Dab Radio With External Aerial Connection

I copy a post I made back in 2010 on the subject, when DAB was more in its infancy...

Some facts of physics, each with pros and cons (usually gain size and impedance) to determine which aerial to "go for" .....

The Theory : ( The "In Practice" bit follows)

- "Good" aerials include aerials that are of a resonant length - a 1/4 wave aerial on say 100 MHz ( 3 mtrs wavelength) is 75 cms long, and matches the feeder impedance, which in turn matches the receiver impedance, so the maximum signal caught by the aerial is transferred into the receiver...

- T he feed point impedance will vary each side of the correct length as the length of the aerial is varied.

- The design input impedance of most transmitter/receivers is 50 ohms, whereas most receivers are 75 ohms (nominal). Co-ax can be bought in 50 ohm or 75 ohm types ( as well as other impedances). It is important to have the correct impedance co-ax else signal will be lost, and this needs to be not old and waterlogged else signal will be lost. It also needs to have good braid coverage ( 100% is best) to resist pickup of interference. Satellite downlink cable is usually both braided and foil wrapped and is 75 ohm ( as the TV and the aerials are also 75 ohm.

- The impedance at the centre of a 1/2 wave dipole in free space (arms each have a length of 1/4 wave) is around 72 ohms (good match to 75 ohm co-ax)

- The impedance of a 1/4 wave groundplane aerial is about 35 ohms ( reasonable match) - tilting the groundplane legs down by 45 degrees brings it up to
approx 50 ohms where it is a good match to 50 ohm co-ax

- A halfwave aerial ( element around 1/2 wavelength long ) is ground plane independent ( needs no / must NOT have) a groundplane ( i.e. boat roof) , as that will mess up the characteristics.

Glazed over yet ?

- Going up in length and gain, is the 5/8 whip and the 7/8 whip, which are actually 3/4 wave, and full wave whips with an 1/8th wave coiled up for impedance matching purposes.

- A 5/8 aerial is approx 3dBd gain (double the gain compared to a dipole - the last "d" is critical) and is a 3/4 wavelength aerial with the bottom 1/8th coiled up for impedance matching to 50 ohms (usually), and will need a groundplane. A groundplane is typically 3 or 4 1/4 wave elements equally spaced and at right angles ( or legs sloping downwards at around 45 degrees) to the vertical whip, and placed at the bottom of of the whip which must be 1/4, 5/8ths or 7/8ths wavelength. The feedpoint will also be at this junction and be around 50 ohms.

Still here ?

- For frequencies up to about 75 MHz, a whip is usually limited to being a 1/4 wave as any larger proportion of a wavelength is too long to physically manage.

- Between approx 75 and 300 MHz people do use 5/8, 7/8 whips at the higher frequency end, and co-linear aerials.

In Practice:

First thing to discover is if your receiver has an aerial input and what its impedance is - probably around 75 Ohm, although I have not seen many DAB radios yet and discovered if any are providing aerial inputs. Assuming there is, and its 75 ohm....

The frequency of the DAB band II transmissions is approx 174 MHz to 240 MHz, so centered on around 200 MHZ. The wavelength of 200 MHZ is 300/200 = 1.5 metres ( the "300" is the speed of light ) (anyone left ?) a half wave is 75 cms and a 1/4 wave is 37.5 cms

So, for a vertical whip on the DAB frequencies it should be 37.5 cms ( or a bit less ), although a vertical dipole would match probably better and use 75 ohm co-ax to the receiver.

So, to make a 75 ohm halfwave dipole for DAB... you would get some 75 ohm co-ax, terminate in the plug that matches the receiver, and at the other end of the cable, you would separate the co-ax into its sheath and the inner, and solder on approx 40 cms of (any light duty) wire to each. You now have a piece of coax terminated in two 40 cms approx "pigtails". Stretch these out in opposite directions from each other, on a light piece of batten / wood / bamboo stick of at least 80 cms length and fasten appropriately ( sticky tape). Trim each leg down to around 37.5 cms from where the coax splits. Ideally the coax should come away from the dipole at right angles for at least a 1/4 wavelength

This is a halfwave dipole for 200 MHZ and will pick up e.g. the DAB frequencies.

Mounting: It may be mounted horizontally or vertically and should match the signal being transmitted's polarisation - If horizontal, a further pole (broomstick) could be fixed to the centre point and the co-ax run away from the centre point at right angles. The aerial will receive most signal from right angles to it ( i.e. broadside on), so will need rotating for best reception.

For FM, exactly the above applies, but as the frequencies are centred on 100 MHz, make the lengths around 75 cms for each leg
For a whip on the roof for FM, the 1/4 wave whip would be 75 cms, but the impedance would be around 37 ohms at the base, and so not a very good match and much signal lost - you could vary the length from 75 ohm ( either way) to try and improve the match, or incorporate a matching section at the base of the aerial to transform it to a better match - probably easier to use a dipole though.

Dab Radio With External Aerial Connection

Source: https://www.canalworld.net/forums/index.php?%2Ftopic%2F88502-connecting-an-external-antenna-to-dab-radio%2F

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