Eliminating 60 cycle hum from antenna feed.

Thread Starter

SamR

Joined Mar 19, 2019
2,006
Just rebuilt and rehung my Vibroplex Par EF-SWL 45' end fed sloper antenna for my Icom R-75 all band receiver. On the radio input, I am also using an MFJ-1040C 1.8 - 54 MHz combination preamp, filter and antenna tuner. My problem is that ~100' from the antenna is a 3 phase power distribution feeder. RG-8 coax to the antenna. It is grounded at the radio through the circuit safety ground and also grounded from the balun to a driven dedicated earth ground. On the low frequencies, the hum is horrible from ~1.25MHz and down. Irritating from 1.25 - ~1.35-1.45. The obvious solution is a Hi-Pass input filter.

For comparison: On my Icom IC-7410 (which is also an all-band receiver) it is not as bad on the 135' off-center fed 6 - 80M dipole and near-zero on the 10M vertical. Yes orientation does make a difference. Much better input filtering.

How much attenuation would this add to the overall input? Any practical advice or recommendations?
 

Papabravo

Joined Feb 24, 2006
13,456
Orientation is not filtering -- it is polarization. Your experiment indicates the 3-phase power feeder has horizontal polarization. The vertical antenna will attenuate horizontally polarized QRM much more than vertically polarized QRM. For short range VHF communication you want your antennas at the transmitting station and the receiving station to be the same. For long distance HF communication, once the signal bounces off the ionosphere the polarization can be anything. So for the signal it doesn't matter what polarization the antenna has. That being the case, try turning the 45' sloper into a vertical and see what happens. Spider Poles are really good for this purpose.
 

Thread Starter

SamR

Joined Mar 19, 2019
2,006
That being the case, try turning the 45' sloper into a vertical
Unfortunately not possible. Lacking a suitable tower, I am left with the tree option and my antennas are placed where I can. The vertical and dipole off-center balun are supported by 35' of 1.5" pipe braced by a bracket to the roof ridge at the end of my house. Yes, I was not surprised at the polarity effect as was expected. Although I really have very little need for the low frequencies, I am afraid extra additional filtering is my only real option to eliminate the interference. Before Hurricane Michael, I also had a ~150' end-fed horizontal long-wire antenna for the Icom R-75 that has not been replaced as yet. The MFJ-1040C has switching ability between 2 antennas.
 

Papabravo

Joined Feb 24, 2006
13,456
The problem you are going to have is finding components with the right values to do the filtering. Blocking low frequencies with realizable components is difficult at best. Try it and see.
 

Tesla23

Joined May 10, 2009
388
Just rebuilt and rehung my Vibroplex Par EF-SWL 45' end fed sloper antenna for my Icom R-75 all band receiver. On the radio input, I am also using an MFJ-1040C 1.8 - 54 MHz combination preamp, filter and antenna tuner. My problem is that ~100' from the antenna is a 3 phase power distribution feeder. RG-8 coax to the antenna. It is grounded at the radio through the circuit safety ground and also grounded from the balun to a driven dedicated earth ground. On the low frequencies, the hum is horrible from ~1.25MHz and down. Irritating from 1.25 - ~1.35-1.45. The obvious solution is a Hi-Pass input filter.

For comparison: On my Icom IC-7410 (which is also an all-band receiver) it is not as bad on the 135' off-center fed 6 - 80M dipole and near-zero on the 10M vertical. Yes orientation does make a difference. Much better input filtering.

How much attenuation would this add to the overall input? Any practical advice or recommendations?
A high-pass filter is straightforward, and there are simple apps that will design them for you, e.g.
https://rf-tools.com/lc-filter/

1584745760838.png

will (ideally) give you hundreds of dB attenuation at 120Hz and almost none at 1MHz and above. If the issue is low frequencies entering the antenna terminals then a filter like this (or even simpler) should stop it.

