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Author Topic: Low-pass filter and 10-meters  (Read 3432 times)

Posts: 83

« on: February 04, 2012, 07:59:34 PM »

I was having what I think was some RFI on the low bands, so I installed a low-pass filter, mostly by the suggestion of another ham, just to see if it took care of the issue.

It seems to have cleaned up the noise, although that doesn't make sense to me because I thought the low-pass filter was to prevent me from causing interference, not the other way around. Perhaps I misunderstand how a low-pass filter works. Either way, the extra noise is gone now.

The low-pass filter allows 1.0-30mhz through, but the filtering starts before 30 mhz. As a result, I can no longer use on the 10-meter band. I can't even monitor it.

I am using a single radio with a tuner (the filter is between the radio and tuner) and a 135' OFC dipole. Before I installed the filter I was able to use 10 meters.

Any suggestions for how I can bypass the low-pass filter and still use just one antenna?

A 10-meter antenna isn't that big, I just don't want a bunch of antennas and lines coming in  the house.

I know, I am one of those odd hams who likes minimalism  Grin


« Last Edit: February 04, 2012, 08:13:33 PM by W0TLP » Logged

Posts: 5917

« Reply #1 on: February 05, 2012, 12:08:39 PM »

A LPF should not affect low frequency noise; so, what is going on? Can you insert and remove the filter to confirm if it is reducing the lower frequency noise.

If it is helpful simply switch it in and out of the circuit using a 2 position coaxial switch. MFJ and others sell these switches.

Posts: 1465

« Reply #2 on: February 05, 2012, 01:28:25 PM »

Just for the record, the purpose of the low pass filter is to greatly reduce transmitted HARMONICS of your HF signals - that's it.  Harmonics could be a serious problem back when there were TV signals on the lower channels and the third harmonic of a 15 meter signal fell within the span of channel 3.  If you're having RFI problems, chances are 99% that the problem is that the device interfered with cannot handle the fundamental of your HF signal.  If your RFI problem was noise you're hearing on the HF bands, then the low pass filter is of no use to prevent your hearing that noise.

Regarding the cutoff frequency of your lowpass filter: These filters are comprised of multiple sections of inductors and capacitors.  I'd guess this is a filter with some age on it, and you have no idea how many times it's been dropped or otherwise abused.  And, unless it was a good quality unit from Drake, B&W, Heathkit, etc., the cutoff frequency might never have been right to begin with.  Also, these filters were designed for 50 ohms input and output.  Presenting it with something other than a 50 ohm load on the output would effect the performance of the filter. 

Posts: 12672

« Reply #3 on: February 05, 2012, 02:32:47 PM »

Unless the noise is caused by some strong signal well above 30MHz overloading the front end of your receiver, the low pass filter should not do anything to reduce the noise. Normally the noise you hear on 40M, for example, is actually on 40M and therefore passes right through the low pass filter right along with the desired signals.

Perhaps your low pass filter has a lot of in-band attenuation and is just reducing the signal strength of everything (including the noise). Tune in a broadcast signal and connect/disconnect the filter and see how much the signal strength changes.

The cut off of an L/C filter like a typical low pass filter is gradual rather than absolute. If the mfg specifies a cut off of 30MHz that will be at some level like -6dB. That means that a 30MHz signal will be reduced by 6dB. It doesn't mean that a 29.99MHz signal will pass through with no attenuation but a 30MHz signal will be totally blocked. Normally a low pass filter designed to pass the amateur HF bands have a cut off frequency well above 30MHz (like 40 or 50MHz) in order to pass a 30MHz signal will little attenuation.

Given what you said about the noise and not being able to hear 10M signals with it connected, I suspect there may be a problem with the filter - or it is not a very good design. Also be aware that such filters only work properly when the line impedance is 50 Ohms. You have it installed in the correct place between the radio and the tuner. Is your tuner finding a good match for your antenna and presenting a 1:1 SWR (or close) to the radio? Does the SWR at the radio change when you insert/remove the filter? The SWR at the radio should be the same with or without the filter and shouldn't require readjustment of the tuner.

As K8AC stated, a low pass filter is designed to keep harmonics generated by your transmitter from reaching your antenna. They were used quite regularly in the good old tube days when transmitters often generated such harmonics. With modern transceivers they are not normally needed, especially if you have a tuner in line which itself will provide some attenuation to harmonics. A properly functioning low pass filter will never attenuate in band receiver noise. If it does then there is something wrong with the filter or you have some other antenna system issue that it is covering up - or as I said your noise is being generated by receiver overload from a signal well above 30MHz.

By the way, an OCF coax fed dipole normally causes common mode currents on your coax shield. Perhaps the noise is being coupled to your feed line and adding the filter and associated jumpers changes the total line length enough to make a difference in the noise pickup.
« Last Edit: February 05, 2012, 02:38:18 PM by AA4PB » Logged

Posts: 31

« Reply #4 on: February 05, 2012, 02:35:13 PM »

I agree with the comments above. The low pass filter should not filter noise that you are receiving on the low bands. Also, your low pass filter should not be filtering out the 10 meter band. Something I'm not understanding here ...

Maybe your tuner is not handling the unbalanced to balanced issue you have going from the unbalanced (coax) output of your radio to your balanced dipole? Maybe your coax shield between radio and tuner is part of your antenna?

I would try removing the low pass filter and make sure the noise comes back. And then put some split ferrite cores on the coax between radio and tuner. Or ferrite beads would be cheaper, if you don't mind replacing the coax connector on one end.

Posts: 12672

« Reply #5 on: February 05, 2012, 02:46:03 PM »

I would try removing the low pass filter and make sure the noise comes back. And then put some split ferrite cores on the coax between radio and tuner. Or ferrite beads would be cheaper, if you don't mind replacing the coax connector on one end.

Good idea. If the noise is due to feed line pick up due to common mode currents I'll bet the noise goes away with the addition of ferrite beads even without the low pass filter - you can loose the low pass filter if it does. You can also make a choke by winding a bunch of turns of coax into a coil.

Posts: 83

« Reply #6 on: February 05, 2012, 08:29:56 PM »

Thanks all.

It turns out the filter was causing all sorts of issues, including SWR that fluctuated with modulation. I removed the filter and everything works fine.

I don't hear the noise I thought I heard.

The noise issue could have been me not knowing what I should or should not hear on the HF bands. I just got into HF a few months ago and had no practical experience with HF before that (even though I had my General ticket for a year). It could also have been something that coincidentally disappeared about the time I put the low-pass filter in the circuit.

So now I am back to my original set up and added a decent speaker, which gives better audio than the radio speaker.

I am pretty sure whatever noise I hear now is just band noise and it's not bad. For a while it actually hurt my ears to listen to 80 meters, but that's just not the case now.

I might try the split ferrite cores on that jumper anyway. Surely it can't hurt anything.

Thanks again for all the feedback. I always learn something here.

I am a happy HFer.
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