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Thread: Using 75 ohn coax with a Slim Jim

  1. #1

    Default Using 75 ohn coax with a Slim Jim

    I need to check a 2m transceiver (Yaesu FT-230R) that's not been used for nearly 20 years. I have a Slim Jim antenna and know this needs 50 ohm coax but I don't have any. All I want to do is check whether the rig is working; if it is, I'll fix up the Slim Jim properly and feed it with 50 ohm. The only coax that I have is 75 ohm. Might I do some damage to the Yaesu by hooking it up with 75 ohm coax?
    Thanks in advance for comments.
    R

  2. #2

    Join Date
    Oct 2009
    Location
    Oulu, Finland
    Posts
    572

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    No and from what I have read, the mismatch between 50 and 75 ohms isn't that bad either - if you have 75 ohm cable into a 50 connector, then there shouldn't be any real problems - just use a SWR meter to make sure it isn't too far out. The only real problem will come is if you use a 75 ohm male connector with a 50 ohm socket - such as BNC. The 75 ohm plug will have a thicker pin that could damage the receptacle part of the socket.

  3. #3

    Default

    It's probably just as easy to find a bit of stiff wire - like old fashioned wire coat hangers, and cut 19" and stick it in the socket - from memory it's the usual PL259. This will easily be sufficient load to see if it fires up and transmits and receives.

  4. #4

    Default

    OK - VMT for the replies and also to the administrators for reactivating my account so promptly.
    Yes, I read somewhere that the connector could be a problem. But I have an unused amphenol plug and guess I can solder in piece of 75 ohm coax. On the other hand, the idea of the wire coat hanger sounds promising. As I say, all I want to do is see if the transceiver still works after all these years.
    R

  5. #5

    Default

    Here's what can happen. Assume a 50 ohm antenna fed with 75 ohm coax. The SWR on the coax is 1.5:1. That SWR can transform the impedance to 1.5(75)=112.5 ohms which is a 50 ohm SWR of 2.25:1 which is high enough to cause some transceivers to fold back on the power.

  6. #6

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    OK - thanks for the latest tip.
    I've found an old telescopic aerial from an indoor TV amp. I have cut this off and bent it to fit into the rear socket; it can be adjusted to 19 inches as mentioned above.
    For starters, I'll fit this in and see what I can hear.
    R

  7. #7

    Join Date
    Nov 2017
    Location
    With my wife
    Posts
    108

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    If I tried that around here, odds are I might think the radio broken because there isn't hardly any 2 meter activity this far away from the big city. So, if it appears broken, it may not be; could be just the wrong time of day for operators to be on at your place. No conclusion jumping, OK?

    No, I do not endorse kerchunking repeaters, but maybe just this once

  8. #8

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    Do US repeaters have idents? Our UK ones still do, with CW ID which is handy. My UHF business repeater has no licence requirement to ID, but it does, which I find very handy when I'm out and about to show things are working.

  9. #9

    Default

    Thanks again for the replies.
    Well, I've been off the air for nearly 20 years but still have the HF equipment in the loft.
    The Yaesu FT-230 does appear to be working --- I heard some faint voices last night --- but it's been scanning away for hours without any signals coming in.
    I might take it to the local club station and hook it up to a proper aerial.
    R

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