Connecting an external antenna to DAB radio

[blackrose]

I just bought a second hand Pure DAB radio from eBay. I thought it was the same as one I used to have but it's a more recent model and they've got rid of the F-type fitting on the back for the telescopic antenna which I used to be able to unscrew and connect an external ariel cable.

 

I opened up the back of the radio but I'm not sure how (or where) I would solder on an F-type connector. I had a look on youtube but can't find anything. Does anyone know?

 

The telescopic antenna connects through the back of the radio with a screw to the metal pole in the picture, but when I tried connecting the core of the external coax to the pole it didn't work.

 

Edited November 16, 2016 by blackrose

[Nickhlx]

You will need to connect both the core and the braid, else the core has nothing to "push" against. The braid needs to be connected to the chassis or screening within the radio - be careful if the aerial is electrically connected to the boat if e.g. a roof aerial, else the radio might be in the earth path if a fault develops on the boat !

 

I have such a radio and connected a 3 ele beam to the aerial and battery earth (runs off ext power) with two short croc clip to croc clip leads - works a treat !

 

Nick

[blackrose]

Thanks Nick, sorry just to clarify, are you saying I connect the core of the coax to that metal pole and the braid needs to be connected to what? The radio is in a hardwood box. What's the chassis exactly?

 

Also what's a 3 ele beam? Mine is a mains radio. I assume you've converted yours to run from 12v?

[Guest]

You will need to connect both the core and the braid, else the core has nothing to "push" against. The braid needs to be connected to the chassis or screening within the radio - be careful if the aerial is electrically connected to the boat if e.g. a roof aerial, else the radio might be in the earth path if a fault develops on the boat !

 

I have such a radio and connected a 3 ele beam to the aerial and battery earth (runs off ext power) with two short croc clip to croc clip leads - works a treat !

 

Nick

Interesting, i wonder if you could explain that a bit further? Sounds like what I need. Cheers.

[Nickhlx]

(For home use) I got a 3 element beam for the DAB band (Like the FM band but approx 1/2 to 2/3rds the size), put it in the roofspace and ran the co-ax down to the radio. Its would probably be a bit too big for installation on the boat unless you are moored up for a while.

 

As the radio doesn't have an aerial socket on it, I retracted the aerial fully, and connected the co-ax core to the aerial with a croc clip, and the braid to the (electrical) chassis or earth point, which was easiest to find as the negative spring pole of the battery compartment. We aren't in a bad area for DAB, but where we wanted the radio in the house was well shielded from the airwaves, and this greatly improved the signal to the radio.

Edited November 17, 2016 by Nickhlx

[Guest]

(For home use) I got a 3 element beam for the DAB band (Like the FM band but approx 1/2 to 2/3rds the size), put it in the roofspace and ran the co-ax down to the radio. Its would probably be a bit too big for installation on the boat unless you are moored up for a while.

 

As the radio doesn't have an aerial socket on it, I retracted the aerial fully, and connected the co-ax core to the aerial with a croc clip, and the braid to the (electrical) chassis or earth point, which was easiest to find as the negative spring pole of the battery compartment. We aren't in a bad area for DAB, but where we wanted the radio in the house was well shielded from the airwaves, and this greatly improved the signal to the radio.

Great thanks.

[Guest]

Great thanks.

Had a go with my DAB radio today, and added a cable to the battery negative as suggested. I then made an "ariel" with copper wire of length 318mm top and bottom (dipole style) on a strip of wood, and connected to aerial and ground with coax as suggested (75ohm.) Works an absolute treat and made the radio so I can actually use it now. Will knock a proper aerial up. possibly car style dab aerial. Many thanks Nick for this idea. Might go to town and fit a socket same as Mike is doing. Just need a headphone socket in it now.

[Guest]

There's more. Attached the adapted radio to the telly aerial in the house and it works a treat. Not technically correct, but if it works in the black hole where I live it should work most places.

[Guest]

Tried the adapted radio on the boat yesterday, using the telly aerial. The DAB radio is much better with the telly aerial, but I intend to make a dipole as I did get a bit of interference now and again. Compared to the built in aerial on the radio, even the telly aerial is stunning.

[blackrose]

I've been away for a couple of weeks.

 

I'm still struggling to understand where to solder the core and braid of the coax cable. There is no battery compartment on my DAB radio. I went to Maplins to see if they could help but they had no idea either.

[Guest]

I've been away for a couple of weeks.

 

I'm still struggling to understand where to solder the core and braid of the coax cable. There is no battery compartment on my DAB radio. I went to Maplins to see if they could help but they had no idea either.

