on top of a hill and LOS to the AM towers.

I'm not really getting any BC interference, its all coming from down lower so I just wanted something with good attenuation from 1.8Mc down to DC.

The bad thing is when Panasonic announced last September or October they were going to stop making Plasma TV's they flew off the shelves at the Best Buy down the street and one of my neighbors must have purchased one. Not sure which one it is, but it’s either the one right across the street or the one right next door. It’s not real bad, but it's strong enough to where its there in the evening after they get home. I know that's what it is because the RFI is in the exact same spots like when my Panasonic plasma is on. I spoke with the guy at Best Buy recently and he said they went on sale after the announcement and they sold them like crazy until they couldn’t get anymore and now they are gone. The problem is Panasonic's last forever. It wasn’t a problem when I was the only one who had one in the area and if they hadn’t announced they were going to stop making them it would still be that way. I had just about solved all of my noise problem too :icon_thumbdown: