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Limited Space Antennas – The Small Transmitting Loop Antenna

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isoloop_aeafullviewThe AEA Isoloop

My basement has a collection of limited space antennas.  They don’t work well in the basement.  Oh yes, I forgot, they are there in the basement in storage.  All these antennas have a story to tell.  They were used, at one time or another, in one of my limited-space or stealth-operating living locations.

The AEA IsoLoop HF Antenna was an antenna I used off the balcony in a Chicago high-rise.  The IsoLoop was a little more stealthy than my Texas Bug Catcher with its 10ft length at about a 45 degree angle hanging over the edge of the balcony within sight of my neighbors.

The AEA IsoLoop is a small loop transmitting antenna that covers 14 Mhz to 30 Mhz continuously tunable.  Being continuously tunable is a good thing and its a bad thing.  It’s a good thing as one antenna can go any where between 20 meters and 10 meters – anywhere.  The bad thing is that it’s tunable and can go any where between 20 meters and 10 meters.  Got that?  It’s tunable – which means you have to make some sort of adjustment for each band on which it operates..  Unlike a multi-band antenna that is resonant on multiple bands at the same time, a small loop antenna, like the screwdriver antenna is resonant on one swath of  frequencies at a time.

Tuning the IsoLoop

isoloop_aeacontrollerTuning these tunable antennas such as a screwdriver or the IsoLoop is the bane of these antennas. 

On a screwdriver antenna, unless you can get to the exact place that you want you are going to have to splatter some portion of the band you want to work with a tuning signal until you get the lowest SWR at the place you want to transmit.  The same is true of tuning a small loop antenna.  Tuning is adjust and check SWR until you get it right.

There are pictures below of what is inside the IsoLoop.  At this point, suffice it to say that the IsoLoop is a  LC(R) tuned circuit – a (fixed) inductance (the loop) in series with a (variable) capacitor plus some resistive loss.

The resonant frequency (formulas below) is changed by changing the capacitatiance via a stepper motor which turns the blades of an air-dialectic capacitor.

The AES IsoLoop is tuned via a control box which controls the  stepper motor. The IsoLoop has a very high Q which means that it has a very narrow resonant bandwidth.  On 20 meters ( 14 MHz ) the tuning is so tight that one step on the stepper motor is going to be the difference between a 1.5 and 2.0 SWR.  It’s that tight.

The tuning procedure for the IsoLoop is as follows.

  1. Set your radio to the frequency on which you want to transmit.
  2. Set the power control to about 10 watts.  You will use this as the tuning signal.
  3. Turn up the volume on the radio and set the stepper motor speed to fast.  Run the stepper motor to tune the antenna for max noise on the receiver.  When you have max signal on your radio you are close to resonance.
  4. Turn the speed control on the stepper motor down.
  5. Key the transmitter and teak the stepper motor (forward/reverse) to fine tune the lowest SWR.  You should be able to get a 1.5 or less SWR across 14 MHz to 29 MHz.  You may need to adjust the coupling loop for band preferences – see below.  You do not need (and should not use) an antenna tuner with this antenna.
  6. You are done.

How long does this take?  Once you get the hang of this it will take you about 20 seconds.  That’s about how long it would take to tune a tube radio – if you can remember what that is like.

Once you have the IsoLoop tuned – you are all set.  QSY? – too bad for you.  You have to do it all over again.  It can be tedious to tune and re/tune the IsoLoop if you are all over the bands.  The only solution is to spend about $100 and get a controller that will show you digitially the position of the stepper motor.  It’s the same challenge and solution with any tunable single resonance antenna such as screwdriver antennas.

My opinion, is that the ability of the IsoLoop to tune continuously 14 MHz to 30 MHz in a compact size antenna out weights the effort to run the Stepper motor to find the resonance point.

A closer look at the IsoLoop

You can take a look at all the parts of the loop in the picture above (click on any image to enlarge)

  1. The loop itself which is made out of aluminum tubing.
  2. The variable air-dielectric capacitor in series with the loop
  3. A stepper motor and pulley to change the position of the blades of the air-dielectric capacitor which changes the capacitance.  This variable capacitance in series with the loop inductance determines the resonance frequency of the antenna.
  4. A control cable that goes from the stepper motor to the control box.  The control box is at your operating position; the control cable is 50 ft in lenght.
  5. The control box which controls the direction and speed of the stepper motor through the control cable.

