Just got a Blake SR18 group B aerial which will be used to replace the
Televes X43 wideband at my mate's holiday home in Ilfracombe.
According to the instructions, the cable should be routed down the
reflector and diagonally across to the mast.  It looks a bit untidy to
me, and even ATV have written 'not sure about this!' on the
instruction leaflet!

Marky P.

Marky P wrote:

> According to the instructions, the cable should be routed down the
> reflector and diagonally across to the mast.  It looks a bit untidy to
> me, and even ATV have written 'not sure about this!' on the
> instruction leaflet!

On the other hand the manufacturers are quite sure that it works better 
that way, particularly in regard to feeder pick-up suppression[*].  The 
same almost certainly applies to the Antiference products discussed 
recently in another thread.

The instructions are there for a reason and if you ignore them you 
_will_ degrade the overall performance of the aerial.


[*] The baluns aren't perfect, so the routing if the feeder does have 
some effect on an aerial's performance.

-- 
Andy

"Andy Wade"  wrote in message 
news:6b73d3F37sgavU1@mid.individual.net...
> Marky P wrote:
>
>> According to the instructions, the cable should be routed down the
>> reflector and diagonally across to the mast.  It looks a bit untidy to
>> me, and even ATV have written 'not sure about this!' on the
>> instruction leaflet!
>
> On the other hand the manufacturers are quite sure that it works better 
> that way, particularly in regard to feeder pick-up suppression[*].  The 
> same almost certainly applies to the Antiference products discussed 
> recently in another thread.
>
> The instructions are there for a reason and if you ignore them you _will_ 
> degrade the overall performance of the aerial.
>
>
> [*] The baluns aren't perfect, so the routing if the feeder does have some 
> effect on an aerial's performance.

Possibly. A quick and dirty test on the Anti TCX showed no difference. It 
certainly goes against the grain flying the cable to the mast with no 
support, and it looks dreadful.

I wonder why would it make much difference with good quality cable?  I first 
noticed the trend with Triax and am surprised that Anti and now Blake are 
advocating it.

Bill, how are you doing it - or are you away on holiday again :-)

In article <qd-dnZxcl8POItPVnZ2dnUVZ8tyqnZ2d@plusnet>, Doctor D
 wrote:

> "Andy Wade"  wrote in message
> news:6b73d3F37sgavU1@mid.individual.net...
> > Marky P wrote:
> >
> >> According to the instructions, the cable should be routed down the
> >> reflector and diagonally across to the mast.  It looks a bit untidy to
> >> me, and even ATV have written 'not sure about this!' on the
> >> instruction leaflet!
> >
> > On the other hand the manufacturers are quite sure that it works better
> > that way, particularly in regard to feeder pick-up suppression[*]. 
> > The same almost certainly applies to the Antiference products
> > discussed recently in another thread.
> >
> > The instructions are there for a reason and if you ignore them you
> > _will_ degrade the overall performance of the aerial.
> >
> >
> > [*] The baluns aren't perfect, so the routing if the feeder does have
> > some effect on an aerial's performance.

> Possibly. A quick and dirty test on the Anti TCX showed no difference. It
> certainly goes against the grain flying the cable to the mast with no
> support, and it looks dreadful.

> I wonder why would it make much difference with good quality cable?  I
> first noticed the trend with Triax and am surprised that Anti and now
> Blake are advocating it.

the theory is that any stray metalwork - and that includes cable - between
the elements is bound to change the polar diagram of the aerial.  This is
also why Antiference suggest tail mounting even the bigger aerials.  The
mounting cradle in VP mode does have a strange effect.  Of course, if
you're using a tail mount, the cable is perfectly supported ;-)

-- 
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

>> Possibly. A quick and dirty test on the Anti TCX showed no difference. It
>> certainly goes against the grain flying the cable to the mast with no
>> support, and it looks dreadful.
>
>> I wonder why would it make much difference with good quality cable?  I
>> first noticed the trend with Triax and am surprised that Anti and now
>> Blake are advocating it.
>
> the theory is that any stray metalwork - and that includes cable - between
> the elements is bound to change the polar diagram of the aerial.  This is
> also why Antiference suggest tail mounting even the bigger aerials.  The
> mounting cradle in VP mode does have a strange effect.  Of course, if
> you're using a tail mount, the cable is perfectly supported ;-)

If they fitted the TCX18 B/K with a tail it may be possible, although I 
wouldn't tail mount something that long.

