- Counterpoise in addition or in replacement to ground connection
- Roof mounted antenna
Or generally, some improvement to the antenna.
Your existing antenna sounds like effectively a vertical with no radials. This means much of the ground current is in the soil. The soil is a resistor. Resistors convert electrical energy into heat.
The ICOM-7300 is capable of 100W which should be sufficient for SSB when conditions are good, but not if 90% of that power is just making warm dirt.
I would see if your signal is even heard on some WebSDRs, in conjunction with VOACAP to see what combination of bands, times, and locations are likely to work. You won't make many contacts if no one can hear you!
Thus, my first recommendation is to focus on the antenna. A good antenna will help you on receive and transmit, and costs very little.
One way to go is a dipole. A dipole requires no radials, is cheap, easy to install, and works great. It needs to be at least a quarter wavelength high (but even higher is better) and a half-wavelength long. Depending on the space you have this might limit operation to higher frequencies. I'd suggest 20m to start, simply because there's a lot of activity there.
Another way is to install radials, at least 16 of them. But it's a lot of work, and in the city there's often not enough space on the ground.
A roof mounted vertical is a way around this. My first station was in a suburban 1/4 acre lot with a 1000 square foot ranch. I had a 4BTV in the center of the roof, with 16 radials which ran to the edge of the roof. This worked well because the roof was the largest flat space where I could lay radials.
Unfortunately this roof antenna was also pretty noisy since the house wiring was only a few feet below. If you can do it, I'd recommend the dipole first: if you can string it up in 50' trees to get it farther from the house that will reduce the RFI. But either will be a big improvement over your current antenna, I bet.
- Addition of feedline chokes at unun or near entrance to house
In the city there's really not much you can do about the ambient RFI, but you can get the antenna as far away as possible from nearby sources. But without a common-mode choke, the feedline is the antenna, and it comes in the house and connects to the radio, which is connected to the house wiring, which is connected to computers, switch-mode power supplies, etc. Without a choke, all these RFI spewing devices are literally directly attached to the antenna.
At a minimum, a choke at the feedpoint is a good idea. Another one at the entrance to the house can't hurt. If you like, measure the common-mode current to quantify the improvement you get from a particular configuration. Remember that by reciprocity, if you measure a common-mode current on something when transmitting, that thing is also part of the receiving antenna.
Your "turn the house breaker off" test is also a good one. Ideally you're receiving RFI only from your neighbors. After getting the common-mode current under control, you might work to identify noisemakers in your own home and put chokes on them, making them less efficient radiators.
If you want:
- Learn CQ [assuming you mean CW]
It takes much less power to get an intelligible CW signal, so if you want to invest the time to learn Morse code, this certainly will extend your range. Many digital modes have sensitivity similar to or better than CW, so that's another route which doesn't involve learning Morse code. The FT8 and JT65 modes provided by WSJT software are a good example, though don't allow for free-form "rag-chewing". If that's your preference, you might look at PSK31 or even old-school RTTY.
I wouldn't bother:
- Swapping to a loop antenna
- Trying one of these DX canceling phasers with active antennas
- Remote tuner with grounding wire
As discussed previously, loop antennas aren't really all that great.
Phasers could work theoretically: with a phased antenna array you have a more directional antenna system and can null out noise sources. But most of them are cheaply made, and I think you'll be disappointed.
A remote tuner might reduce feedline losses, but if you do the math on the actual losses on 50' of LMR-400, you'll probably find they aren't more than a few dB. A tuner won't provide any significant improvement to receive performance. A grounding wire is irrelevant because there should be no common-mode current to ground in a proper antenna system.