multicoupler using RG-6 (75 ohm coax)?

[Ubbe]

Use an SDR to get a visual of the dB results. Even if you use LMR-400 for the test and regular old RG6. Remember, this test is for receiving.

It's both costly and time consuming doing double installations and the differences will be so small that the weather and time of day will probably have a bigger impact on the result. You will have to test most of your frequencies and then quickly switch coax and any other devices in line before external conditions changes.

But if having dual coaxes you probably also get another antenna of a different type or model and as antennas are frequency dependent you shouldn't replace the old antenna, keep it and install the new one next to it. It will be one antenna that works best on some frequencies and the other will hopefully work best on other frequencies to complement each other.

Use one coax with one scanner and the other coax with another scanner, or have a coax switch or perhaps a diplexer to one single scanner.

/Ubbe

 

[G7RUX]

So this is quite a complex area to consider.

Firstly, very few scanner receivers have a 50 Ohm input impedance across anything like an appreciable portion of their operating range.

Secondly, 75 Ohm coax is actually lower loss than 50 Ohm, all other things being equal.

Thirdly, and perhaps most importantly, you already have the 75 Ohm feeder in place so there would have to be a significant advantage to support swapping it out. Personally, if I was swapping it then I would use the absolute best, lowest-loss cable I could get my hands on.

Finally, even if the receiver input were 50 Ohms the mismatch losses from 75-50 Ohm are less than half a dB so you would be really hard pressed to notice that, even with decent test kit to measure it.

Using a low-ish gain multicoupler/splitter fed with 75 Ohm feeder will be absolutely fine and won't introduce any noticeable effects; most LNAs that work over a reasonably wide band similarly don't have terribly flat input impedance across that range. Just make sure to keep the gain low, just to overcome the splitter losses (3 dB for a 2-way split, 6 dB for a 4-way, and so on) so you don't introduce other issues. There are decent quality, cheap TV/receiver splitters and distribution amplifiers which should do the job nicely.

 

[vagrant]

@Ubbe - Ohhh…that’s right about the time of day. Yes, testing in the morning, afternoon and night makes a difference on the results. I should have noted that. It was a slight pain, but I tested and then swapped the coax several times one day and logged the numbers. For another test I even compared LDF4-50A against LMR-400, LMR-240UF and RG6 over the course of several days, but noted the times to keep things fair-ish.

In summary, looking at the results there were no surprises, but if one wanted to keep their costs low RG6 definitely works. No need to chase gain when one monitors local repeaters/transmitters already blasting in. Alternatively, I enjoy listening to military aircraft chatter, so LMR-400 works for me. LDF4-50A has a premium cost and its weight along with low flexibility makes it a solution I would use if I was ”transmitting“ on 33cm and up. Receive only allows one to use 75 Ohm RG6 and have plenty of fun.

* The easier and better solution is using the right antenna, or multiple appropriate antennas for the tasks. A diplexer, switch and or multiple receivers may the prudent solution depending on one’s needs. Some inline filters will also keep things friendly regardless of antenna or coaxial cable used.