Cable for DAB aerial (sorry if it's a wee bit OT)

Post by Jim
I plan on buying one of these outside aerials
http://www.screwfix.com/p/labgear-labdabo-s-omni-directional-dab-radio-a
erial/23595?_requestid=375662
However i'm a little unsure on what cable/coax would be needed for
this, I'm going to try installing the aerial inside first so will only
need about 10 feet, but if that fails to work i'll mount it outside and
will need about double that.

Whatever cable you use, be sure to mount that aerial VERTICALLY - or it
won't work very well at all.

--
Ian

Bill Wright

2014-10-07 02:13:05 UTC

Whatever cable you use, be sure to mount that aerial VERTICALLY - or it
won't work very well at all.

Off topic? Have you seen this newsgroup?

You need normal TV/satellite coax. Ideally it should be the good quality
stuff (copper foil; copper braid), not the type with a silver-coloured
foil or no foil at all.

http://www.screwfix.com/p/labgear-pf100-satellite-coaxial-cable-50m-black/21318

The price is outrageous; about twice what it should be. If you only need
about ten or fifteen metres you might be better buying it off the roll.

Here's a better deal:

http://www.amazon.co.uk/Black-Satellite-Coaxial-Cable-Webro/dp/B008VEN2JS

Mount the aerial with the rod vertical, as far as possible from masonry
or metal, and as high as you can get it.

You'll need an 'f' connector at the aerial end of the cable, and some
insulating tape to fix the cable to the short mast.

Bill

Woody

2014-10-07 07:24:46 UTC

Post by Jim
I plan on buying one of these outside aerials
http://www.screwfix.com/p/labgear-labdabo-s-omni-directional-dab-radio-a
erial/23595?_requestid=375662
However i'm a little unsure on what cable/coax would be
needed for this, I'm going to try installing the aerial
inside first so will only need about 10 feet, but if
that fails to work i'll mount it outside and will need
about double that.

Whatever cable you use, be sure to mount that aerial
VERTICALLY - or it won't work very well at all.

Off topic? Have you seen this newsgroup?
You need normal TV/satellite coax. Ideally it should be
the good quality stuff (copper foil; copper braid), not
the type with a silver-coloured foil or no foil at all.
http://www.screwfix.com/p/labgear-pf100-satellite-coaxial-cable-50m-black/21318
The price is outrageous; about twice what it should be. If
you only need about ten or fifteen metres you might be
better buying it off the roll.
http://www.amazon.co.uk/Black-Satellite-Coaxial-Cable-Webro/dp/B008VEN2JS
Mount the aerial with the rod vertical, as far as possible
from masonry or metal, and as high as you can get it.
You'll need an 'f' connector at the aerial end of the
cable, and some insulating tape to fix the cable to the
short mast.
Bill

Interesting that a domestic DAB aerial is 75R but a car DAB
aerial is 50R. I know the latter makes for easier connection
in terms of professional connectors (usually SMA) but it
does not do well for continuity or comprehension on behalf
of the user.

It doesn't help either when a FM/DAB radio is fitted in a
caravan or motorhome (OK, comparatively small market) and it
is fed from the TV aerial system which is all 75R!

--
Woody

harrogate three at ntlworld dot com

Ian Jackson

2014-10-07 07:44:26 UTC

In message <m104g3$101$***@dont-email.me>, Woody <***@ntlworld.com>
writes

Post by Woody

Post by Jim
I plan on buying one of these outside aerials
http://www.screwfix.com/p/labgear-labdabo-s-omni-directional-dab-radio-a
erial/23595?_requestid=375662
However i'm a little unsure on what cable/coax would be
needed for this, I'm going to try installing the aerial
inside first so will only need about 10 feet, but if
that fails to work i'll mount it outside and will need
about double that.

Whatever cable you use, be sure to mount that aerial
VERTICALLY - or it won't work very well at all.

Off topic? Have you seen this newsgroup?
You need normal TV/satellite coax. Ideally it should be
the good quality stuff (copper foil; copper braid), not
the type with a silver-coloured foil or no foil at all.
http://www.screwfix.com/p/labgear-pf100-satellite-coaxial-cable-50m-bla
ck/21318
The price is outrageous; about twice what it should be. If
you only need about ten or fifteen metres you might be
better buying it off the roll.
http://www.amazon.co.uk/Black-Satellite-Coaxial-Cable-Webro/dp/B008VEN2JS
Mount the aerial with the rod vertical, as far as possible
from masonry or metal, and as high as you can get it.
You'll need an 'f' connector at the aerial end of the
cable, and some insulating tape to fix the cable to the
short mast.
Bill

Interesting that a domestic DAB aerial is 75R but a car DAB
aerial is 50R.

It will depend on the type aerial. A quarterwave vertical with a good
groundplane all around it (say, in the centre of the car roof) will be a
notional 37.5 ohms. A 5/8th wave vertical (with series inductance to
make it impedance-wise a 3/4 wave) will around 50 ohms. At around
200MHz, with the short lengths of coax involved, the impedance of the
coax and the connector will hardly make a scrap of difference.

Post by Woody
I know the latter makes for easier connection
in terms of professional connectors (usually SMA) but it
does not do well for continuity or comprehension on behalf
of the user.
It doesn't help either when a FM/DAB radio is fitted in a
caravan or motorhome (OK, comparatively small market) and it
is fed from the TV aerial system which is all 75R!

See above.

--
Ian

Brian Gaff

2014-10-07 08:17:06 UTC

Hmm, this impedance mismatch is not that bad in practice, as I suspect,
domestic gear is not that accurate in this area in the first place, so as
long as its not made up of lots of bits of different coax etc, it usually
works.
In any case, Aerials are only the right impedance at one frequency, so are
going to have mismatches across the band in any case.

