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Author Topic: Coaxial antenna connection question  (Read 310 times)

Offline Treehouse SWL

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Coaxial antenna connection question
« on: March 12, 2019, 1915 UTC »
Question, as the technical side of antennas and feed lines, etc., is just at the edge of my understanding ...

I currently have my antenna feedline strung through a window, which keeps the old window cracked open just a bit year-round, which is less than ideal.

But I've located an old hole in my exterior wall for a cable TV connection, and there's an installed coaxial outlet on the inside of the wall there. It's even in just the right place on my house for my listening spot!

So if I use this to attach my outdoor feedline to the antenna on one side and then screw in a coaxial cable on the other side, leading to my receiver (which will use a coaxial-to-3.5mm adapter), is that whole setup likely to cause any adverse noise effects or significant signal loss?
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Offline Josh

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Re: Coaxial antenna connection question
« Reply #1 on: March 12, 2019, 1935 UTC »
If the coax is good and no shorts in the connectors, I doubt you could measure the loss at vlf to hf.
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Offline redhat

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Re: Coaxial antenna connection question
« Reply #2 on: March 12, 2019, 2356 UTC »
As long as the shield integrity is intact, I suspect you'll have no issues.

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Offline ChrisSmolinski

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Re: Coaxial antenna connection question
« Reply #3 on: March 13, 2019, 0955 UTC »
I don't see any major problems, as others noted the coax loss will be insignificant at HF.

You may want to use a transformer to connect the antenna to the coax.
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Offline Treehouse SWL

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Re: Coaxial antenna connection question
« Reply #4 on: March 16, 2019, 0259 UTC »

Thanks to all for the advice! I'm always learning something new from the experts here.

In the meantime, though, I figured out I can get my feeder line straight outside and directly to my antenna without putting a coaxial connection in between. With that part now figured out, I'm just a couple weeks away from stringing my new, longer random-wire antenna.
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Offline i_hear_you

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Re: Coaxial antenna connection question
« Reply #5 on: May 02, 2019, 1922 UTC »
You may want to use a transformer to connect the antenna to the coax.

^^Seconded!

I recently hooked an outdoor skywire to the center conductor of an old cable TV (rg59) run that makes ingress into my listening room to a female F-connector wall plate.  How lucky! 

That was a great first step. It brought in a huge signal, but also some huge noise.  It turns out the noise the shield is supposed to protect against still needs to go somewhere, and most likely it is directly (DC-) coupled to your receiver and your antenna.

I wound a 1:1 isolation transformer on a binocular ferrite core, and clipped the primary to the antenna/ground terminals on my receiver, and the secondary to the center conductor/shield of the cable. Now the coax shield cannot pass common-mode directly into the receiver.  It seems to have cut the noise down (but maybe it's just that psychological trick we use to make ourselves feel better.) However, that common mode still has a path to the antenna.

To defeat this, there is another refinement I intend to make:  another 1:1 isolation transformer for the antenna side with the antenna and an Earth ground (currently the wire just terminates to the center conductor of the cable and has no ground) working against center conductor and shield.  This leaves the coax "floating" so that common-mode cannot directly flow onto the antenna or into the receiver. 

A final refinement involves grounding the shield to its own Earth ground (NOT the antenna's Earth Ground!) to give the common mode current a sink.

I got these ideas from http://www.hifidelity.com/w3eee/ under "reducing noise."

My understanding is that this also provides some spark protection to one's receiver's frontend.

Amidon sells all the ferrite and wire you need to do this if you are interested, OP.


Offline Josh

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Re: Coaxial antenna connection question
« Reply #6 on: May 02, 2019, 2025 UTC »
I wanted to try the balanced input on the RSP2 so spliced in a coax to 300ohm balun onto the coax fed ocfd, and it was worse (as in noise and signal levels overall) than direct coupling to one of the 50ohm sma connectors on the RSP. Gatta test each system to see what helps and what hurts.
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