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Author Topic: Radio stops receiving during QSO  (Read 2736 times)
KV4PR
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Posts: 13




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« on: July 30, 2013, 05:21:42 AM »

I have an interesting problem in the car that I suspect others have seen as well.

Here is my setup:
  Yaesu FT-11R (old 2M HT capable of 5 watts out, but I'm only running 1.5 watts usually)
  RF Concepts MiniAmp 144P
  Commet dual 2M / 440 antenna

I'm seeing an SWR of 1.5 between the amp and the antenna and am showing about 7 watts reflected back to the radio.

After a few minutes of a QSO, my radio stops receiving.  I'm not sure what the other station is hearing, but the SWR meter shows that the power is still getting to the antenna when I transmit.

Also, at this point, the radio has gotten rather warm (though I'm reluctant to call it "hot").

I suspect what's going on is that the high SWR is due to the dual band antenna and the reflected power is what's causing the radio to stop functioning correctly.  If I let it cool down, everything is fine again.

I'm guessing that the amp doesn't do anything to protect the radio from the reflected power.

Has anyone else had this kind of an issue?

I believe the answer is to get a single band antenna that'll give me a better SWR, but I got the dual band antenna because once I buy a new car (late this year), I'm going to get a dual band mobile rig.  If you guys think a 2M antenna will do the trick I'll get one, but wanted a bit more certainty before making the trek down to HRO (a bit of a haul from my house).
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73,

Frank
KV4PR
K5LXP
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Posts: 4474


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« Reply #1 on: July 30, 2013, 06:06:35 AM »

I doubt it's your antenna.  When the receive poops out, can you receive anything else, e.g. NOAA weather?  Can you even get squelch noise?  Does the headphone output work?  Try bypassing the amplifier too, to eliminate that as a variable.


Mark K5LXP
Albuquerque, NM
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KV4PR
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Posts: 13




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« Reply #2 on: July 30, 2013, 06:15:39 AM »

I doubt it's your antenna.  When the receive poops out, can you receive anything else, e.g. NOAA weather?  Can you even get squelch noise?  Does the headphone output work?  Try bypassing the amplifier too, to eliminate that as a variable.

I've tried switching to other repeaters where I'd just heard a QSO moments before and those don't receive either.  I believe that it's just not receiving anything at all.  Have not tried NOAA, but I'll give that a shot the next time I see this issue.

As for trying without the amp, I'd have to get fairly close to one of the repeaters to key it up at only 5 watts, but 2 of the ones I use are on a mountain that's rather close to my gym, so I can give that a shot at some point.

Part of me wonders if I shouldn't just bite the bullet and get a decent mobile rig now, rather than waiting for the new car.  Just dread having to install it twice. . .

The next time I'm at the gym I'll try it out barefoot and see if it's the amp, however, if my original theory is correct, ampless I won't have the 7 watts reflected back to the radio so it might be an invalid test.  Just not sure.
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73,

Frank
KV4PR
G8YMW
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Posts: 220




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« Reply #3 on: July 30, 2013, 06:16:33 AM »

SWR of 1.5:1 I cannot see that causing any problems.
What is the SWR between the HT and the amp?

Quote
I'm guessing that the amp doesn't do anything to protect the radio from the reflected power.

Wrong, that 7watts is reflected to the PA transistors in the amp

Does the handheld behave when running direct to the aerial?
If it fails this test then the problem is deffo the handheld (Which is my suspicion)

Regards Tony
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73 details Tony
Sent by WW2 Royal Navy signal lamp
KV4PR
Member

Posts: 13




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« Reply #4 on: July 30, 2013, 06:27:08 AM »

SWR of 1.5:1 I cannot see that causing any problems.
What is the SWR between the HT and the amp?

Quote
I'm guessing that the amp doesn't do anything to protect the radio from the reflected power.

Wrong, that 7watts is reflected to the PA transistors in the amp

Does the handheld behave when running direct to the aerial?
If it fails this test then the problem is deffo the handheld (Which is my suspicion)

Regards Tony


The SWR alone wasn't much of a concern.  It was more that 7 watts was being reflected back to a radio that can only output 5 to begin with.  It sounds like, from what you're saying though, that the amp IS acting as a bit of a buffer between the reflected power and the HT.

