In article , Ian Jackson
wrote:
In message , Jim Lesurf
writes
In article , Woody
wrote:
One thing that everyone has missed is that DAB is vertically
polarised. Thankfully all FM transmissions in the UK are now of mixed
polarisation (i.e. they have both horizontal and vertical components)
so turning your FM aerial vertical (if necessary) will not be a
problem (electronically - might be physically!)

You beat me to commenting on the polarisations. I suspect that having a
VHF antenna 'work' for DAB may simply be using its coax as the DAB
antenna because the actual antenna is a hopeless match at DAB
frequencies. :-)

Aerial impedance mismatch doesn't cause pickup on the coax.

I'm afraid that it does when the cause is essentially an irregular bunch of
bits of metal connected to the end. The point here is that the mismatch
comes from an arrangement that is nothing like a correctly unbalanced and
matched arrangement.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

In message , Jim Lesurf
writes
In article , Ian Jackson
wrote:
In message , Jim Lesurf
writes
In article , Woody
wrote:
One thing that everyone has missed is that DAB is vertically
polarised. Thankfully all FM transmissions in the UK are now of mixed
polarisation (i.e. they have both horizontal and vertical components)
so turning your FM aerial vertical (if necessary) will not be a
problem (electronically - might be physically!)

You beat me to commenting on the polarisations. I suspect that having a
VHF antenna 'work' for DAB may simply be using its coax as the DAB
antenna because the actual antenna is a hopeless match at DAB
frequencies. :-)

Aerial impedance mismatch doesn't cause pickup on the coax.

I'm afraid that it does when the cause is essentially an irregular bunch of
bits of metal connected to the end. The point here is that the mismatch
comes from an arrangement that is nothing like a correctly unbalanced and
matched arrangement.

If a balanced (or even sort-of balanced) aerial is connected to an
unbalance feeder, indeed there will be pickup (or radiation) from the
feeder. However, that doesn't really have anything to do with impedance
mismatch. Connect a perfect, balanced 75 ohm dipole directly to perfect
75 ohm coax, and you will get feeder pickup/radiation effects. Connect a
perfect, balanced 300 ohm dipole via a balun 75 ohm to coax, and you
won't. [However, your signal will be 8db lower than if it was a 75 ohm
dipole.]
--
Ian

"Ian Jackson"


If a balanced (or even sort-of balanced) aerial is connected to an
unbalance feeder, indeed there will be pickup (or radiation) from the
feeder.

** Which merely makes the polar pattern a bit lop sided.


However, that doesn't really have anything to do with impedance mismatch.
Connect a perfect, balanced 75 ohm dipole directly to perfect 75 ohm coax,
and you will get feeder pickup/radiation effects. Connect a perfect,
balanced 300 ohm dipole via a balun 75 ohm to coax, and you won't.
[However, your signal will be 8db lower than if it was a 75 ohm dipole.]

** That is ambiguously worded.

Is the balun a 300ohm to 75 ohm type ?

If so, there is no significant loss of signal level.

If it is a 75ohm to 75 ohm type, then the loss is 8dB compared to using to
correct balun.

NB 1:

If the antenna has passive elements ahead and behind the folded dipole - all
bets are off regards correct matching. See the maker's specs for advice.

NB2:

Long as the co-ax cable is matched at the receiver, there is no problem with
standing waves.


..... Phil

In message , Phil Allison
writes

"Ian Jackson"


If a balanced (or even sort-of balanced) aerial is connected to an
unbalance feeder, indeed there will be pickup (or radiation) from the
feeder.

** Which merely makes the polar pattern a bit lop sided.


However, that doesn't really have anything to do with impedance mismatch.
Connect a perfect, balanced 75 ohm dipole directly to perfect 75 ohm coax,
and you will get feeder pickup/radiation effects. Connect a perfect,
balanced 300 ohm dipole via a balun 75 ohm to coax, and you won't.
[However, your signal will be 8db lower than if it was a 75 ohm dipole.]

** That is ambiguously worded.

Is the balun a 300ohm to 75 ohm type ?

If I had meant a 300 to 75 ohm balun, I would have said so.

If so, there is no significant loss of signal level.

If it is a 75ohm to 75 ohm type, then the loss is 8dB compared to using to
correct balun.

Which is why I said "8dB".

NB 1:

If the antenna has passive elements ahead and behind the folded dipole - all
bets are off regards correct matching. See the maker's specs for advice.

