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By Brad Tuttle

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📡Using AI to Diagnosis Tricky Problems

If you haven’t used AI to help you diagnose tricky problems, (automotive, electrical, computer, anything technology, etc.) you’re missing a great diagnostic tool. Here’s one example of my 2-meter band ham radio going on the fritz. Although I had a good idea of the problem, having already eliminated a faulty radio and power supply, AI confirmed my suspicions and saved me a lot of time and possible wasted actions. Hopefully, readers will adopt AI for diagnosing their own tough problems.

Chat history

You said:

I’m not being heard on multiple 2m repeaters that up until now I was hitting with ease. I’ve checked shift direction, shift frequency, and PL tone. When I touch the antenna coax connection, the signal strength on RX greatly increases until I transmit again. My radio is a Yaesu FT-2980.

ChatGPT said:

Yeah… that symptom set is classic, and you’re not imagining things. You’re being heard less and your RX jumps when you touch the coax — that’s a huge clue.

Short version first, then we’ll narrow it down.

What your symptoms are screaming 📢

“When I touch the antenna coax connection, RX signal strength increases — until I transmit again.”

That almost always means a bad RF ground or a failing coax/connector, not a repeater or programming issue.

Your body is temporarily acting as:

  • a ground plane
  • a counterpoise
  • or a capacitively-coupled antenna extension

As soon as you transmit, RF heats/arcs/changes the bad connection and it stops behaving again.

Most likely causes (ranked)

1️⃣ Bad coax connector (PL-259 / SO-239)

This is by far the most common failure point.

  • Cold solder joint
  • Broken braid right at the connector
  • Center pin cracked or barely making contact
  • Shield not actually bonded anymore

Why RX improves when touched:
Your body is completing the shield/ground path.

Why TX makes it worse:
RF current + heat + vibration = intermittent open.

2️⃣ Coax shield broken internally

Often happens:

  • right at the radio
  • right at the antenna base
  • after repeated flexing or door pinches

RX can limp along.
TX needs a solid return path — and you don’t have one.

3️⃣ Antenna mount or ground plane failure

Especially if mobile:

  • Magnet mount corrosion
  • Set-screw mount loosened
  • NMO mount not actually grounded to vehicle body
  • Paint or rust isolating the mount

Touching the coax gives it the missing counterpoise.

4️⃣ Final-stage self-protection kicking in

Your  FT-2980 is a tank, but:

  • High SWR can cause power foldback
  • You may think you’re transmitting full power, but you’re not

This would explain “I was hitting repeaters easily before.”

What this is not

Just to clear the deck:

❌ Not tone / shift / offset (RX behavior wouldn’t change by touch)
❌ Not repeater issues (your RX physically changes)
❌ Not desense or front-end overload
❌ Very unlikely to be the radio itself

Do these tests in order (10–15 minutes total)

🔧 Test 1: Wiggle test (no tools)

  • Put radio on a steady signal (NOAA WX is perfect)
  • Gently wiggle:
    • PL-259 at radio
    • First 6–12 inches of coax
    • Antenna base
  • If signal jumps or crackles → found it

🔧 Test 2: Bypass the antenna system

If you have any spare antenna or dummy load:

  • Connect it directly to the radio
  • Try local repeaters or simplex

If it works normally → antenna/coax/mount is 100% the problem

🔧 Test 3: Continuity check (multimeter)

Disconnect both ends of coax.

You want:

  • Center → center: ~0 Ί
  • Shield → shield: ~0 Ί
  • Center → shield: OPEN / infinite

Now flex the coax while measuring.
If it jumps → broken conductor.

🔧 Test 4: SWR check (if available)

  • High or unstable SWR confirms feedline/mount failure
  • Watch SWR while lightly touching the coax — if it changes, that’s decisive

The “almost guaranteed fix”

If this were my rig:

  1. Cut off the PL-259 at the radio
  2. Install a new, properly soldered connector
  3. If mobile: clean and re-ground the antenna mount
  4. If base: inspect lightning arrestor, bulkhead, or feed-through

Coax and connectors fail far more often than radios.

One important safety note ⚠️

If touching the coax improves RX, you may be radiating RF on the coax shield. That’s:

  • inefficient
  • unpredictable
  • hard on finals
  • sometimes painful (RF bite)

So it’s good you caught this early.

Bottom line

You’re almost certainly dealing with:

An intermittent coax shield or connector failure

The radio is probably fine.
The repeaters are fine.
Your instincts are spot-on.

If you want, tell me:

  • mobile or base?
  • magnet mount, NMO, or fixed antenna?
  • how old the coax is

I can narrow it down to exactly where it’s broken.

You said:

I’m using my FT-2980 as a base and manually switching coax between two other radios. (I’ve ordered a three-way switch). Vertical antenna on top of twenty-foot mast. I think I’ll replace the connector at the radio first as that is the easiest.

ChatGPT said:

Yep — your instincts are exactly right. 👍
Given base station + manually swapping coax + RX improves when touched, the odds are overwhelming that the failure is right at the radio-end connector.

You’re choosing the right first move.

Why the radio-end connector is the prime suspect

In a base setup like yours:

  • That connector gets the most handling
  • Repeated twisting stresses:
    • the braid right at the ferrule
    • the center conductor solder joint
  • The antenna and mast don’t move
  • The feedline run is otherwise static

So replacing the connector at the radio end is:

  • the fastest
  • the cheapest
  • the highest-probability fix

A couple of practical tips before you solder 🔧

✔️ Connector type

If you’re using PL-259:

  • Use a quality silver-plated connector
  • Avoid the cheap crimp-only types unless you have the proper tool

If your coax is:

  • RG-8X → make sure the PL-259 is specifically for RG-8X
  • RG-213 / LMR-400 → different connector entirely

Mismatched connector = hidden failure.

✔️ Prep matters more than soldering skill

Common failure I see:

  • Center pin soldered perfectly
  • Braid barely wetted, or only touching mechanically

Make sure:

  • Braid is fully bonded to the connector body
  • No stray whiskers near the center pin
  • Center conductor isn’t just “tinned” — it should flow

✔️ Strain relief (quietly important)

After installing the connector:

  • Add heat shrink over the rear
  • Leave a gentle service loop at the radio
  • Don’t let the coax hang from the connector

This alone can add years of life.

Quick validation after replacement

Before you button everything up:

  1. Tune to a steady signal (NOAA WX)
  2. Touch the connector
  3. Touch the coax
  4. Wiggle gently

If the signal does NOT change, you’ve likely nailed it.

Then:

  • Transmit briefly into the repeater
  • Ask for a signal report
  • Watch for stability

Your  FT-2980 is a beast — once the feedline is solid again, it’ll be back to hammering repeaters like before.

About the 3-way coax switch

Good move — two notes for later:

  • Get one rated for VHF power (not HF-only)
  • Ground the unused ports if the switch supports it

Once installed, it’ll also:

  • reduce connector wear
  • make future troubleshooting much easier

One last sanity check (optional but smart)

After the connector fix:

  • Swap radios again
  • If the problem follows the coax → confirmed
  • If it stays with one radio → then we look deeper

But honestly?
Your plan is exactly what I’d do standing in your shack.

Report back after the connector swap — I’d bet a good cup of coffee that fixes it ☕📡

Note: I replaced the coax connector at my radio and the problem was solved. Taking apart the old, faulty connector, I noticed that the braid had not been soldered to the connector. I wished I had simply soldered the braid on the old PL-259 before going to a completely new connector. At any rate, the problem is solved and I’m happy again.

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