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[K8CEB]

I have a 40m OCFD antenna in my attic (HOA) with 100 ft of LMR-240 coax into my shack. I get 40, 20, 10 and 6 meter bands. However, on 20m and 40m my noise floor is about S-6 to S-7. This noise is from RFI but I have so far been unable to eliminate it. My question is if I put up a folded dipole (40 & 20 meter) would this configuration offer reduced noise floor?

[W9IQ]

Do you have common mode chokes installed on your coax?

- Glenn W9IQ

[K8CEB]

Yes. I have two CMCs at the antenna end.

[K9AO]

Not if the antenna is in the same physical space.

The very first thing you need to do is to make sure that your coaxial cable is not allowing the noise ingress. To do that disconnect the connector at the antenna end and using a barrel connector and using an adapter if needed, terminate the antenna end of the cable into 50 ohms. You can use an all metal dummy load for this, a CB type 5 watt all metal dummy load in a PL-259 connector body, or even a 75 ohm CATV all metal terminator with an F connector adapter. It's got to be totally shielded. Now, tune across the bands with the rig and the interference should be gone if the coaxial cable is not allowing noise ingress. If you see noise still then you can address your coax. There are 3 surfaces for RF on a coaxial cable. These are the outside of the center conductor, the inside of the shield, and the outside of the shield. This is because the RF flows on the surface of the conductors only due to the skin effect. If the connectors are installed properly and there is sufficient shielding on the cable (98%) and possibly a foil shield too, then the noise cannot make the transition from the outside of the coaxial cable shield to the inside of the shield and then to the rig.

If the coax is RF tight then you look to the antenna. How is it fed? You will want a choke balun at the antenna end of the coax to handle any ingress issues. But even with that in place if the noise is actually there and being picked up by the antenna in the attic, then the only thing you can do is to eliminate the sources. If your unit is clean and the neighbors are not cooperative you are going to want to think about ways to mitigate the noise.

An outside magnetic loop would be a good possibility. Diversity receive using a noise antenna would be another.

We can toss ideas around once we know exactly what you have there in the environment.

Rick, K9AO

[W9IQ]

Yes. I have two CMCs at the antenna end.

You also need one towards the radio end of the coax in order to minimize RFI coupling. What brand or construction are you using for the chokes?

- Glenn W9IQ

[WB6BYU]

What type of balun are you using at the feedpoint?

That makes a big difference in noise pickup for an
OCFD.  A common 4 : 1 voltage balun doesn’t
provide much noise reduction.  The best solution
is probably to add an effective 1 : 1 current balun
(i.e. feedline choke) between the existing balun
and the feedline.

The reason I bring that up first, before answering
your original question, is that noise pickup on
the outside of the coax is probably the most common
source of such noise.  If so, the solution isn’t
necessarily to change the antenna.

Changing antennas may change the noise pickup,
but most likely due to a change in the balun used
rather than the antenna itself.

There is nothing inherent in a folded dipole that
would cause the antenna itself to pick up less
noise.  Well, not for a conventional monoband
folded dipole (which won’t work on both 40m
and 20m).  If you are looking at one of the
wideband terminated folded dipoles, then expect
about 1 S-unit reduction in noise, along with
the same loss in other signals, due to losses.

If the antenna has an ineffective balun, then a
folded dipole may be less prone to noise pickup
on the outside of the coax than the OCFD,  but
that is due to the balun, not the antenna itself.

[K8CEB]

Thanks for the great comments guys. I’ll dig into this over the next few days and get back to you.

[KO4QLC]

Thanks for the great comments guys. I’ll dig into this over the next few days and get back to you.

I have operated a 80M OCFD.  NOISE IS REALLY BAD ON 80 AND 40 METERS 1/3 - 2/3 FEED POINT.  FED WITH A 4:1 BALON AND 50 FEET OF LMR-400.  LIKE I SAID VERY NOISY ON 80 METERS AND 40 METERS.  WITH A GOOD "TUNER IT WILL WORK ON THE ENTIRE LIST OF BANDS.  ( WITH A GOOD ENOUGH TUNER ANY THING WILL "WORK" )

IN A THGHTLY PACKED NEIGHBORHOOD, WITH ALL THE ELECTRONIC POWER SUPPLIES WITHIN THE NEER FIELD, YOU ARE GOING TO EXPERIENCE A GREAT DEAL OF NOISE BEING GENERATED.  ALL THE LED LAMPS BEING USED AS WILL AS THE TELEVISIONS AND THEIR NOISY DISPLAYS, ALL THE WALL WARTS, AND DONT FORGET THE CFL LAMPS.