Also odd that this has only come to vogue in the past few years. Before that 
the instruction was to run the cable along the boom and down the cradle, 
even before decent double screened cable became commonplace.

In article <sYKdnZ1gHvxiRtPVRVnyiwA@posted.plusnet>, Doctor D
 wrote:

> >> Possibly. A quick and dirty test on the Anti TCX showed no difference.
> >> It certainly goes against the grain flying the cable to the mast with
> >> no support, and it looks dreadful.
> >
> >> I wonder why would it make much difference with good quality cable?  I
> >> first noticed the trend with Triax and am surprised that Anti and now
> >> Blake are advocating it.
> >
> > the theory is that any stray metalwork - and that includes cable -
> > between the elements is bound to change the polar diagram of the
> > aerial.  This is also why Antiference suggest tail mounting even the
> > bigger aerials.  The mounting cradle in VP mode does have a strange
> > effect.  Of course, if you're using a tail mount, the cable is
> > perfectly supported ;-)

> If they fitted the TCX18 B/K with a tail it may be possible, although I
> wouldn't tail mount something that long.

> Also odd that this has only come to vogue in the past few years. Before
> that the instruction was to run the cable along the boom and down the
> cradle, even before decent double screened cable became commonplace. 

Perhaps, because double screened cable has more metal than cheap stuff, it
has a worse effect.

-- 
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

...snip...


>> Also odd that this has only come to vogue in the past few years. Before
>> that the instruction was to run the cable along the boom and down the
>> cradle, even before decent double screened cable became commonplace.
>
> Perhaps, because double screened cable has more metal than cheap stuff, it
> has a worse effect.
>

If the cable is tightly taped to the boom, will it really have much affect? 
Time for one of Bill's investigative reports I feel ;-).

Paul DS

In article ,
   Paul D.Smith  wrote:
> ...snip...


> >> Also odd that this has only come to vogue in the past few years. Before
> >> that the instruction was to run the cable along the boom and down the
> >> cradle, even before decent double screened cable became commonplace.
> >
> > Perhaps, because double screened cable has more metal than cheap stuff,
> > it has a worse effect.
> >

> If the cable is tightly taped to the boom, will it really have much
> affect? 

but at some point it will leave the boom and go off at an angle.  That's
when it causes the effect on the polar diagram.

-- 
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

In article , charles <charles@charl
eshope.demon.co.uk> scribeth thus
>In article ,
>   Paul D.Smith  wrote:
>> ...snip...
>
>
>> >> Also odd that this has only come to vogue in the past few years. Before
>> >> that the instruction was to run the cable along the boom and down the
>> >> cradle, even before decent double screened cable became commonplace.
>> >
>> > Perhaps, because double screened cable has more metal than cheap stuff,
>> > it has a worse effect.
>> >
>
>> If the cable is tightly taped to the boom, will it really have much
>> affect? 
>
>but at some point it will leave the boom and go off at an angle.  That's
>when it causes the effect on the polar diagram.
>

In broadcasting these days you have to model wit a NEC programme all metallic
objects within usually a wavelength, sometimes more, that might alter the
radiation pattern. I once did one for a tower block roof in SW London and what
should have been an ommni pattern off a vertical dipole was nothing but in
some directions it had peaks and nulls of around 15 dB!...
-- 
Tony Sayer

On Wed, 11 Jun 2008 21:24:53 +0100, tony sayer 
wrote:

>In article , charles <charles@charl
>eshope.demon.co.uk> scribeth thus
>>In article ,
>>   Paul D.Smith  wrote:
>>> ...snip...
>>
>>
>>> >> Also odd that this has only come to vogue in the past few years. Before
>>> >> that the instruction was to run the cable along the boom and down the
>>> >> cradle, even before decent double screened cable became commonplace.
>>> >
>>> > Perhaps, because double screened cable has more metal than cheap stuff,
>>> > it has a worse effect.
>>> >
>>
>>> If the cable is tightly taped to the boom, will it really have much
>>> affect? 
>>
>>but at some point it will leave the boom and go off at an angle.  That's
>>when it causes the effect on the polar diagram.
>>
>
>In broadcasting these days you have to model wit a NEC programme all metallic
>objects within usually a wavelength, sometimes more, that might alter the
>radiation pattern. I once did one for a tower block roof in SW London and what
>should have been an ommni pattern off a vertical dipole was nothing but in
>some directions it had peaks and nulls of around 15 dB!...