Add to that the fact that where you mount an aerial, ie close by buildings
and other metal items also affect this and the effect of a little mismatch
is not usually noticed much.
Brian
--
From the Sofa of Brian Gaff Reply address is active

Whatever cable you use, be sure to mount that aerial VERTICALLY - or it
won't work very well at all.

Off topic? Have you seen this newsgroup?
You need normal TV/satellite coax. Ideally it should be the good quality
stuff (copper foil; copper braid), not the type with a silver-coloured
foil or no foil at all.
http://www.screwfix.com/p/labgear-pf100-satellite-coaxial-cable-50m-black/21318
The price is outrageous; about twice what it should be. If you only need
about ten or fifteen metres you might be better buying it off the roll.
http://www.amazon.co.uk/Black-Satellite-Coaxial-Cable-Webro/dp/B008VEN2JS
Mount the aerial with the rod vertical, as far as possible from masonry
or metal, and as high as you can get it.
You'll need an 'f' connector at the aerial end of the cable, and some
insulating tape to fix the cable to the short mast.
Bill

Interesting that a domestic DAB aerial is 75R but a car DAB aerial is 50R.
I know the latter makes for easier connection in terms of professional
connectors (usually SMA) but it does not do well for continuity or
comprehension on behalf of the user.
It doesn't help either when a FM/DAB radio is fitted in a caravan or
motorhome (OK, comparatively small market) and it is fed from the TV
aerial system which is all 75R!
--
Woody
harrogate three at ntlworld dot com

Johny B Good

2014-10-07 13:33:34 UTC

Post by Woody

Post by Jim
I plan on buying one of these outside aerials
http://www.screwfix.com/p/labgear-labdabo-s-omni-directional-dab-radio-a
erial/23595?_requestid=375662
However i'm a little unsure on what cable/coax would be
needed for this, I'm going to try installing the aerial
inside first so will only need about 10 feet, but if
that fails to work i'll mount it outside and will need
about double that.

Whatever cable you use, be sure to mount that aerial
VERTICALLY - or it won't work very well at all.

Off topic? Have you seen this newsgroup?
You need normal TV/satellite coax. Ideally it should be
the good quality stuff (copper foil; copper braid), not
the type with a silver-coloured foil or no foil at all.
http://www.screwfix.com/p/labgear-pf100-satellite-coaxial-cable-50m-black/21318
The price is outrageous; about twice what it should be. If
you only need about ten or fifteen metres you might be
better buying it off the roll.
http://www.amazon.co.uk/Black-Satellite-Coaxial-Cable-Webro/dp/B008VEN2JS
Mount the aerial with the rod vertical, as far as possible
from masonry or metal, and as high as you can get it.
You'll need an 'f' connector at the aerial end of the
cable, and some insulating tape to fix the cable to the
short mast.
Bill

Interesting that a domestic DAB aerial is 75R but a car DAB
aerial is 50R. I know the latter makes for easier connection
in terms of professional connectors (usually SMA) but it
does not do well for continuity or comprehension on behalf
of the user.
It doesn't help either when a FM/DAB radio is fitted in a
caravan or motorhome (OK, comparatively small market) and it
is fed from the TV aerial system which is all 75R!

The aerial seems to be a common or garden folded half wave dipole
with a 2:1 balun transformer (misleadingly referred to as
'electronic', implying an active amplifier - but there's no mention of
the need for a 5 or 12v biassing supply).

A 'straight' centre fed dipole in free space has a feed impedance
that approximates to 72 ohm (a perfect groundplane quarter wave
equates to 36 ohms). These impedance 'mismatches' are a close enough
match to 75 (50) ohm co-ax feeder as to be swamped by the real life
effects on practical aerial installations.

The impedance of a folded halfway dipole is four times that of a
straight dipole (centre fed) becoming 300 ohms instead of 75 ohms. A
2:1 turns ratio balun makes an ideal impedance matcher, as well as a
means of connecting a balanced 'load' / 'source' to an unbalanced
feeder.

It's the 'folded' element that needs to be vertical (parallel to the
mast it's clamped to - the mounting arm looks to be the good quarter
wave in length required to keep mast effects[1] to a minimum so it
seems to be an honest to goodness, no nonsense antenna (far far better
than the halo I was expecting to see).

[1] Assuming the antenna is kept clear of other nearby conductive
structures which can so easily swamp out the 'mast effect' anyway.

--
J B Good

Bill Wright

2014-10-07 14:35:21 UTC

Post by Johny B Good
it
seems to be an honest to goodness, no nonsense antenna (far far better
than the halo I was expecting to see).

I don't think anyone makes halos for DAB, since it's VP.

The fact that the dipole is folded broadens it's bandwidth, which is
good considering it needs to work on pretty well all of BIII.

Bill

Johny B Good

2014-10-08 12:38:48 UTC

Post by Johny B Good
it
seems to be an honest to goodness, no nonsense antenna (far far better
than the halo I was expecting to see).

I don't think anyone makes halos for DAB, since it's VP.
The fact that the dipole is folded broadens it's bandwidth, which is
good considering it needs to work on pretty well all of BIII.

I was thinking of that but thought I'd already given the aerial
enough praise. :-)

Making the 'driven element' on a multi-element Yaggi a folded dipole
is a rather neat way to raise the feedpoint impedance to compensate
for the impedance reduction effect by the reflector and director
elements (using a 1:1 balun if the manufacturer has any respect for
his design engineers).

--
J B Good

Bill Wright

2014-10-07 14:36:50 UTC