When I said a 75 ohm dipole, I meant a 75 ohm dipole.
When I said a 300 ohm dipole, I meant a 300 ohm dipole.
That's why I said a 75 ohm dipole and a 300 ohm dipole.

NB2:

Long as the co-ax cable is matched at the receiver, there is no problem with
standing waves.

No one mentioned problems with standing waves.

You're missing the whole point about the effects of connecting a
balanced aerial directly to coax.



--
Ian

In article , Ian Jackson
wrote:
In message , Jim Lesurf
writes


I'm afraid that it does when the cause is essentially an irregular
bunch of bits of metal connected to the end. The point here is that the
mismatch comes from an arrangement that is nothing like a correctly
unbalanced and matched arrangement.

If a balanced (or even sort-of balanced) aerial is connected to an
unbalance feeder, indeed there will be pickup (or radiation) from the
feeder. However, that doesn't really have anything to do with impedance
mismatch. Connect a perfect, balanced 75 ohm dipole directly to perfect
75 ohm coax, and you will get feeder pickup/radiation effects. Connect a
perfect, balanced 300 ohm dipole via a balun 75 ohm to coax, and you
won't. [However, your signal will be 8db lower than if it was a 75 ohm
dipole.]

The theory is fine, based on presuming only one factor isn't optimised. But
the reality is that the two factors will tend to be wrong, in a related
way, and each then has an impact on the results from the other. Hence the
reality can be that signals may be picked up from the coax acting as a
vertical antenna.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

"Ian Jackson"
Phil Allison "Ian Jackson"


If a balanced (or even sort-of balanced) aerial is connected to an
unbalance feeder, indeed there will be pickup (or radiation) from the
feeder.

** Which merely makes the polar pattern a bit lop sided.


However, that doesn't really have anything to do with impedance
mismatch.
Connect a perfect, balanced 75 ohm dipole directly to perfect 75 ohm
coax,
and you will get feeder pickup/radiation effects. Connect a perfect,
balanced 300 ohm dipole via a balun 75 ohm to coax, and you won't.
[However, your signal will be 8db lower than if it was a 75 ohm dipole.]

** That is ambiguously worded.

Is the balun a 300ohm to 75 ohm type ?

If I had meant a 300 to 75 ohm balun, I would have said so.


** Pedantic crap.

Your post has a glaring error that creates the ambiguity.



If so, there is no significant loss of signal level.

If it is a 75ohm to 75 ohm type, then the loss is 8dB compared to using to
correct balun.

Which is why I said "8dB".

** No fooling ????

Who woulda thunk .....



NB 1:

If the antenna has passive elements ahead and behind the folded dipole -
all
bets are off regards correct matching. See the maker's specs for advice.

When I said a 75 ohm dipole, I meant a 75 ohm dipole.
When I said a 300 ohm dipole, I meant a 300 ohm dipole.
That's why I said a 75 ohm dipole and a 300 ohm dipole.

** More autistic, boring pedantic crap.

Can you spell the word "ambiguous" for me?



NB2:

Long as the co-ax cable is matched at the receiver, there is no problem
with
standing waves.

No one mentioned problems with standing waves.

** I did.

To head off those with the usual misconceptions before they ****ing posted
them.


You're missing the whole point about the effects of connecting a balanced
aerial directly to coax.


** You have yet to post anything non-ambiguous about that too.

But I'm not holding my breath waiting.



..... Phil

In article , UnsteadyKen
scribeth thus

Gordon MacPherson wrote...

The DAB has an F type connector. I would like to
be able to use both FM and DAB from the single cable.

1. If it is possible, what would be the most effective way of setting this
up?

It depends on your existing signal level, if it is good then you could
use a simple passive splitter but you may need to use a signal booster
to compensate for the loss incurred in the splitter.

I'm in a similar situation, in that our communal aerial system provides
a single combined (very good) FM/DAB feed.
I tried various ways of getting this to to my FM & DAB tuners
including a couple of passive splitters and they were fine for DAB but
degraded the FM signal too much compared to a direct connection.

In the end I got one of these 1 in 2 out amplifiers
http://www.philex.com/catalogue/prod...d=116&cat=1069
which works well for both DAB and FM. It has a variable gain control
which allows you to fine tune things.

Here are the results of using that on the end of a much derided FM
halo.
http://goo.gl/maps/sF9Yh


Think how much better it would be with a decent Vertical dipole;-)..

In fact we did some tests on one of they and they did do better than a
coat-hanger .. which incidentally wasn't resonant;-!..


