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Author Topic: Heathkit HM-102 and HM-15 comparison  (Read 1594 times)
K6RQR
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Posts: 222




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« on: November 03, 2012, 09:53:27 AM »

Hello All -
 Does anyone have any experience comparing the sensitivity of these two meters? I currently have an HM-15 which seems insensitive on 75 meters and especially so on 160. On 160 even if I apply 100 watts into it I cannot get the meter needle to reach the Set point even when the sensitivity control is set to maximum. On 75 I can barely get it to the Set point when using about 40 watts of carrier. Even on the higher bands like 12 and 10 the sensitivity control is always set to at least 2 o'clock. I've checked the components and they seem fine and within spec. My question is - have others experienced this lack of sensitivity with the HM-15 and is the HM-102 more sensitive to RF power for tuning purposes?

73,
Bruce  K6RQR

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W6EM
Member

Posts: 900




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« Reply #1 on: November 04, 2012, 06:25:42 AM »

Bruce:  How does the HM-15 couple to the coax passing through?  If it uses the old "one wire under the shield" approach, you might try using instead an FT-43 toroid core on an unshielded end and then put two wraps of the coupling wire through the toroid to double the pickup sensitivity.

73,

Lee
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W6EM
Member

Posts: 900




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« Reply #2 on: November 04, 2012, 01:48:59 PM »

Bruce:  One thing I didn't think of, since it used the parallel wires approach for coupling would be to add a very small amount of capacitance at the "hot" end of each of the coupling wires.  Maybe a couple of pieces of insulated wire, wrapped a couple of turns.  A few pF more of coupling is what it probably needs to get a little more voltage at 160m.  If that's not enough, you could try some silver mica caps in place of the twisted wires.

Of course, at higher frequencies, you'll get more than you need.  :-)

Brings to mind the reason Bird in their Model 43 uses so many different slugs for frequency ranges for the same power range.  They adjust the capacitive coupling.  Less as they go up in frequency.

73,

Lee
W6EM
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K6RQR
Member

Posts: 222




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« Reply #3 on: November 04, 2012, 05:06:03 PM »

Hello Lee -
 Thank you for the suggestion. I looked more closely at the sensor unit and the two wires that run parallel to the copper tube I described to you cannot be adjusted. They are held in place. It would also be impossible to get a toroid in there so your suggestion of wires with turns or mica caps is a good one. I will attempt that mod and see what happens.

73,
Bruce  K6RQR

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