50 OHM coax cable to 75 OHM coax cable


#1

I am looking for any antenna guru’s to help me with this current conundrum I am having. Currently I have the flightaware 1090 antenna in the perfect spot on the roof. The thick 50 ohm coaxial cable it being fed to the outdoor cable box where all the 75 ohm TV/Internet cables can be tapped into and used on in the house. I have correctly identified which outdoor connection leads to where my Raspberry PI is. My issue is when I spliced the coax cables together to allow for the change in cable size, I seem to have lost a significant amount of capability in picking up aircraft in the area. Anyone have thoughts on how to best approach this? My whole goal is to avoid drilling any holes into my house and tap into the coax infrastructure already in place. If you are a master in this topic, please email me your ideas at edwikman@gmail.com, as I seem to always forget to check back on my posts in forums.

Note: The coax cable that is already leading from outside the house to the inside does not connect, split, or interfere in any way shape or form within the walls/crawlspace.

Note 2: I am aware the long length of the system I am trying to implement is long (approximately 100 feet total, 1/2 is the 50 OHM cable, and the other half is the 75 TV/Internet style cable in the walls) and will reduce the power of the antenna. If you have suggestions on how to boost this as well, that would be much appreciated.

Thank you,
Erik


#2

The 50/75 ohm impedance mismatch is not a big deal - something that’s nice to avoid, but the reflection loss is <1dB IIRC.

Probably your issues are either a poor connection somewhere along the cable, or some of the cable is not suitable for use at 1GHz. Satellite cable is probably OK but generic TV cable may not be. What type of cable are you using?


#3

I agree with Obj --a clean splice shouldn’t make much of a difference. On the other hand, cable quality will!

Try to keep the splice compact, say half an inch or so, smaller if you’re comfortable with it.

But again, the cable has to be good quality, both the 50 Ohm section and the 75 Ohm section. Good satellite RG6 is easy to get and cheap. Good 50 Ohm cable that’s low loss at 1 GHz is more expensive.

Let us know what you’re using, and what your results are.

bob k6rtm


#4

I did some redesigning and am just using the LMR-400 50’ directly to the FlightAware dongle. I want to go into the prexisting infrastructure and use the RG6 already fed through the walls. I use my Pi as the feeder and the display using their 7" touch screen. Everything works great, but once I tried to make everything permanent and pretty, everything went downhill. Have it back to original ugly temporary set up, just to keep feeding, as I am in the motherland of feeder areas. 4 major international airports all within 100 nm


#5

There is a large signal loss from using splitters. If you have 2 way splitter then half the signal power goes one side and half on the other side of the splitter. If you have a 4-way splitter then each cable has 1/4 the signal power. Etc.

If you want to use a splitter I suggest you boost the signal with an amplifier before the split. I think a 10dB-20dB amplifier might work fine. There are so many variables the more parts that are added to the system and quality of components start to matter a lot.

perfect splitter have losses of:
2-way splitter - 3dB of loss
4-way splitter - 6dB of loss
8-way splitter - 9dB of loss
The splitter’s connector + other losses add around 0.2 to 0.5dB to a perfect splitter.

I found this diagram that explain how it the signal gets weaker after a split.


#6

I’m liking this amplification suggestion. I didn’t even think to look into that. I am luckily not splitting anything. It is a splice into an already in place infrastructure that is unused. It’s going from LMR-400 to an RG6.

Do you have any recommendations on amplifiers? I would still like to put one at that splice junction.


#7

I will let you know the outcome. Ordered a different splicing kit that will hopefully make it even better. Looking at amplifiers to place at the junction as well.

Thank you all for the great ideas!


#8

Ed–
You didn’t say what you’re using for an antenna, SDR, and filters.

LMR-400 is good stuff. I use it for ham stuff as well as ADS-B. Hopefully you’ve got correctly installed type N connectors on both ends.

If you’re using a simple SDR without filtering and a simple antenna inside your house, and then moving that antenna (or a better antenna) to the roof on the end of your LMR-400, I would expect your ADS-B numbers to go to practically zero, as your SDR is overloaded by local signals (outside of the ADS-B band at 1090 MHz) when you move the antenna to a much richer environment.

If you’re using an unfiltered SDR, you can verify this by running spectrum sweeps (rtl-power) with an inside antenna and an outside antenna to see how much “stuff” (interfering signals) you have locally. I’d bet on a lot of digital TV and cellular, which is going to swamp signals on ADS-B without good filtering between antenna and SDR.

cheers

bob k6rtm


#9

i’m wondering why you do not simply place the raspberry pi in your attic instead of all this coax-tinkering just connect the raspi via wifi to your network …

no coax is better than short coax :smiley:

cheers
tom

p.s. here is all what you need to max out your site:
- antenna on roof flightaware or jetvision
- uputronics filter/amplifier
- nooelec standard dongle is simply the best
- fa sd-card image

#10

Apologies, that would probably have been good information to give.

Currently I have an antenna on the roof with full 360 degree clear view,
1090 filter built into the antenna, N type connector going to 50 ohm
LMR-400 going to F type through an N to F adapter. F RG6 coax 75 ohm cable
(normal TV cable) goes approximately 50 feet to another adapter bringing
the size to SMR that goes into my FA Pro-stick +. I have 4 major
international airports within 100 nm of my antenna, and more if I can boost
it even further. It is my belief that as a feeder it is my duty and
obligation to maximize the data I can provide in exchange for the services
I get from those sites.


#11

@ Tom:

Due to bizarre architectural design, there is no attic. That was my first
thought as well.


#12

erik,

if ‘bizarre’ does not mean that your roof hovers about 60 feet above your flat - i’d really try everything possible to install the pi and dongle as near as possible to the antenna. if there is no way the fa-dongle is not a good choice because amplification should be next to antenna - especially if you have a long cable run. so again - use the uputronics filter/amp at least there where you connect the two antenna cables. you find my installation here.

cheers
tom