Stepper Motor and Air-Dielectric Capacitor

This is the business end of the antenna showing the variable air-dielectric capacitor, drive mechanism and the stepper motor.  The picture below shows the antenna with half of the protective (black) shield removed.  Since the IsoLoop can be mounted outside this black shield protects the critical parts from the environment.

isoloop_aeadrive
 

Feed Point

isoloop_aeafeed

At the other end of the loop is the feed point.  The main loop (aluminum tubing) is driven via coupling to the small loop. 

According to AEA, the loop gets it’s name (IsoLoop) from the fact that the freed line is isolated from the resonant loop thus reducing RF into the operating position.

The coupling loop can be adjusted for lowest SWR.  This loop can be rotated to be in the plane of the main loop, perpendicular to the main loop, or any place in between.

If the coupling loop is in the plane of the main loop then the lowest SWR will be at the top end of the loop design (10 meters).  If the coupling loop is perpendicular to the main loop the lowest SWR will be at the bottom of the loop design ( 20 meters).  AEA recommends the compromise position at a 45 degree angle to the main loop.

Small Loops in General

isoloop_schematicsmallloopThere is plenty of technical information written about small transmitting loops.  There is no value in saying it all over again as other writers have done a much better job than I can do here. 

isoloop_schematicsmallloopequivOne of the best  sites I found on small transmitting loop antennas is by Steve Yates AA5TB.  On this site you will find technical information, formulas, and about 50 links to other sites on the web that contain information on loops and loop construction projects. 

Of note on the AA5TB site is a very useful excel spreadsheet that has all the formulas for small loop design.  This sheet will also work with the free open source software OpenOffice if you do not have Microsoft Excel.

isoloop_aa5tbcalc

The AEA IsoLoop – OPL ( Other Peoples Labor )

If you have not figured it out by now, I am an advocate of OPL – Other Peoples Labor.  That’s how I got my small transmitting loop antenna.  I purchased mine from AEA – at a considerable cost.

AEA  is no longer in business and the IsoLoop is no longer available.  However, there is a MFJ product – the MFJ 1786 10-30 Mhz Loop Antenna at $419.

Your Labor – A (perhaps) fun project

After owning the IsoLoop and doing some research on the Internet on small transmitting loop antennas there is quite a large (and cult-ish – in a good way) community of loop builders.  $419 for the MFJ 1786 is a lot of money in the context of looking at numerous web sites where people have built their own loops – at considerably less cost.

Even if you only have some copper tubing or an old aluminum bicycle rim along with a few junk variable capacitors  - these folks have built transmitting loops with claimed good results.

isoloop_loop_75cm_var_capisoloop_dl2hrg4isoloop_loop1

isoloop_sample_copperisoloop_sample_coppercap

isoloop_hb9abxisoloop_fig3vhfloop

Conclusion

Many people reading this posting have plenty of space to put up a full size antenna for a number of different amateur radio bands – and some don’t.  Even if you have enough space then maybe a full size outdoor antenna would be an eyesore in a particular neighborhood.  If this is the case then you need to make a compromise.

According to the technical documentation on magnetic loops and the folks that have the necessary expertise and equipment to measure antenna performance, the small transmitting loop antenna approaches the performance of a full size dipole on the higher HF bands.  Yet the size of a small loop is much less than a full size dipole even on 10 meters.

hirampercymaximThere was a time in the history of ham radio that amateurs would experiment with radio technology and antennas just to do it – it was part of the hobby – experimentation was something you did simply because it was radio and you were an amateur radio operator and it was interesting to do these things.  The “need” or “justification’ of time or money was not needed.  It was all about the satisfaction of building something that worked – whether you had a demonstrated need or not.  In a sense, the journey was the reward.

I unfortunately took the route of OPL (Other Peoples Labor) and bought the AES Isoloop.  Maybe I missed out on something in not building a loop myself.  But time was tight and I wanted to get on the air in a limited space environment.

