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I have the following installation:

Dipole antenna <------> Coaxial cable (3 m) <-----> DVB receiver

If I connect my dipole antenna directly to the DVB receiver, removing the coaxial cable (the cable of the antenna is long enough), the signal has very bad SNR. The connectors are OK.

If I connect a useless coaxial cable between the antenna and the DVB receiver, the signal becomes far better and I can watch TV without issue.

Can somebody try to explain to me why? How can this make sense? What kind of magic happens inside my coaxial cable?

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    \$\begingroup\$ Best guess: very strong signal and you’re overdriveing the input stage. Adding a cable will add losses (attenuation) and you are within limits. \$\endgroup\$
    – winny
    May 16 at 20:24

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A dipole antenna requires a balanced RF input. A standard coax is an unbalanced cable but, over a certain length it can, under some circumstances, offer a more balanced impedance to the dipole antenna than wiring the dipole directly (via a shorter cable) to your unbalanced antenna input.

Can somebody try to explain me why ? Can this make sense ? What kind of magic happens inside my coaxial cable ?

Try looking up transmission-line theory and the telegraphers equations to understand how transmission lines work and how they can produce effects (especially at a quarter wavelength) that appear magical. Try also looking up quarter wave impedance transformers.

If you don't get-on with math, you may soon be out of your depth and find your brain hurting; it's not an easy subject. Making a coax into a balun (balanced to unbalanced converter): -

enter image description here

Image from here.

Balanced antenna and unbalanced antennas: -

Balanced dipole: -

enter image description here

Unbalanced monopole (possibly more suitable for your receiver): -

enter image description here

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  • \$\begingroup\$ oh true yeah, that might be it more than a simple mismatch \$\endgroup\$ May 16 at 20:18
  • \$\begingroup\$ Thank you very much for this explanation. I didn't know about balanced/unbalanced cables. That makes sense. I will have to keep my coaxial cable that permits to better match the impedence of my unbalanced antenna input. (it seems i'm quite lucky with this) \$\endgroup\$ May 16 at 21:29
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You get a very bad SNR when connecting the antenna directly to the DVBT receiver and a better SNR when using a longer cable?

Of course antenna impedance, cable impedance and receiver input impedance should be matched. A naked antenna dipol is not matched to an asymmetric coaxial cable but to a parallel wire symmetric cable.

May be you have a very strong signal due to a short distance to the DVBT transmitter.

We need the cable length for both cases, the SNR values and signal strengths in dbµV. What is the maximum specified signal strength of the receiver? May be you need a signal attenuation plug.

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Probably an effect of impedance mismatch.

Things with an interface where electromagnetic waves leave or enter have a wave impedance. That is just a number that denotes how the magnetic and the electric field of the electromagnetic wave propagating in the device relate. By how Maxwell's Equations work out, the unit of that is Ω, like resistors!

How large that impedance is, is defined by the shape and material of the waveguide. Some devices even act as an impedance transformator. For example, an antenna takes the wave impedance of free space (ca. 370 Ω) and transforms it into the impedance of the antenna cable (for example, 50 Ω, 70 Ω, 100 Ω, sometimes something complex if we're talking tuned systems…).

And if you want to send power from one device port into another, you will only achieve full power transfer if these two impedances are the same. Otherwise, you get reflections. The amount of reflection depends on the kind of mismatch.

Now comes the fun thing: a cable that is not the same impedance as the port at its end functions as impedance transformer. As a crass example: leaving a piece of cable open and connecting it with a piece of cable that is one quarter wavelength transforms that "open end" (\$\infty\$ Ω) to a "short circuit (0 Ω). Funky!

That way, your antenna might be better matched to your receiver by transforming the impedance through some length of cable.

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