Need suggestions to reduce signal pick up of coax cable.

On a sunny day (Sat, 9 Jun 2018 07:31:35 -0500) it happened amdx wrote in :

First thing: What sort of noise do you get with that co-axe?. If it is the usual MW RF impulse noise, better find the source. put a second antenna near the noise source, and use one of those noise subtracter circuits:

2) Is the grounding of the coax at the receiver, so the receiver's input circuit, correct? This is easy to check, short the co-axe at the receiver side, and the noise should disappear, if not then the receiver's input circuit is shit. Best circuit is likely just a single turn and then an LC, note the galvanic isolation

--------- FET gate ===-- | | | | |( |/ Ctune | )|( === |___| |( /| |______| | ///

3) In any case it is better to have a preamplifier at the antenna side, and a less sensitive RF input stage in the receiver.

Semi-rigid coax - where the outer conductor is thin tube of solid copper - is probably the nicest, but 200 feet wouldn't be cheap

covers RG402. 200feet is 61 metres, and for that much it's $A11.84 per metre in Australia.

Solder-on connectors aren't cheap either.

It's going to be cheaper in the US - Newark will sell you a 50 metre spool for $US184.14

--
Bill Sloman, Sydney

er.

Don't know, but just a thought, try installing a common mode choke. At FM frequencies this can be a few turns of the coax on a 2 liter coke bottle. Normally this deals with issues created by a balanced antenna on an unbalan ced line, but really it has to do with currents on the outside of the coax. The fact that grounding your shield at the remote end increased the noise makes me think this could be your problem.

Rick C.

Does the signal go away if you disconnect the center conductor at the radio?

I had to check, I did not use the word noise, I'm receiving radio signals on my coax. How do I quiet it?

When I installed my active mini whip I had terrible noise, I have already conquered electrical impulse noise. Mostly outdoor lighting with sensors. Mikek

I addressed that here, "I tried 3 different Chokes on the Coax at the radio, one monster choke is a very large potcore that I wound 15 turns of coax on, it has 25kohms of reactance at 1MHz. The chokes didn't help." They were all CMCs.

The fact that grounding your shield at the remote end increased the noise makes me think this could be your problem.

I'll try that later today. Mikek

JOOI, did you try touching the tip (or screen - it shouldn't matter which in this case) of the co-axial plug to the screen/ground of the receiver?s co-axial aerial socket? The interference could simply be due to ground currents in the receiver's input circuit leaking into its input filter.

If you just connect that co-ax to the centre pin on the aerial socket as an end fed long wire aerial, you're likely going to see S9 plus 30dB or higher signals on the Medium Wave band (I've assumed you mean the MW broadcast band with your AM reference). Trying to attenuate a 30dB over S9 signal down to the S1 level, assuming 6dB per S point, corresponds to a total attenuation factor of 78dB. That level of attenuation is tricky enough to achieve in the best of circumstance and no mean feat at that!

Considering your project (200 feet of co-ax *and* a directional aerial for the MW band), I guess you must be well clear of any conurbations and out in open country. Even though you might not be so troubled by QRM as most of us unfortunates usually are, QRN is likely to be the limiting factor in this case. That S6 level of unwanted leakage from local MW broadcast stations is likely to be drowned out by the QRN from a decent MW antenna array anyway. You may be worrying about nothing if those interfering signals from such an antenna array are going to appear as S9 plus 30dB or higher anyway.

I suspect the problem is less to do with a deficiency of the co-ax and more to do with a deficiency of receiver's input screening, hence my suggestion above.

--
Johnny B Good

How much would 200' of aluminum foil cost you? You could just foil-wrap the whole run. And depending on the width of the foil, you could probably get 3x or 4x the length by cutting the foil in strips first.

Would be messy to install.

Also, are you concerned about lightning damage? (i.e., a lightning strike being conducted into your house/receiver? Something to consider?

