OBSERVATIONS FROM SCOTLAND 28 May 2008 GM1SXX
YASTL!
Yet Another Small Transmitting Loop.
Well I had to do it. It's a wee while since I last built a Small Transmitting Loop ... S.T.L or as is more commonly known, magloop. I think I prefer STL. It's an altogether better description. I believe the term mag-loop is a misnomer, because the magnetic component ONLY predominates in the near-field (where it doesn't matter a hoot anyway, on transmit). It's only the far field that really matters!
I wanted one that would be usable on several bands while having a low visual impact. (Yes, and I too believe in flying pigs... LA2QAA!). This one was designed using the excellent Pascal software from G4FGQ. I've found it in the past to be both reliable and repeatable. Use RJELOOP2.exe from http://www.observations.biz/G4FGQ_Archive/page3.html#S301 to model rectangular loops. Do it for yourself. I won't insult your intelligence by including a worked example.
This particular loop is in the form of a rectangle of size 1.5M wide by 1M high. As HF aerials go, that's small. Sandra, on the other hand ,(my 'better half'), thinks it's monstrous! Oh ye of small aerials!
Back to business! The loop was constructed from 15MM copper water pipe then primed with a water based primer for non-ferrous metals. The primed metal was then given a sprayed coat of cellulose car paint followed by two coats of exterior wall paint, of the sort the outside of our house is painted with. This was done for the benefit of the neighbours and it has no special electrical properties. Well, perhaps it does have some interesting properties in that it's helping 'keep the peace'.
The primer and cellulose paint DO have 'special' properties. They prevent corrosion!
It IS very important with small loops not to allow the external surface to corrode or tarnish because ALL the circulating RF current flows in the outer skin of the loop. Any corrosion buildup will lead to a drop in efficiency. Small loops carry very big circulating RF currents. Put some numbers into RJELOOP2.EXE and you'll quickly see what I mean. Ergo, keeping the 'skin' free of corrosion and making the connections have as low a resistance as possible is very important.
You'll notice that I used Compression elbows... don't do this! The solder ring types (Yorkshire fittings) do a better job and are both neater and stronger. I just didn't have any!
The loop structure is supported on a plastic 'former' made from common-or-garden MARLEY brand square PVC downpipe of the type that can be found in most DIY stores. The plastic former is attached to the rear wall of our garden shed using wooden 'stand-off' blocks and the correct MARLEY fixings for the drainpipe. A pair of short self-tapping screws are screwed through the bracket and downpipe to keep the pipe from slipping dowwards. As with my other loops, this one is fed from a sizeable type 41 ferrite toroid with three turns of wire on it. The toroid is housed inside the square water pipe for weather protection and the loop itself is then fed through the toroid to complete the transformer. A dollop of sealant on the end of the co-ax to help prevent water ingress is a good idea!
Here's how the loop was attached to my garden shed.
I siliconed some battens to the inside of the shed into which the fixing screws go.
The trailing bit of co-ax you can see in the photo goes to another aerial. Note the liberal application of silicone sealant where the element enters the plastic pipe.
At the open (top) end of the loop is a DC motor that drives a 3000:1 reduction gearbox (yes, 3000... not 300!) and the output shaft drives a variable capacitor via an flexible insulated coupler. For 20M use, a loop of this size needs a mere 11pf to pull it to resonance while on 80M, that rises to 406pf on the 80M band. I used a 600pf PA tuning capacitor salvaged from a scrapped rig for this loop. If you want to run 100W, then it's best to use a wide-spaced type.
Why is the tuning capacitor at the top of the loop and not at the bottom? Well, for safety reasons. At even modest power levels, there is enough RF voltage present across the capacitor to kill. This is also why an insulated spindle coupler is necessary to tune the capacitor. By comparison, the bottom of the loop is 'cold' in voltage terms. Safe to have closer to the ground!
The motor/gearbox/variable capacitor assembly were built on a strip of perspex sheet that slides neatly inside the Marley downpipe. The capacitor had a couple of heavy wire 'tails' soldered to it which were soldered to the open ends of the loop using a 90W 'electric soldering bolt'. As a stiffener, a piece of fibreglass rod was expoxied between (into) the open ends of the loop leaving a one inch gap. I had pre-tinned the ends of the loop to make soldering the capacitor connections easier. It's best if you can to use soldered 'yorkshire' elbows on the loop rather than compression elbows. they are both neater and stronger. The gap in the loop for the capacitor is a mechanical weak spot, hence my use of a fibreglass stiffener epoxied in place.
A word of advice.... DC brush-fed electric motors are electrically noisy! Use three capacitors in parallel across the motor leads of 10/100/1000pf and thread the power supply cable through a ferrite toroid (split the wire pair into separate single wires and wind several turns onto the toroid as a pair of coils wound in opposite directions to make a common-mode choke). I used a 12V motor and found that it still works (at reduced speed) at far lower voltages. Using lower voltages results in less interference!
The piece of perspex carrying the motor/gearbox/coupler & tuning cap was slid inside the MARLEY piping and attached with plenty of silicone sealant plus a single pop-rivet and plated washer at the top end. The wire tails from the capacitor were then quickly soldered with the large iron and the points at which the copper pipe exit the PVC pipe (both top and bottom runs) were liberally slathered in solicone sealant to help keep water out. A small sheet copper 'cap' was formed to keep the rain out. I just snipped the edges with scissors, bent the edges over and soldered them up to form a rain-hat. This was then epoxied to the open top of the PVC pipe after the loop was tested.
The control cable to the tuning motor and the co-ax to the transformer run inside the pipe and the base of the pipe was deliberately left open to allow air flow and to let out any water that gets in.
Loop PROs.
1 They are quiet compared to most wire aerials or verticals.
2 They don't need an ATU. Just tune and go.
3 Low visual impact. They can even be disguised as everyday objects.... like a Hula-hoop or a picnic table!
4) They are multiband. 4 bands are possible from a modest sized loop.
5) They are positively ideal for the town dweller with limited space.
6) Efficiency on the higher bands can be excellent.
7) Helps eliminate 'pregnant cow' syndrome on HF. (He means 'Awooooooollllaaahhh! LA2QAA!)
Loop Cons
1 They are compromise aerials... but see #6 above.
2 Efficiency can be poor (like lots of other aerials!) on the LF bands.
Noisy Motors.
I discovered after completing the construction of this loop that there IS an advantage to having some noise leakage from the motor..... because the noise is extremely broadband, it assists in quickly tuning the loop on a specific frequency if band conditions are quiet. No more 'pregnant cows on HF with this aerial! Tune for max signal and go. It's taken me 50 odd years, but I HAVE discovered something useful about 'problems' like motor noise (and other sorts! ... LA2QAA). Problems almost always have a positive side, should you be willing to look for it.
Loop controller
Just a box with some batteries that feed either 4.5 or 9V to the motor (fast/slow) via a switch. Another switch (spring loaded centre off rocker) selects the tuning direction. Nothing to it.
Why a loop?
If you live in the middle of town and feel that you can't put up a decent HF aerial, think again. Try a loop. Preferably a DIY one you can make for £30-50 using new parts, and for considerably less if you have a decent junkbox. Should you need help or advice, my email address is below. Just take a note and email me. I don't bite!
And if you have a *serious* question about aerials, I'll pass them to LA2QAA who actually KNOWS what he's talking about! (He does 'real' aerials, and I do STL's)
Dummy loads.
Useful for setting up hybrid radios. ALWAYS a good investment. Build yourself one today!
73 AL.
GM1SXX