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Thread: best coax cable 50 or 75 ohm for dx r8t receive only

  1. #1

    Thumbs up best coax cable 50 or 75 ohm for dx r8t receive only

    I just got my alnico dxr8t in and also my qso king 117 foot end fed antenna with there 9:1un/un from bruce at maple leaf which coax would be the best im thinking of drf 400 but since its receive only receiver I am hearing 75 ohm is just as good and cheaper but if im right the end fed should match the receiver which I believe is 50ohms any advice would be so appreciated thanks!!

  2. #2

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    The specs give the input impedance at 50 ohms (link: https://www.universal-radio.com/cata.../0084spec.html), so I would stick with 50 ohm. Since you're not putting power into the cable, you're not going to damage anything, but from what I've read, 75 ohm stuff is generally much more lossy than 50 ohm cable. I think it is because the 75 ohm cables are usually made for broadcast receivers such as TV and FM radio, where power levels are much, much higher that trying to pull in an amateur station running 500 mW the other side of the globe on 20 metres.

    Also, I'm not sure about PL259/SO239 (laughingly called "UHF") connectors, but with N-type connectors (N, BNC & TNC), the centre pin for the 75 ohm type is wider than the 50 ohm version. So, if you put a 75 ohm plug into a 50 ohm socket, you risk damaging it.

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    Oh yeah, and welcome to the forum!

  4. #4

    Smile coax possibilities

    Quote Originally Posted by OH8GAD View Post
    The specs give the input impedance at 50 ohms (link: https://www.universal-radio.com/cata.../0084spec.html), so I would stick with 50 ohm. Since you're not putting power into the cable, you're not going to damage anything, but from what I've read, 75 ohm stuff is generally much more lossy than 50 ohm cable. I think it is because the 75 ohm cables are usually made for broadcast receivers such as TV and FM radio, where power levels are much, much higher that trying to pull in an amateur station running 500 mW the other side of the globe on 20 metres.

    Also, I'm not sure about PL259/SO239 (laughingly called "UHF") connectors, but with N-type connectors (N, BNC & TNC), the centre pin for the 75 ohm type is wider than the 50 ohm version. So, if you put a 75 ohm plug into a 50 ohm socket, you risk damaging it.
    thank you very much!! would you recommend rg8x or rg213/u or lmr 240 I heard rg213 i.e. great for the lower bands because of the solid core copper wire albeit because of transmission but should work for signal reception I think but really just want to get the right coax, almost 100 foot run but more like 70 I have 2 chokes mfj915 and my antenna cmc 0510r I was going to put a 20 foot or so jumper from antenna feedpoint of balun then at 20 foot mark the mfj 915 then a 50 foot run to a 3 foot jumper to receiver with the cmc 0510, also soldered versus crimped abr has crimped but good cable mp digital and dx have soldered any info on those? thanks again!!

  5. #5

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    A mismatch between 50 and 75 ohms will yield an SWR of 1.5. In other words, 4% of whatever wattage encounters the mismatch will be reflected. At an SWR of 1.5, the mismatched loss in dB is 0.178, which is probably better than the loss in any broadband transformer you might buy. Go for it!

    Most 75ohm coax I am familiar with is used for TV and satellite hookup. Knowing these signals reach into the UHF region, I would assume the loss in these cables would be insignificant at HF, but the power levels they can handle are not great as they were designed for reception only. A quick look at the coax list in the ARRL handbook shows that typical 75ohm coax has a maximum Vrms of 300V. Since forward signals add to reflected ones to make the standing wave, we can assume a maximum forward voltage on these cables should be kept under 150Vrms. Since E^2\R=P, 300w at a total mismatch would yield a maximum standing voltage at the hot spots of 300V. So, to be safe, if you intend to TX on that cable, I would recommend keeping the power to half that, or 150w max.

    EDIT: 75ohm cable rarely fits into the PL-259 connector as the jacket is a tiny bit bigger than standard 50ohm cable. You might need to shave a bit off the jacket to get it into the connector. And most 75ohm tv cable has an aluminum braid/foil. This makes soldering tough...
    Last edited by brandon lind; Wed 20th Mar 2019 at 00:55.

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    Quote Originally Posted by brandon lind View Post
    EDIT: 75ohm cable rarely fits into the PL-259 connector as the jacket is a tiny bit bigger than standard 50ohm cable. You might need to shave a bit off the jacket to get it into the connector. And most 75ohm tv cable has an aluminum braid/foil. This makes soldering tough...
    I just hog out the adapter a little bit to fit the size difference with a drill bit. I also just use friction for the shield connection. It works great for me!

    I've run up to 400 watts SSB/CW into RG-6 for quite a while now without detrimental effect that I can determine.

    I'm thinking you will have more trouble using an end fed antenna than using RG-6 coax (the catv quad shield stuff). Check with your local catv installers to see if they have a spool end they would rather give you than go to the dump with it.

  7. #7

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    Quote Originally Posted by WZ7U View Post
    I've run up to 400 watts SSB/CW into RG-6 for quite a while now without detrimental effect that I can determine.
    The 150w limit I suggested is for an infinite SWR. I agree, 400w should be fine if everything is fairly matched.

    I've often wondered if just a friction contact is sufficient. I suppose there is plenty of capacitive coupling even if the braid oxidizes. Never had a bad one to find out...

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