Issues with FM broadcast interference (6400)

NT1K

Purchased a 6400 earlier in the year and it's been a really nice radio. However, I haven't really used it on 10m because of the solar cycle and lack of activity. When I went on to participate in the ARRL 10m contest. I noticed a big signal (150kHz+) centered on 28.180MHz. I didn't know what it was until I opened up the filters as wide is it could go and started hearing music. It was a FM Broadcast signal.

I live very close to a 5000w AM BC tower and closer to a 50kW FM BC tower.  The FM tower is xmitting at 94.7Mhz

So I am going to assume the FM signal is overloading the front end of the flex.  (94.7Mhz (FM BC) + 28.18Mhz (10m Band) = 128.88 (Direct sample). I attached an image showing the flex and the RTL-SDR at 94.7MHz (https://i.imgur.com/T9M0zQa.jpg)


Is there anything I could do with the flex to solve this issue or do I have to build some kind of stub filter/filter to attenuate?

Thanks,
Jeff (NT1K)

Duane_AC5AA

Perhaps the station is emitting a strong sub-harmonic?  Might want to check with the station's engineer to see what he says. 

Mike-VA3MW

Hi Jeff

You may have to build a notch filter for 94.7Mhz and see if you can knock it down.  I think there are some designs in the ARRL Antenna Books.  I know I have seen them in some places.

That is assuming the RF signal you are looking at is actually that main carrier frequency.

Others may have some ideas.

Mike

NT1K

Thanks Mike.

It's certainly the same FM signal on 28.180MHz as 94.7MHz

I have a Low Pass TVI filter somewhere the cuts everything above 40MHz that I'll try when I get back. I didn't think about that until after I posted this.

I was just curious if there was anything I could do with the flex itself before looking at other options

- Jeff

bahillen

If you or a fellow ham have an old low pass transmitter filter that may tell you a lot about the source of the mixing. Put it in line with ant 1. You may also try RX 1 that may take some circuitry out of the path for testing. Make sure the radio is not in the wide mode that provided RX protection. Use the pan adapter to measure strong signals from 60 khz to 54 MHz. This will identify RF that you don’t expect. Try measuring you interference while changing your amplifier gain. Also if you have an attenuator to put in front of the RX. (Just for evaluation. ) if it’s a simple, first order mixing the interference will change dB for dB. If a 3 rd order mixing it will change several times that amount. It seems the direct sample is simply chopping the FM signal getting into your radio. What is the effect of different antennas, maybe a beam or other of your antennas as an analysis tool. These are just some tools that can be used to isolate interference. 73 Bill W9JJB

Mike-VA3MW

Hi Jeff

You could try putting a tracking notch filter on it, but that will not allow you to hear any signals underneath it.  The only way to do that would be to somehow **** it before it gets to the radio.  It might take more RF rejection that the Low  Pass TVI filter can provide.  I suspect you want to see at least 30 db or more

How far away is that transmitter?  

The other challenge is that RF might not be coming down the feedline but it might be on the DC power lead, especially if the DC power lead is 1/4 wave of the 94Mhz signal (about 79cm or 31")

Mike 

Rick WN2C

Jeff, you say you live 'close' to both those station's transmitters, How close is close?

Chuck - W9WLX

I thought the 6000 series had a deep notch filter eliminating the FM broadcast band at all times. Needed to eliminate any nyquist wrap-around issues. You may want to reach out to the help desk before getting too far into this. 

NT1K

1/2mi from the 50kW FM xmitter (not sure if it's ERP or PEP) and 3/4mi to the AM BCB.

I have another ham that lives a block away with a 7300 and it faintly shows up on their rig.

NT1K

Thanks bill. I have a "TVI" LPF (Bell Industries C-514-T) that I will try tonight when I get home. It clips starting at 30Mhz with 80db of attenuation above 40Mhz.

As of right now it's S-8 no matter where the beam goes. 9+ when the beam is pointing at the xmitter (Between me and Africa). Similar when I switch to the vertical.
If I take the pre-selector out it's slightly worse.

SteveM

Chuck,


The issue Jeff is experiencing is nothing out of the ordinary for Flex 6K radios. An external broadcast-band filter was the first thing I needed before I could even try to use s 6500 at my QTH. I found a flat-topped signal in the pan-adapter at AM1490 and images of that signal dispersed throughout the amateur bands. So I bought a DLW filter and it fixed me right up.

I suspect the inadequacy of the preselector filters for mine and Jeff's purposes is related to, either:

1. Depending on the operating band, the filters operate in a low-pass or high-pass mode - thus, allowing the broadcast signals to pass straight through, unimpeded.

Or,

2. The stop-band of the preselector filters provides for only a modest amount of attenuation, say a maximum of 50dB. Whereas, the strong out-of-band signals from a nearby broadcast station may require more than 90dB attenuation to prevent receiver overload.

The situation at Jeff's QTH is similar to that of a repeater site where there is a transmitter and receiver operating simultaneously. At the repeater site you would probably employ a duplexer to provide the necessary isolation (+90dB) between tx/rx.

NT1K

Just an Update for those reading. I added a LPF  (Bell Industries C-541-T) inline that attenuates anything above 41Mhz by 80db or so. These were commonly used to deal with "TVI" back in the day. 

It appears to be working quite well to remove the offending FM station. 



https://imgur.com/a/R0Dt1Hd

Consider the matter closed. Thanks to those who replied

Ken Wells

I would also check to see if there are some dis-similar metals in the area generating RFI in the presence of the high power transmitter.  a rusty fence connection, just the right length, or some other bad metal to metal joint can create galvanic semi-conduction (a passive mixer, in effect) that can cause havoc in the presence of two high power transmitters. 

If you put a high pass or band-reject filter on the front end and you still  have the interference, then you need to go looking outside the shack.

Perhaps JUST outside the shack.  We have seen partially fried lightning arrestor MOVs or Gas discharge elements fail in such a way that they became perfect RFI generators......