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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 16th Nov 2014, 9:01 pm   #1
G4_Pete
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Join Date: Feb 2014
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Default Mathematical HRO Coil setup and tracking

Hi,
Having an HRO 1.7 - 4 Mhz coil that was badly out of calibration I thought I would apply a mathematical tuning method I developed and had published in wirless World around 2001. I did not get any feedback positive or negative at the time so I shelved the idea as a dead duck. However having tried it sucessfully on a 1.7-4 Mhz coil again today I thought I would share it and see if anyone can verify usefulnes or otherwise!

The details are attached but in overview you take the desired scale readings near but not too close to the ends say 20 and 480 from the HRO chart or any other reciever or oscillator and then you take 4 measurements of the actual calibration with the pad C meshed in and out. The exact pad setting is not important but MUST span the calibration above and below desired fMax/fMin ratio.
Then with a calculator or Excel plug in the numbers and the resultant is a single frequency value, this is used to set the Pad C, only once you don’t touch it again you then set the inductor , via the brass disk on my 1.7 - 4.0 coil to the scale reading, no iterations between L/C needed. You then find the dial tracks the graph spot on .

However Having got the Oscillator spot on I now can’t get the RF to track it is either full sensitivity mid-scale rolling of at the ends or ok at high rolling of to Low or vice versa. However I plotted the RF Response by pulling the Oscillator and measuring the RF mV at the Mixer valve throughout the range. This gave me the exact frequency response of the RF stages at the 20 and 480 dial settings and several checks through the range. All that was then needed was to add the 455 Khz IF to give the desired oscillator range.

The 4 measurements tests were repeated and using the new oscillator requirements the new frequency calibration value for the pad Capacitor calculated. With the dial at 480 this frequency was set via the pad and then the required oscillator frequency at 480 dial set via the inductor.

A few minutes work, no repeat adjustments and the receiver now tracks perfectly noise and sensitivity was constant across the range.

If you need an exact chart for the newly calibrated coil entering the values into excel and via the graph regression tools you can plot a graph to match the original or any format you want, attached is an example of a 14 Meg bandspread coil.
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Old 17th Nov 2014, 12:40 pm   #2
Bazz4CQJ
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Default Re: Mathematical HRO Coil setup and tracking

Thanks for your post, which I've added to my HRO folder. I'm not sure that I've fully appreciated what's going on, but suspect it becomes clear when you actually use the procedure. Having owned my HRO for decades, I've recently come to suspect that the my coil set and Rx were not an original combination, but the product of a mix and match job.

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Old 17th Nov 2014, 9:02 pm   #3
GW3OQK Andrew
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Default Re: Mathematical HRO Coil setup and tracking

Pete, congrats on that work.

I might as well add my writeup to the thread. Here's how I modified my HRO and bandspread 6 of the coils. It's the online calculator that made it possible for me to try different values in a flash to find the ideal result.
73
Andrew
HRO 5T MODS.pdf
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