Ferrite Cores

The basic idea with ferrites is that they form a lossy 'RF Choke' disconnecting the interfering signals from the 'antennas' (leads) connected to them, and disrupting RF ground-loops. Assuming the core material is appropriate for the frequencies involved, simplistically the bigger the core and the more turns the lead is wrapped through it, the better this choking will be; a single split-bead clamp type over a cable isn't going to help much at all. Typically for low-frequency and short-wave kind-of frequencies big cores with multiple turns and/or multiple cores and as part of a grounding/choking strategy are required to have meaningful effect. Again, not recommended for starters, but for after basic major stuff is dealt with.

Warning: At this stage in the game ferrites will probably not be terribly effective. They are excellent, indeed can be a game-changer where the last few droplets of EMI are being wrung out of a system, but until the big ugly gross noise offenders have been identified and dealt with and a decent antenna decently fed remote from them is in place and working well, their use can be inconclusive and relatively expensive for the results. It generally takes a lot more ferrite than one would think to make a real difference and in many more places than one would think; it is not a 'one-shot-deal' where putting one clamp-core on something causes the heavens to open and angels to sing.

This is particularly true at low frequencies, where the inductance required to form any meaningful choke demands BIG cores with LOTS (but the right number) of turns; that single clamp-core made of VHF-relevant core material such as 43 (the usual) will do worse than nothing. They're not cheap.

It's easy to get misled by seeing the little ferrite lumps on keyboard leads, video feeders etc. into thinking that's all that is necessary or involved. They are there solving the manufacturer's entirely different problem, which is to reduce the generated interference by a few dB up in the VHF region just enough so that it'll pass FCC Part 15 testing. It is emphatically not there to eradicate huge swaths of energy in the LF/MF region, which is what WE have to try to do. Although the techniques are broadly similar, the scale of the operation is very different.

Jim Brown, W9YC, has done the definitive analysis of ferrites in this sort of application.

It's worth repeating: Don't start down the ferrite route until the nasty noise sources are nailed, and you have a decent well-fed remote antenna.

Towards Quieter Receive Antennas

Almost any antenna is going to give better results than a bit of wire stuck in the back of the radio. There are two discrete reasons:

So what?

Well it means that you have a live antenna within inches, certainly within a couple of feet, of likely all sorts of EMI noise-makers directly (e.g. power supplies) and certainly and particularly the AC distribution, which carries and re-radiates all the noise made by countless EMI noise generators within not only the room, and your house, but your neighbours' houses, and just maybe the whole street. How can it NOT hear all those noises? How can it be reasonably expected to be quiet?

There is a further mechanism, too, which is less intuitive but actually more important: It springs from the basic manner in which antennas work and receivers extract energy from them. The bit of wire poked into the antenna socket is only HALF THE ACTUAL ANTENNA. It can only work by the current from it passing through the input impedance of the receiver's front-end so developing the voltage which the receiver 'hears', thence out into a complementary 'ground', in this case the chassis of the radio. Now: Consider what is connected to that receiver chassis - the power supply, which is thence coupled more-or-less directly to the AC distribution system.

Yes, Virginia. A full HALF of your effective antenna is indeed the AC power distribution, home to all the noises of all the EMI-generating crap on this side of the known galaxy. How can the receiver NOT hear all those noises? How can it be reasonably expected to be quiet?

It is thoroughly plain that having the AC power distributon system as part of the antenna is a really bad idea. It is also a given that placing the active part of an antenna system as far way from it as possible is a good idea. So what are good, simple ways of achieving this?

Feedline Basics

Althouh there are plenty of different feedline types and variants, by far and away the most common and useful nowadays is coaxial cable, or coax. This consists of an inner wire totally surrounded by a shield which screens the inner wire from the outside. It is very common; TV antenna / Cable TV feeder is a household example. For receive-type activities there is no need to look any further than commonly available TV/Cable coax: it is readily available, thin, light, inexpensive and actually better for the purpose than thicker 'proper' radio-type coax such as RG-8, being less lossy. There are two general type numbers - RG-59 and RG-6 - the latter being preferable since its screening is superior. The rated impedance of 75 ohm is an irrelevance - 'matching' antenna-to-cable-to-radio impedances (although of import with transmitting systems) has very low impact on receiving.

