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Varicaps vs old fashioned variable capacitors

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  • n3ikq
    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
    Message 1 of 13 , Apr 30, 2014
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      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!
    • Chris Trask
      ... Varicaps can produce serious IMD products when used for remote antenna tuning, especially in the presence of strong nearby broadcast stations. Variable
      Message 2 of 13 , Apr 30, 2014
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        >
        >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!
        >

        Varicaps can produce serious IMD products when used for remote antenna tuning, especially in the presence of strong nearby broadcast stations. Variable capacitors will always give far better performance in those circumstances. Varicaps will also degrade the equivalent antenna noise temperature due to finite bulk resistance, whereas variable capacitors have little if any series resistance.

        I have not gone to the trouble of doing a comparison between the two, I just know from experience about the IMD problems of varicaps. I have recently devised a single variable capacitor approach to tuning a loop antenna, and if I can ever get my act in gear I'll publish it online as I have done with other designs.


        Chris Trask
        N7ZWY / WDX3HLB
        Senior Member IEEE
        http://www.home.earthlink.net/~christrask/
      • Richard (Rick) Karlquist
        ... Varicaps definitely can rectify and create QRM from strong signals, be they AM BCB stations or nearby ham stations or if you are running SO2R. The back to
        Message 3 of 13 , Apr 30, 2014
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          On 4/30/2014 7:39 AM, n3ikq@... wrote:

          > Put another way, Does the mechanical complexity of a remotely tuned
          > variable capacitor provide any advantages over varicap diodes with all

          Varicaps definitely can rectify and create QRM from strong
          signals, be they AM BCB stations or nearby ham stations
          or if you are running SO2R. The back to back configuration
          helps but you can still have rectification.

          You said you were using a preamp. Preamps also create
          their own intermod so it can be hard to determine whether
          any given intermod is due to the preamp or varicaps.
          This includes built in preamps in radios. The IC706
          preamp is bad for this.

          Rick N6RK
        • n3ikq
          Wow answers from the living legends themselves! I have a few of your documents in my files that I have been educating myself from! The balanced (differential)
          Message 4 of 13 , Apr 30, 2014
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            Wow answers from the living legends themselves! I have a few of your documents in my files that I have been educating myself from! The balanced (differential) preamp I am gathering parts for is from a paper by Lyle Koehler. It seems to be a variation on a common design as it appears to be used by at least two of the three builders of commercial loopstick antennas. I am guessing the balanced, center tapped loop design allows the loop (or interchangeable coils) to be grounded thereby protecting the FETs from ESD. Anyway thanks for the great feedback. I was guessing that the varicaps had some sort of Achilles heel that keeps the antenna builders looking for good old fashioned variable caps. My remote variable capacitor design is nothing earth shattering but I have made it to use standard cat5 cable and cheap eBay stepper motors from China. I use the three left over cat5 wires for positional feedback from a pot attached to the cap. All 8 wires get used and cat5 cables are everywhere so no custom cables need to be built.
          • Chris Trask
            ... Is that the one with the balanced MPF102 input stage and a balanced 2N2907 output stage? I have reservations about that approach as the low impedance of a
            Message 5 of 13 , Apr 30, 2014
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              >
              >Wow answers from the living legends themselves! I have a few of your documents in my files that I have
              >been educating myself from! The balanced (differential) preamp I am gathering parts for is from a paper
              >by Lyle Koehler.
              >

              Is that the one with the balanced MPF102 input stage and a balanced 2N2907 output stage? I have reservations about that approach as the low impedance of a loop pretty much dictates a low input impedance amplifier with series tuning. His design is more appropriate for use with a short dipole, which can be a pretty good small antenna. But, his amplifier is fairly simple.


