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Hi to all.

I'm in a kind of similar situation here and this is my experience about it:

I live in Cuba, yes, THAT island in the Caribbean sea, any in the SWL community knows what that means: Cuba has a military program to interfere radio signals in the SW bands from USA.

What ever you tune to a frequency of "Radio Martí" or some times "VOA" you may heard a "wrack, wrack, wrack, wrack... " signals on top of it, that's what I'm taking about.

But happens that I live near (~120 meters) from one of that little interfering stations, (infamously) knows by the Cuba ham's community as "las sombrillitas" (the "little" umbrellas), I can't talk about the transmit station but yes about the antennas, the name came from the discone antennas they use to broadcast, as per my calcs tuned as low as 4 Mhz... hence the "little" nickname.

Why I tell you this?

Because every day at 6:00 pm local time they switch from a ~12-13 Mhz frequency (ruining 20m) to a 7 Mhz one, exactly on 7.435 kc most of the time.

Now imagine with me a powerful (don't know how exactly how much, but I would say about more than 100W) at 120 meters from your antenna and just ~300 Kc away, in the "pass" part of your passband filter.

Tip: around the end of 2015 they changed/perfected the transmitter technology. Before that, they transmitted a very broad signal of about 100 to 200 Khz one (at least on my reception point and looking it with a SDR) and now is just about 15 Khz. Before 2015 I have to turn my rig off and do anything else then they came to 40m...

Even my older FT-747GX has troubles with that, mind the little bitx40...

My task was research, experiment and test; and the result is this:

1. Don't use a COAXIAL to feed your antenna, the external part of it is a third wire and will act as a marconi antenna, at that power levels any fraction of RF will ruin your reception. (Having a heart connection just will worst the problem, current balums over the coaxial helps but not as much as the following item)
   
2. Use a twisted open wire line, and a balanced tuner if possible, if not, don't even try any type of voltage balums (Rutherfort, the kind almost any manual tuner has) use the current type of balums (Guanella) at least in my case they are more efficient fighting that interference.?
   
3. Put your antenna in the line formed by your position and the position of the emitter (whit the tip facing the transmitter, hence the null, not the lobe)

This kind of things will help to minimize the impact, not eliminate it, just the first two items will bring the interference a few S units down, the third will lower it a bit more but may ruin your DX as you may need to point your antenna to an ocean and not a continent as in my case...

Just a few cents of my experience.

73 de Pavel CO7WT.

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