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lowpass TX Filter




 
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Author Topic: lowpass TX Filter  (Read 2197 times)
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Steve W8TOW
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« on: August 11, 2005, 09:59:36 AM »

I acquired individual transmit low pass filters for 160,m, 80m, & 40m..
These are well constructed able to handle 1KW AM no problem...
I think  many years ago they were referred to as "Harmonic Traps"?
How useful are these with single band tx and single band antennas???
Are they worth trying/??

An expampe of the circuit:
INPUT > .002 uF to ground, series inductor, .002 uF to gnd, .002 to ground, series inductor, .002 to gnd  OUTPUT>

73 steve
8tow
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Always buiilding & fixing stuff. Current station is a "Old Buzzard" KW, running a pair of Taylor T-200's modulated by Taylor 203Z's; Johnson 500 / SX-101A; Globe King 400B / BC-1004; and Finally, BC-610 with SX28  CU 160m morn & 75m wkends.
73  W8TOW
WA1GFZ
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« Reply #1 on: August 11, 2005, 03:28:04 PM »

Very helpful if you qro and want to be clean on other bands.
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K1JJ
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"Let's go sailing, Tommy!" - Yaz


« Reply #2 on: August 11, 2005, 03:45:11 PM »

I acquired individual transmit low pass filters for 160,m, 80m, & 40m..
These are well constructed able to handle 1KW AM no problem...
I think  many years ago they were referred to as "Harmonic Traps"?
How useful are these with single band tx and single band antennas???
Are they worth trying/??

Steve,

As you said, it depends a lot upon your antenna. Or if your amplifier final is too low-Q.

If it's a single band coaxial fed antenna, then rejection of the second harmonic is very high. It's the same for a good antenna-tuner fed openwire system.  These antennas  can give you an additonal 40+ db of rejection.

However, an antenna like a log periodic, or any of the numerous multiband [G5RV gimmick] or trap antennas can be a problem since they offer no rejection, assuming they are working as designed. Also, if your amplifier is not up to snuff, or you are using a solid state mobile amplifier without any LP filters in it... :-)

I've tried low pass filters twice. One was a 75M LP filter that was very well built. It got slightly warm as would be expected, since the roll off is not brick wall... it will rob SOME power from the fundamental. I got rid of it after finding no one cud hear my 2nd harmonic without it anyway. The same for 6M where they are commonly used. I tried one and got rid of it too. A 6M Yagi is totally mismatched at the 2nd harmonic and radiates very little.

Bottom line is that unless you have a specific problem you are trying to cure, like a neighbor who is being RFIed by a 2nd harmonic, or you use a multiband antenna [with no ant tuner] and have had someone actually HEAR your problem on the band, then I would not waste my time or additional watts into them, OM.

Using one without really needing it is like following your dog around the  woods with a pooper scooper.  Cheesy

73,
T
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Steve W8TOW
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« Reply #3 on: August 12, 2005, 08:10:13 AM »

Sounds good, since I either use a coax fed antenna, single or dual band loaded,
(one for 40m/160m) or use ladder line to my double zepp with the trad Johnson KW mbox,
I guess there really isn't much use in incorporating these into the system...
thanks
es 73 steve
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Always buiilding & fixing stuff. Current station is a "Old Buzzard" KW, running a pair of Taylor T-200's modulated by Taylor 203Z's; Johnson 500 / SX-101A; Globe King 400B / BC-1004; and Finally, BC-610 with SX28  CU 160m morn & 75m wkends.
73  W8TOW
KB2WIG
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« Reply #4 on: August 12, 2005, 01:24:19 PM »

Steve, 
       Why not put them on e-pay and sell them as 11m signal enhancers?        klc
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