OT: High RF currents in small TX'ing lopps

[Ken G Kopp]

Small transmitting loops by their very nature involve high levels of
current flow and this can/will cause "ordinary" variable caps to heat,
which results in tuning drift.  Most home builders use vacuum caps
and relatively large copper pipe in their designs.  Mechanical joints
(clamps, wiping contacts, etc.) are to be avoided.

This tuning drift problem plagued the AEA loops of some years ago.
(I had the prototype here for early testing.)  Does anyone know how
well MFJ's loop performs in this respect?

On Tue, Jan 1, 2013 at 10:03 PM, Ron D'Eau Claire <[hidden email]> wrote:

> **
>
>
> ** **
>
> "Eliminating every stray milli-Ohm counts towards achieving highest
> efficiency!"****
>
>
> http://www.ahars.com.au/documents/the_underestimated_magnetic_loop_hf_antenna_vers%201.1.pdf
> ****
>
> ** **
>
> wunder****
>
> K6WRU****
>
> ** **
>
> On Jan 1, 2013, at 12:17 PM, Jim Beard wrote:****
>
>
>
> ****
>
>   ****
>
> ** **
>
> I've been playing around with A Magnetic Loop I built and have found
> Several good Air Variable capacitors in old radios or test equipment at
> antique stores. Most of the old stuff can be bought for 15.00-30.00 and you
> get some great spare parts. I've found singles and double air variable
> capacitors and 20-80 pf up to 56-560 pf air variables and so far the
> antenna has worked quite well. I believe The Alex loop Walkham is the best
> by far. But I spent very little using old spare parts to make a loop. My
> goal is to get a Alex loop but have had fun playing with mine. There are
> some good math formulas on the web on what size versus what pf for what
> frequency. Happy New Year to all and good DX in 13. ****
>
> Jim Beard****
>
> Sent from my iPhone****
>
> AE5ZZ****
>
>
> On Jan 1, 2013, at 11:15 AM, Walter Underwood <[hidden email]>
> wrote:****
>
>   ****
>
> ** **
>
> On Jan 1, 2013, at 4:25 AM, David Wilcox wrote:****
>
>
>
> ****
>
> Also, not "fiddly" to build if you are handy. I am some but no time.  I
> have a neighbor who whips up stuff like that in an afternoon.  Coax, a
> varicap with 10:1 reduction gear off eBay (with a simple search he found
> the same one Alex uses with the same reduction drive..... looks just like
> the one in my AlexLoop), PVC pipe from Ace Hardware, some RG-174 and a RS
> box is all they are.****
>
> ** **
>
> You can't use just any capacitor. The resistance in the wipers will kill
> the performance of the loop. You need a butterfly capacitor with no
> rotating contacts. Those aren't easy to find, but MFJ does sell two models.
> ****
>
> ** **
>
> You also need really low resistance material for the loop. For a home
> setup, I'd start with the flexible copper tubing used to hook up
> refrigerators with icemakers. After you get the details worked out, you
> could switch to something better.****
>
> ** **
>
> Loops are usually limited to a 2:1 frequency range, so you need multiple
> loops to go from 80-10.****
>
> ** **
>
> This article is really useful:
> http://www.mfjenterprises.com/antennatalk6.php****
>
> ** **
>
> The instructions for the MFJ-935 have good info on loop sizes and RF
> exposure limits: http://www.mfjenterprises.com/pdffiles/MFJ-935B.pdf****
>
> ** **
>
> wunder****
>
> --****
>
> Walter Underwood****
>
> K6WRU****
>
> ** **
>
> ** **
>
> ** **
>
> ** **
>
> ** **
>
> ** **
>
> --****
>
> Walter Underwood****
>
> [hidden email]****
>
> ** **
>
> ** **
>
> ** **
>
> ****
>
>  __._,_.___
>   Reply via web post<http://groups.yahoo.com/group/KX3/post;_ylc=X3oDMTJybnF2aDlvBF9TAzk3MzU5NzE0BGdycElkAzc0NTIwOTQ3BGdycHNwSWQDMTcwNTA2MzEwOARtc2dJZAMyMTE3OQRzZWMDZnRyBHNsawNycGx5BHN0aW1lAzEzNTcwNzc4Nzk-?act=reply&messageNum=21179>  Reply
> to sender
> <[hidden email]?subject=RE%3A%20%5BKX3%5D%20Receive%20noise%20%2F%20ground>  Reply
> to group
> <[hidden email]?subject=RE%3A%20%5BKX3%5D%20Receive%20noise%20%2F%20ground>  Start
> a New Topic<http://groups.yahoo.com/group/KX3/post;_ylc=X3oDMTJmOWJhcnBwBF9TAzk3MzU5NzE0BGdycElkAzc0NTIwOTQ3BGdycHNwSWQDMTcwNTA2MzEwOARzZWMDZnRyBHNsawNudHBjBHN0aW1lAzEzNTcwNzc4Nzk->  Messages
> in this topic<http://groups.yahoo.com/group/KX3/message/21026;_ylc=X3oDMTM3cG9hOGptBF9TAzk3MzU5NzE0BGdycElkAzc0NTIwOTQ3BGdycHNwSWQDMTcwNTA2MzEwOARtc2dJZAMyMTE3OQRzZWMDZnRyBHNsawN2dHBjBHN0aW1lAzEzNTcwNzc4NzkEdHBjSWQDMjEwMjY->(34)
> Recent Activity:
>
>    - New Members<http://groups.yahoo.com/group/KX3/members;_ylc=X3oDMTJndDU1OGIzBF9TAzk3MzU5NzE0BGdycElkAzc0NTIwOTQ3BGdycHNwSWQDMTcwNTA2MzEwOARzZWMDdnRsBHNsawN2bWJycwRzdGltZQMxMzU3MDc3ODc5?o=6>
>    40
>    - New Photos<http://groups.yahoo.com/group/KX3/spnew;_ylc=X3oDMTJndmFwdDRzBF9TAzk3MzU5NzE0BGdycElkAzc0NTIwOTQ3BGdycHNwSWQDMTcwNTA2MzEwOARzZWMDdnRsBHNsawN2cGhvdARzdGltZQMxMzU3MDc3ODc5>
>    2
>    - New Files<http://groups.yahoo.com/group/KX3/files;_ylc=X3oDMTJobXB2cGdpBF9TAzk3MzU5NzE0BGdycElkAzc0NTIwOTQ3BGdycHNwSWQDMTcwNTA2MzEwOARzZWMDdnRsBHNsawN2ZmlsZXMEc3RpbWUDMTM1NzA3Nzg3OQ-->
>    6
>
>  Visit Your Group<http://groups.yahoo.com/group/KX3;_ylc=X3oDMTJmcTNrZnA3BF9TAzk3MzU5NzE0BGdycElkAzc0NTIwOTQ3BGdycHNwSWQDMTcwNTA2MzEwOARzZWMDdnRsBHNsawN2Z2hwBHN0aW1lAzEzNTcwNzc4Nzk->
>  [image: Yahoo! Groups]<http://groups.yahoo.com/;_ylc=X3oDMTJlN2NlZHViBF9TAzk3NDc2NTkwBGdycElkAzc0NTIwOTQ3BGdycHNwSWQDMTcwNTA2MzEwOARzZWMDZnRyBHNsawNnZnAEc3RpbWUDMTM1NzA3Nzg3OQ-->
> Switch to: Text-Only<[hidden email]?subject=Change+Delivery+Format:+Traditional>,
> Daily Digest <[hidden email]?subject=Email+Delivery:+Digest>•
> Unsubscribe <[hidden email]?subject=Unsubscribe> • Terms
> of Use <http://docs.yahoo.com/info/terms/> • Send us Feedback
> <[hidden email]?subject=Feedback+on+the+redesigned+individual+mail+v1>
>    .
>
> __,_._,___
>

