[PPRAANet] transmission line question

[John Bloodgood]

Because of the length of the cable and RCL constants and losses involved, when you are measuring run using RG8X you are mostly measuring the cable, not the antenna.  By putting better cable on, you may be reducing the cable issues and seeing more of the antenna.  I would recommend checking the antenna out with just 10 ft of coax and see how it responds.  Get your SWR down to an acceptable level just using the 10 ft coax.  If you have it working good at 10 ft, then if you add the longer coax and the SWR goes up, then you may need to check that coax.  This is where a good, resistive dummy load can come in handy -- in my opinion a dummy load or 50 Ohm test load should be part of every ham shack.





John Bloodgood, KD0SFY

________________________________
From: ppraanet-bounces at mailman.qth.net <ppraanet-bounces at mailman.qth.net> on behalf of james.f.gibson at gmail.com <james.f.gibson at gmail.com>
Sent: Wednesday, August 4, 2021 11:13 AM
To: ppraanet at mailman.qth.net <ppraanet at mailman.qth.net>
Subject: [PPRAANet] transmission line question

Transmission line experts, I have a question.



After installing a MFJ-2389 antenna (currently used only for 2M operation) I
temporarily connected a 120 ft RG8X cable between the rig and the short ( <
10 ft ) span of RG8U attached to the antenna.  The measured SWR was 1.48.



Then I removed the RG8X cable and attached the intended permanent cable
which is 50 ft of MPD400 (LMR400 equivalent) to the RG8U.  Surprised to get
a SWR of 2.8.



These are all 50 ohm coax cables.  Any idea why the MPD400 swap-in would
give me an unacceptable SWR?



Jim KS4F













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