I bought this HT back in '94 but it's not had a whole lot of use in that time (I was dormant in ham radio for about 15 years).  Certainly could be a bad radio though.

The next time I'm in the parking lot at the gym I'll stick the rubber ducky on it and see how it goes.
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73,

Frank
KV4PR
K5LXP
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Posts: 4474


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« Reply #5 on: July 30, 2013, 06:33:47 AM »

Let me be more specific about bypassing the amp.  Operate until the problem presents itself.  Then immediately take the amp out of the circuit, or switch a different antenna to the HT.   Amps can oscillate or do other things into mismatch loads, so this will help show the problem as being in the HT or the amp.  It'd also be a good data point to see if you could receive through the headphone jack or not when this problem presents itself.


Mark K5LXP
Albuquerque, NM
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KV4PR
Member

Posts: 13




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« Reply #6 on: July 30, 2013, 06:36:59 AM »

Let me be more specific about bypassing the amp.  Operate until the problem presents itself.  Then immediately take the amp out of the circuit, or switch a different antenna to the HT.   Amps can oscillate or do other things into mismatch loads, so this will help show the problem as being in the HT or the amp.  It'd also be a good data point to see if you could receive through the headphone jack or not when this problem presents itself.


Mark K5LXP
Albuquerque, NM


Ah, ok.  That sounds like an easy test.  Once the receiver stops working, I'll just put the rubber ducky on and see if I can receive then.  If so, you're saying it's likely the amp?

Also, I use a separate speaker/mic.  I've tried with and without and got the same results. 
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73,

Frank
KV4PR
N3HFS
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Posts: 211




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« Reply #7 on: July 30, 2013, 08:58:03 AM »

Possible T/R switching (PIN) failure?
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K1CJS
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Posts: 6008




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« Reply #8 on: July 30, 2013, 10:05:03 AM »

If the radio gets warm as you state, the radio is probably shutting something down to protect itself.  The amp shouldn't feed any power back to the HT, but if its gone bad, or if you're powering both from the same source it could. 

If you're powering both from the car, try using the radio on battery power and the amp on the car battery.  Interaction between the two on their power connections shouldn't happen--but it could!
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W5DXP
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« Reply #9 on: July 30, 2013, 12:01:27 PM »

Amps can oscillate ...

I once built a Heathkit 2m amp that oscillated every time I went from XMT to RCV and I couldn't hear a thing. In addition, I wiped out all the local VHF antenna TV reception in a quarter mile radius around my QTH. It was an angry neighbor knocking on my front door that gave me a clue to the actual problem. Changing the length of the connecting coax cable between the amp and HT from 1/2WL  to 1/4WL cured the problem, having something to do with the phase of the feedback.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W5WSS
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Posts: 1694




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« Reply #10 on: August 03, 2013, 06:15:05 PM »

N3hfs yes it is possible that a t/r switching is not releasing back to Rx as a function of abnormal heat.
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KQ6Q
Member

Posts: 973




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« Reply #11 on: August 05, 2013, 12:11:30 AM »

another possibility, since this is a mobile setup,that the HT is not well shielded, and the RF from the amp is getting into the HT and causing the amp to oscillate. Is the HT running on its internal battery when this happens ? If so, then the problem could be in the length or the shielding of the coax from HT to amp, as mentioned earlier. I've found it safe to use an amp with an HT only when operating from a location in a building with a long run to an outdoor antenna. I stopped using the amp in the mobile situation altogether.
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KV4PR
Member

Posts: 13




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« Reply #12 on: August 05, 2013, 03:07:37 AM »

Thanks to all for your help!  It does appear that the problem is with the amp not switching between T/R correctly once the amp heats up (though at low power, it wasn't even heating up that much) so it looks like it was the amp itself that was the issue.

Rather than waiting until I get a new car, I went ahead and bought a mobile rig this past weekend (Kenwood TM-V71A) so the problem is now resolved another way.  I just couldn't justify replacing the amp when I know it's only going to get used for 6 months.
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73,

Frank
KV4PR
G8YMW
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Posts: 220




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« Reply #13 on: August 05, 2013, 05:28:07 AM »

Pleased you're sorted

Cheers
Tony G8YMW
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73 details Tony
Sent by WW2 Royal Navy signal lamp
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