[mythtvnz] Freeview-T UHF Antenna in strong signal area

[Stephen Worthington]

On Fri, 20 Apr 2012 22:59:50 +1200, you wrote:

>On 20 April 2012 22:45, Johan Schuld <johanschuld at gmail.com> wrote:
>>
>> Hi All,
>>
>> Continuing on my recent post about making my Nova-TD500 work properly, I have now observed the following.
>>
>> 1: HVR4000 = Perfect reception on all Channels
>> 2: Nova-TD500 = Poor reception, barely watchable
>> 3: HVR2000 = Reasonable reception, however TV3 is very bad or does not even work.
>> 4: Set Top Box = Perfect reception on all Channels
>>
>> Which makes me think that possibly it's "Dual" cards that need a better signal than a single tuner card, although to me it seems a bit unlikely.
>>
>> Anyway, as I have identified that my antenna, antenna position and cable are all in average condition, I decided to do the whole thing properly and replace the antenna and possibly add a masthead amp.
>>
>> I live in Te Atatu Peninsula, Auckland where UHF signal strength is supposed to be quite strong as it's close to the transmitter. Our house is fitted with a medium/low-gain antenna (kinda like this one, but maybe even smaller: http://www.freetv.co.nz/webapps/p/61418/52840/306850). No masthead amp, no diplexer fitted (so just one antenna connected). However, an VHF antenna above it. Line of sight very blocked by a big and dense tree 10m from the antenna.
>>
>> Now I'm not an antenna expert but I know enough about them to know these things can be quite critical. Simply fitting a high-gain antenna (a long one) with a masthead amp might just create a too strong signal. Well, at least with analog that was the case so I assume digital will have similar issues(?)  so I'm wondering what the right plan of attack is.
>>
>> If I look around the area, some houses have the really small antenna like we have, and some have a big high-gain antenna. I wonder if the big ones are placed because of similar DVB receiving issues? But they are a bit too expensive to just try it out. So I was thinking the following, in order of preference/trying out.
>>
>> 1: Get a medium/high gain antenna and mount and cable it properly. Get rid of the soon to be useless VHF antenna.
>> 2: Put a mast head amp in. (I like the Kingray ones)
>> 3: Relocate the antenna to the other side of the house which has line of sight and effectively divides the required cable length by 3 at least.
>>
>> Are there any antenna experts here which can comment?
>>
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>
>
>I'm no antenna expert, but a couple of thoughts:
>1) It's more than just signal which can be affecting the various tuner
>cards' performances - drivers and firmware being a potential biggie.
>2) Getting rid of VHF aerial and tree could help. I however, I assume
>you don't want the VHF aerial for FM radio reception, and that the
>tree won't be movable.
>3) Options 1 and 3 sounds reasonable and cheap. Probably save cash on
>option 2 for now.

I would have to agree with option 3.  UHF needs line of sight much
more than VHF, and is worse affected by trees.

You have to realise that digital channels rarely have "poor" reception
- it is usually good enough or none, that being the basic difference
between anything digital and anything analogue.  So "poor" reception
is usually a period of digital reception where the data is good enough
to produce a valid signal, then a period of dropout where the digital
data is too bad to be repaired, and then another good period.  The
difference in signal level and quality between receiving 100% and not
can be quite small, and wind in your tree is entirely sufficient to
cause that level of change.

The TD-500 seems to have less sensitivity (ie needs a higher signal
level) than some other tuners.  So a signal that is OK for the
HVR-4000 may not be for the TD-500.  It may only be just OK for the
HVR-4000, but just OK works for digital signals where for an analogue
signal you would get a degraded picture.  One explanation for the
lower sensitivity is that the TD-500 has dual tuners, and therefore
must have a splitter on the board.  Hence the signal level is
necessarily going to be reduced to each tuner.  The HVR-4000 only has
a single tuner, so that gets the full signal from its antenna cable.

You should also consider your cabling.  Do you have a splitter and
then cables from that to all of the tuners, set top boxes and the TV?
If so, then even a fairly good signal from the antenna will be
significantly reduced at each of the tuners.  Does the TD-500 work if
you directly plug it into the antenna without anything else connected
and no splitter?  Have you tried swapping around the cables?  Cables
are often the weak point in any setup and can be damaged (eg by
running the vacuum over them, or kids or the dog playing).

When you had the UHF antenna installed, did the installers replace the
cable from the roof, or just use the existing cable from the VHF
antenna?  If so, it may not be cable rated for UHF.  Check and see if
it is labeled RG-6, which is what it should be for UHF.

Also, the different UHF channels can be far enough apart that the
antenna is better tuned to some than others - typically the highest
frequency channel is received worst.  Since TVNZ is usually the lowest
frequency multiplex, it is usually best received.  Reception testing
should be done on the worst multiplex, usually Kordia.  You need to
check on the frequencies from your transmitter and find out which is
the highest.

Here in Palmerston North, before the recent frequency change, I used
to be able to receive TVNZ OK from rabbits ears, but always needed the
full antenna for Kordia and usually for Mediaworks.  I have not tested
with rabbits ears since the change, but I would expect better
reception on the new lower frequencies.