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Ham Radio - The Chameleon Antennas power compensator for magnetic loops.

Discussion in 'Amateur Radio News' started by KB7TBT, Nov 3, 2019.

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  1. KB7TBT

    KB7TBT Ham Member QRZ Page

    Ham Radio - The Chameleon Antennas power compensator for magnetic loops.

     
    KK4HPY, WW1WW and 4L5P like this.
  2. PA0MHS

    PA0MHS Ham Member QRZ Page

    You should check the bandwidth of the loop antenna at a set frequency, with and without this device. This lump of dielectrical material at the "hot" end of the loop will probably lower the Q of the loop and thus lower the peak voltage across the tuning capacitor. This thing is nothing more than a high voltage capacitor connected at one end. So it either lowers the Q of the loop by introducing a lossy material close to the loop or by adding a small parasitric capacitance which loads the resonant circuit.
     
    Last edited: Nov 4, 2019
    KE0EYJ, KR3DX and AK5B like this.
  3. KF5FEI

    KF5FEI Ham Member QRZ Page

    Looks like a 500 pF 10KV doorknob cap to me.
     
    KR3DX, VE7DXW, AK5B and 1 other person like this.
  4. KD0CAC

    KD0CAC Ham Member QRZ Page

    He said it , it's magic ;)
     
    AK5B likes this.
  5. KK7Y

    KK7Y XML Subscriber QRZ Page

    Did you check the field strength meter when transmitting 30W? The real question is whether the radiated power increased proportionally or if the compensator simply prevented a voltage increase in the loop.
     
    KE0EYJ, DL2MEW, AA6RE and 4 others like this.
  6. AK5B

    AK5B XML Subscriber QRZ Page

    There's no free lunch to be had with any antenna; I am especially leery of these commercially-made STLs as their manufacturers invariably need to cut corners in one way or another (often in the most crucial areas of tuning capacitor and element materials and size) to make a profit in selling to the public, covering overhead and marketing expense, etc. I know that with only one or two exceptions most STL designs can improved upon by the ardent DIYer who is really willing to go the extra mile and be extra mindful of construction techniques.

    I may refer to my own home-brewed loops as "magic loops" because of their apparent magic from time to time but I would never attempt to fool others about their performance or worthy attributes. They are fractions of a wavelength in physical size and while they are a godsend for hams like me who have little space for antennas they are still limited and governed by the laws of physics.

    (but that doesn't mean they are necessarily "dummy loads" or shabby performers, either---they simply need to be "done right "and built carefully to achieve relative greatness considering the diminutive antenna that they are (1/10 of a wavelength in diameter or less!).

    I encourage those curious about small transmitting loops to read up about them first. If they are right for you it will soon become apparent---but they are not for everyone. They are finicky and need to be understood before using effectively.

    If intrigued as I was several years ago upon considerable internet research,---go ahead take the DIY plunge---and find out firsthand.
    Building one from "scratch" can be quite rewarding, especially after working some rare dx or nulling out pesky noise that could not be so possible otherwise. They have a lot of selectivity built into them and that can be another hidden bonus, too.
     
    VA7LDT, K5VZD, KR3DX and 1 other person like this.
  7. K7LZR

    K7LZR Ham Member QRZ Page

    My STL product is likely one of the exceptions. Has never claimed to be the holy grail of magnetic loops and all of the posted feedback is from honest users. It is also currently the lowest priced - by far - antenna of its type available. Big profit? Sure. At our pricing and expense levels, average profit for us is $20-35 per unit. Less if we run a sale on them. They are VERY labor intense to build and since we really do use quality materials they're expensive too. In terms of hourly wage for building one from start to finish - even with the uniform, efficient and predictable production techniques which we have in place - works out to about $4-6 per hour. Less if we are running a sale.

    I make much, much more of my living from my job in the fitness industry.

    The whole idea here was never to make a lot of money - it started as a labor of love and turned into a low-cost good quality antenna offering.

    After 4+ years, It is becoming likely that our STL antenna will not be produced for much longer.
     
    W7SJP likes this.
  8. KD5U

    KD5U XML Subscriber QRZ Page

    That is exactly what it looks like.
     
