Question of the Week

Am I a human TV antenna?

Sun, 1st Nov 2009

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Question

Andrew Hawthorn, Arkansas asked:

Why does my television signal improve when I hold the aerial?

Answer

We posed this question to Phil clark from the Particle Physics Laboratory in Edinburgh...

Phil - Essentially, when you grab the aerial, you're effectively increasing the size of the aerial The 25 metre steerable antenna at the Chilbolton Observatory.by making use of your body's conducting electrolytes. 

A good example of this would be, in a laboratory, if you have an oscilloscope and you look at the pick-up from the [electrical] mains frequency, you can see a 50-Hertz signal, which is the pick-up from the resonance circuits in the room. 

And if you hold the probe in your hand, the amplitude of the pick-up increases dramatically. [Because your body is also picking up the electrical signals from the mains, producing small currents that the oscilloscope can detect.]

The same thing happens, effectively, when you grab the TV aerial: youíre improving the pick-up and itís then using you to resonate within the circuit to produce the signal in the television.  

The TV has to be tuned quite closely to the right frequency, but when you touch the aerial you will improve the signal dramatically.  

The other thing to bear in mind is, when you grab on to the aerial, the connectivity between your fingers and aerial; the tighter you squeeze on the aerial, or if your fingers are wet, the conductivity between you and the aerial will be improved.  So [the picture improving effect] also often depends on how hard you squeeze the aerial.

Diana -   And what about the difference between analog TV signals and digital?

Phil -   The digital signal is a sequence of 'on' and 'off' bits that come down the aerial and effectively it will either be receiving those or it wonít; you may, with the aerial, be able to increase the chances of it picking out 0ís and 1ís correctly, but you have to get it all right for it to work.  This is also why you need quite good signal for digital antennas to work correctly.

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Here's my idea, and it's only a slightly educated guess. I think when you hold the aerial you effectively be come part of it making it larger. The bigger your aerial the better. I've noticed the same effect when you touch the antenna of a radio. mountaineirc1969, Wed, 28th Oct 2009

This depends on many factors. Assuming the TV is operating in the UHF band then, if the signal improves when you touch the aerial, the aerial is not operating optimally. This could be because of a number of combined effects: the aerial is a low-gain aerial which is not very directional, the wrong aerial for the band being received, not functioning properly or in a position which is not conducive to good reception (ideally a UHF aerial should be directional - perhaps some sort of Yagi array to have some gain - and in direct line-of-sight to the transmitter). A properly setup, correct, aerial will usually result in a worse signal if you touch it. graham.d, Wed, 28th Oct 2009

I used to think it was because your body was grounding static electricity that built up in the antennae Madidus_Scientia, Wed, 28th Oct 2009

But what's actually happening when you do touch it? chris, Sat, 31st Oct 2009

When you touch an aerial, your body becomes substantially electrically connected to it - your body effectively becomes part of the aerial. Unsurprisingly, this changes how well it receives signals.

In "free space" (the chimney-stack or a small mast is a good approximation) the radio or TV signal is a nice plane wave, coming from a single source (the transmitter) with few significant reflections (the signal won't depend on position) and it's relatively easy to design an efficient aerial - such as your typical Yagi. With such an optimal aerial, if mounted on the rooftop you should find that if you were to touch it the reception will almost certainly get worse.



Indoors, with TV signals (with a wavelength of about 50cm) the radiofrequency field can be very complicated, with many reflections off of walls and metallic/conducting objects (including people). These reflections will interfere constructively (increasing the signal) or destructively (weakening it) to different extents at every point in the room. (See also "standing waves".) The relative hotspots and deadspots will be of the order of half a wavelength distance apart, ie. ~25cm.
People moving in the room can cause this mess of hotspots and deadspots to move around chaotically. Touching the aerial (changing its effective size, shape, and location) will change the mix of hotspots and deadspots it 'feels', and so may improve reception (but it could equally make it worse).

I would also make the suggestion that if the reception was ok to start with, you probably wouldn't bother touching the aerial (and therefore you've indvertently dismissed the cases where touching the aerial makes things worse).


Each station broadcasts on a different frequency, with a different wavelength, which means each has a unique pattern of hotspots and notspots - which is why you may need to reposition the aerial for different stations.
techmind, Sat, 31st Oct 2009

I know that electromagnetic radiation is "bent" by going through matter.  (For example, it's very difficult to catch a fish in a lake since the light bends in the water since it travels differently than it does in air.)

So maybe as we are big bags of water, the waves are moving through you and are refracting the waves somehow. Jessica H, Tue, 3rd Nov 2009



Plenty of diffration effects occur with radiowaves around obstructions; refraction certainly can happen but I suspect it's not such a dominant effect in the context of the vaguaries of indoor TV reception. (Largely because of the salt and other ions which make the body substantially conducting.)

There's an 'A' level physics practical where you use a big perspex prism to refract microwave (3cm wavelength) radiowaves. No doubt you could do the same with cleam water. techmind, Tue, 3rd Nov 2009

So sorry to revive a very old thread ... but how do I emulate this? I am using an antenna to bring in OTA digital signals, and it is affected by weather. Touching the antenna improves the signal strength. How do I emulate this so I can continue to have the improved signal without requiring that I continue to stand and hold the rabbit ears? Jenn, Sun, 6th Apr 2014

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