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Author Topic: HM-15 SWR/Power meter sensitivity problem  (Read 1062 times)
K6RQR
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Posts: 222




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« on: October 21, 2012, 09:10:12 PM »

Hello All -
 I have a Heathkit HM-15 SWR/Power meter. I am using it to monitor the SWR and power output on 80-10 meters. I am using a doublet antenna (fed with 450 ohm twinlead) that is tuned with a Dentron Super Tuner connected to an Icom 735. The manual says that this tuner should be used with 50-75 ohm antennas and is not meant for balanced antennas with tuners. I have noticed that in order to set the HM-15 for an SWR reading I have to apply power and adjust the sensitivity control to the "Set" mark. The problem is that when I do this in the AM position to get a tuneup carrier (about 25 watts) I can easily get a full scale on 20 and above but on 40 and especially 80 meters I can't achieve a full scale reading even with the sensitivity up to maximum.
 After looking at the schematic I noticed that there are two resistors connected to the sensor unit. They have certain values if 50 ohm operation is intended and somewhat lower values if a 75 ohm feedline is intended. I'm wondering if I can increase the sensitivity by lowering the values of these resistors to compensate for the higher impedances this meter was not designed for? Any advice appreciated.

73,
Bruce  K6RQR

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WB6BYU
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Posts: 13567




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« Reply #1 on: October 21, 2012, 09:38:32 PM »

The instructions mean that you shouldn't use it in the balanced feedline, or on
the output of a tuner.  But using it between the rig and the tuner is exactly what
they are intended for.

Is that one of the older designs with some conductors in parallel with the feedline?
Those designs will be frequency sensitive:  some of my meters take 50 watts or
so to get full scale deflection on 80m, but on 2m I can driven them with the output
of my signal generator.

But you don't need to be able to drive it to full scale to adjust the tuner for low
SWR:  usually it is sufficient to monitor reflected power and adjust the tuner
for a minimum (though I find it more convenient to use devices with two separate
meters rather than a single switched one.)  Adjust the tuner for minimum reflected
power at your standard output level, and that should get you close enough that
you can increase the drive to where you can reach full scale.
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KA4POL
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Posts: 2124




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« Reply #2 on: October 21, 2012, 11:08:02 PM »

(though I find it more convenient to use devices with two separate meters rather than a single switched one.) 
It would be easy to hook up a second instrument of 100 µA.
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K6RQR
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Posts: 222




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« Reply #3 on: October 22, 2012, 12:11:03 AM »

 Yes, I think that it has those parallel conductors near the feedline. Thanks for the info about needing 50 watts to get a full scale reading on 80 meters on your meter. The circuit of the HM-15 is certainly frequency sensitive. I was just looking for a way to make it more sensitive on 80-40 meters. I was confused about the balanced feedline application because, even on the higher bands like 15, I still have to crank the sensitivity control up to 3 o'clock to get full scale. At no frequency when it is in the first half of its rotation does it provide enough sensitivity to get a full scale reading. This was a pretty inexpensive power meter if I recall so this may be typical.
As you suggested, I can use the less than full scale reading as a precursor to getting the SWR but I'd like it to be more accurate.
 Thanks for the replies.

73,
Bruce  K6RQR

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