An 18 element aerial is quite big, and the cable will look quite
hideous coming down from the reflector to the mast.  I've looking at
other roofs with similar aerials and not one has it in this fashion.

Marky P.

"Paul D.Smith"  wrote in message 
news:1213178407.656.0@proxy02.news.clara.net...
> ...snip...
>
>
>>> Also odd that this has only come to vogue in the past few years. Before
>>> that the instruction was to run the cable along the boom and down the
>>> cradle, even before decent double screened cable became commonplace.
>>
>> Perhaps, because double screened cable has more metal than cheap stuff, 
>> it
>> has a worse effect.
>>
>
> If the cable is tightly taped to the boom, will it really have much 
> affect? Time for one of Bill's investigative reports I feel ;-).

Time for one of Bill's rambling woffles, I think you mean . . .

Regarding the Blake SR18, I take the dipole off before I fit the reflector, 
turn it round, then fit the reflector. This lets me bring the cable off in a 
tidy manner. It doesn't seem to affect performance.

Now I'm going to be a bit controversial. I have a suspicion, based on nowt 
concrete, that the need for a balun is much less if the cable leaves the 
dipole cleanly and is tightly fixed to the boom and mast all the way. I 
offer that as a topic for discussion, nothing more.

Bill

Bill Wright wrote:

> Now I'm going to be a bit controversial. I have a suspicion, based on nowt 
> concrete, that the need for a balun is much less if the cable leaves the 
> dipole cleanly and is tightly fixed to the boom and mast all the way. I 
> offer that as a topic for discussion, nothing more.

Well taken literally that's a nonsense because you'd effectively be 
connecting one side of the dipole to the boom, whereas the aim is to 
make the boom and the outer surface of the feeder as 'neutral' or 
'RF-cold' as possible.  A quarter-wavelength of coax in free air with 
the outer then  bonded to the boom might work though.

There's no getting away from the fact that an antenna with centre-fed 
dipole(s) and a coax feeder simply has to use some form of balun in 
order to work properly, and not radiate from, or pick-up on, the outer 
surface of the coax.  Any arrangement of the coax which achieves the aim 
of decoupling RF current on the outside surface of the coax from the 
wanted signal path _is_ /ipso facto/ a balun - e.g. feeding the coax 
though one of the 'booms' of a log-periodic.

It's quite instructive to experiment with this sort of thing.  You need 
to find a way of coupling RF to the outside of the feeder with some form 
of longitudinal current transformer.  For the aerial benchmarking tests 
an EMC absorbing clamp is used because it's a commercially available, 
standardised and calibrated coupling mechanism, but you could probably 
rig up a crude alternative by taping a few inches of wire to a piece of 
plastic tube just big enough to slide along the coax.  The clamp is used 
to inject RF current onto the outer of the feeder, the aerial is mounted 
out in the clear and pointed up at the sky, and the level of signal 
coming back down inside the coax is measured.  (Or in reverse - transmit 
RF normally through the feeder and aerial and use your clamp to sense 
how much RF there is on the outer of the coax.)  You can use your 
spectrum analyser and tracking generator to do swept measurements of course.

Doing this you'll soon discover that the baluns on commercial Yagi-oid 
TV aerials are far from perfect, and that the routing of the cable away 
from the driven element does have quite a significant effect on 
susceptibility to feeder pick-up - and hence, potentially, to impulsive 
interference (II) problems on DTT.  How much this matters in practice is 
a moot point though.  If there's plenty of signal (45+ dBuV, say) then 
II isn't likely to be too much of a problem and you can safely ignore 
any cable routing instructions.  If you're trying to make the most of a 
weak signal then it could be a different matter.  NB a masthead amp 
won't help here since feeder pick-up getting in due to imperfect balun 
arrangements gets in ahead of the amplifier.  (Though the amplifier does 
help swamp downstream II, getting in though poor coax, outlet plates and 
flyleads, etc.)

