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Author Topic: EF Johnson Low Pass Filter Specs?  (Read 9544 times)
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n4wc
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« on: June 14, 2009, 06:14:34 AM »

I have the 250-20 low pass filter and was wondering what is the max continuous power rating. 
Also, is there a good way to set the two variables without a spectrum analyzer?
I assume the tuning range of these caps is limited.

Thanks for any information.

Bill Cook
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Bill Cook
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« Reply #1 on: June 14, 2009, 09:52:08 AM »

I have the 250-20 low pass filter and was wondering what is the max continuous power rating. 
Also, is there a good way to set the two variables without a spectrum analyzer?
I assume the tuning range of these caps is limited.

Thanks for any information.

Bill Cook

250 watts of carrier.

Adjust both caps for minimum reflected power, into a 50 ohm dummy load using a CLEAN exciter.

You can also use the MFJ 259 or any other VNA. 

BUT, a low pass filter will work best when the in and out networks are tuned for the freq designed for.

Alternatively, a TV on a set of rabbit ears next to the thing is also a decent way, and one I'd used in the past at "customers" houses.

--Shane
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ke7trp
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« Reply #2 on: June 16, 2009, 03:16:12 PM »

I have set them with the 259B like shane suggested. I tuned for 50 ohms at the input and output.   I ended up taking mine out of line. I have no complaints from the people around me on the low bands and to me, a filter and another coax jumper is just something else inline.  Everyone in the area has cable or sat now so TVI is just not an issue on the low bands for me.

Clark
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Pete, WA2CWA
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« Reply #3 on: June 16, 2009, 03:40:18 PM »

I would also open it up and inspect it completely to make sure components haven't been blown apart or damaged. If, in it's previous life, it was in line, and there was an antenna issue (wrong antenna, no antenna, dog chewed through the coax, etc.) very high RF currents through the thing could have caused damage. I haven't had a need for a low pass filter in over 30 or more years even when operating on 6 meters.
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
w5gnb
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« Reply #4 on: February 18, 2020, 12:53:17 PM »

The Johnson 250-20 Filter is actually rated for 1-KW at 50 ohms but substantially less than that if there is any SWR present at the filter.  I would keep it below 250-Watts considering peak to peak on AM would be 1-KW.

The cut off frequency is roughly 40-mhz.  I have measured a good attenuation of about 30db at 45-mhz and above that it will provide up to 50db attenuation say in the FM broadcast band.

Tuning of the two variable caps on the ends will float this cutoff frequency up or down and it is really not possible to set them correctly without some sort of spectrum analyzer.  Set them at about mid scale and you should be just fine.
 
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KA3EKH
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« Reply #5 on: February 18, 2020, 01:22:43 PM »

I ran one on my RCA BTA-1MX running 375 watts of carrier, never had an issue and the person I sold the transmitter too still has it in line. I liked it because it cleaned up any potential junk above forty or fifty Megs.
Always tried to keep the spurious down, think I had second and third harmonic down below fifty and any other products below eighty, no reason other than good engineering practice. Have to wonder how many others using rebuilt broadcast transmitters can make that spec?
The filter never got hot or anything.
 
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AG5UM
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« Reply #6 on: February 18, 2020, 09:56:11 PM »

There is a spec. sheet and ad page you can find on-line for the Johnson 250-20 low pass filter, it states:
" more than 1000 watts RF, amplitude modulated"
" Cut -off frequency is 45 mcs., with "M" derived end sections adjusted to provide maximum attenuation
at 57 mcs." Etc. alot of good info.
I think I used my Military Grid-Dip meter (AN/PRM-10  Test Oscillator Set) to test mine.
 as I remember, I think it worked well for testing the filter frequencies.
AG5UM
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kc2we
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« Reply #7 on: February 26, 2020, 09:26:20 PM »

The old 1 KW rating was based on AM when the transmitter was running 1000 Watts DC INPUT to the finals. That equates to about 700 watts AM output. This is how the older boat anchor manufacturers specified their equipment based on DC INPUT.

 Now, the duty cycle of AM is about 4 x in PEP, so 700 watts output x 4 (from the old standard FCC limit) is about 2800 Watts PEP. AM has a carrier and two sidebands. This is all ICAS rating.

Since the PEP level of AM based on output is the same as Continuous carrier that's also 2800 Watts.

***Actually by today's FCC OUTPUT limit, AM of 375 watts output is 1500 watts PEP or close to it*** In that case FCC rules limits the AM power, not the filter rating per se. Again 375 watts AM is about 1500 PEP.

Older gear was all rated  by ICAS ratings. Since ICAS ratings are about 40% higher than CCS, I'd say that the filter can handle 2000 watts RF continuous like RTTY which is above the legal limit. CW having a 50% duty cycle, The filter would be OK at today's legal limit of 1500 watts.  CW is no problem with it's 50% duty cycle.

I used one of these for years with a pair of 3-500Z's running 1400 watts OUT and the filter never failed, never got warm in all modes but not AM.

I wouldn't use it above 30 MHz. Above 30 MHz it's starts absorbing power and that's a problem.

Seth KC2WE ex KZ5BA (Canal Zone)
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Seth Taylor
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« Reply #8 on: February 29, 2020, 12:40:01 AM »

I had one in which the 'hot' capacitor plates were disk-like pads on top of some kind of plastic. The 'cold' plates being the frame of the thing. Coils went from one to the next. I tested with a KW DC input to the 4-1000 and 80-90% modulation, with a dummy load, and punched holes in the plastic sheeting. It may have been a Drake, but it was a copper colored long rectangular box as usual, not round. More than one LPF has been ruined. Among other things. Need a smaller transmitter.
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