It's possible your multiple ground connections are causing low frequency currents to flow between the receiver ground connections, and that these are causing hum to enter the receiver circuits. If this was the case, though, I guess it would affect all frequencies.
 

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wayneh

Joined Sep 9, 2010
16,384
If you were a noob I'd suggest confirming that the signal is truly entering via the antenna and not the power supply of the receiver. Running a radio or audio amp off a battery instead of a mains supply can often fix the latter and thus diagnose the actual source of noise. Turning off nearby fluorescent bulbs is another magic fix.

But I'm guessing you've covered that stuff already.
 

Thread Starter

SamR

Joined Mar 19, 2019
2,006
It's possible your multiple ground connections are causing low frequency currents to flow between the receiver ground connections, and that these are causing hum to enter the receiver circuits. If this was the case, though, I guess it would affect all frequencies.
I already had the noise before adding the dedicated antenna/radio ground. Nice tool, thx for the link. Before they upgraded the power line feeder from single to three phase a few years ago, plus increasing feeder size to accommodate the new home construction over the years the 60 cycle hum wasn't so bad.

I also use Alpha Delta TT3G50 gas tube lightning suppressors on the antenna feeds to try and prevent lightning-induced EMF from entering the house wiring. The coax surge suppressors are also grounded to the dedicated antenna ground.
 
Last edited:

Tesla23

Joined May 10, 2009
388
I'd go with Wayne's idea and try separating it from the mains, run it from a car battery or similar. If that's the problem, seriously filter the DC line from the power supply
 

Papabravo

Joined Feb 24, 2006
13,456
You do realize that you will lose 6 dB insertion loss with that filter. It will knock both the signal and the noise down that much for starters.
 

Thread Starter

SamR

Joined Mar 19, 2019
2,006
Yes, my main concern is overall signal attenuation. Eliminating the noise with minimal insertion loss is the goal if at all doable. I have some coils I can experiment with or make my own. I have no clue as to how to calculate insertion loss, however.
 

Papabravo

Joined Feb 24, 2006
13,456
It still has a 6 dB loss because of the 50Ω source impedance and the 50Ω load impedance. You can't make a lossless passive filter. To maintain the signal levels you need a gain stage. This has absolutely nothing to do with the L's and the C's. Possibly some of you learned filter design at Overunity University
 

Tesla23

Joined May 10, 2009
388
It still has a 6 dB loss because of the 50Ω source impedance and the 50Ω load impedance. You can't make a lossless passive filter. To maintain the signal levels you need a gain stage. This has absolutely nothing to do with the L's and the C's. Possibly some of you learned filter design at Overunity University
A real source has a source impedance, we are assuming the antenna is matched to 50ohms, and a real load has an impedance, we are assuming the receiver is matched to 50ohms. In that case, at 1MHz there will be an insignificant excess loss using that filter compared to simply connecting the source to the load.
 

Papabravo

Joined Feb 24, 2006
13,456
Isn't that merely the coax impedance?
It is the coax and the termination at the other end. The termination of a transmission line can be open, short, or any impedance in between. Usually a transmission line is terminated with it's characteristic impedance. Each possible termination will have different characteristics with regard to reflections. Termination with the characteristic impedance guarantees there will be minimal reflections.
 

crutschow

Joined Mar 14, 2008
24,708
Termination with the characteristic impedance guarantees there will be minimal reflections.
So since those 50 ohm impedances on the filter schematic are the normal coax and receiver termination impedances already there, the filter won't add any significant high frequency attenuation.
 

Tesla23

Joined May 10, 2009
388
I redid it for 20% standard values and will play with it more tomorrow.
View attachment 201960
If you want lots of attenuation at 120Hz you would be far better with a 500kHz cutoff freq. The components will be smaller and lower loss at RF as well, as well as cheaper.

Your ideal 500Hz highpass only has about 35dB loss at 120Hz whereas the ideal 500kHz one has hundreds of dB.
 
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