It is designed to be terminated in 'F'-style connectors, being easy to crimp onto the ends; it is generally wise to use these connectors rather than attempt to solder on other kinds - often the cable material is aluminium which does not solder well, or at all. This may seem to present a problem if your receiver has a dfferent connector, say the usual SO-239 screw type. Adaptors are commonly available (Radio Shack do or at least used to carry PL-259-to-F adaptors), or one can get artsy making convertor cablets. An alternative is to use 'Euro-style' screw-block connectors (RS again) on the ends of the cable, which makes adapting to a PL-259 jumper for the radio end, or wiring directly out to the antenna at the other end easy. Sealing the exposed coax ends with silicone goop or hot-melt glue is a very good idea.

Phase 1 - the easiest remote antenna

Not recommended, but I know that because it's so easy you'll go ahead and do this anyway!

This is to hang your antenna wire up outside as long and as high as you can, then feed one end of it with coax, connecting the wire JUST to the inner wire of the coax. The coax plugs into the back of the radio.

"Doesn't this fix all the problems?" you may ask. "It's up high, outside, and away from all the buzz-makers. It should be clean, right?"

Well, some. It improves the antenna wire as a reaper of radio signal (this alone should improve the signal-to-noise ratio), and this is a plus. But what the addtion of the coax has done is merely extend where the antenna input of the receiver is; the outside of the coax is the antenna system's 'ground', which runs back to the receiver, which is still connected by the power supply to the AC power system.

Half the antenna is STILL the AC power system, with all the gark and spit on it that entails.

However, two positive moves; as remarked the antenna wire is hearing better, and it itself is no longer draped over or around noise-makers in the radio room. There will be an improvement in signal-to-noise.

Phase 2 - REALLY starting to move the antenna outside

So, how do we stop the feedline-borne interference from entering the antenna system? A good first step is to give it somewhere easier to go to. Ground (real ground) can work well.

A nice high wire in the air has one end brought down to a convenient patch of dirt. Into this dirt is stuck/hammered/driven the longest piece of 'ground rod' one can muster. Farm stores, Lowe's etc sell 4' and 8' ground rods but as rule they're hopeless, being merely copper-clad steel which degrades in a depressingly short time. I find thick-walled household copper plumbing tubing to be best, 1/2" or 3/4", doesn't matter. It can be knocked in quite easily, and if it gets stuck (hits a rock, say) can be easily sawn off, the remainder being driven in a couple of feet away - the RF grounding effect of a number of shorter rods linked together is as good as one big long one.

Connect the shield (outer) of your feedline to the earth-ground ground rod(s); the inner wire to your skywire.

The antenna wire itself is still up and in the open, and you may have been able to extend it somewhat, bringing the tail down to the grounding spot. Good, it'll hear well. A plus. Yes, AC system noise is still on the coax outer BUT a significant amount of it will be shunted by the relatively low impedance provided for it by the (literal) earth-ground connection at the antenna end. The receiver's 'virtual' input is still at the coax end, but since at least some of the EMI has been diverted into your shiny new earth-ground system, it should be a bit less troublesome. There should be a significant improvement in signal-to-noise.

That said, there is still a direct connection between AC line noise / coax outer / receiver input, and some cross-injection (technically 'common-mode to differential-mode transformation') is inevitable.

SAFETY ALERT... From the above instructions, the feedline screen is connected directly to earth ground. This in itself is not bad, BUT if the other end of the feedline in the shack is connected to mains AC power ground (e.g. via the receiver itself, or its power supply is connected to AC ground) then the feedline screen and your outside ground have just become part of the mains AC safety grounding scheme, and this is mighty bad ju-ju. Firstly, and most benignly, a 'ground loop' has been formed which may well worsen your noise reception; Secondly and way more important is that if a major electrical fault were to occur in your household, your feedline becomes part of the safety ground and may be expected to deal with a proportion of hundreds of amps fault current. Or, if the house's AC ground is coincidentally faulty, ALL OF IT. For this reason, most nations' electrical codes forbid multiple or incidental grounding points, such as the one you might have just made.