There are more sites on the Internet on building small transmitting loop antennas than I expected to find.  So there is a community of people where these small loops have caught on as a fun building and operating experience.  Given that it only takes a chunk of aluminum or copper tubing, an air dielectric capacitor, and a few more parts it seems like an easy and fun building job.  The MFJ loop at $400 seems like a secondary option at best.

So enjoy my hypocrisy – go build a loop and have some fun.  Keep the image of Amateurs as experimenters and innovators alive.  Using OPL I think I lost my opportunity.

Resources

Schematic for the stepping motor controller for the AES Isoloop and radiation patterns
http://frrl.files.wordpress.com/2009/03/aea_isoloop_radiationpatterns.jpg
http://frrl.files.wordpress.com/2009/03/aea_isoloop_schematic.jpg

Related Article – Life on HF – The MFJ-1796 6-Band HF Antenna for Limited Space

Highly technical presentation on small antennas
All sorts of small antennas – they are better than you think – heuristics shows why!
By Professor Mike Underhill – G3LHZ

Short video demo of loop polarization and directionality

The journey is the reward – An excellent adventure in loop building

Similar to my AEA IsoLoop – The MFJ 1786 Loop Antenna Theory of Operation

One of the best sites I found on small transmitting loop antennas is http://www.aa5tb.com/loop.html
This is a PDF version of AA5TB small loop page in case the page goes away for some reason.

Here is a collection of links from the AA5TB page (the PDF does not have live links).

isoloop_myattic

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Written by frrl

March 21, 2009 at 5:56 am

17 Responses

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  1. Hi, nice article.
    can i have the excel calculator file ?
    thanks. :)

    Faizul

    April 13, 2011 at 12:30 pm

    • I got that picture from this site -
      http://www.aa5tb.com/loop.html

      and at the time I wrote the article the excel file was on the aa5tb site. I looked and it seems it’s gone. WordPress does not allow me to post xls files or I would have grabbed it and posted it when I wrote the article.

      So I don’t have it. You might want to contact Steve Yates – http://www.aa5tb.com/index.html

      frrl

      April 13, 2011 at 4:46 pm

  2. Hi, nice article.
    can i have the excel calculator file ?
    thanks.

    Faizul

    April 13, 2011 at 12:29 pm

  3. Do you have a schematic for the LC-1 controller? I just got one of these loops and would like to get it going again. It is the older squarish style with the yellow cross bar like yours.
    Thanks.
    Andrew

    VE1FX

    November 17, 2010 at 7:29 pm

    • Let me take a look over this weekend – I think I have the schematic. Will let you know – will post the schematic as part of this posting.

      frrl

      November 18, 2010 at 6:41 am

      • I added the AES Isoloop radiation patterns and schematic for the controller to this posting

        frrl

        November 23, 2010 at 6:23 am

  4. Hi All,

    I am an apartment ham too. With my loop, I have been on JT-65A lately. I have Japan, Germany, and lots of places inbetween in the log book. Last night I just missed Sakalin Is. and the Kamchatka Peninsula (who indicated he copied my TX, but apparently he was off to breakfast and then the band failed). This is all with 25 watts RF. Now if the Power Line noise will get low again, I might have a chance at some DX tonite!

    WB9MJN

    June 6, 2010 at 8:51 pm

  5. And, it keeps getting better and better….

    Using an MFJ-1786 Magnetic Loop antenna in my apartment, I’ve recently begun using PSK-31 / 63. With only 20-25 watts, I just worked Russia, the Netherlands, Guatemala, and Germany. I’ve also recently discovered some local QRN that kicks up from time to time that almost obliterates the 20M digital sub band. No problem; just turn the loop a few degrees one way or the other and the noise is nulled out.

    I think it’s time to begin homebrewing a Mag Loop to take backpacking!

    73′s,
    Pete

    AA8GK

    July 20, 2009 at 4:59 pm

  6. Hi All,

    I have finished the Bell-Crank Butterfly capacitor turner on my 1 1/4 meter diameter magnetic loop, and its working great. Listening to Radio New Zealand International (RNZI) as I type this on 13.730 MHz DRM (Digital Radio Mondial) through a modified Drake TR-7 – S 9 solid, 100% decode! The careful adjustment of the capacitor is good for an S unit or two of extra signal over adjusting it by ear from the other room.

