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Author Topic: 2 meter simplex -- Help me diagnose a prob pse  (Read 1855 times)
AJ4IK
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Posts: 16




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« on: August 12, 2007, 08:16:19 PM »

I just got a friend into the hobby and we've been experimenting with FM Simplex between our houses (about 2 miles apart).

I loaned him my Yaesu VX-6R dual band handheld along with a 19" flexible whip and a mag mount antenna.

On my end I've used an FT-8800M Mobile (Comet SBB-5 on NMO hard mount) and my FT-857D Base (Comet GP-3 on mast just over roof level outside).

What works:
We have good communication thru several repeaters, UHF simplex (446.000), and I hear him *great* on about every antenna and power combination he's tried.

What doesn't:
He has garbled copy from me on 2m Simplex. We've tried every bit of the 2m simplex band plan in 15 kHz steps. Says my sig is very strong but I sound garbled like talking through a paper cup or Charlie Brown's teacher.  We've had this problem with me TX'ing on both rigs mentioned and power from 5w to 50w. Increasing power helps some, but not completely.

What's weirder:
He can copy me *fine* by tuning off my op freq 5kHz either direction. If I'm on 146.550, he copies me better on .545 and .555.

Initially I thought I was competing with interference at his location, BUT we tried one other thing -- (briefly) operating simplex on the output freq of a local repeater. He said it sounded fine then. And how would his "tuning off" by 5kHz improve the clarity of my signal?

I haven't changed anything on my rigs from before we started, and I use them both regularly (granted, for repeater work mostly) without any such complaints. He's new so he might not be able to identify what he's hearing by the sound of it.

Thanks for reading and any suggestions you may have.
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ONAIR
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Posts: 3536




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« Reply #1 on: August 12, 2007, 11:44:42 PM »

     You may be off frequency, or the problem could be with his receiver out of tune.  Have him tape record your signal at his location, or switch positions (you at his rig and QTH and vice-versa).
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K5LXP
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Posts: 5342


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« Reply #2 on: August 13, 2007, 06:25:10 AM »

Could be multipath.  Have him move the antenna around a few feet and see if it improves.  This phenomena seems more noticeable on simplex for some reason.  When you have a really strong carrier but the modulation is garbled and "crunchy", odds are it's either off-frequency or multipath (phase distortion).

Mark K5LXP
Albuquerque, NM
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AJ4IK
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Posts: 16




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« Reply #3 on: August 13, 2007, 07:34:23 PM »

All set now. He'd accidentally hit the "mode" button which cycles through NFM, WFM, and AM. Had him cycle it to Auto and he was receiving normally. Musta been on WFM.

Thanks to all for ideas!
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KE3WD
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Posts: 5689




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« Reply #4 on: August 13, 2007, 08:45:24 PM »

More likely on AM.  

You were on NFM, when he detuned slightly he "slope tuned" the edge of your modulation.  

.
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K5LXP
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« Reply #5 on: August 14, 2007, 06:23:34 AM »

A VX-6 won't transmit AM.

Mark K5LXP
Albuquerque, NM
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AJ4IK
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Posts: 16




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« Reply #6 on: August 14, 2007, 08:12:01 AM »

The issue was the *receiver* was likely set to AM. Transmitter was NFM.
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KE3WD
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Posts: 5689




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« Reply #7 on: August 14, 2007, 09:48:27 AM »

Yes, an AM receiver can indeed slope tune demodulate an FM transmission.  

Something known to the older Clegg owners if nothing else.  


"Slope Tuning" describes how an AM demodulated receiver can be fooled into demodulating an FM signal when tuned to either edge of it, on the slope.  

The AM detection will be able to see a little bit of Amplitude Modulation there, usually enough to be able to hear intelligence.  Depending upon the type of detection circuit, this can sometimes be crystal clear demodulation, but most of the time will still sound slightly distorted.  

Sometimes more properly called, "slope detection".


.
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AA4PB
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« Reply #8 on: August 14, 2007, 12:09:10 PM »

More likely he was set to receive NFM and you were transmitting WFM.
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Bob  AA4PB
Garrisonville, VA
AJ4IK
Member

Posts: 16




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« Reply #9 on: August 14, 2007, 12:29:26 PM »

"More likely he was set to receive NFM and you were transmitting WFM."

Nope. We've already determined I was xmitting normal good ol' NFM (on 2 different radios).

The AM slope detection theory others have posted is the only thing that makes sense.

Anyhoo, it's solved now (by correcting the mode on his RX). Thanks again for the feedback.
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KE3WD
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Posts: 5689




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« Reply #10 on: August 15, 2007, 10:02:59 PM »

>>More likely he was set to receive NFM and you were transmitting WFM.<<

Dunno, if that were the case, moving the receiver frequency slightly likely wouldn't make it sound any clearer...


.
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