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

[Kenneth Christiansen-2]

Hi Ken

I have had the MFJ 3 foot loop for several years. I am now using it to receive only as it has a better signal to noise ratio than my Hy-Tower vertical. I found the SWR drifted badly due to changes in temperature caused by sun light and the environment. I felt it was much more stable at night so I did not feel the heat caused by 100 watts of RF was making a problem The changes in SWR are not noticed by my receiving experience. I hope my observations are helpful to you

73

Ken W0CZ

Sent from my iPad

On Jan 1, 2013, at 4:19 PM, Ken G Kopp <[hidden email]> wrote:

> Small transmitting loops by their very nature involve high levels of
> current flow and this can/will cause "ordinary" variable caps to heat,
> which results in tuning drift.  
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

[k6dgw]

Small TX loops are a moderately unexplored section of the radio universe
by hams.  Impedances are very low, hence very high currents, I^2 * R
suggests small resistances really count.

I sold my Buddipole, which I've used in many field and SOTA excursions
and got an AlexLoop.  Much lighter, sets up in 1/10th the time, QSY's
almost instantly, doesn't need elevation, 1/10th the weight or less, and
seems to be more or less equivalent to anything I could do with the
Buddipole.  And, it's pretty much insensitive to "ground."

In the field, it is generally on a tripod over my left shoulder so I can
just reach up and tune it.  K2, 5W, no tuning drift, and I don't line my
hat with tinfoil.  Not sure I'd sit that close running power into a loop
designed for 100+ watts however.  A good friend and retired EE pointed
out recently, "All the magnetic lines of force from everywhere in the
universe where I'm heard, go through that small loop." :-))

I tried just about every BuddiPole configuration and came to two
conclusions:

1.  It doesn't matter how you extend the ends of the elements ... up,
down, one up-one down, in a "Y'.  If the sections close in to the feed
point are horizontal, it's a horizontal antenna and elevation matters.
I guess I knew that from theory but it's nice when you see theory in
practice.

2.  Any vertical configuration that includes some form of reasonably
good ground plane [2+ radials?] will be less sensitive to elevation.
Without a good ground plane, it will behave like a dummy load [on which
you *can* make a few Q's :-)].

The BP is a good field antenna, very well made, very sturdy, and well
supported, with lots of options.  The full version is probably more
"luggable" than "portable."  I think I just got tired of lugging it as I
get older.

73 and a happy and peaceful New Year to all,

Fred K6DGW
- Northern California Contest Club
- CU in the 2013 Cal QSO Party 5-6 Oct 2013
- www.cqp.org


On 1/1/2013 2:19 PM, Ken G Kopp wrote:
> Small transmitting loops by their very nature involve high levels of
> current flow and this can/will cause "ordinary" variable caps to heat,
> which results in tuning drift.  Most home builders use vacuum caps
> and relatively large copper pipe in their designs.  Mechanical joints
> (clamps, wiping contacts, etc.) are to be avoided.
>
> This tuning drift problem plagued the AEA loops of some years ago.
> (I had the prototype here for early testing.)  Does anyone know how
> well MFJ's loop performs in this respect?


______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html