Getting poor performance from antenna. Bad cabling? Does feed length matter significantly?
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reddit.comPosted Sep 1, 2020, 1:17 AM UTC
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reddit.comI recently started dipping my toe in APRS and discovered that my transmit was really bad. After putting a tester on the cable, I found it wasn't great. 30-70 ohms depending on the feed. Tested SWR @ 146Mhz and it came back in the 30's. Swapped the antenna out and was much better, but still up at 3. I used PL259/SO239 connectors. I'm regretting that now as I've read that they reflect back a lot over 300Mhz (and I'd like to use 70cm from time to time). How should I have done the cabling? N connectors? Was LMR-400 overkill? Ditch the NMO idea? (I like removable antennas though). I'm using a Yaesu FTM-400DR.
edit: speeeling
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How did you measure the 30-70 ohms? You should have either zero or infinite ohms depending how you're measuring it. The 30 swr tells more. You have either a short or totally open coax.
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I tested using a RigExpert AA-600. The 'Select all" function had a display for Ohms. I tried it both with and without an antenna to see how the numbers changed. Just to establish a baseline, I put it on a line connected to a known-good antenna and got an SWR of 1.01/50 ohms via 2m.
Now I'm wondering if I should re-crimp all the connections. I thought I was careful, but it was my first time crimping LMR400 with PL259 ends. Might be shorting there. I guess with the right test equipment I could find out.
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be aware that LMR400 is copper plated aluminum
it will not take flex and vibe
nice work on the install though
NMO is the only mount worth using
but I am a bit leary of those right angle things
I recently tossed out a few Amphenol 90deg elbows
they ruined a other wise proper match on VHF/UHF
even testing them into a proper dummy load
they may have worked out on HF, but they are gone
even Mother Moto uses a soldered connection onto the NMO mount even at 900 Mhz
https://i.imgur.com/BkXuKxI.jpg
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30-70 ohms depending on the feed
Note that the impedance "rotates around" when you measure it at the bottom of different length feedlines.
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Yes. It was overkill. Big, big overkill.
If I had to guess, either bad connectors (as you’ve surmised) or it turns a corner too tightly. LMR-400 is a foam dielectric, so it likes a gentle curve. The official spec is 1” radius, but on a vehicle I would consider it a “repeated” bend and go for 4”. If it’s stressed, the wire will begin to migrate through the foam. RG-213 is way better for that kind of application if you must have a low-loss RG-8 type.
Unless you’re making your truck into an EME rig, use a LMR-240 or RG-58 and try not to lose too much sleep over it. I think I used 17’ of RG-58 on my Tacoma, so the difference between -400 and -58 is... about a dB at 440MHz. That’s 1/6th of an S-Unit. At 144MHz it’s about half a dB. You’ll run out of horizon with 15w in a vehicle and still have 35w of ice melter left to go in your FTM-400dr.
But do not ditch the NMO idea. It’s a good connector.
This is awesome. Thanks!
I also used rg58 on a third gen tacoma. Radio is under driver seat, antenna is on a mount over the drivers side taillight. Rg58 through a drain plug in the floor and up between the bed and fenders.