-- 
Andy

"Andy Wade"  wrote in message 
news:6bf3j7F3bsfokU1@mid.individual.net...
> Bill Wright wrote:
>
>> Now I'm going to be a bit controversial. I have a suspicion, based on 
>> nowt concrete, that the need for a balun is much less if the cable leaves 
>> the dipole cleanly and is tightly fixed to the boom and mast all the way. 
>> I offer that as a topic for discussion, nothing more.
>
> Well taken literally that's a nonsense because you'd effectively be 
> connecting one side of the dipole to the boom, whereas the aim is to make 
> the boom and the outer surface of the feeder as 'neutral' or 'RF-cold' as 
> possible.  A quarter-wavelength of coax in free air with the outer then 
> bonded to the boom might work though.

My suspicion is based on nothing more than the fact that when the downlead 
from an aerial is basically loose and is flapping about, the behaviour of 
the aerial is unpredictable. The effect is far greater than I would have 
thought possible from the mere presence of the cable itself. Fixing the 
cable to the boom and mast results in a tremendous 'tightening' of the 
aerial's behaviour. Likewise, many times I've done nothing more to an aerial 
than tape the loose cable to the mast and boom, resulting in a surprising 
improvement in performance.

>
> There's no getting away from the fact that an antenna with centre-fed 
> dipole(s) and a coax feeder simply has to use some form of balun in order 
> to work properly, and not radiate from, or pick-up on, the outer surface 
> of the coax.
Leaving aside the issue of pick-up on the cable, what effect do you think a 
balun has on gain, pure and simple?

I wonder if my remarks above are actually referring to the question of the 
phase relationship between signals received on the cable and those received 
on the active element(s)? If the strenngth of the former is significant this 
would explain why taping the cable to something metalic seems to have such a 
beneficial effect, in terms of making the aerial behave more 'rationally'

Bill

Bill Wright wrote:

> Leaving aside the issue of pick-up on the cable, what effect do you think a 
> balun has on gain, pure and simple?

That's not really a valid question.  You can't leave aside the issue of 
pick-up on the cable because without a balun the cable becomes part of 
the aerial.  I think that about all you can say is that the gain of the 
aerial without the balun could be either more or less than with it. 
Then there are other issues such as the insertion loss of the balun and 
the change in impedance conditions with it present or absent.  In 
principle you could go even further and include the effect of source 
impedance on the receiver's noise figure.

> I wonder if my remarks above are actually referring to the question of the 
> phase relationship between signals received on the cable and those received 
> on the active element(s)?

Yes, undoubtedly that's what's happening.  The feeder pick-up 
interferes, constructively or destructively with the signal from the 
intended aerial elements.  As Charles has noted this usually has a 
deleterious effect on the radiation pattern, often skewing the main lobe 
slightly off-axis and generally destroying any symmetry in the side and 
back lobes.  It also degrades the cross-polar discrimination - 
particularly for the most common case of a horizontally polarized aerial 
with a (mostly) vertical feeder.

> If the strenngth of the former is significant this would explain why 
> taping the cable to something metalic seems to have such a beneficial
> effect, in terms of making the aerial behave more 'rationally'

It certainly explains why taping the feeder to something _fixed_ 
stabilises the situation, but I'm not so sure why is should always give 
an improvement.  When you said

> many times I've done nothing more to an aerial than tape the loose
> cable to the mast and boom, resulting in a surprising improvement in
> performance.

did you mean (mainly) an increase in forward gain?

-- 
Andy

"Andy Wade"  wrote in message 
news:6bk0djF3bodu4U1@mid.individual.net...
> Bill Wright wrote:
>
>> Leaving aside the issue of pick-up on the cable, what effect do you think 
>> a balun has on gain, pure and simple?
>
> That's not really a valid question.  You can't leave aside the issue of 
> pick-up on the cable because without a balun the cable becomes part of the 
> aerial.  I think that about all you can say is that the gain of the aerial 
> without the balun could be either more or less than with it.
So it is a valid question, because you've just given a valid answer.

>
> did you mean (mainly) an increase in forward gain?
In the old days ghosting would often be reduced.

Bill