    KR3DX and AK5B like this.
  9. VE3VXO

    VE3VXO Guest

    Every magnetic loop contains a stray capacitance in addition to the variable tuning capacitor. All he has done is shunt some of the RF current around the air variable through the increased stray capacitance. If this trick also introduces loss it will further reduce the peak voltage on the air variable. If the experiment is repeated several times but with a different orientation of the "compensator" he would find that the amount by which the antenna resonant frequency is shifted would vary in each case because the exact value of the total stray capacitance would vary by each different orientation (all other things being held constant). It goes without saying these tests should be done outdoors away from everything as well. Indoors you are just chasing your tail! Even changing where you are sitting would skew the data. The comment of PA0MHS is bang on. Check the bandwidth between the ponts where the VSWR rises to ~2.6. Depending on the dielectric losses in that Chinese HV cap and the PVC cover they put on it, the "compensator" is likely also adding loss and that will be apparent in a widening of the bandwidth (lower Q). A well made home brew loop can meet the Q predicted by the various calculating utilities for small transmitting loops. Many of the comercially made ones can't make that claim. I can even show anyone a home brew loop that exceeds the predicted Q because the calculators aren't perfect in their assumptions. Q factor is an indication of efficiency in these antennas. If you can't hit the predicted Q for a given size loop at a given frequency then you have additional and excess loss which can originate in several key areas. The only way to know what you really have got is to make careful measurements of the quality factor as suggested by PA0MHS. The very high Q loop I built was measured without a rain cover on the vacuum variable and then again with a cover made of polypropylene which was carefully chosen for its low dielectric loss factor. The cover lowered the Q a little bit. Painting it to protect it from sunlight also lowered it a bit. Buyer beware. A chameleon can look pretty amazing but it is still just a lizzard.
     
    K3JLS, KR3DX and PA0MHS like this.
  10. K3FHP

    K3FHP XML Subscriber QRZ Page

    Exactly, no free lunch as you said. You may bend the laws of physics but you may not break them. It is up to the Amateur to understand the applicable science and use it accordingly. To stay in business ANY manufacturer must sell for 3X the cost...sorry just the facts.
     
    K3JLS, KR3DX and AK5B like this.
  11. AK5B

    AK5B XML Subscriber QRZ Page

    Kudos for your astute observations; many good points in your meaty post. ( The emphasis added above is mine)

    Via no-holds barred optimization it is possible to create a small "lizard-like loop" that can roar like a Komodo Dragon---but that requires the best components at greater expense and size not always feasible with the vast majority of manufacturers.

    The sole exception might be Ciro Mazzoni's massive TIG-welded Midi Loop---perhaps Ciro could rename his "The Dragon" since it practically breathes flames by comparison!

    73,

    Jeff
     
    Last edited: Nov 5, 2019
  12. N4RL

    N4RL Ham Member QRZ Page

    It seems like the point of this is to make a trade off that lowers Q, and thus allows for minor frequency adjustments without retuning, while increasing power to try and offset diminished performance. Is that how others see it?
     
  13. PA0MHS

    PA0MHS Ham Member QRZ Page

    Well, the lower Q also lowers the radiated field strength. The relation between produced field strength and applied power is linear. So if this device lowers the Q to allow you to push more power into the antenna to eventually reach the same field strength, then the only thing gained is the extra money in the pockets of the manufacturer...
     
    N6SPP likes this.
  14. VE3VXO

    VE3VXO Guest

    "while increasing power to try and offset diminished performance. Is that how others see it?"

    He said his air variable could handle up to about 15 watts of RF power before it would arc. With the addition of the hockey puck capacitor he could run 30 watts. So this "compensator" made a 3db improvement in the power handling capacity, but the question is did he realize a 3db increase in the far field strength? It's fine to dump more power into an antenna but if he added 3db of loss then it isn't going to make any difference in being heard. In terms of QSY without re-tuning, until you get north of 1/4 wave circumference for the size of the radiator, the Q is going to be so high that you won't be able to move much at all without re-tuning, hockey puck or no. It is a fact of life with small transmitting loops that you are always re-tuning them. Where broadening can help is when the Q gets so high that you can only use CW because the bandwidth is too narrow for phone. In that case the answer is to raise the radiation resistance by using a larger circumference, NOT broadening the response by adding loss.
     
    KK4HPY likes this.
  15. W7SJP

    W7SJP Ham Member QRZ Page

    I sure appreciate what you do for the hobby--I know it's mostly a labor of love. It'll be sad to see you stop producing this, but I can understand the reasons.
     
    KK4HPY and K7LZR like this.

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