[Lowfer] L.F. LOOPS

[Chris Trask]

     I was using single-ended varactor and then a preamp in my earlier designs, but this time I tried balanced varctors and wideband transformers to match the loop to the cable, sending the control voltage up the cable rther than by a separate set of wires.  Turned out to be much quieter with almost as much gain, so the S/N ratio is actually better this way.

     I'm using MVAM109 varactors same as you, and I can get close to two octaves at HF frequencies.  

>
>I am using a balanced loop, with the balanced varactors accross the loop 
>feeding a preamp.  The preamp sends the amplified signal over twisted pair 
>using CAT-5 cable.  Power and varactor control goes out over the CAT-5 using 
>other twisted pair.  Depending on the preamp gain, you may have problems 
>using twisted pair in the vicinity of the loop due interaction with the loop 
>magnetic fields.  I initially had feedback.  I settled mine down by dressing 
>the return leads along the support pole (steel) and reversing the wires from 
>the preamp to the loop.
>
>It's a cheap solution as the CAT-5 is a lot less costly than coax.
>
>Not sure you can get that full tuning range as the loop's distributed 
>capacity my limit it.  If you tap the loop, you will reduce the signal 
>level.  I am using MVAM109 varactors and I think 26 turns on 4 foot loop 
>arms (roughly 5 foot on the side) spaced over roughly 4 inch stringers.
>
>Pete
>
>----- Original Message ----- 
>From: "Chris Trask" <christrask at earthlink.net>
>To: "Discussion of the Lowfer (US, European, & UK) and MedFer bands" 
><lowfer at mailman.qth.net>
>Sent: Tuesday, August 26, 2008 11:10 PM
>Subject: Re: [Lowfer] L.F. LOOPS
>
>
>> Rick,
>>
>>>
>>> Hi all,
>>> I was just looking for opinions on feedlines for small loop antennas.I
>> built a
>>> 3 ft square loop with about 24 turns .I plan to to tune it from  about
>> 160 -500
>>> khz. It wil be placed outdoors.I havent decided how to feed it yet.Should
>> I link
>>> couple or direct couple it?Should I coax feed it or use twisted pair.? I
>> have
>>> some nice 22 ga. shielded twisted pair.
>>> I will also be remote tuning it using varicaps.
>>> Any input would be greatly appreciated.I have read too many loop articles
>> recently
>>> only to become more confused!
>>>
>>
>>    Funny you should bring this up as I'm going to put a paper about
>> varactor tuned loops for HF on my webpage soon, maybe as early as tomorrow
>> if everything goes well.  I'll post a message on this group when I do. 
>> Even
>> though it's oriented towards HF receiving loops, you may find information 
>> in
>> it that will be useful for what you're doing.
>>
>>    I found that coupling to the loop balanced, operating the varactors
>> balanced, and keeping the impedance stepping balanced until the very last
>> stage before going to coaxial cable does wonders for reducing harmonics 
>> and
>> intermodulation problems with AM BCB stations.  The transformer 
>> construction
>> is a critical item for good performance, and I used a combination of
>> powdered iron toroids and ferrite balun cores to get good results and keep
>> the cost down.
>>
>> Chris
>>
>>     ,----------------------.       High Performance Mixers and
>>    /    What's all this     \    Amplifiers for RF Communications
>>   / extinct stuff, anyhow?  /
>>   \  _______,--------------'           Chris Trask / N7ZWY
>>  _ |/                                  Principal Engineer
>> oo\                                  Sonoran Radio Research
>> (__)\       _                             P.O. Box 25240
>>  \  \    .'  `.                     Tempe, Arizona 85285-5240
>>   \  \  /      \
>>    \  '"        \                 IEEE Senior Member #40274515
>>     .       (  ) \
>>      '-| )__| :.  \              Email: christrask at earthlink.net
>>        | |  | | \  '.       http://www.home.earthlink.net/~christrask
>>       c__; c__;  '-..'>.__
>>
>>                       Graphics by Loek Frederiks
>>
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>
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Chris

     ,----------------------.       High Performance Mixers and
    /    What's all this     \    Amplifiers for RF Communications
   / extinct stuff, anyhow?  /
   \  _______,--------------'           Chris Trask / N7ZWY
  _ |/                                  Principal Engineer
 oo\                                  Sonoran Radio Research
(__)\       _                             P.O. Box 25240
  \  \    .'  `.                     Tempe, Arizona 85285-5240
   \  \  /      \
    \  '"        \                 IEEE Senior Member #40274515
     .       (  ) \
      '-| )__| :.  \              Email: christrask at earthlink.net
        | |  | | \  '.       http://www.home.earthlink.net/~christrask
       c__; c__;  '-..'>.__

                       Graphics by Loek Frederiks