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I live in the city, with 50’x 100’ lots. I use an 80 meter doublet at 30 feet, fed with ladder line to a tuner. My noise floor ranges from S5 to S9 depending on the time.

Would a coax feed reduce the noise level, while eliminating my ability to tune all the HF bands? Or is this just the nature of city living and the HF bands? I tried a loop antenna and it actually has more noise! I miss the suburbs for my ham radio hobby!

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  • $\begingroup$ The answer might depend on the noise generator and its location. Please describe the noise; is it atmospheric static or is it produced by devices like TVs, wall-warts and modems? $\endgroup$ – Brian K1LI Mar 9 '19 at 15:31
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There are a few things to think about.

Coaxial cable will likely not have a positive impact on your RFI.

Unfortunately, urban locations are loaded with RFI. Antenna direction and distance are the primary ways of reducing the received noise. On a small lot, this is not always possible but improvements should at least be evaluated. For example, an electrically shortened antenna may allow the antenna to be located further away from local noise sources and oriented such that its directionality attenuates these noises.

Don't overlook the opportunity to identify and potentially eliminate noise sources. Start with your own house by powering your radio off of a battery and shutting down the house power and other battery powered devices within the house. If the noise level drops, start turning on individual breakers to help identify the sources of your local noise. Go RFI hunting in the neighborhood to identify other noise sources that might be correctable.

If the antenna is not well balanced due to factors such as uneven heights or trees or buildings near the antenna then the feedline can become unbalanced and be a point of ingress for noise. A 1:1 balun at the feedpoint may help mitigate the noise ingress.

Connecting a balanced feedline to an unbalanced tuner can also couple local RFI into the antenna system. The antenna side of an unbalanced tuner should have a 1:1 balun between it and the balanced line. A belts and suspenders approach is to also put a 1:1 balun on the coaxial side of the tuner going to the transceiver.

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No, coax wouldn't be expected to improve the noise, unless your ladder line is running very close (within 1-2 times its conductor spacing) to a specific noise source, in which case you would be better off fixing that by finding a way to provide better spacing. The only thing that will change with coax is that your losses will become quite a bit higher, and more of your transmit power will be spent on heating the coax instead of making radio waves, especially when operating the doublet far away from a match with the coax.

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  • $\begingroup$ Agreed. And, unless specific steps are taken, coax shield can be a very effective "antenna" to pickup noise. $\endgroup$ – Brian K1LI Mar 9 '19 at 15:28
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    $\begingroup$ Coax losses are not necessarily higher. LMR-400 has about the same loss as dry plastic window line. The window line is cheaper, but it does not have lower loss. $\endgroup$ – Phil Frost - W8II Mar 10 '19 at 12:49
  • $\begingroup$ @PhilFrost-W8II LMR-400 appears to come within about 2-3x the loss of 450-ohm ladder line at HF (0.2 vs. <0.1 dB/100ft 3.5MHz; 0.5 vs. 0.2 dB/100ft 14MHz; 0.9 vs. 0.4 dB/100ft 50MHz). I'd hesitate to call it the same, but I suppose there are times when it would do just fine, yes :) $\endgroup$ – hobbs - KC2G Mar 10 '19 at 13:18
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    $\begingroup$ I'm sure it depends on the ladder line. For example this 300 ohm line from DX engineering gives the dry loss at 30 MHz as 0.688 dB/100 ft compared with 0.7 dB/100 ft for LMR-400. DX Engineering don't seem to provide a datasheet for the 450 ohm line. And this is the problem with a lot of ladder line: there's no data sheet, and it's not necessarily low loss. In fact it can be quite poor. $\endgroup$ – Phil Frost - W8II Mar 10 '19 at 15:58
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Coax with a good 1:1 current balun at the antenna feedpoint could reduce your received noise. The function of that balun is to provide a high impedance to common mode signal (mostly nearby noise) and a matched impedance to differential mode signals.

I did a simple S-meter test when I put a balun on my dipole. The noise floor dropped by 6 dB on the 20 meter band.

The next step is to hunt down and eliminate nearby noise sources.

For understanding baluns and RF chokes, I highly recommend "A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing" by Jim Brown, K9YC. This is the best publication on the subject and it is free.

http://audiosystemsgroup.com/RFI-Ham.pdf

Jim Brown also wrote two good articles on "killing receive noise" for the National Contest Journal in 2016. Those are available here:

http://ncjweb.com/features/mayjun16feat.pdf
http://ncjweb.com/features/sepoct16feat.pdf

Finally, thinking of ladder line as "balanced" doesn't help understand the noise problem. Like coax, ladder line carries a mix of differential mode and common mode signals. Your goal is to reduce the noise generated near the antenna and reduce the noise on the feed line.

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