Common Mode Choke On Receiving Ant.: Do They "Work" ?

[BOBRR]

Hi,
Happy Holidays to all.

Very weak with ant. theory, so let me ask for a few opinions.
Simplest is of course to just try it out, but would be interested in any thoughts on also before I do.

Have an outside long wire ant., which feeds some kind of Balun (from many, many years ago) to
a 50 ohm coax there, that then runs to the house and upstairs into my study where i have a new RSPdx.

Have found an old WinRadio Common Mode Choke module, an WR-CMC-30

For sw listening <30 MHz, think it would improve things if I placed
between the coax's end (in Study) and the 50 ohm input on the RSPdx ?

It's from an old WinRadio Excalibur G-31 receiver, and if I remember right, I could never
discern any difference in using it, or not using it.

Think it would make any difference with the SDRPlay RSPdx ? Why ?
Any "trick" in utilizing it ? Grounding, etc. ?

Thanks,
Bob

 

[prcguy]

A common mode choke with ferrite will try and absorb RF currents riding on the outside of your coax without affecting signals inside the coax. I'm not familiar with the WinRadio choke and they do not provide any meaningful specs and its very small, so it could be a low performer.

In my station I have measured noise and interfering signals with a spectrum scope before and after inserting an effective common mode choke at the antenna and the results were very good. My noise floor in general was lowered by several dB over a large frequency range and many birdies were reduced or eliminated. Placing another choke at the radio end further reduced some birdies.

In my situation most of my coax runs past computers, monitors, LAN and USB cables for a distance where that equipment was probably inducing RFI onto the shield of my coax and it was traveling up to the antenna and being received. The chokes did a good job of stopping the RFI on the shield and improved my reception. Every setup is different and may experience different results. Common mode chokes sometimes known as 1:1 baluns come in all sorts of designs with different results. Most are junk and do basically nothing.

Finding useful specs like how much common mode attenuation it has or what its common mode impedance is will help weed out the trash. I've found chokes from MyAntennas to have the highest levels of common mode attenuation and highest common mode impedance of any other mfr. Many if not most chokes are simply a ferrite bead over some coax or a string of beads over coax. At best, a long string of correct beads for your target frequency range might approach 20dB of common mode attenuation and 20dB is a 100:1 reduction. More complex chokes will have coax wound around a ferrite ring or bead and will use different ferrite mixes with a different number of windings in one housing to cover a wider frequency range with more attenuation. Most of the MyAntennas chokes are above 30dB attenuation (1000:1 reduction with a few reaching over 40dB or a 10,000:1 reduction of RF currents or RFI riding on the outside of your coax. You should not have to ground anything on a common mode choke for it to work.

Here is a link so some of my favorite common mode chokes. Common Mode Choke

 

[vagrant]

Well prcguy's answer simplified my response. At least one thing to take away from that is "...every setup is different and may experience different results." You have the choke already, so put it inline and test it with and without grounding it. Theory is fine, but one must test as every setup really is different. A MyAntennas choke did not make a difference for my setup, but I still leave it inline as things can change.

 

[BOBRR]

Hi Guys,

Just a quick thanks for info.
Really helpful.

Appreciate the time to write it all out for me.

Thanks again,
Bob

 

[Ubbe]

I use clip on ferrite chokes that's specified for 25MHz-600MHz and I have them at the receiver end of the coax and at 400Mhz they reduce spurious signals by 10dB or more and the signals that aren't affected comes from my SDR dongles themselves.

The higher the frequency the more effecient the ferrite will be, until it reaches it's maximun frequency where it quickly drops off completly. On shortwave a choke with a frequency range like this one will be of no use at all.

Higher frequencies have shorter range so at short wave the interferencies from other devices will reach further and be stronger and needs the choke to be very efficient. The outside of the coax can actually be a very good AM broadcast antenna that could need a very good working choke to stop the signal from interfering with shortwave reception.

If you see no improvement then the interference you see probably enters the antenna and then no chokes will help at the receiver end of the coax. But interferencies ride on the outher coax braid and also up to the antenna where it enters the inside of the coax. Chokes at the antenna end of the coax are then helpfull. Try and move the choke you have to the antenna and see if makes any difference. Ideally one should have chokes at both ends of a coax.

/Ubbe

 

[BOBRR]

Hi Ubbe:

Good info.
Sure is a lot to know. At least for me.

Thanks,
Bob

 

[mancow]

I have one of the my antennas brand chockes, the one for 80-6 meters or whatever. It's at the antenna. I guess I should put it near the radio?

 

[vagrant]

It depends on the antenna. If you TX and use an OCF, it would typically be at the antenna feed point or close to it. Adding another near the radio may or may not help. With or without a choke at the feed point and or near the radio made no difference on my particular OCF antenna setup at home. Also, most of that coaxial run is buried. Still, a choke makes a huge difference when I am portable and my coax run is not as long as it is at home (40 meters), as well as the distance of the antenna from my home (18 meters). Additionally, I have read that some operators that use an inverted V OCF antenna may have several meters of coaxial cable drop straight down before adding a choke in order to acheive an omni-ish type lobe.

For an end fed antenna, a choke a few feet from the radio is needed. Well, I have always needed a choke when transmitting using an end fed, even if it has a separate ground wire.

Everyone's setup is different so test it at the feed point and near the radio whether you TX or only RX.

I have one of the my antennas brand chockes, the one for 80-6 meters or whatever. It's at the antenna. I guess I should put it near the radio?

 

[mancow]

I'm using an Sgc tuner at the bottom of a 43' vertical.

 

[prcguy]

That's a nice setup and the choke is better off near the tuner. Are you running separate power wires to the tuner or using a bias T and running power up the coax? If you have separate power wires you would want to choke those also near the tuner. About six turns through a large 31 mix snap on ferrite bead is usually good for that. For a bias T setup the choke in the coax will take care of everything.

I'm using an Sgc tuner at the bottom of a 43' vertical.

 

[mancow]

Separate power from a wall wart in an outdoor outlet box on the house. I hadn't thought of that vector for noise. Thanks. I'll try it.

 

[prcguy]

A bias T is easy to make and you need one near the radio and one in the SGC tuner box. Its nice to have only the coax to worry about and the power is then inside a shielded cable. I can send info on the bias T if you need it.

Separate power from a wall wart in an outdoor outlet box on the house. I hadn't thought of that vector for noise. Thanks. I'll try it.

 

[prcguy]

Is the SGC tuner exposed to the elements or inside something for protection? I found water can get in to these tuners and even SGC recommends putting a plastic container over them. Here is a pic of my 43ft vertical and you can see some RG-142 coax wrapped around a ferrite ring for a coax choke near the tuner. The other pic shows a Home Depot bucket over the tuner for protection and the whole thing cammo painted to disappear in the trees.

This particular 43ft is by DX Engineering and its fed by 125ft of LMR400 buried in conduit to the house.

Separate power from a wall wart in an outdoor outlet box on the house. I hadn't thought of that vector for noise. Thanks. I'll try it.

 

[mancow]

It seems to work fine. I was just curious what the best placement was. The filter is above it on the fence rail where the cable runs around to the house.