LF roll-off problem

"Ian Jackson" wrote in message
...
In message , Bill Wright
writes

"Ian Jackson" wrote in message
...


It would
therefore be unusual for the aerial itself to cause a sudden drop-off of
signal level at the LF end, below Ch23.

But see a special case:

http://www.wrightsaerials.tv/article...r-syndrome.pdf

I think what you're saying is that, at the LF end, where the reflector
becomes less than a halfwave, the forward gain suddenly 'starts to go into
reverse' (if I may use such a highly technical description). Interesting.

Yes, well, it seems that the aerial just stops being a yagi and becomes . .
.. err . . . a chunk of metal.

Bill

In message , Bill Wright
writes

"Ian Jackson" wrote in message
...
In message , Bill Wright
writes

"Ian Jackson" wrote in message
...


It would
therefore be unusual for the aerial itself to cause a sudden drop-off of
signal level at the LF end, below Ch23.

But see a special case:

http://www.wrightsaerials.tv/article...r-syndrome.pdf

I think what you're saying is that, at the LF end, where the reflector
becomes less than a halfwave, the forward gain suddenly 'starts to go into
reverse' (if I may use such a highly technical description). Interesting.

Yes, well, it seems that the aerial just stops being a yagi and becomes . .
. err . . . a chunk of metal.

Well, maybe not quite. When you have a too-small reflector, I suspect
that the dipole will think that the reflector is another director 'on
the other side', so the back-to-front ratio will fall rapidly,
coinciding with a marked reduction in forward gain. Taken to extremes,
the aerial would become bi-directional.
--
Ian

Getting at the aerial is no real problem, so I'll try moving it to see if
that gives improves things. By the way, I installed the aerial in 2004 (not
1994 as I said before - a senior moment..!).
Furthermore the aerial is a Gp A Televes X-range 8042, which I now see is
not generally regarded as a 'good buy' by many of you... Relevant?
The roll-off (poor ch22) appears at every outlet of the amp that feeds a
Freeview receiver. Thus I tend to think the problem is before the amp. Hence
with your advice - many thanks - the aerial is now my prime suspect.
-Roy-
in Ligh****er GU18...
..

"Ian Jackson" wrote in message
...
In message , charles
writes
In article , Roy
Hammond
rhammondATieeDOTorg wrote:
Before I start delving into this problem in earnest, I thought I'd see
if
anybody here can give me any pointers...Here goes:

The signal and quality readings on the lowest channel 22 for DTV mux2
from Crystal Palace are poor (the picture degrades from time to time),
whereas all other channels from 23 upwards are fine (signal and quality
readings perfect or near perfect, and no picture degradations seen).
This
problem with those digital services (especially affecting the ITV
programmes) is common to all the Freeview receivers I have.

I'ver been living with this problem for years now. I really ought to try
to sort it out! The advent of a new digital TV set this year really
prompted me to do something: we're watching digital now rather than
analogue!

I've a somewhat interesting setup here. Aerial (installed in 1994) is on
the only chimney that can 'see' Crystal Palace properly. Unfortunately
that chimney is at the other end of the house from the living room, so
there are some longish runs of coax to and from a distribution amplifier
in the loft feeding the living room, bedrooms and kitchen.
Significant...?

My key question is, is there any common factor that could cause a
drop-off just at the band edge? Could the aerial be the main suspect?
Could a poor joint somehere result in such an effect?

Certainly the aerial is the most likely cause of the poor signal. The
long
runs of co-ax will, generally, attenuate the lowest frequency the least.
However if there was somewhere on that long run with a severe kink in the
cable, that could cause probles with just the one channel - it would
depend
on the distance between the kink and the source point (aerial or
amplifier). It could also be that due to lack of screening, one of the
long
runs is picking up sufficient signal on one channel to partly cancel the
signal that is being picked up buy the aerial.

If you haven't any test equipment, I'd suggest trying to look with your
receiver at the signal before the aerial. That might mean taking the set
into the loft which might be difficult. If so, try joining the input from
the amplifier to any outlet - ideally the one with the shortest cable - to
see if the error still occurs on Mux2. At least, by trying the various
outlets you can eliminate the distribution amplifier and cables.

The gain of a UHF TV aerial is usually 'escarpment-shaped'. Starting at
the LF end of the frequency range, the gain rises progressively by a few
dB, reaching a peak at (say) a couple of channels below the HF end of the
frequency range. Above the peak, the gain drops very rapidly. It would
therefore be unusual for the aerial itself to cause a sudden drop-off of
signal level at the LF end, below Ch23.

However, what you might have is a 'null' in the RF field surrounding the
aerial. This happens where reflections partially null out the direct
signal. This effect can be very frequency-dependent, and moving the aerial
position by only a few inches may change things considerably. In some
circumstances, the null can be very sharp and deep. You can usually see
this effect on a spectrum analyser, if you observe simultaneously the
signal levels of all the channels 22 to 34 (or even 37). As you move the
aerial position and direction, the individual signal levels usually go up
and down like yo-yos, with some being high when others are low , then vice
versa. If this is causing your problem, then the most obvious cure would
be to move the aerial a little (if you can).
--
Ian

"Ian Jackson" wrote in message
...
In message , Bill Wright
writes
Yes, well, it seems that the aerial just stops being a yagi and becomes .
.
. err . . . a chunk of metal.

Well, maybe not quite. When you have a too-small reflector, I suspect that
the dipole will think that the reflector is another director 'on the other
side', so the back-to-front ratio will fall rapidly, coinciding with a
marked reduction in forward gain. Taken to extremes, the aerial would
become bi-directional.
--
Ian

I took the trouble to plot the response of one of these aerials on ch21 and
honestly it was hard to tell which was the main lobe. It was like a
starfish! Worse than you'd imagine. This was for VP; I didn't bother with HP
because my problem was Crosspool which is VP.

No balun of course.

Bill

"Graham." wrote in message
...


I'm going to go back to the 70s here, back to the infancy of varicap uhf
tuners. There were two makes we used, The mullard and the telefunken

The original ELC1043 did tune down to the 70cm ATV band without
modification
Pity about its performance though.

Can I nominate the TAA550 as the world's most unreliable Zener?

(Zener with delusions about being an IC)


The ZTK33B was certainly better.