I DID FIND THAT A BOUBLE BAZOOKA WORKS WELL TO HELP WITH SOME OF THAT NOISE.  I AM RUNNING A TRPPED DOUBLE BAZOOKA FOR 80, AND 40 METERS.  STILL REQUIREING A LITTLE TOUCHUP WITH MY TUNER, BUT I GOT ABOUT A  S3 REDUCTION IN OVERAL NOISE  FROM ABOUT A S9 TO A S6, AND A LITTLE BETTER SIGNAL REPORTS ON THE OTHER END

[N3HKN]

Could it be coming in via the USB port on the radio IF it is PC connected?
N3HKN

[AI5BC]

I know quite a few hams hiding in their attics, and most have this the same problem. We have found two main culprits, and it has nothing to do with a choke.

1. I assume your attic space is being used distribute your house electrical wiring, and if like around here, also has your HVAC blower and heat exchanger located up in the attic. Is that you? If so, very good chance your antenna is just to close and picking up noise from the wiring and HVAC systems in your attic. Really simple to test, turn the power off at your main breaker, and power your radio from a 12-volt battery. If the noise goes away, you are the source of the noise, and no ham foolery choke will fix it. 

2. If you tried to ground anything, get rid of it because it is wrong. With an attic antenna you have no need for any grounding other than the AC ground in your DC power supply. So, remove anything you might have added.

Do those two things, and you will find your problem.

[WA3SKN]

Now here is where you build that small receive loop or ferrite rod antenna and try to get bearings and/or null out the offending signal!
Cal you radio separate the transmit and receive antenna?

-Mike.

[N9TYI]

I also have a ocfd in my attic , i used DX Engineering Ferrite DXE-CB31-500-10 at the feed point and it cut my noise in 1/2. Mine is set up as a inverted V at about 150 degrees angle suspended at the feed point .

[K8CEB]

Here’s more details about my attic antenna system. I have a 4:1 balun with a 1:1 choke (home made on 240-43 ferrite cores and 18 AWG wire) at the OCFD antenna. I have also added an additional common mode choke at the antenna. It consists of 11 turns of RG-58c/u coax on a 240-43 ferrite. Near the antenna I have a remote antenna switch module that switches my RG-8x coax between the OCFD and my discone and 2m dipole antennas. From the remote ant. switch I have 100 ft. of RG-58 down to my basement shack, with a ferrite bead at the radio end of the coax.

With this system I am getting a swr of 2.3 to 2.5 on 40m and 1.1 to 1.4 on 20m. I mostly use my Yaesu FT-920 on FT8 digital and get successful operation on 40, 20, 17, 10 and 6 meters using the rig’s tuner.

I have tried isolating the RFI by turning off my circuit breakers and have found that with all the breakers off my noise goes down to about s-1 to s-2 on 40 and 20 meters. I isolated the phone and cable modems as a source and added ferrite beads to their cables without success.

I am considering adding more ferrites to the modems as well as the coax at the radio end.

Also, my attic does not contain any hvac or house wiring.

Suggestions?

[AC2RY]

I have tried isolating the RFI by turning off my circuit breakers and have found that with all the breakers off my noise goes down to about s-1 to s-2 on 40 and 20 meters. I isolated the phone and cable modems as a source and added ferrite beads to their cables without success.

You said that turning everything off your noise level is down. Now you need to find what is connected to power line branches and fix noise there. Do it one branch at a time. You should be able to find what is connected to each.

[KO4QLC]

MOST NEW APPLIANCES LIKE REFRIGERATORS, AIRCONDITIONERS, DISH WASHERS HAVE VFDs TO MAKE THEM MORE EMERGY EFFICIENT.  UNLESS WELL SHEILDED AND ISOLATED THEY ARE EXTREAMLY NOISY.  MOST NEW HOMES, UNLESS ONE IS ABLE TO SPECIFY APPLIANCES, WILL BE FULL OF THE CONTRACTOR MODELS.

WITH A HAND FULL OF SNAP-ON FERITES, AT LEAST SOME OF THE NOISE CAN BE REDUCED.  AND THANKS TO BIG BROTHER  WE WILL JUST BE REQUIRED TO LIVE WITH THE NOISE!