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10622Re: Varicaps vs old fashioned variable capacitors

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  • jfarley44@...
    Jun 16, 2014
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      Hello all;

      Name is Joe and this be my first post here.

      The question of whether to use tuning diodes or a mechanical cap is extremely interesting, and I think that many of the concerns ment here are valid.  I also think that there is another consideration, and that is "What is your expectation for the Q of your resonant loop in the design frequency range?"  One has to consider which tuning means would have appropriate Q for the desired frequency band, and you might have to answer questions such as "Which approach will give me higher Q", if Q is what you are after.

      My current workhorse resonant receiving loop tunes between 2500 and 7800 kHz, and is used primarily for tropical band and SW pirate DXing.  This is a four turn loop built on a square PVC frame which gives a 20" side length.  The winding is 10AWG type TEW untinned; a 3" center gap (the length of a standard PVC cross) is maintained between turns 2 an 3 to depress parasitic capacitance.  This is tuned by a pair of BTB NTE618s or MVAM115s, and buffered by a balanced JFET (MPF102 and U310 pairs)  cascode amp.  Amplifier Vdd, tuning voltage, and RF are multiplexed on one 180 foot continuous run of F6 coax between the amplifier/tuner head on the mast and the power supply/signal splitter in the shack.  The loop is mounted on a light duty rotator which is primarily used for depressing local QRN.

      Using the BTB tuning diodes in this tuning scheme gives me a fair-weather loaded Q of between 200 and 250 over the mentioned frequency range.  Using the highest quality mechanical variable caps I own (I have not tried  vacuum caps yet), the best Q I have observed is less than half of the Q exhibited by the diodes in the lower SW frequencies.

      BTW, I do agree that a diode tuned resonant loop may favor colder weather; on the two mornings here in Chicago where the temp bottomed at -17F, I was able to measure a loaded Q of approximately 330 on this loop.   I have yet to bang through the numbers to find the most likely cause (or whether the measurements were bogus!).  In the hotter weather, it's harder to tell (and possibly moot) given the generally higher summertime noise levels in these bands.

      ---In loopantennas@yahoogroups.com, <n3ikq@...> wrote :

      Hi all, I'm experimenting with various remotely tuned open air loops and ferrite loopsticks. I have made a remotely tuned varicap diode assembly and a remotely tuned variable capacitor assembly (using a stepping motor w/arduino). I intend to experiment with each on my 4x4 open air box loop and soon a loopstick as well. I am also building up a differential preamp so I will not have to bother with a second pick up loop. My question is: What is the disadvantage of using varicaps (in pairs) compared with using a variable capacitor to tune a loop?  Do varicaps have any tendency to overload or buckle under strong noise conditions? Put another way, Does the mechanical complexity of a remotely tuned variable capacitor provide any advantages over varicap diodes with all other things being equal? According to circuits I have obtained on the net, I can drive a differential preamp with either setup. A thorough search of the net has not provided me any data concerning my question. My only conclusion to date is that my stepping motor imparts significant noise into the antenna when it is operating. This is not actually a bad thing in that it tends to serve as a tuning aid when using a traditional receiver. Of course when using an SDR with a spectrum scope, the noise is a distraction because one can see the antenna peak moving along the frequency domain as it is tuned. Any thoughts welcome!
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