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Messages - jFarley

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Equipment / What's all this regenerative loop stuff?
« on: April 20, 2018, 1311 UTC »
I have just verified the operation of a resonant NDB loop which uses positive feedback to enhance the Q which can be attained with commonly available real world components, namely, tuning diodes and Litz-wound ferrite rods.  A working prototype is up and running in the basement here and is showing great promise.

I have re-tooled my ferrite loop so that it is now tuned remotely by a pair of back to back NTE618 tuning diodes (4 diodes total) via a control signal of 0 - 15 VDC.  These diodes resonate a wound ferrite rod (160 turns of Litz) over the NDB band.  The loop output is amplified by a balanced JFET differential amp.  Tuning diodes work great with one caveat: they have a Q which increases with frequency, and vicey-versey.  At the top end of the band (around 400 kHz) they have enough Q to produce a reasonable but not great loaded Q of around 180.  I would really like this to be higher; Q = 250 would be acceptable .  At the low end of the band (around 200 kHz) the loaded Q is only around 90, and the loop, well, the loop sucks.  There is just too much loss due to series resistance in the loop tank, and this needs to be reduced for the loop to perform as desired.  One could use lower loss components in the tank, but this is not practical.  Instead, one can trick the loop into believing that it has lower losses.  If one replaces some of the energy burned up in the lossy tuning diodes and ferrite rod with some energy derived from the loop amp output, then the loop Q should increase.  The loop now becomes a regenerative loop which employs positive feedback for Q augmentation.

The best way to see this is through an understanding of what Q represents.  We have all probably seen that the Q of a resonant loop is the ratio of the loop's resonant frequency to the width of the response curve between its -3 dB points.  As the Q is increased, the loop tunes more sharply, and its -3 dB points move in towards the center frequency.  This is really a result of Q and is not a formal definition of Q.  In electrical engineering, the Q of an LC circuit is defined as the ratio of the energy stored in the L and the C per cycle to the energy dissipated in losses per cycles.   If one can replace some or all of the losses dissipated as heat, then the loop Q must increase.   As one adds controlled positive feedback to the LC tank, one is in effect adding negative resistance to the loss elements in the LC tank.

In practice, I take the output of the loop amp and split it using a toroidal splitter.  One port goes to the coax back to the SDR.  The other port is attenuated in a 75 ohm 20 dB attenuator and passed to a variable attenuator comprised of a PIN diode and load resistor.  This voltage is then applied to a one turn tickler winding on the ferrite rod.  The biasing of the PIN diode is set up so that a 0.7 - 5V control signal will cause a current of 0 to 150 uA to flow through the PIN diode, thus changing its effective AC series resistance.

I have observed 3 regions of operation.  Below a PIN diode current of 1 uA, the loop behaves like the underlying loop without regeneration, and has a generally depressed Q due to the Q of the NTE618 diodes.  If the PIN diode current is increased to a value of typically 100 uA, the loop breaks into oscillation (as it should) and the SDR waterfall goes bat-guano crazy.  Between these 2 points, one can observe the loop Q increasing smoothly above the default level with increasing PIN diode current.  The response in the spectrum pane of the SDR begins to peak higher, and the response skirts narrow.  The object viewed near the resonant peak (carrier or offset) in the waterfall begins to brighten with increasing PIN diode current.  I can typically see an increase of up to 20 dBm in this middle region with increasing PIN diode current before it breaks into oscillation.

The regeneration process seems to be well behaved and stable as the Q operating point is varied, and this bodes well for the future. I am not anticipating running the loop at insanely high Qs such as 10000; I simply would like to enhance the Q of a deficient loop to around 250 which experience has shown is about what is required to make the loop competitive with the amplified whip which I also employ.

Interesting sidebar: if the sense of the tickler winding is reversed - negative feedback - then the regeneration will produce increasing attenuation of received signals as the PIN diode current is increased.

What I now have is a resonant NDB loop with software programmable (and knobless) control of:
    - The loop azimuthal position, with an accuracy of better than 1 degree.
    - The loop resonant frequency with a resolution of typically 100 Hz.
    - The loop Q and thus its Effective Height.

Further details will be posted after it is sited outdoors and fully shaken down.

Equipment / Re: Some more adventures in RFI reduction
« on: April 20, 2018, 1305 UTC »
It's a never ending battle; congratulations!