That design, if you can call it that, was used for some IBA transmit
aerials waay back before they devised better mixed polarisation ones but
it has got a relative loss compared to the straight half wave dipole..

As regards matching alluded to elsewhere its worth remembering that a
simple half wave dipole has a typical impedance at resonance of 73 ohms
but when folded this rises to close on 300 hence the need for a
transformer to wind that down to a more useful value and also to an
unbalanced cable as 300 ohm.. does anyone use that anymore in the UK I
rather suspect not.

Sadly most all FM aerials made in the UK dispense with any attempt at a
balun device apart from some Triax ones. Antiference use another odd
arrangement.

If you NEC model a half wave fm dipole whilst the SWR does go high at
DAB frequencies the gain oddly enough improves around the DAB sector.
I have seen reports by Bill Wright of Wright's aerials saying that they
very often get decent DAB signals off vertical FM dipoles in the UK.

Whilst on the humble Halo and V and H polarisation it is true to say
that most all main stations and higher power relays do use Mixed
polarisation but some relays and most all community stations use
Vertical only hence another reason for the implementation of a vertical
dipole RX aerial..

--
Tony Sayer

In message , Jim Lesurf
writes
In article , Ian Jackson
wrote:
In message , Jim Lesurf
writes


I'm afraid that it does when the cause is essentially an irregular
bunch of bits of metal connected to the end. The point here is that the
mismatch comes from an arrangement that is nothing like a correctly
unbalanced and matched arrangement.

If a balanced (or even sort-of balanced) aerial is connected to an
unbalance feeder, indeed there will be pickup (or radiation) from the
feeder. However, that doesn't really have anything to do with impedance
mismatch. Connect a perfect, balanced 75 ohm dipole directly to perfect
75 ohm coax, and you will get feeder pickup/radiation effects. Connect a
perfect, balanced 300 ohm dipole via a balun 75 ohm to coax, and you
won't. [However, your signal will be 8db lower than if it was a 75 ohm
dipole.]

The theory is fine, based on presuming only one factor isn't optimised. But
the reality is that the two factors will tend to be wrong, in a related
way, and each then has an impact on the results from the other. Hence the
reality can be that signals may be picked up from the coax acting as a
vertical antenna.

The screening effect of coax 'works' because as far as the impinging RF
signals (hitting the coax) are concerned, at any position on the coax,
the voltage on the inner conductor is (or should be) the same as the RF
voltage on the outside of the shield (ie there is RF potential between
inner and the outside of the shield).

By itself, well-screened coax only delivers RF signals to the receiver
input if the shield isn't properly (solidly) connected to the reference
ground/chassis of the receiver. If there is any impedance in the way
(isolation capacitors, lead lengths etc) at the receiver end, the
outside of the shield can be at an RF voltage (from RF fields impinging
on the length of the coax shield).

As the RF voltage on the inner is the same as that on the outside of the
shield, any RF voltage on the outer of the shield also appears on the
inner - and this gets fed (whether by direct connection or by stray
coupling) into the 'RF live' side of receiver input.

[Well, that is my understanding things!]
--
Ian

tony sayer wrote...

Think how much better it would be with a decent Vertical dipole;-)..

Who knows why they use these halos?, DAB is a V dipole, it is the
standard council communal installation round here for sheltered housing
and flats, consisting of a large sat dish, FM halo, DAB dipole and two
TV aerials; one pointing north for Waltham and one south for Sandy
Heath all mounted on a sturdy pole.

Receiving two different TV regions with equally strong signals confuses
a great many Freeview boxes and TV sets, plus their elderly owners.

Equipment which detects this and allows you to choose a region cope
fine, but lots end up with the whole 70 channels duplicated in the
800's

In fact we did some tests on one of they and they did do better than a
coat-hanger .. which incidentally wasn't resonant;-!..

It gets those results through the use of massive amplification, whole
swathes of the VHF band are obliterated by the resulting harmonics from
the local main transmitters, (Peterborough and Geddington)



--
UnsteadyKen

tony sayer wrote...

I have seen reports by Bill Wright of Wright's aerials saying that they
very often get decent DAB signals off vertical FM dipoles in the UK.

We seem to be in a sweet spot for DAB reception round here, especially
since the Northamptonshire service started earlier this year from the
transmitter at Geddington. The communal system also pulls in the
Leicester, Nottingham and Cambridge muxes as well as the locals, A
portable DAB radio will work on a bit of wet string and I can only
clear the memory of my DAB tuner by shorting the aerial input.
Bubbling mud, wossat?


--
UnsteadyKen