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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.

     
    WW1WW and 4L5P like this.
  2. PA0MHS

    PA0MHS XML Subscriber 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
    NH7RO likes this.
  3. KF5FEI

    KF5FEI Ham Member QRZ Page

    Looks like a 500 pF 10KV doorknob cap to me.
     
    KR3DX, VE7DXW, NH7RO and 1 other person like this.
  4. KD0CAC

    KD0CAC Ham Member QRZ Page

    He said it , it's magic ;)
     
    NH7RO likes this.
  5. KK7Y

    KK7Y Ham Member 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.
     
    AA6RE, PE5I, W7MCK and 2 others like this.
  6. NH7RO

    NH7RO Ham Member 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.
     
    KR3DX and AJ6KZ 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 NH7RO like this.
  9. VE3VXO

    VE3VXO Ham Member QRZ Page

    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.
     
    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.
     
    KR3DX and NH7RO like this.

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