In article , pamela
scribeth thus
On 20:27 30 Nov 2017, Bill Wright wrote:

On 30/11/2017 17:11, pamela wrote:
http://support-uk.pure.com/en/kb/art...aerial-lengths
-for- dab-fm

Schoolboy error. What about velocity factor? How do we know this
quarter wave will be connected to something that matches its
theoretical 50ohm impedance? Anyway, even a perfectly resonant
and matched length of wire will only collect signal that exists,
and it will not produce a better signal to local noise ratio
than a random length.

Bill

So those naughty boys are Pure are misleading us.

A bit more Googling shows that DAB Digital Radio magazine once
suggested 31.5 cm which doesn't match the 53 to 86cm suggested by
Pure.

"the perfect length for a DAB digital radio aerial is 31.5 cm
(based on the frequency used to broadcast the national
commercial digital radio stations)"

Now I am extra confused.

By the way, wouldn't a resonant aerial produce a greater signal for
the same level of background noise and therefore improve the S/N?

Well in principal yes. The DAB band is now quite wide frequencies from
174 to 239 odd as per;

http://www.wohnort.org/dab/freqs.html

are getting used, so for a simple plain old vertical whip mounted on a
ground plane thats say from 300/174 divide 4 x.9 velocity factor say 387
mm to 281 mm.

Not a very wide band device so cut to resonance is a good idea. However
a folded dipole much better, in fact we have a now unused dipole that
used to carry network rails band 3 service on 206 MHz it's now carrying
a 163 MHz channel with no reflected power!.

--
Tony Sayer