JohhnyBgood, could you repost, on both of my computers your post is empty. Mikek

I haven't read the replies yet but the first thing that comes to mind is to find or wind baluns and use 2 lines of coax cut to the same length. Like a home brew LVDS. Of course the shields are connected directly and you might even want to strip it to the shield in a few places in the run and clamp t hem together - electrically of course but the right clamp is probably what you'll need. This will further reduce pickup, making more sure that it is c ommon mode even if it comes to ground gradients.

I think the way you described what you're getting with the current setup is the shield is picking up the signal inductively. Even if you leave one end open it is still RF so that doesn't necessarily mean shit.

Now I will read the rest and either find out that I am an asshole or (EEK ! ) that I am right. You did say you had a bunch of RG-6, and I am a firm pro ponent of using what you got.

Is the receiver mains powered and especially double insulated ? If there are just an antenna connect tion but no ground connection, the antenna grounding happens through the signal ground through the power supply bridge rectifier and mains transformer capacitance to mains neutral. This can cause mains buzz (and possibly intermodulation products), if the rectifier diodes are not bypassed with small capacitors.

What is the radio input stage like ? Is it part of a ferrite rod antenna or perhaps a source follower intended for a whip antenna ? Or perhaps a proper 50/75 ohm input stage with a pi-section as in some car radios ?

While impedance matching is not critical for reception especially on MW band, but proper impedance matching is critical for proper transmission line operation (such as interference intrusion) of the coax cable.

Make sure that the receiver shows about 50 ohm termination resistance to the cable. If the receiver input impedance is much higher, you may need a step-up isolation transformer or a 50-100 ohm resistor across receiver input, with some loss of wanted distant signal.

If I understand--- I have taken the coax loose and connected just the shield to the shield connection of the radio. No signal. Connecting the coax shield to the center pin of the radio does give plenty of signal, but I think that's not a problem.

Naw, highest signal is about an S 9, plus a few db.

Trying to attenuate a 30dB over S9

I guess you must be well clear of any conurbations and

Not seeing nearly as much noise as you. I just put two very large CMCs on the power cord of the radio, An Icom

71A btw. This did reduce the signal pick up. So, now with the power cord CMC and a short at the far end of the coax, I have 5 S units on 590kHz, but 1260kHz and 1430kHz only have 0.5 S units, that is a huge reduction. This leads me to think I need to run my radio from a battery. 590kHz has been the highest signal with every iteration. Both transmit from the same location, 590kHz at 1700watts and 1430kHz at 5000watts.

You may be worrying about nothing if those

Now with my Power cord CMC and the coax shorted at the far end. I can get 14 stations 9 are 1 S unit or below 4 stations below 3 S units and

590 is 5.5 S units. I'm going to spend some effort getting the coax below any leaves and in contact with actual as much dirt as I can. ?

Mikek

How did you shield the chokes and transformers? Can we assume that when you put shorting connector right at the receiver, the interference went to zero? And with the inductive gizmos connected and the shorting connector at the INPUT to the inductive gizmos? What happened with the short and connecting the coax shield to the radio ground?

Yes, you are on to something, (see previous post) I added CMC to the power cord and it did a lot.

I'm using an Icom R71A, it's old but a quality receiver.

Using 75 ohm coax and a 50 ohm input receiver. I have a 1:1 isolation transformer, putting in actually increases signals, BUT, I may need to put it in a shielded box.

What noise level do you get if you attach a 75R dummy load at the antenna end?

Is there any change if you touch earth to coax earth to antenna earth?

If you use an attenuator, does the noise level drop more that the attenuator value?

I'm wondering if there is some strong signal you're inadvertently picking up.

--
Mike Perkins 
Video Solutions Ltd 
www.videosolutions.ltd.uk

The 75 ohm termination causes an extra 2 S units increase from 980kHz to

1480kHz, from 870 down the signals are about the same. This is verses the Short.

None that I have noticed.

Lightning surge protectors handle nearby strikes.

Your insurance agent handles direct strikes, provided you survive.

73,

--
Don Kuenz, KB7RPU

Hmm, I'm not a radio guy, but isn't ground part of the antenna you'll hang on the end? If so, then why is picking up radio signals when you ground it, bad?

George H.