Yet another coax question

[dave3825]

Looking for advice on something. I have a desk on one of a side of a room and run about 20 feet of coax to an lna with a moxon antenna in the front window.

I have read the moxon is 50 ohm. The 20 feet of cable I am using is F677TSVV (spec sheet) Its listed as 75ohm. I am only receiving for scanner and sdr and mostly use the moxon.

Question is for anyone can read the spec sheet and let me know if this stuff is any good. I don't know about this stuff. It's what I had at the time but if it sucks, I will replace with what's recommended. Like I said, I do not transmit at all, just scanner / sdr for 7/800, 470, airband and vhf. I just want the best possible reception, like everyone else.. What would my coax be compared to?

Thanks.

 

[217]

Quick answer is this. The first chart is your specs. Notice the loss of 5.65 at 750. The second chart is for quad shield that I currently use. Its a tad bit better. Last chart is Belden 9311 which is 50 ohm. These are just two examples to show you how other types compare with loss. If you are receiving everything you intend to monitor with your 20 ft coax, then its all good. If not, you will have to invest in some better coax. Others here can recommend the best for your needs because I only use what I can get for free.

 

[Ubbe]

It's only 20 feet of that coax and that's 1.1dB loss at 750MHz and 1.2dB at 865MHz which is almost nothing. You also write that you have a LNA at the antenna so coax loss are non important for you as the LNA increase the signal way more than the coax loss. The difference between a 50 ohm and a 75 ohm being connected together are something like a 0.2dB loss and that is IF it really are those impedances. A scanner certainly not have a 50 ohm impedance over its frequency range.
https://forums.radioreference.com/threads/bcd536hp-front-end-filter-measurements.371406/post-2942675

/Ubbe

 

[buddrousa]

If you want to get picky like stated you do not have to but if you want to look at this like.
How to build a 75 ohm to 50 ohm converter

 

[Ubbe]

Try to do a search for "common mode choke" in the forums. When you have coax going from one end of your room to the other, and your room are probably filled with a lot of different electronic devices that emit RF interference, that coax shield works as a wire antenna and picks up all the interferencies and it travels to the end of the coax where you have your antenna and it will receive it and feed your receiver with interferencies that might make it receive worse. If you stop those interferencies on the coax at the antenna end of the coax, and probably also need to do so at the receiver end if your receiver isn't completly shielded from the outside world, the level of interference will be drasticly reduced and you'll get a much cleaner signal to receive.

The least one could do are to coil up the coax at both ends, 4-5 turns will be enough to let the signal at one turn meet the signal at another turn a half wave lenght later and they will phase each other out. The coil will also add some inductance and capacitance to the outer braid and will act as a poor mans choke to help reduce the interferencies. The best thing are of course ferrit chokes put around the coax starting with a 1/4 wavelenght from the antenna element at the highest frequency you gonna receive, using ferrits for that frequency range, and then up to about 1/4 wavelenght of the lowest frequency/longest wavelenght from the antenna you are gonna receive with ferrits handling that frequency range, to make the ferrit perform at its best for the most suitable frequency.

/Ubbe

 

[cmdrwill]

Yikes ferrits on your coax. Should be ferrite choke.

 

[questnz]

Another Coax calculator here Coax Calculator