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Author Topic: strange swr issue  (Read 1991 times)
KJ4MYY
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Posts: 34




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« on: July 01, 2012, 02:01:16 PM »

I have a strange SWR meter issue, and am not sure what to trust.  I am running a FT-450 rig with a HM-102 SWR meter/power meter in line before my tuner and antenna.  The issue is once I tune up using 10W both my HM-102 and FT-450 both show 1:1 SWR.  The interesting thing is if I increase power the SWR on my FT-450 stays flat at 1:1 but the HM-102 SWR seems to always increase with power.  Any ideas of why this is?  Should I just trust the SWR of the rig, or should I be concerned and trust the SWR of the HM-102 that sometimes goes over 3:1 or higher depending on power output and mode. 
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G4IJE
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« Reply #1 on: July 01, 2012, 02:14:08 PM »

It's normal for many SWR meters to show an apparent slight increase in SWR as you increase power, but if your reading is going from 1:1 at 10 watts to 3:1 at 100 watts there must be something wrong. I'd be looking for connector/cable problems, or even a fault in the HM-102. Can you try a different meter?
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AC5UP
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« Reply #2 on: July 01, 2012, 02:16:31 PM »

...and when you increase power from 10 watts to 100 watts do you also adjust the sensitivity control on the HM-102 to compensate?

The Heathshkit meter is calibrated to a specific power level. Any change in power requires a re-cal of the sensitivity knob to match.

One Other Thot:
The HM-102 is old enough (1970 thru 1981) that blanket statements like "there are better diodes available today" and "the meter might be due for a rebuild" could be applicable. Meaning.... A set of modern Schottky detector diodes can increase both the sensitivity and linearity of the meter at a modest cost. Even a matched set of 1N270's would be an upgrade, and they can be had for maybe a nickel each. (Translation: Buy more than you need and choose the best match in terms of forward drop)

Opening the case also affords an excellent opportunity to clean the contacts on the rotary switch. Which can improve the consistency of your readings...........   Grin
« Last Edit: July 01, 2012, 02:44:13 PM by AC5UP » Logged

Never change a password on a Friday                
N9EJS
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Posts: 218




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« Reply #3 on: July 01, 2012, 02:21:55 PM »

I agree with the two previous replies.  If your HM-102 has a calibration function, you  need to perform that when you increase power  (or decrease from a higher power) to get a more accurate reading on the meter.
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M5AEO
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Posts: 273




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« Reply #4 on: July 02, 2012, 08:31:45 AM »

I might be wrong, but isn't this because your external SWR meter is monitoring the SWR at a point before your internal ATU?  So it is showing SWR without any antenna tuning, but the built in SWR meter is showing the SWR reading after it has tuned the anenna?
I had the same experience when I had an IC-746 with its internal ATU.
M5AEO

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G3TXQ
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« Reply #5 on: July 03, 2012, 10:40:02 AM »

I might be wrong, but isn't this because your external SWR meter is monitoring the SWR at a point before your internal ATU?  So it is showing SWR without any antenna tuning, but the built in SWR meter is showing the SWR reading after it has tuned the anenna?

From the OP I take it that both meters are before the tuner!

Steve G3TXQ
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G4IJE
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« Reply #6 on: July 03, 2012, 01:02:31 PM »

I am running a FT-450 rig with a HM-102 SWR meter/power meter in line before my tuner and antenna. 

Sounds like the meter is in the right place and it must be an external tuner, not built into the transceiver.

Obviously the HM-102 needs to be adjusted when increasing power - I'm not familar with that model but presumably it has a "forward" or "cal" position where you adjust for full scale reading then switch to "reverse" or "SWR" position.
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KJ4MYY
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« Reply #7 on: July 03, 2012, 07:09:20 PM »

Here is some more info that answers some of the questions that everyone is asking.  I actually purchased the HM-102 as a non working unit.  I purchased all the components with the exception of the meter and the large rheostats.  I have calibrated the meter and it matches exactly the meter in my rig at low power.  Also, my FT-450 has no internal tuner and both the rig and the HM-102 are before the external tuner, a dentron super tuner.  The HM-102 does have a sensitivity knob on it, on the front that is helpful when trying to use the tuner for bands that are far off, you can turn the sensitivity down and find that you are coming down in SWR, until you dial it in.  At full scale it matches the SWR meter in the rig.  I just don't understand why at higher power it's SWR rises, but the rig does not.  My Amp seems to have no issue with the load when I run it, and the rig has no problem when I run it, I just don't think the high SWR rise with power is correct, just wish I knew why it did that. 
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WB6BYU
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Posts: 13486




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« Reply #8 on: July 03, 2012, 07:45:34 PM »

Quote from: KJ4MYY

...The HM-102 does have a sensitivity knob on it, on the front that is helpful when trying to use the tuner for bands that are far off, you can turn the sensitivity down and find that you are coming down in SWR, until you dial it in.  At full scale it matches the SWR meter in the rig.  I just don't understand why at higher power it's SWR rises, but the rig does not...



The SWR meter in the rig is designed to be independent of output power:  it contains a circuit that
compares the forward and reflected power and calculates the SWR from that.

The HM-102 gives an indication of forward and reflected power, rather than SWR.  You have to
set the sensitivity control so it reads full scale on forward power, then switch to reflected power
in order for the SWR readings to be accurate.

When you change output power, the meter in your rig automatically compensates because the
ratio between forward and reflected stays constant for a given SWR.  But the HM-102 just
shows you the relative reflected power on the SWR scale.  As your output power increases,
the reflected power increases.  That makes the SWR reading higher.  But if you readjust
the sensitivity control so the meter is full scale in the forward position again, the SWR
readings should still be good.


So each time you change output power level, you have to readjust the sensitivity control
on the HM-102 to full scale in the forward position, then the SWR readings will be accurate.
If you don't, they won't be.
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