    This was allot of work, including machining the plastic crank, motion nut, mount end and motion screw (3/8 inch diameter brass, about 8 inch threaded 24 tpi length, and integral 1/4 inch hex end for drive from a electric screw driver motor). Crank and Mount were made from home recycled black plastic from pop bottles, that was melted down in a bundt pan, and milled to rectangular blocks and then bored for the shafts. The butterfly cap needs only 90 degree of motion, and with approximately 2 1/2 inch crank center to center distance the 250 RPM cheap electric screwdriver is reduced so that it takes about 45 seconds to make the 90 degree travel. The crank and motor are mounted away from the capacitor end of the loop, with about a 1 1/2 meter long non-conducitve drive shaft. This avoids running the power wires within the loop, and avoids coupling to these power wires.

    The motor and screw are held in place with a combination steel pipe and PVC pipe right angle mount. A PVC Tee is used. The motor and screw are held in line by the top of the “T” and the leg of the “T” has a threaded adapter which spins the on a pipe nipple. A PVC adapter was borred for a tight fit to 3/8 inch ball bearings, and the adapter fit into one end of the Tee.

    The 3.6 volt screwdriver is powered with a 5 volt supply through about 35 feet of 18 ga power cord.

    WB9MJN

    July 3, 2009 at 3:55 am

  7. Magnetic loop antennas rock! I live in an apartment in SE Michigan. The other night I worked the U.S. Virgin Islands with an MFJ-1786 and five watts from an FT-817. The antenna is inside my second story apartment. I am really fascinated by these radiators.

    73,
    Pete AA8GK

    AA8GK

    June 22, 2009 at 12:20 pm

    • I think a bell-crank design is very good for a Butterfly cap. The Butterfly cap only requires a 90 degree turn, and the bell-crank with a motor driven nut at its tip can provide a large multiplication. Say if you do 90 degrees in 2 1/2 inch movement of the crank, and use a 24 turn per inch screw to run the crank, then you have 120 turns. On my homebrew antenna, 7-15 MHz, that is .067 MHz per turn. With a 200 step/revolution, that is a .3 KHz per step.

      The disadvantage of the bell-crank is that its not colinear with the tuning shaft, and requires some careful fabrication to get the screw in line withthe nut.

      WB9MJN

      June 29, 2009 at 5:43 pm

  8. Great blog!!! Using the MFJ-1786 myself (and being an apartment ham) I especially like all of the info on small transmitting antennas. Keep up the great work!

    AA8GK

    June 11, 2009 at 11:08 am

  9. I have searched about the word “stepper motor” and you blog appeared to me.
    where can I read about stepper motor in details in your blog?!

    microcontroller

    March 30, 2009 at 5:05 pm

    • The real answer to your question is that, Yes, sometimes we cross-promote other sites.

      You can read about stepper motors here -
      http://en.wikipedia.org/wiki/Stepper_motor

      And as a practical word of advice on loop tuning… there is a reason why a stepper motor is used in the AEA IsoLoop Design.

      The tuning on this antenna is so critical that 1 step on the stepper motor on 20 meters is going to make the difference between 1.1, 1.5, and 2.0 SWR.

      I ran the stepper motor with the antenna shield off and 1 step on the motor is almost visually imperceptible.

      What this means is that if you build one of these loop antennas and try to hand tune it you may have a very difficult time – unless you design in some sort of reduction gearing (take a look at antique radios for some ideas on reduction gearing)

      What the stepper motor adds is precision to small changes on the capacitor – which perhaps you can not reproduce manually. Depending on the design for your loop – this could end up being a critical issue when it comes to tuning the antenna to resonance on a particular band.

      If you have experience with “hand tuning” a loop – let us know what you discovered.

      frrl

      March 31, 2009 at 3:08 am

  10. Very good information on loops dear friend. Keep up the good work. God bless.

    Sai, VU2SGW

    March 23, 2009 at 6:55 am

  11. [...] You Don’t Say placed an observative post today on Limited Space Antennas – The Small Transmitting Loop AntennaHere’s a quick excerptEven if you only have some copper tubing or an old aluminum bicycle rim along with a few junk variable capacitors  - these folks have built… [...]

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