[mythtv-users] Antenna advise

[Mike Thomas]

On Thu, 23 Jan 2014 10:58:18 -0600
Barry Martin <Barry_Martin_3 at Q.COM> wrote:
...snippety snip...
> Each input requires a connection to the antenna, so a splitter will
> probably be required.  Splitters will reduced the signal level to the
> tuner, so if possible use the proper splitter -- don't use an 8-way
> splitter with four tuner inputs, for example.
> Device                                 dB Loss
> 
> Splitter, 2x                             - 3.5 dB
> 
> Splitter, 4x                             - 7 dB
> 
> Splitter, 8x                             -11 dB
> 
> Coax, RG-6/U QS, 100'         - 5 dB
> 
> Coax, RG-59/U, 100'             - 7 dB

Dear Chaps,

If you're getting into this level of detail you might like to think
about the cabling and connectors. I haven't any experience of the parts
you use in the USA, but here in the UK we're stuck with the awful
Belling Lee connectors. These babies are not impedance matched so if
you're desperate for signal level you might consider using proper RF
connectors everywhere you can.

I've tried F connectors but they are pretty crummy. I've had the
lowest loss from soldered BNC connectors, although you might prefer
higher-frequency variants like TNC or even N-series. The key is to
solder them neatly.

On the cabling front, you should appreciate that a digital signal will
be degraded by multi-modal transmission in an unsuitable co-ax. I've
had the best results with RG59B/U which has a solid inner, a foil outer
and an overall braid. If you can find a lower-loss cable which is not
too thick for your installation, is constructed in the same fashion as
RG59B/U and for which you can obtain soldered RF-matched connectors
for your frequency range you should use that in preference. Never bend a
cable more tightly than the manufacturer specifies (look for minimum
bend radius).

Use stranded cables for the final link from the wall socket to the
computer; anything else will fracture (and degrade the signal) long
before it breaks.

Barry is correct about the splitters. Use splitters which have the same
connectors as the cables you make up and only buy splitters which have
the same number of outlets as you're going to use. (They are available
in many different arrangements).

Some RF boosters produce voltage gain, which at RF yields
intermodulation products. These can result in lower-frequency
transmissions being super-imposed on the high-frequency channel you're
listening for. It's best if you can do without boosters, but if you
can't, buy one multi-output booster and put it at the first fork
in your cable or at the mast head. This means you won't need a splitter
at that point.

Now that mobile phones have started using TV frequencies, you might
benefit from inserting a filter before your first splitter/amplifier to
attenuate them.

But far and away the best advice for simple installations is to buy a
proper Yagi (the more elements the better), mount it on your roof with
good strong bracket designed for the purpose and point it towards the
transmitter. The more elements you have the greater its gain and
directionality. Hopefully this will result in less interference. A blob
of non-corrosive goo over the aerial terminals will help resist
corrosion.

In your case, I'd go with a two-aerial solution as proposed using
Yagis with fewer elements. If you choose wide-band aerials like
log-periodics you risk picking up too many other (possibly strong)
transmissions. I suggest mounting them on separate poles, but if not,
then spacing them out as much as possible on the one pole. I think it's
best to treat the aerials independently and connect them to their own
tuner cards. Any combining risks picking up interference, especially if
you use a booster.

You might need to borrow a signal level meter to set the direction
accurately enough. If you do, you should insert your mobile phone
filter before the signal level meter to avoid spurious results.

Hope this helps.

Cheers,

Mike.