*** Just be sure that if you are connecting the feeder's screen to earth-ground that there is no direct connection at the receiver end to safety ground. ***

Thank you for your attention - Yours, Captain Sensible.

Phase 3 - Transformer Isolation, your new best friend

Here we introduce the transformer. In this kind of application it is often mistakenly called a 'Balun' (for Balanced-Unbalanced), even though we're not specifically balancing or unbalancing anything. For receive-only applications RF transformers can be very small, even down to about 1/4" cube, but are often small toroidal cores say 1/2" - 1" diameter with a couple of windings on them, one for the feedline side, one for the antenna side. Often they are 1:1 in ratio (same number of turns on each winding) but different ratios can be better suited to the job. In particular for this application 1:9 or 1:16 impedance ratio is better (1:3 and 1:4 turns ratio respectively. With a simple receive-only broadband antenna design such as this, the ratio is delightfully non-critical - it will work regardless.

A minor stumbling-block is availablility - such small RF transformers are not an off-the-shelf item at Radio Shack or such. Most hobbyists build them, acquiring the raw cores and hand-wiring the turns. There are some industrial types available, but they are subject to (fairly modest) minimum order values and quantities. Some options (of many):

FT50-J 0.5" cores or FT50-77 0.5" cores. The former are better for lower frequencies, but either will get you going. If you think these are going to be fiddly to work with, get larger diameter cores, but they don't NEED to be bigger to work any better. For starters, assuming 0.5" cores, try 5 turns per winding for a 1:1, 5 turns and 15 turns for a 1:9 ratio transformer. For LF (100kHz region) try double the number of turns.

Although it seems strange at the outset making your own 'components', winding toroids for specific applications becomes slightly addicting... order plenty, you'll use them! Wire-wrap wire, or enamelled magnet wire (both available from RS) is suitable for the windings themselves.

Oh, yes: Avoid like the plague surplus, unmarked mystery-meat cores, however tempting, however cheap. It is vital to know what the core material is to determine the needed number of turns, and surplus cores are invariably ever-so-slightly-exactly completely wrong.

They do an utterly bewildering variety of transformers, but ones I've successfully used are:
T1-1T (1:1 with centre-tapped secondary)
T16-6T (1:4 with centre-tapped secondary)
This latter one is versatile, offering impedance ratios of 1:4 and 1:16. Get a handful of each: the T16-6T is most useful for antennas, the 1:1's being good for general-purpose isolation (see later). Tiny and as delicate as they may seem, despite using dozens I've never lost any to lightning except for a direct hit, where I couldn't even find it any more!

This is to put windings on the two parts of a clip-together anti-EMI core; this can sort-of work, but be aware that the core-material used is designed to DISSIPATE not transform RF energy, and at frequencies which may not be directly of interest; as such they might not be terribly efficient. Try 10 turns per winding as a start for a 1:1, or 10 on one side, 30 on the other for a 1:9 ratio; be prepared to greatly alter the number of turns depending on how it turns out and what the specific core actually affords.

Any of these options will need 'beefing up' for the real world; little plastic boxes with any of: pig-tails coming out, 'proper' connectors, or simply wired out to 'Euro' screw connectors are all viable. Liberally seal with silicone or hot-melt.

Ah, sorry. Back to the Phase 3 antenna

Connect the antenna skywire to one end of the higher-impedance winding (more turns) of the transformer, the other end of that winding to the earth-ground. Connect the coax across the lower-impedance winding. DO NOT connect the coax screen to the earth ground. Read that last sentence again. The coax should be just connected across the lower-turns side of the transformer, nowhere else.

We are relying on the 'common-mode rejection ratio' (CMRR) of the transformer to decouple the feedline from the antenna; noise on the feedline has a hard time leaking over to the 'pure' antenna side and is radically reduced. It isn't (can't ever be) perfect isolation, but is very significant. It will make a substantial improvement to the signal-to-noise performance.

