Limited Space Antennas – The Small Transmitting Loop Antenna
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
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.
- Set your radio to the frequency on which you want to transmit.
- Set the power control to about 10 watts. You will use this as the tuning signal.
- 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.
- Turn the speed control on the stepper motor down.
- 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.
- 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)
- The loop itself which is made out of aluminum tubing.
- The variable air-dielectric capacitor in series with the loop
- 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.
- 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.
- 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.
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
One 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.
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.
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.
There 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.
Schematic for the stepping motor controller for the AES Isoloop and radiation patterns
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
The journey is the reward – An excellent adventure in loop building
Similar to my AEA IsoLoop – The MFJ 1786 Loop Antenna Theory of Operation
Here is a collection of links from the AA5TB page (the PDF does not have live links).
- Magnetic Loop Antennas - by PY1AHD (a superb loop site!)
- Stealth ST-940B Mobile HF NVIS Magnetic Loop Antenna- by Stealth Telecom
- The Loop-Forum - English and German Forum
- The Loop Antenna - The ARRL
- Antenna Research - Click on Miniloops
- HF LOOP AND HALF-LOOP ANTENNAS in a FEW WORDS - by STAREC
- PA3CQR Magnetic loop antenna page - by PA3CQR
- 80m Frame Antenna - by SM0VPO
- Packing Crate Antenna - by SM0VPO
- REMOTE ANTENNA TUNER (for loops) - by SM0VPO
- Magnetic Loop Antennas - by ON4CEQ
- A Magnetic Loop Antenna - by GM3MXN (via G3YCC)
- CT1ETT’s Home-brew Loop Picture - (via G3YCC)
- THE ROCKLOOP - by W9SCH (via G3YCC)
- A magnetic loop antenna for HF - by Peter Parker VK3YE (ex. VK1PK)
- An Unusual Two Band Magnetic Loop Antenna - by Jindra Vavruska, OK1FOU
- The G3BGR Magnetic Loop - by G3BGR
- Practical Experiments with Magnetic Loop Antennas - by David Reid PA3HBB / G0BZF
- Magnetic Loop Antennas and Other Indoor Antennas - by Radio Habana Cuba: Dxers Unlimited Special Edition
- GW0TQM’s Magnetic Loop Site - by GW0TQM
- Magnetic Loop Antenna References - by Chris Trask
- DJ3TZ’s Small Tuned Loop Antenna Page - by DJ3TZ
- Magnetische Antennen – Magnetic Antennas, by DH4FAW (was by DK5CZ, now SK)
- DL6KBG`s Magnetic-Loop Page - by DL6KBG
- Meine Magnetic-Loop mit zwei Windungen für 80 und 40 Meter - by DL7AWL
- Magnetic Loop-Antennas - WiMo Antenna Ltd.
- HF Loops and Half-loops - The World of Chelton Antennas
- Magnetic Loop Antenna - A Magnetic Loop by 7N3WVM
- My magnetic loop antenna - A Cool Apartment Loop by KR1ST
- LowCost Magnetic Loop Antenna - by Oliver A. Durm, DL3SDW
- A Portable Magnetic Loop Antenna - by G4FON
- Magnetic loop antenna - by HB9ABX
- ML-90 Vehicle Roof Rack Magnetic Loop Antennas - Q-MAC Electronics
- W6OAV’S LOOP - Looks like a box fan.
- Small loop antennas (magnetic loops) - by G4HJW
- VHF Antenna in a Lunchbox - The Magnetic Loop on Two Metres, by Lloyd Butler VK5BR
- A Small Transmitting Loop Antenna for 14MHz and 21MHz - by Lloyd Butler VK5BR
- SMALL LOOP ANTENNAS -
- The New Magnetic Loop Antenna… - by VE3GK
- Magnetic Loop - Plans, by Piotr Balcerzak
- Magnetic Loop - My Projects, by EA5XQ
- Magnetische LOOP Antennen - by HB9CRU (BIG capacitor!)
- Magnetische Antennen – by DL7JV
- Adventures in Stealth Radio - by Art Heft
- Magnetic Loop Antenna - LA6NCA
- Portable HF Transmitting Loop Antenna - by N5IZU
- Experiences with Loop antennas - by G3YMC
- Antenna Projects and More… - by HB9MTN
- Loop Antenna - by I3VHF
- I3VHF – Baby Loop Antenna - A small loop antenna on Charlie Ho’s, VR2XMT, blog.
- French Site - by F5NGZ
- PE2FOX - Some good magnetic loop photos.
- HF Magnetic Loop Antennas - by KI6GD