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Author Topic: Odd SWR and behavior on HF  (Read 409 times)
KG6SII
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Posts: 27




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« on: February 12, 2012, 03:26:03 PM »

Hello, Elmers,

I recently upgraded to General and am trying to get my HF station working.  I am experiencing some behaviors that seem non-intuitive for me, but probably make sense to you.  Can you please help me understand what may be going on?

My Station:  a Yaesu FT-747GX Transceiver going into an MFJ-949E tuner, then I'm feeding a horizontal dipole with about 50-60ft of Radio Shack's 20gauge 300ohm twin-lead. The dipole is a random 40ft length (the length of the peak of my roof), sitting on top of concrete roof shingles.  Height above ground is just under 20ft.  My power supply is an Atsron RS-50A, and my microphone is a Kenwood MC-60.

According to the meter on my antenna tuner, on 40m it tunes to 2:1 SWR, and on 15m it tunes to 1.3:1 (tuned using CW solid carrier on very low-power).  I then switch to SSB mode, hit the PTT on my mic, start calling TEST or CQ while I turn up the transmitter's power.  It begins putting out about 10watts (avg.), and the SWR stays near what it was on CW, but power won't go much beyond 20 or 40 watts when I talk.  It quickly reaches a point at which I can turn up the drive a lot and get no additional forward power on the meter.  The backlight lamp on my tuner flickers, and I cause RF interference on household amplified speakers, so I know I'm putting out some power.  I listen to the audio quality of my transmission on a shortware receiver, sitting right next to my transceiver.  I sound great with low-to-medium MIC gain, but begin to sound grainy/distorted past half-gain.  It behaves such that I need to turn up the MIC gain into the "splatter" range to get my forward power to increase.  It just seems strange that with such a low SWR, and a 100watt transceiver, I'm not putting out more than a few dozen watts.  I have a RigExpert AA-54 antenna analyzer, and it says my SWR is 4 on 40m, and 30 on 15m (measured at the transceiver feedpoint to my balanced line)

I have not received any responses to my CQ's.  Any idea what may be going on here?

Thanks in advance!
Glenn
KG6SII
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KE3WD
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Posts: 5689




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« Reply #1 on: February 12, 2012, 03:55:46 PM »

That sounds about what is to be expected reading in SSB and modulating with voice while looking at an average reading wattmeter. 

CW mode can show you the full power.  Voice does not drive to full power all the time, hardly at all, actually. 


73
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K8AC
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Posts: 1475




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« Reply #2 on: February 12, 2012, 04:24:18 PM »

Commenting on another point: You might want to consider increasing the length of the dipole to be a half wave on 40, by bending the ends at a right angle and running the new ends down the roof or hang them over the ends.  It might be a bit easier to match and you might have less radiation from your feedline.
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N4CR
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Posts: 1668




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« Reply #3 on: February 12, 2012, 04:42:49 PM »

Hello, Elmers,

I recently upgraded to General and am trying to get my HF station working.  I am experiencing some behaviors that seem non-intuitive for me, but probably make sense to you.  Can you please help me understand what may be going on?

My Station:  a Yaesu FT-747GX Transceiver going into an MFJ-949E tuner. My power supply is an Astron RS-50A and my microphone is a Kenwood MC-60. I'm feeding a horizontal dipole with about 50-60ft of Radio Shack's 20 gauge 300 ohm twin-lead. The dipole is a random 40ft length (the length of the peak of my roof), sitting on top of concrete roof shingles.

Concrete is a pretty good conductor. You need to elevate the wire off the roof so it's not so closely coupled to the concrete. Effectively, your wire is just about the same as laying it on the ground. See if you can get it up a couple of feet minimum. Especially the feed point. Also, it's a bit short for 40 meters. You need to add some wire to the ends and zig zag them in opposite directions down the roof edge. It'll make the antenna much easier to tune on 40 meters. Try to get each end of the antenna up to 32.5 feet long.

Quote
According to the meter on my antenna tuner, on 40m it tunes to 2:1 SWR, and on 15m it tunes to 1.3:1 (tuned using CW solid carrier on very low-power).  I then switch to SSB mode, hit the PTT on my mic, start calling TEST or CQ while I turn up the transmitter's power.  It begins putting out about 10watts (avg.), and the SWR stays near what it was on CW, but power won't go much beyond 20 or 40 watts when I talk.

That rig does not have a peak reading watt meter. Since it's an averaging meter that can't read the really fast voice peaks while you talk, it reads much lower than peak. Usually around 25% of your peak would be a normal reading. Cranking up the mic gain to get an average reading watt meter to read high on the scale generally leads to you not being understandable to anyone who might hear your signal.

There are many watt meters available that you can put inline after your rig that will give you a real picture of your wattage peaks while using ssb, but a good starting point would be for your average power to read about 25 watts. And getting your antenna closer to the correct length and elevated will make a huge difference in how you will be heard.

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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
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