At this point on can proudly state that for the first time you have an antenna, and you have a feedline, and can treat and regard them separately.

Phase 4 - If one transformer's so good, how about two?

This involves putting a 1:1 isolation transformer (same number of turns on each winding, or a 1:1 Minicircuits) at the receiver input. Build a little plastic box with the transformer in it and your connection method of choice, and insert it in the antenna feedline close to the receiver - within a couple of feet is fine.

What this does is help prevent all that AC power-line gark from getting onto the feedline. This mitigates another mechanism of this noise finding its way into the antenna, which is radiation from the feedline close to the antenna itself being 'heard' by the antenna; if there's less noise on the feedline, there's less of it to radiate and less to be heard.

This does leave the coax effectively 'floating', and it is in fact working as a balanced transmission-line, if it isn't too much of a head-hurter to imagine an obviously physically unbalanced construction behaving as electrically symmetrical. Don't worry, it does, and will do so just fine.

Phase 5 - Oh, no. Not more grounds...

Often additionally useful - once transformer isolation is in place at both ends, and not before - is to find a convenient third place to ground the feedline along its travels. NOT to the AC ground, and NOT to the antenna's earth-ground. The best place typically seems to be at the threshold, i.e. where the feedline leaves the house and first reaches dirt, say below the window, just outside the garage door, or wherever it exits. Create another entirely separate earth-ground right there, and connect the outer of the coax to it.

There is considerable latitude as to what, where and how this feedline ground-point can be and be implemented, just that it must be a THIRD and entirely separate ground from the AC power ground and the antenna's earth-ground. My friend Lawrence, who travels the world and expends extraordinary effort to play radio from high-rise hotel rooms and apartments, has reported surprising and consistent luck with feedline-grounding to the building's metal framework and structure, by various means such as an aluminium slidey-door frame and such.

Experiment. Just remember the rule: NOT to the antenna's earth-ground and NOT the AC power ground.

What this additional technique affords is 'potentiometric attenuation' of the AC power-line borne noises. What still makes it across the receiver-end isolation transformer (there will be some) added to that which the feedline acting as an antenna 'hears' from being in close proximity to noise-sources directly or power lines along its travels, is being 'offered the opportunity' to at least be partially diverted to ground before it can re-radiate into the antenna, or leak through its isolation transformer.

To Recap:

Taking stock: The antenna is its own subsystem, it is transformer decoupled from the feedline, which itself is isolated being also transformer decoupled from the receiver. The feedline may also be additionally earth-grounded en route. This is good, solid, belt-and-braces EMI-mitigation engineering.

It is pretty much as good as it gets: There is now every liklihood that any EMI-type interference being heard is ACTUALLY BEING HEARD BY THE ANTENNA ITSELF and not by some fuzzy, mungey, half-assed combination of wires and feedline masquerading as one.

It is actually pretty easy to confirm what is being heard by the antenna, assuming the ownership of a battery-powered receiver and a sunny day: Take the portable radio out to the antenna feedpoint, disconnect the feedline to the indoor receiver, connect the portable to the antenna's transformer feedpoint, and listen. If the noise is still there, then guess what? It's being heard by the antenna itself as a 'genuine' signal; obviously the feedline system is blameless.

The only options at this point are to invesigate moving the antenna further away from buildings/powerlines, or investigate different kinds of antennas which may be less subject to EMI reception, or have intrinsic directional capabilities that may help null it out. Whatever - any remote antenna will require a feedline treated as described here in order to be optimally quiet, whether it be this simple longwire, a loop variant, or an active antenna. Especially active antennas. The principles and basic techniques are exactly the same.

But all that, and more in-depth usage of ferrites, is the subject of many books, websites and glossy brochures; maybe eventually a 'Part 2' to this. I'm just glad for now to have dragged you along the path of how to engineer a feedline that acts as a feedline and not as an antenna.

© Steve Dove, W3EEE, 2003,4