Not sure about that one, Mike. Mine had a battery compartmnt so was very easy.

Does you radio run off a transformer type power supply that you plug in, or is the 240V cable direct into the case?

[Arthur Marshall]

I use a standard car aerial on the boat with a splitter to the DAB socket on the car radio. Should I connect the earth bit of the aerial (which just connects to the boat roof) to the radio? There must be a way of getting a better signal...

[blackrose]

Not sure about that one, Mike. Mine had a battery compartmnt so was very easy.

Does you radio run off a transformer type power supply that you plug in, or is the 240V cable direct into the case?

The cable has a transformer which plugs into the mains.

[Guest]

The cable has a transformer which plugs into the mains.

Same as mine. The low volatge supply into the set was the same negative as the battery in mine, so maybe connecting the brading to the low voltage input negative on the socket would work? Hopefully somebody with more knowledge might know better than I.

I use a standard car aerial on the boat with a splitter to the DAB socket on the car radio. Should I connect the earth bit of the aerial (which just connects to the boat roof) to the radio? There must be a way of getting a better signal...

A bit different than my setup, but I once tried a wire on the existing radio aerial and it made no difference whatsoever. I think if you connect the earth in this manner you will need a capacitor in circuit, others will know better. When I connected the earth brading as suggested by NickX in post 2, the difference was amazing. I recently made the simplest aerial ever out of a piece of tV aerial coax and it works a treat in the window at home, and we live in a reception black hole. I intend to try this simple aerial on the boat, as well as a home made dipole. So easy to make, here it is:

http://www.motorhomefun.co.uk/forum/threads/how-to-make-a-dab-radio-antenna.34914/

 

This simple aerial can be sealed in a plastic tube with silicone, and people report they work brilliantly on campers and caravans as well as houses, so I suspect they are OK for boats.

Edited December 17, 2016 by Guest

[Nickhlx]

Hi Guys,

 

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...

- The 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.

 

Hope that gives some insight and ideas to put something together,

 

Nick

Edited December 17, 2016 by Nickhlx

[blackrose]

Cor blimey, I couldn't get through all that technical mumbo jumbo! My question seems simple in comparison.

[Iain_S]

Same as mine. The low volatge supply into the set was the same negative as the battery in mine, so maybe connecting the brading to the low voltage input negative on the socket would work? Hopefully somebody with more knowledge might know better than I.

A bit different than my setup, but I once tried a wire on the existing radio aerial and it made no difference whatsoever. I think if you connect the earth in this manner you will need a capacitor in circuit, others will know better. When I connected the earth brading as suggested by NickX in post 2, the difference was amazing. I recently made the simplest aerial ever out of a piece of tV aerial coax and it works a treat in the window at home, and we live in a reception black hole. I intend to try this simple aerial on the boat, as well as a home made dipole. So easy to make, here it is:

http://www.motorhomefun.co.uk/forum/threads/how-to-make-a-dab-radio-antenna.34914/

 

This simple aerial can be sealed in a plastic tube with silicone, and people report they work brilliantly on campers and caravans as well as houses, so I suspect they are OK for boats.

 

With this one, I don't see how stripping the outer sheath from the braid, and leaving it like that, can make a difference. I'd prefer to pass the inner core plus its sheath through the braid, converting it into a dipole. Still, if it works .....

[Guest]

 

With this one, I don't see how stripping the outer sheath from the braid, and leaving it like that, can make a difference. I'd prefer to pass the inner core plus its sheath through the braid, converting it into a dipole. Still, if it works .....

Weird isn't it? But yes it works well and cost nowt (used an old lead off a VCR!) I am also puzzled as to what the loops are for?

I am certain a dipole will be better, so going to make one on Christmas Day to kill the time.

[Nickhlx]

You can make a "sleeved dipole", or co-axial dipole, by removing the outer plastic jacket for a quarter wavelength, and folding back the braid over the existing outer jacket, and trimming both the inner core to the desired length and the folded back braid to the same length... Both the exposed inner core and the outermost portion of the braid then become the active elements...

 

 

 

Nick

Edited December 18, 2016 by Nickhlx

[Guest]

You can make a "sleeved dipole", or co-axial dipole, by removing the outer plastic jacket for a quarter wavelength, and folding back the braid over the existing outer jacket, and trimming both the inner core to the desired length and the folded back braid to the same length... Both the exposed inner core and the outermost portion of the braid then become the active elements...

 

 

 

Nick

I can see how that would work, it is just a simple dipole. But I don't understand how the one I copied works, even though it does. I'll try and get a pic of it; as they say worth a thoucand words.