Longwave Loggings / Re: Good news for you NDB guys
« on: March 29, 2018, 1813 UTC »

"If itís what you say I love it especially later in the summer."

Only modest changes here in 2018.  If New Bern and Bobo go off then a Brasilian and an Oregonian (both unlogged here) should be uncovered.

Can't wait until Meguon 304 goes off as it covers a pair of Canadians (and who knows what else) which are pure unobtanium in Chicago.

DGPS / Re: DGPS Logs 17-18 Mar 2018
« on: March 19, 2018, 1902 UTC »
Sounds like a good plan.  I am going do overnite recordings looking for these guys.  All antennas are down for some PM right now, and I hope to be back up and running in a week or so.

I gotta NDB Jones big time.

DGPS / Re: DGPS Logs 17-18 Mar 2018
« on: March 19, 2018, 0219 UTC »
I need to find something to repurpose this SDR for,  I think DGPS season is about over.

Fishin' Beacons!

Longwave Loggings / Spring NDB Logs
« on: March 14, 2018, 1740 UTC »
I begrudgingly accept the fact the the NDB season is just about over.

2018-03-14  1153  332.0  VT  Buffalo Narrows, SK, CAN  2152 Km

NDBs this report 1  New NDBs 0

Longwave Loggings / Re: Winter NDB Logs
« on: March 10, 2018, 1725 UTC »
Recent logs from Downers Grove, Illinois

2018-03-10  0402  358.0  NL  Signal Hill, NL, CAN  2845 Km
2018-03-10  0002  407.0  FR  Farmingdale, NY, USA  1224 Km    New

NDBs this report 2  New NDBs 1

Longwave Loggings / Re: Winter NDB Logs
« on: March 10, 2018, 0202 UTC »
Yukon, cool.   :)

Yeah; I was beginning to wonder.  Up to last nite I had logged 330 Canadian beacons this season, and not one log from YT.  Then, 2 in one nite.

Go figger...

Longwave Loggings / Re: Winter NDB Logs
« on: March 09, 2018, 1555 UTC »
Last nite's logs from Downers Grove, Illinois.  Great opening to the NW.  TR is usually covered by a DGPS which was late coming up.

2018-03-09  1220  242.0  ZT   Port Hardy, BC, CAN  3152 Km
2018-03-09  1002  269.0  ZW  Teslin, YT, CAN  3639 Km    New
2018-03-09  0047  299.0  TR   Bristol, TN, USA  763 Km    New
2018-03-09  1213  332.0  WC  White Rock, BC, CAN  2807 Km    New
2018-03-09  0932  365.0  MA   Mayo, YT, CAN  3897 Km

NDBs this report 5  New NDBs 3

DGPS / Re: DGPS Logs 8-9 Mar 2018
« on: March 09, 2018, 1528 UTC »
Very good opening to BC, YT, and NT this morning from the NDB side of things.

Longwave Loggings / Re: Winter NDB Logs
« on: March 07, 2018, 1808 UTC »
Recent logs from Downers Grove, Illinois

2018-03-05  0048  239.0  TN     Menominee, WI, USA  367 Km  reactivated
2018-03-03  1157  305.0  ONO  Ontario, OR, USA  2367 Km
2018-03-07  0029  329.0  IA     Niagara Falls, NY, USA  767 Km    New
2018-03-07  0056  385.0  ZNS  Halifax, NS, CAN  2014 Km

NDBs this report 4  New NDBs 1

Longwave Loggings / Re: unid on 504
« on: March 04, 2018, 2358 UTC »
It remains of unknown purpose, but related to the military.

Earliest reports in 2007 had this as being a portable NDB under test at Ft. Rucker, AL.  It seems to have moved to Ft. Eustis, VA around 2017 after a hiatus, and some do not consider this to be a portable any more.

At any rate, it counts as "another one logged" in your totals!

Good signal here in an SDR recording.

Thanks PV!

SDR Recording catch (out  last nite!)

Great signal with steady props in general, tending to be around S9+10 throughout the show.  Audio I've heard sounds pretty good, and will listen to the entire show while cleaning out the garage this afternoon.


Longwave Loggings / Re: unid on 504
« on: March 04, 2018, 1534 UTC »
Hey pv;

This is most likely NEED 505 in Ft. Eustis, VA.  Good catch!

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