              Chris Trask
              N7ZWY / WDX3HLB
              Senior Member IEEE
              http://www.home.earthlink.net/~christrask/
            • n3ikq
              Yes, that s the circuit. http://www.lowfer.us/k0lr/bal-pre/bal-pre.htm http://www.lowfer.us/k0lr/bal-pre/bal-pre.htm. I thought the point was to have such a
              Message 6 of 13 , Apr 30, 2014
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                Yes, that's the circuit. http://www.lowfer.us/k0lr/bal-pre/bal-pre.htm. I thought the point was to have such a high impedance that the loop/inductor was not effected by the directly attached preamp. Another interesting design is from the late Ralph Burhans. I bought and used his preamp way back in the 80's and lost it. I was happy to find the design again. However, it is not a tuned design and I'm not quite sure if I can just slap a tank circuit on the transformer input. http://members.shaw.ca/ve7sl/burhans.html
              • Chris Trask
                ... I ll look at that later when I have a WiFi connection. The loop antenna is best viewd as being a current source with a series inductance and a low series
                Message 7 of 13 , Apr 30, 2014
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                  >
                  >Yes, that's the circuit. http://www.lowfer.us/k0lr/bal-pre/bal-pre.htm
                  >http://www.lowfer.us/k0lr/bal-pre/bal-pre.htm. I thought the point was to
                  >have such a high impedance that the loop/inductor was not effected by the
                  >directly attached preamp. Another interesting design is from the late
                  >Ralph Burhans. I bought and used his preamp way back in the 80's and lost
                  >it. I was happy to find the design again. However, it is not a tuned design
                  >and I'm not quite sure if I can just slap a tank circuit on the transformer
                  >input. http://members.shaw.ca/ve7sl/burhans.html
                  >http://members.shaw.ca/ve7sl/burhans.html
                  >

                  I'll look at that later when I have a WiFi connection. The loop antenna is best viewd as being a current source with a series inductance and a low series resistance. When seen that way, series tuning with a low input impedance amplifier becomes obvious.

                  I think I placed a 1- or 2-page PDF files in the group files section that showed schematics for simple active antenna amplifiers using the CA3046 transistor array. If not, I'll look around the see where I put it. It's not on my office computer, so it must be on the laptop.


                  Chris Trask
                  N7ZWY / WDX3HLB
                  Senior Member IEEE
                  http://www.home.earthlink.net/~christrask/
                • T. Hills
                  I ve been using varactors for MW and LW loops for years. There are a couple of things to be careful of. When using a back to back configuration I ve found it
                  Message 8 of 13 , Apr 30, 2014
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                    I've been using varactors for MW and LW loops for years. There are a couple of things to be careful of.

                    When using a back to back configuration I've found it best to match them as closely as possible. An imbalance increases the chances of IMD and other problems.

                    The lower the signal level across the varactors the better. I use set of 4 pairs (8 total) MVAM109s to tune a ferrite loop and have used as many as 10 pairs (20 total) for a LW air core. The higher capacitance allows for fewer windings and lower AC voltage across the junctions and matching is easier if 4 parallel varactors are matched to 4 others. I got lucky and bought 300 of them for next to nothing, rejected lot with 50 of them being defective but I still see those or similar in bulk on Ebay from time to time.

                    I also use a second center tapped pickup loop with a balanced MOSFET preamp feeding a balanced line to the receiver. Even with 3 BC stations close enough to hear on the toaster I don't get any noticeable intermod.

                    I prefer varactors mostly because I stick to the KISS principle as much as possible. A well regulated reference voltage, a 10 turn pot, a single op-amp, a few resistors and caps and one wire is all that's needed for control.

                    I've heard of people having problems with varying ambient conditions. The attic above the garage where the loop was mounted goes from -30F in winter to over 140F in the summer. The antenna is "noisier" the hotter it gets but I can say the same for any MW antenna since atmospheric noise in this region is very high from April to October so it may be junction noise or it may just be background/thunderstorms. Probably a combination.

                    HTH
                    Tim Hills
                    Sioux Falls, SD
                    On 4/30/2014 9:39 AM, 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!

                    _

                  • Richard (Rick) Karlquist
                    ... One other problem with varactors is testing. The loop design I published in the National Contest Journal cannot be tested with the MFJ-259 impedance meter
                    Message 9 of 13 , Apr 30, 2014
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                      On 4/30/2014 7:39 AM, n3ikq@... wrote:

                      > Put another way, Does the mechanical complexity of a remotely tuned
                      > variable capacitor provide any advantages over varicap diodes with all

                      One other problem with varactors is testing. The loop design
                      I published in the National Contest Journal cannot be tested
                      with the MFJ-259 impedance meter (and probably similar units)
                      because the drive level is too high. I only recently discovered
                      this. I did all the development work originally with an
                      HP 4815 vector impedance meter. I also found that if I
                      used an Agilent E5071C VNA, I had to reduce the drive level
                      below the default level of the instrument.

