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low pass filter or low swr?

space cowboy

space cowboy

Quack Quack
Oct 16, 2012
1,368
452
93
Michigan
any thoughts on which is the better option?
I just got my starduster up, around 35' to the feedpoint.
without the low pass filter I have a flat swr from 1-40, with it installed my swr jumps to 1.3 on 1 and 1.5 on 40.
I'm assuming it's because the lpf is 52 ohms instead of 50, or am I wrong?

the filter is a 20 year old drake with 80 db atten. minimum above 47 mhz. good for 1000W, but I'll never push more than 80, if that.
even without it I haven't seen any signs of tvi. my computer speakers remain quiet on both am and ssb. they usually pick up people modulating while driving past.
 

From the numbers, you aren't experiencing any significant SWR. There's no practical/significant difference between a 1:1 and a 1.5:1 SWR. It would seem like the LP filter is the reason for that change and that's not all that unusual.
If you didn't need that LP filter but want to use it anyway, why not? Sort of like insurance...
- 'Doc
 
Try turning the filter around and reversing the connections. The coil on the opposite side may be tuned closer to 50 ohms. If that doesn't give satisfactory results, don't be afraid to remove the screws and open the filter. "Tweak" the coil on the input side of the filter. Bending it together just a little bit and checking the input SWR to the filter. If the SWR goes up, you know you need to spread the coil a little bit. This coil can adjust the input impedance to the filter. Do not touch the other coils in the filter.
 
when changing jumper length changes vswr readings significantly it IS changing the vswr, there is no fooling the meter with jumper lengths,

it helps to know what you are talking about instead of posting cb myths mr i don't need my clarifier unlocked radio expert,

post your technobull explanation of how changing jumper length is fooling the meter.
 
If it were me I would run the filter in line just for peace of mind.

But if you are not having any known issues with it not in line the choice is yours.

The difference in SWR readings is nothing to be concerned with.if the SWR had jumped to over a 2:1 then it would be time to start looking for the problem,but with the 1.3 to 1.5 you have you are good to go
 
I don't use a filter unless a last resort. Wrap a ugly balan with the coax at the antenna. Doesn't. Changeling length of coax tell you the coax is radiiating like part of the antenna. Is that the same with a jumper.
 
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Grogan said:
I don't use a filter unless a last resort. Wrap a ugly balan with the coax at the antenna. Doesn't. Changeling length of coax tell you the coax is radiiating like part of the antenna. Is that the same with a jumper.
Click to expand...

While this is true, we should not confuse coax radiation with harmonic radiation. The cure for both of these problems is not the same. While there is no more analog TV channel 2 to worry about here, there is still the 6 meter ham band and many operators have complained recently that CB is getting into 6 meters. Because of the excellent DX propagation your harmonics go further too and low pass filters still make sense.
 
Thanks for the replies, guys.

I just got this all set up Saturday evening, and the operating periods have been short, and followed by "before the storms rolled in", so I have no idea if I'm causing problems elsewhere.
I'm running TM LMR-400 with an Alpha Delta TT3G50 lightning arrestor onto a 8' ground rod, if it matters. there is no ground on the low pass other than the coax.

I'd prefer not to go ask the nearest neighbor, who's a talker, since her husband just died and I can see it quickly becoming a weepy hour+ discussion. I get it, but that's already happened once, so I'll wait a few months on that.

I tried flipping the filter around like shockwave suggested, and now I'm at 1.2:1 for the whole band. I'll try going inside and messing with the coils after the next storm front or two passes by.

I came across this in my search for answers-
dudmuck said:
You must ask yourself, do I really need a low-pass filter?
Tube amplifiers have a Pi-L output network, which inherently reduces harmonic output to antenna. You should never need a low pass filter with any decent tube amp.
Click to expand...
is this true? if so I'll be running the low pass right off the radio, since I wouldn't need it farther on.

On a less problem solving note - I made contact with New York(911 - says I need to turn it up) and New Hampshire(237 - just let me know he heard me before turning it off "before the storms rolled in") last night around 10 pm, nothing but my northstar.
 

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9C1Driver said:
All that does is fool the meter, it does not address the problem. That's a smoke and mirror fix.
Click to expand...

'zactly correct,.....

Joe Is testing a new Yagi beam antenna. It seems like no matter what Joe does to the antenna, the SWR is 3 to 1 or more. Joe tries changing the length of the coax because he read somewhere that doing that tunes the antenna. And guess what,....... the SWR drops to 1.2 to 1! Obviously Joe has confirmed that the length of the coax tunes the antenna, right?

Wrong. Joe had a problem before, and still has a problem, although his SWR meter suggests that he fixed it.

SWR is the ratio of transmitter impedance to antenna impedance,

if changing the length of coax changes the SWR READING, then the the coax is acting as the LOAD and NOT as a transmission line.
 
To add to this, if changing the coax length alters the SWR, the antenna was not tuned to 50 ohms and this should be your first clue that you must adjust the antenna. If changing the coax has no noticeable effect on SWR, you can feel fairly confident your antenna is correctly tuned and matched to the coax.
 
To add yet more: 50 ohm coax works fine with a 50 ohm antenna. If your antenna's feedpoint impedance is closer to 70 ohms (not at all unusual), a good piece of RG-11 or other 70/75 ohm feedline might make you feel happier.

Then again, most SWR meters are specifically designed and calibrated for 50 ohm systems....

Don't lose sleep over 2:1 unless your rig cuts back power before that point.
 
bob85 said:
when changing jumper length changes vswr readings significantly it IS changing the vswr, there is no fooling the meter with jumper lengths,

it helps to know what you are talking about instead of posting cb myths mr i don't need my clarifier unlocked radio expert,

post your technobull explanation of how changing jumper length is fooling the meter.
Click to expand...


Sorry Bob but the "cb myth" here is that changing coax length fixes the problem. If there is an impedance mis-match with a device you must correct the problem not add coax. This topic usually comes up when someone adds an amplifier and the SWR goes up. The band aid fix is always fiddle with jumper length. That's all I am saying Bob, i'm no X-pert. Take care.
 

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