A revolutionary new aerial, that will double the range of your wireless mike system, recently caught my eye.  

Alas, there is nothing new about the helical aerial.

Check out this stunning 1958 NASA photo of helical aerials.

First published in the late forties by radio ham John Kraus W8JK, the helical is a specialized aerial that works best in matched pairs.  The helical is not the solution to the world’s communication inadequacies.  In our industry, they are most likely to appeal to the same types who think that adding masthead preamps is the 'silver bullet' for every wireless mike challenge.

The main feature of the helical is that it employs circular polarization.  In space, fixed polarization is a very difficult animal to tame.  When Apollo 13 went into severe rotation, due to a venting oxygen tank, the steerable high-gain aerials were rendered useless and comms were dependant on the omni-directional aerials (omnis). 

In space, not only can nobody hear you scream, they can’t hear you at all without radio communication.  Massive helical arrays (as used by NASA) on the ground can be a real lifesaver in these situations.  If you are planning to launch the wife and/or kids into orbit, buy up big on them by all means. 

Circular polarization is not new; you benefit from it every day as you listen to your favorite FM station in your car or home.  The vast majority of commercial FM broadcasters employ either circular or mixed polarization.  The reason being that receiver aerial polarization (vertical or horizontal and everything in between) is mostly indeterminable.

From the radio alarm clock beside your bed or the transistor on the beach, the only time you can be reasonably sure of receive polarization is in the case of the automobile.  The ubiquitous telescopic aerial is mostly vertical (or near enough to).   As car listeners comprise a very sizeable slice of total audience, especially during the lucrative breakfast and ‘drive’ shifts, vertical polarization was once the preferred option.

When FM broadcasters first experimented with circular polarization, they quickly arrived at two broad conclusions.

 1.   The maximum range of the station was NOT improved.

This was not surprising as the original power, say 50 kilowatts, was now spread over 360 degrees and the signal arriving at any discrete polarization is therefore lessened.

 2.   The reliability of reception, within the prime coverage area, WAS improved on most receivers, other than in automobiles.

This was also unsurprising as domestic aerials are usually only vertical if they are the telescopic type. 

 By transmitting signal in circular polarization, you improve your chances of hitting the target.  It is analogous to shooting a rabbit with a shotgun.  Not an exercise requiring great finesse, but effective all the same.

That helical aerials work well in an environment where polarity is variable is certainly true.  No argument there. 

That gain comes with a trade-off, narrow beam-width.  This is true for many aerials, Yagis, corner reflectors, phased arrays, etc. One notable (apparent) exception is the vertically polarized collinear.  The collinear (aka co-linear) looks like a slim vertical pole with no protrusions.  They also have narrow beam-width but theirs is in the E-Plane, i.e. the signal radiates narrow and horizontal to the ground. Virtually none of the signal goes up or down. This is usually not a disadvantage unless you are trying to receive skydivers, or communicate with The Matrix   

Why haven’t we been using the helical for years?  First let us examine the claim that they work ‘better’ and give “twice the range”:

They work better than what, a coat hanger? 

Twice the range of what, a Sears crescent wrench perhaps?

When manufacturers publish specifications, we need to interpret them against standard references.  Aerial gain is universally expressed in either dBd or dBi.  dBd gain is referenced to a dipole aerial, hence the lower case ‘d’ suffix.  dBi gain is referenced against an isotropic Radiator, hence the ‘i’ suffix.

What the ‘beep’ is an isotropic radiator? 

Before you rush out to your local Tandy store… the isotropic radiator is a theoretical aerial that would transmit, or receive, in all directions equally.  If they did sell them at Tandy, believe me, you wouldn’t want one.

As a general rule, a dipole has 2.15dB gain over an isotropic radiator.  An aerial that exhibits 3dB gain over a dipole, i.e. 3dBd, could also be expressed as having 5.15dBi gain.

Some marketers like to express gain in dBi as it makes their numbers look better.  Using the dBd reference makes more practical sense as you can readily buy and compare a dipole. 

If the Helical Aerial has a gain of 12dBd, surely that must make a big improvement? Perhaps, but an improvement over what?  There is a plethora of aerials on the market which offer 12dBd gain. 

The vertical collinear for example.  Unlike the helical, the gain of the collinear is omni directional.  Maybe that could be an advantage if you get the same gain over all points of the compass?  They are also much cheaper.

Long Yagis will readily get you 12dBd gain. Alike the helical, Yagis are also directional – but much cheaper.  Even better, what about two Yagis crossed at 90 degrees?  Tilt one 45 degrees left of vertical, tilt the other 45 degrees right of vertical and you now have 90 degrees difference in polarization with neither aerial being more than 45 degrees away from either the vertical or horizontal plane. Wonderful! Heaven on a stick

Compare that configuration against your helical and then tell me, with your hand on a bible, there was any difference in drop-out free range.

Choosing the ‘right’ aerial is more than a simple technical choice.  It’s a value judgment based on what bang you expect for your buck.   

Q Do helical aerials reduce drop-outs in wireless mike

    applications?  Yes.

Q Can the same improvement be achieved using other

    aerial configurations?   Yes.

Q Is the helical the most price-effective solution?  No.

Q Is the helical more robust and compact to ship.  No.

Q Being a broadband aerial, does that make the helical more susceptible to ‘out of band’ interference from cell phones, walkie-talkies, etc?  Yes.  

The main claim for the helical is that it works for signals of all polarizations.  Sure, but no wireless mike on the market transmits a helically polarized signal.  If the TX aerial is oriented vertically, most of the received signal polarization will be vertical.  Conversely, if the TX aerial is oriented horizontally, most of the received signal will be horizontal.  Because of reflections from surrounding surfaces, the received signal may be a multi-path mix of vertical and horizontal.  Due to phase cancellations, it may constantly vary across many polarizations.   

Helical aerials work best when both the TX and RX aerials are both helical.  Interestingly, if you reverse sense the link by making one end a left-hand helix and the other a right-hand helix (why would you?), that combination works woefully bad. 

If you wanted to send signal from A to B, over extreme distance, you could use a pair of helicals; but why would you?  Fixed links usually have fixed polarization and the simple Yagi would perform the same function at a fraction of the cost.  The only time the helical would excel would be if one aerial was constantly changing polarity.  When would that happen?  You could kit your talent up with a helical but they're damn hard to keep out of shot.  The DoP would go ballistic! 

While the helical copes with polarity variations well, so do crossed Yagis, i.e. two simple Yagis inclined at plus and minus 45 degrees from the vertical.  There are many other aerial options that can be configured for variable polarization.

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