                      Rick N6RK
                    • T. Hills
                      ... When I measured the batch I got I built a buffered VCO with the L.O. coil from an old MW radio using a socket for the varactors to test the frequency
                      Message 10 of 13 , Apr 30, 2014
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                        On 4/30/2014 5:54 PM, Richard (Rick) Karlquist wrote:
                         



                        On 4/30/2014 7:39 AM, n3ikq@... wrote:

                        > Put another way, Does the mechanical complexity of a remotely tuned
                        > variable capacitor provide any advantages over varicap diodes with all

                        One other problem with varactors is testing. The loop design
                        I published in the National Contest Journal cannot be tested
                        with the MFJ-259 impedance meter (and probably similar units)
                        because the drive level is too high. I only recently discovered
                        this. I did all the development work originally with an
                        HP 4815 vector impedance meter. I also found that if I
                        used an Agilent E5071C VNA, I had to reduce the drive level
                        below the default level of the instrument.

                        Rick N6RK

                        _
                        When I measured the batch I got I built a buffered VCO with the L.O. coil from an old MW radio using a socket for the varactors to test the frequency output at 12 discrete voltages using a 12 position switch and resistors.

                        Fed the output to my scope and the output from the scope to a cheap frequency counter. A bit of Math (and a spreadsheet) gave me the cap. values at different voltages and monitoring the waveform on the scope showed if they were noisy or unstable. Neither the scope or the counter had to be NBS traceable or anything since I was comparing apples to apples so to speak and in circuit stray capacitance would be more than the measurement error anyway.

                        Still have the test jig around here somewhere....

                        Tim Hills
                        Sioux Falls, SD
                      • Chris Trask
                        ... I found that file in the group files section. The title is: Active Antenna Amplifiers for Low Voltage Operation RevA.pdf Although the designs are for
                        Message 11 of 13 , May 1, 2014
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                          >
                          > I think I placed a 1- or 2-page PDF files in the group files section that showed schematics for
                          >simple active antenna amplifiers using the CA3046 transistor array. If not, I'll look around the see
                          >where I put it. It's not on my office computer, so it must be on the laptop.
                          >

                          I found that file in the group files section. The title is:

                          Active Antenna Amplifiers for Low Voltage Operation RevA.pdf

                          Although the designs are for low-voltage operation, they can be used for supply voltages up to 12V. For practical purposes, the schematic on the second page is probably the most suitable, and the one on the third page is for demanding situations where even-order distortion from strong signals is a serious problem.


                          Chris Trask
                          N7ZWY / WDX3HLB
                          Senior Member IEEE
                          http://www.home.earthlink.net/~christrask/
                        • jfarley44@...
                          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
                          Message 12 of 13 , 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!
                          • n3ikq
                            Hi Joe, thanks for the post. I m quickly amassing a small army of loops so I can learn for myself all of the good advice I ve been getting. The first little
                            Message 13 of 13 , Jun 20, 2014
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                              Hi Joe, thanks for the post. I'm quickly amassing a small army of loops so I can learn for myself all of the good advice I've been getting. The first little tidbit of reality comes from comparing my recently homebuilt loopstick (Palomar clone) with my newly acquired Quantum loopstick from eBay. I needed a "standard" by which to compare my home built attempts. So far, my homebuilt model compares very favorably with the commercial version. I need to quiet things down electrically by getting some matched JFETs and paying attention to critical component values to keep the balanced amp really balanced. If I substitute a regen type preamp circuit I can improve the Q as well. My plans for the loopsticks are to build a remotely tuned outdoor model and possibly springing for a monster ferrite rod from Stormwise to replace the current Amidon 7.5" rod. I've become a big fan of remote tuning as I've been playing with stepping motors for the past year or two and I've built a few that use simple cat5 wire making hookup easy.

                              The big surprise is how much better even my 30" passive air loop performs against the loopsticks. Of course I was aware of this but there nothing like actually observing it myself! My next steps are to compare tuning the loop with varicaps vs the air variable cap and to try getting the best inductive match to my 50 ohm feed line. Additionally I want try connecting a balanced preamp directly to the resonate loop and observe the performance of that arrangement vs the passive coupling approach. Another few months and dollars later I'll finally be up to speed on what everyone else already knows! Well at least I'm having fun doing it!
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